Novel Boron Subphthalocyanines for Organic Electronic Devices

by

Jeffrey Stephen Castrucci

A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy

Department of Chemical Engineering and Applied Chemistry University of Toronto

© Copyright by Jeffrey Castrucci 2015

ii

Novel Boron Subphthalocyanines for Organic Electronic Devices

Jeffrey Castrucci

Doctor of Philosophy

Department of Chemical Engineering and Applied Chemistry University of Toronto

2015

Abstract

Boron subphthalocyanines (BsubPcs) are a class of organic semiconductor materials that have

been identified as having desirable properties for use in photovoltaic devices due to their strong

light absorbance and the flexibility to develop tunable chemical derivatives. In particular, a lack

of variety in available electron acceptors is an area where BsubPc derivatives can be readily

substituted into existing photovoltaic device architectures. There are, however, no metrics to

facilitate the rapid screening of different BsubPc derivatives. In this thesis, admittance

spectroscopy is used to measure charge carrier mobility of these BsubPc derivatives, and

photovoltaic cells are fabricated to evaluate these derivatives’ performance in devices. We find

that the measured electron carrier mobilities in thin films of BsubPc correlate with the single

crystal structural parameters determined by X-ray diffraction. We also find that for BsubPcs,

electron mobility measured by admittance spectroscopy is insufficient to predict photovoltaic

performance when BsubPcs are used as an electron accepting layer in a device. BsubPc

derivatives, however, are discovered as a new class of versatile molecules that can be designed

and synthesized for use in photovoltaic devices to harvest singlet fission derived triplet excitons

and consequently boost photovoltaic device photocurrent. This thesis also reports vacuum system

design and construction to address experimental challenges arising from dealing with low

solubility, high molar mass materials and limited amounts of high purity material.

iii

Dedication

This work is dedicated to the future users of MARI-KATE and to all the voidsmiths who struggle

daily to (as Trevor irreverently puts it) "keep the nothings in," so that humans can continue to

learn new things about the universe through vacuum enabled experiments.

The Rime of the Tired Vacuum System Student

Day after day, day after day,

We fab, yet not fast, no success, oh;

As idle as a fractured blade

Upon a busted turbo.

Nothing, nothing, every where,

But all the gaskets leaked;

Nothing, nothing, every where,

Yet still too much pressure to proceed.

Pump down, pump down, bake out again,

What I would give for ten to the minus seven;

Pump down, pump down, bake out again,

If only this system worked, I’d be half way to heaven.

Frequency shift, frequency drift, the Hertz tick by,

Cursed damnation;

Frequency shift, frequency drift, the Hertz tick by,

I question my QCM calibration!

By Jeffrey S. Castrucci (with apologies to Samuel Taylor Coleridge’s The Rime of the Ancient Mariner)

iv

Acknowledgments

While many of the experiments in this document happened in a vacuum chamber, my learning

process did not happen in isolation. I was gifted with the opportunity to learn from, and with,

extraordinary individuals that I am most thankful to have met in the Bender Lab and the Lu

Group, most especially: Dr. Brett Kamino, Dr. Graham Morse, Dr. Michael Helander, Dr. Benoit

Lessard, Grayson Ingram, Jessica Virdo, Katie Sampson, Jeremy Dang, Hasan Raboui, Stephanie

Nyikos, and my fellow voidsmiths Trevor Plint and David Josey. I also grateful to Peter

Brodersen for keeping the profilometer running and Phil Milczarek for processing innumerable

purchase orders and shipments, including the shipments that ended up obstructing the loading

bay.

I am most thankful to my parents and brother for their love through the inevitable ups and downs

of research, to my roommate for superhuman patience and for dispensing sage insights that I

sometimes managed to act upon, and to my friends for being there for a chat, a meal, board

games, or a run. Finally, I thank my supervisors Professor Zheng-Hong Lu and Professor

Timothy Bender for their guidance and without whom I would never have had this opportunity to

learn in so many areas beyond where my previous experiences lay.

v

Table of Contents

List of Tables...................................................................................................................................v

List of Figures.................................................................................................................................vi

List of Abbreviations......................................................................................................................ix

1 Introduction..........................................................................................................................1

2 Admittance Spectroscopy Experimental Procedures.........................................................26

3 Charge Carrier Mobility in Fluorinated Phenoxy Boron Subphthalocyanines: Role

of Solid State Packing....................................................................................................................35

4 Acceptor Properties of Boron Subphthalocyanines in Fullerene Free Photovoltaics.......53

5 Challenges of Mobility Determination and The Case for TPBi........................................79

6 Considerations for the Physical Vapor Deposition of High Molar Mass

Organic Compounds....................................................................................................................121

7 Construction of a Vacuum Deposition System...............................................................139

8 Characterization of µ-oxo-(BsubPc)2 in Organic Planar Heterojunction

Photovoltaic Devices...................................................................................................................160

9 Boron Subphthalocyanines as Singlet Fission Harvesting Materials within

Organic Photovoltaics..................................................................................................................181

10 Future Work.....................................................................................................................193

Appendix A: Chapter 3 Supplementary Information..................................................................A-1

Appendix B: Chapter 4 Supplementary Information..................................................................B-1

Appendix C: Chapter 6 Supplementary Information..................................................................C-1

Appendix D: Chapter 7 Supplementary Information..................................................................D-1

Appendix E: Chapter 8 Supplementary Information..................................................................E-1

Appendix F: Chapter 9 Supplementary Information...................................................................F-1

vi

List of Tables

Table 2-1........................................................................................................................26

Table 3-1........................................................................................................................39

Table 3-2........................................................................................................................45

Table 4-1........................................................................................................................63

Table 4-2........................................................................................................................68

Table 5-1........................................................................................................................85

Table 5-2........................................................................................................................91

Table 5-3........................................................................................................................94

Table 5-4......................................................................................................................102

Table 7-1......................................................................................................................155

Table 7-2......................................................................................................................158

Table 8-1......................................................................................................................169

Table 8-2......................................................................................................................172

Table 9-1......................................................................................................................186

vii

List of Figures

Figure 1-1.........................................................................................................................2

Figure 1-2.........................................................................................................................7

Figure 1-3.......................................................................................................................10

Figure 1-4.......................................................................................................................11

Figure 1-5.......................................................................................................................18

Figure 1-6.......................................................................................................................20

Figure 2-1.......................................................................................................................29

Figure 2-2.......................................................................................................................30

Figure 2-3.......................................................................................................................31

Figure 2-4.......................................................................................................................32

Figure 2-5.......................................................................................................................33

Figure 3-1.......................................................................................................................38

Figure 3-2.......................................................................................................................43

Figure 3-3.......................................................................................................................44

Figure 3-4.......................................................................................................................48

Figure 4-1.......................................................................................................................59

Figure 4-2.......................................................................................................................60

Figure 4-3.......................................................................................................................61

Figure 4-4.......................................................................................................................62

Figure 4-5.......................................................................................................................66

Figure 4-6.......................................................................................................................67

Figure 4-7.......................................................................................................................69

Figure 4-8.......................................................................................................................71

Figure 5-1.......................................................................................................................80

Figure 5-2.......................................................................................................................81

Figure 5-3.......................................................................................................................82

Figure 5-4.......................................................................................................................83

Figure 5-5.......................................................................................................................84

Figure 5-6.......................................................................................................................86

Figure 5-7.......................................................................................................................88

viii

Figure 5-8.......................................................................................................................89

Figure 5-9.......................................................................................................................90

Figure 5-10.....................................................................................................................92

Figure 5-11.....................................................................................................................93

Figure 5-12.....................................................................................................................97

Figure 5-13.....................................................................................................................98

Figure 5-14...................................................................................................................100

Figure 5-15...................................................................................................................101

Figure 5-16...................................................................................................................103

Figure 5-17...................................................................................................................104

Figure 5-18...................................................................................................................106

Figure 5-19...................................................................................................................108

Figure 5-20...................................................................................................................109

Figure 5-21...................................................................................................................111

Figure 5-22...................................................................................................................112

Figure 5-23...................................................................................................................113

Figure 5-24...................................................................................................................114

Figure 5-25...................................................................................................................115

Figure 5-26...................................................................................................................116

Figure 6-1.....................................................................................................................124

Figure 6-2.....................................................................................................................126

Figure 6-3.....................................................................................................................130

Figure 6-4.....................................................................................................................133

Figure 6-5.....................................................................................................................135

Figure 7-1.....................................................................................................................143

Figure 7-2.....................................................................................................................147

Figure 7-3.....................................................................................................................151

Figure 7-4.....................................................................................................................153

Figure 7-5.....................................................................................................................155

Figure 7-6.....................................................................................................................157

Figure 8-1.....................................................................................................................164

ix

Figure 8-2.....................................................................................................................166

Figure 8-3.....................................................................................................................170

Figure 8-4.....................................................................................................................173

Figure 9-1.....................................................................................................................184

Figure 9-2.....................................................................................................................188

Figure 10-1...................................................................................................................197

Figure 10-2...................................................................................................................203

x

List of Abbreviations

AFM atomic force microscopy

AS admittance spectroscopy

BHJ bulk heterojunction

CT charge transfer

EL electroluminescence

EQE external quantum efficiency

FF fill factor

FRET Förster resonant energy transfer

HOMO highest occupied molecular orbital

ILC injection limited current

JSC short circuit current density

LED light emitting diode

LUMO lowest unoccupied molecular orbital

MARI-KATE Most Awesome Restored Instrument - "Kontraption" for the

Assembly of phThalocyanine Electronics

OLED organic light emitting diode

OPV organic photovoltaic

OTFT organic thin film transistor

PCE power conversion efficiency

PHJ planar heterojunction

PL photoluminescence

PV photovoltaic

PVD physical vapour deposition

QCM quartz crystal microbalance

SCLC space charge limited current

SD standard deviation

TFT thin film transistor

TOF time of flight

UPS ultraviolet photoelectron spectroscopy

VOC open circuit voltage

xi

Chemical Names

AlPc aluminium phthalocyanines, the class of material

Alq3 tris(8-hydroxyquinolinato)aluminum

6T alpha sexithiophene

NPD N,N'-bis(1-naphthyl)-N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine

BCP bathocuproine

BsubPc boron subphthalocyanines, the class of materials

CBP 4,4'-N,N'-dicarbazole-biphenyl

Cl-BsubNc chloro boron subnaphthalocyanine

Cl-BsubPc chloro boron subphthalocyanine

Cl-Cl6BsubPc chloro hexachloro boron subphthalocyanine

Cl-Cl12BsubPc chloro dodecachloro boron subphthalocyanine

Cl4Phth-BsubPc tetrachlorophthalimido boron subphthalocyanine

F5-BsubNc pentafluorophenoxy boron subnaphthalocyanine

F5-BsubPc pentafluorophenoxy boron subphthalocyanine

F12-BsubPc phenoxy dodecafluoro boron subphthalocyanine

F16-CuPc hexadecafluoro copper phthalocyanine

F17-BsubPc pentafluorophenoxy dodecafluoro boron subphthalocyanine

ITO indium doped tin oxide

MoOx molybdenum oxide, indeterminate oxidation state

MsO-BsubPc mezyl boron subphthalocyanine

MTDATA 4,4',4"-tris[N,-(3-methylphenyl)-N-phenylamino]triphenylamine

Pc phthalocyanines, class of materials

PEDOT:PSS poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)

Pent pentacene

Phth-BsubPc phthalimido boron subphthalocyanine

SiPc silicon phthalocyanines, the class of materials

Tc tetracene

TiPc titanium phthalocyanines, the class of materials

TIPS-Pentacene 6,13-bis(triisopropylsilylethynyl)pentacene

TPBi 2,2′,2"-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)

1

Chapter 1 Introduction

The world is made of materials. Materials behave differently when exposed to electrical

stimulus, leading to classification in one of three categories. If a material readily moves

electrical charge in response to a difference in electrical potential, then it is classified as an

electrical conductor. If a material is very resistant to the motion of electrical charge when a

difference in electrical potential exists, then it is classified as an electrical insulator. If a

material's properties can be readily modified between a conductive state and an insulating state,

then it is classed as a semiconductor. The insight that the change in property of a semiconductor

could be used to perform logical operations is the fundamental insight behind the electronic

computer and the entirety of the modern electronics industry. In addition to their use in

computing, semiconductors find application in photovoltaic (PV) devices, where light is

converted into electricity, and light emitting diodes (LEDs), where electricity is converted into

light. Since its founding the middle of the twentieth century, the semiconductor electronics

industry has been dominated by inorganic semiconducting materials. The most important

inorganic semiconductor is silicon, used in the fabrication of all modern computers and most

commercial PV systems.

In the latter half of the 20th century, it was recognized that inorganic materials do not hold a

monopoly on semiconductor properties. The development of the first semiconducting organic

materials led to Alan J. Heeger, Alan G. MacDiarmid, and Hideki Shirakawa jointly receiving

the 2000 Nobel Prize in Chemistry. Their pioneering work in this field opened the door for an

entire universe of new materials to serve as semiconductors in electronic devices. Organic

materials are much greater in variety and options for modification of properties than inorganic

materials, meaning that a material can be custom designed to meet the demands of a specific

application, greatly expanding the toolkit available for designers to develop devices for specific

applications.

In the fifteen years since then, the number of organic semiconductors developed has greatly

increased and a variety of applications have been identified where these materials are optimally

advantageous. Three main types of electronic devices are the focus of developments for organic

semiconductors: organic photovoltaic devices (OPVs), organic light emitting diodes (OLEDs),

2

and organic thin film transistors (OTFTs). The structures are shown in Figure 1-1 and the

operation of each will now be briefly described. In all cases, the layer thicknesses for organic

semiconductors are generally on the order of tens to hundreds of nanometers, orders of

magnitude thinner (and less material) than the hundreds of micrometers used in inorganic

semiconductor devices.

Figure 1-1. Example structures for a planar heterojunction and bulk heterojunction organic

photovoltaic device (a), a planar heterojunction and an emitter layer containing organic light

emitting diode (b), and a top and bottom gate thin film transistor.

There are two typical OPV structures, the planar heterojunction and the bulk heterojunction.1 In

the planar heterojunction, an electron donating semiconductor (donor) and an electron accepting

semiconductor (acceptor) are present as distinct layers in the device with a clearly defined planar

interface. An alternate configuration is the bulk heterojunction, where the donor and acceptor

are mixed together in a single, interpenetrating, phase separated layer.2 This phase separated

mixture greatly increases the interfacial surface area between the two semiconductors. In either

case, at least one of the electrodes must be transparent to allow light to enter the device. Light

enters the device through a transparent electrode and is absorbed by one of the semiconductor

materials. This results in an excited state, referred to as an exciton, that can be passed from

molecule to molecule within the material. If this exciton reaches an interface between a donor

molecule and an acceptor molecule before relaxing back to the ground state, then an electron

transfer event occurs, resulting in a charge transfer (CT) state, where there is an additional

electron on the electron acceptor and an unoccupied electron state on the electron donor. This

3

CT state then dissociates into an electron charge carrier that moves among the acceptor

molecules and a hole charge carrier that moves among the donor molecules. The charge carriers

move away from the interface and migrate toward their corresponding electrodes where they are

extracted to do electrical work.3

The structure of a simple OLED can be quite similar, almost working in reverse to an OPV.

Charge carriers are injected at the electrodes (again, at least one electrode must be transparent to

allow light to exit the device), and migrate toward the donor/acceptor interface, where they

combine, eventually generating an exciton on one of the donor or acceptor molecules. When this

exciton relaxes, it generates a photon that may be emitted from the device. In more complex

OLED configurations, an additional semiconducting molecule, referred to as a dopant, is mixed

into one of the layers. The properties of the dopant are selected to make it a preferable location

for the excitons to relax to ground state and emit a photon.4

Finally, OTFTs are a rather different sort of semiconducting device, as they are three terminal

devices in contrast with the two electrodes of OPVs and OLEDs. OTFTs serve as logic gates,

the flow of charge from a source electrode through a semiconductor to a drain electrode is

controlled by the potential applied to a gate electrode that is separated from the semiconductor

by an insulating layer. The gate can either be on top or below the source to drain channel, but it

influences the number and sign of the charge carriers present in the semiconductor by inducing

an image charge in the semiconductor of the opposite polarity of the gate charge. The image

charge facilitates the flow of charge between the source and drain and is turned on or off by

modification of the gate electrode voltage magnitude and polarity.5

The unifying concepts for these three types of electronic devices are that semiconducting

materials are integral to their operation and the polarity of the charge carriers greatly impact the

performance of the device. The proper polarity charge carrier in the proper location makes the

device work, while the opposite polarity in the same place interferes with device performance.

Additionally, in all cases, the movement of charge carriers is fundamental to how effectively the

device will serve its function,6 so a quantification of this movement would be most helpful to

facilitate selection of the optimal material.

4

1.1 Purpose

This thesis was ultimately driven by the desire to develop quantitative screening techniques for

the evaluation of newly synthesized semiconductor materials for use in organic electronic

devices. The vision was that such a method would allow facile and prompt evaluation of the

potential of new materials without the requirement of extensive and time consuming iterative

device fabrication and engineering to identify the optimal layer configuration and thickness for a

new material. Further, a quantitative measure of electron mobility or hole mobility would be able

to guide future synthetic and crystal engineering efforts by helping to identify material properties

that impact the material's electronic properties and material properties that leave existing

electronic properties intact.

Our initial belief was that charge carrier mobility would be the material parameter most helpful

to quantify. Another parameter of interest would be quantification of exciton dynamics, possibly

by a technique such as pump-probe fast laser spectroscopy7 or transient absorption

spectroscopy,8 but these techniques are optical in nature, face challenges when probing complete

electronic devices, and we seek to examine the material as similar as possible to its state in a

functional device. While material energy levels are also a consideration, strategies to modify

energy levels are well known, so a knowledge of charge carrier mobility gives much greater

insight into the key electrical process of charge transport operating within a photovoltaic device

or other organic electronic device. For instance, if one observes a material where the electron

mobility was significantly greater than the hole mobility, this material would be a candidate for a

good electron acceptor material as it would transfer a much greater number of electrons from the

exciton dissociating interface to the anode than holes. Reduced hole mobility would mean fewer

holes present in the acceptor, which would lead to fewer recombination processes.

Recombination process rates are typically directly proportionate to the concentrations of the two

carrier species and are undesirable because they reduce the device's photocurrent. Conversely, a

material with a ratio of electron to hole mobility near unity (i.e. an ambipolar material) would be

a desirable interlayer in a cascade style photovoltaic architecture. Further, knowledge of carrier

mobilities for each layer within a device would allow detailed charge balance calculations to be

performed, enabling the prediction of layer thicknesses and ratios that would yield the most

diode-like properties. A computational approach using mobility might enable experimentalists to

minimize the need for iterative device fabrication to identify optimal device configurations for

5

each new material. The iterative approach is the current best practice in the field. There are

several techniques available for the measurement of charge carrier mobility, the relative merits of

these are explored below

1.2 Fundamentals of Charge Transport

To answer our hypothesis, outlined earlier in this chapter, we sought material dependent

properties that describe the electrical conductivity of newly synthesized semiconductor materials.

Thus, a brief discussion of charge transport in organic semiconductors is needed.

Fundamentally, the transport of charge in a semiconductor bears strong similarities to the

transport of mass, heat, and momentum that chemical engineers are much more broadly familiar

with.6 In all these processes, there is a flow of a property in response to a gradient in a related

property. Mass flows in response to a concentration gradient, heat flows in response to a

temperature gradient, and momentum flows in response to a velocity gradient. Similarly, charge

flows in response to a voltage gradient (a gradient in voltage is referred to as an electric field),

and can also flow in response to a concentration gradient. Starting from first principles, the most

basic equation we can consider to describe charge transport is:

(1-1)

where is the current density vector normal to the plane where we are describing the current

density, is the number of charged particles flowing past the plane where we are describing the

current density, is the charge per particle, and is the velocity component of the particles'

total velocity normal to the plane. Formally, we could describe and as vector quantities as

flow can occur spatially in three dimensions, but to simplify the analysis we will model the

devices as only involving current flow in one dimension, (perpendicular to the substrate flow

without any parallel to substrate flow). This avoids the complications of field concentration and

non-perpendicular current flow at the edges of the electrodes, which requires two additional

spatial dimensions to be included in the calculations. If the device area is large relative to the

device perimeter, this is a reasonable approximation. We shall adopt a sign convention of a

negative current density corresponding to a flow of negative charge in the positive direction of

our one dimensional system, see Figure 1-2.

6

The use of thin film transistors (TFT) or the van der Pauw method to determine charge carrier

mobility are fundamentally multiple spatial dimension charge transport devices that are

discussed in detail elsewhere.9 We contend that TFT are unsuitable for characterizing

photovoltaic materials because they measure charge transport parallel to a substrate and are

highly surface treatment dependent. Conversely, photovoltaic devices involve charge transport

perpendicular to a substrate and transport is a bulk process that primarily occurs far from any

surface treatment. For the van der Pauw method, carrier mobilities and concentrations are too

low to make this a viable technique.

1.2.1 Mobility and Diffusivity Transport Processes

As with the analogy in transport processes between mass, heat, and momentum, the phenomena

of charge transport also has its own idiosyncrasies. The most notable of these is that there are

two species of charge carriers present in a semiconductor that contribute to current flow:

negatively charged electrons in the conduction band and positively charged holes in the valence

band, both contribute to the flow of charge within a device.

One can thus identify two transport processes and two carrier species, so a more explicit

description of the current density than that offered by Equation 1-1. It should be a function of

four terms: 1. the flow of electrons due to the electric field, 2. the flow of electrons due to

concentration gradient, 3. the flow of holes due to electric field, and 4. the flow of holes due to

concentration gradient. This brings us to the more explicit Equation 1-2:

(1-2)

Where again, is the current density, is the charge carrier density, is the fundamental charge,

is the charge carrier mobility, is the electric field, is the charge carrier diffusivity, and is

the axis perpendicular to the plane where we are describing the current density. The subscript

corresponds to electron charge carriers and the subscript corresponds to hole charge carriers.

The four processes are illustrated schematically in Figure 1-2. Depending on circumstance, the

relationships are not necessarily linear, they is merely illustrated as such here for demonstrative

purposes.

7

Figure 1-2. Illustrative schematic of the four charge transport processes of electron and hole

flow in response to a voltage gradients and a concentration gradients. The sign convention

adopted in this work is also illustrated.

Of these variables, charge carrier density, electric field, and charge carrier density gradients are

device operating condition dependent, and the fundamental charge is a fundamental property of

the universe, so we are left with two mobilities and two diffusivities to yield four material

dependent properties influencing the current density achieved in a device. However, our task is

further simplified by the fact that charge carrier mobility and diffusivity are indelibly linked by

the Einstein Relation,

(1-3)

8

(discussed extensively in any introductory device physics or solid state electronics textbook),10

meaning that if we know the charge carrier mobility we can calculate the diffusivity or vice versa

by way of Boltzmann's Constant, , and the thermodynamic temperature, . This fact, coupled

with the fact that charge carrier densities are inherently low in organic electronic devices means

we will exclude charge diffusion and the currents resulting from those processes for the

remainder of the discussion. That leaves us with:

(1-4).

By considering Equation 1-4, we observe that if we attempt to determine mobilities from simple

current-voltage measurements, we are still faced with a significant barrier to determining the

charge carrier mobilities because we must deconvolute the contribution of the two types of

carriers to the current. This requires either a way to independently modify the charge carrier

concentrations (traditionally achieved by the creation of dopant and/or trap states when studying

inorganic semiconductors), or a way to ensure a single charge carrier is species is present in

orders of magnitude excess of the other, so that only a single species makes a meaningful

contribution to the current density. This second approach is the one we adopt, beginning with an

explanation of charge injection/extraction between a semiconductor and an electrode.

1.2.2 Injection Limited Current

Thus far we have only considered the transport of charge within a single material, but another

important charge transport phenomena is the transfer of charge across a material heterojunction.

For instance, between an electrode and a semiconductor. This process, charge injection and

extraction, is governed by the injection limited current (ILC) equation:

4 exp ⁄ (1-5)

where is the density of states in the organic film, is the barrier height, is the Boltzmann

constant, is the thermodynamic temperature, and is

⁄ 1 2 ⁄ ⁄ (1-6)

a function of reduced electric field, ,

9

f (1-7)

where ε is the dielectric constant of the semiconductor and is vacuum permittivity (a universal

constant).11 Equation 1-5 shows how charge injection is an exponential function of the injection

barrier height. If an electrode is selected so that it has a very small barrier for the injection of

one charge carrier, and a very large barrier for the other species of charge carrier, then a Single

Carrier Device (SCD) can be created where charge transport is facilitated almost exclusively by a

single species of charge carrier.12

1.2.3 Single Carrier Devices

1.2.3.1 Injection Limited Single Carrier Devices

Thus, in a single carrier device we actually have two processes functioning in series to cause

charge transport. The charge carriers are injected from the electrode and then transported across

the semiconductor. Thus in an electron only single carrier device:

⁄

(1-8),

where the first denominator is the resistance from the injection process (Equation 1-5) and the

second denominator is the resistance from the transport process (Equation 1-4). So if the

injection barrier denominator is large, Equation 1-8 reduces in the limit to Equation 1-4, while if

the transport denominator is large, Equation 1-8 reduces in the limit to Equation 1-5. These two

processes in series are illustrated in Figure 1-3.

10

Figure 1-3. A time series schematic depiction of the injection of a negative charge carrier,

followed by its transport through the bulk of a semiconductor. In (a), the electron (filled circle)

is in the electrode. In (b), the electron is then injected so it moves to the organic layer. In (c),

the electron then travels through the bulk of the organic layer toward the other electrode.

This then suggests two architectures for single carrier devices that could be used to determine

charge carrier mobility, an injection limited architecture or a transport limited architecture.12

Either architecture is hypothetically viable to allow us to again avoid further deconvolution

challenges when only having current-voltage measurements available. An injection limited

architecture would require a moderate barrier for the injection of one charge carrier species and a

large barrier for the injection of the other species, then knowledge of the density of states and

injection barrier would be the only other variables required to supplement a current-voltage

measurement to yield the mobility. The density of states can be estimated to within an order of

magnitude by the density of molecules in a single crystal of the compound, but the injection

barrier height is much harder to accurately determine. While order of magnitude is good enough

for the density of states to allow an order of magnitude estimate of the mobility, the barrier

height must be known with greater precision as its presence within an exponential function

means small differences in barrier height can change the calculated mobility from current-

voltage measurements by orders of magnitude. See Figure 1-4 for an illustrative example.

11

Figure 1-4. Three fits of the same current-voltage characteristic for an F5-BsubPc electron only

single carrier device. Depending on what the assumed barrier height is, the results vary by

orders of magnitude.

Here, measurement limits play a hindering role. The use of ultraviolet photoelectron

spectroscopy (UPS) is a powerful tool for measuring the energy levels of the occupied states of

electronic materials. However, these measurements only provide a precision of at best +/- 0.05

eV, so the comparison of a suitable electrode work function to the valence energy of a new

semiconductor material will have an error in the offset energy calculation of 0.1 eV. When an

entire material's band gap is only 2 eV, an instrument uncertainty of 0.1 eV to estimate an energy

level offset is significant.

For electron charge carriers, the uncertainty in energy level offset for injection barriers is even

larger. Electron charge carriers move through the mostly unoccupied conduction band energy

states. Since photoelectron spectroscopy can only probe occupied states, mostly empty

conduction band states are much more difficult to detect and the imprecision of estimates for

their energies are even larger. This makes the determination of valence band transport energy

12

level injection barriers essentially meaningless as the imprecision of the measurement (> 0.5 eV)

exceeds the range of relevant values (< 0.3 eV)

This problem of charge carrier mobility determination without prior knowledge of injection

barrier height has been discussed previously in the literature,13 and it was noted that injection

barriers of about 0.3 eV or less were typically small enough that bulk transport processes were

the dominant resistance with injection resistance being of negligible magnitude in comparison. It

has also been shown for hole injection barriers that if the work function of the electrode exceeds

the valence energy, then the smallest possible barrier is achieved, regardless of the magnitude of

the excess14 and it is anticipated that a similar trend exists for electron injection.

1.2.3.2 Transport Limited Single Carrier Devices

1.2.3.2.1 Space Charge Limited Current

Seeing the challenges of using injection limited architectures for single carrier devices, we now

turn to consider transport limited architectures. Transport limited architectures benefit from not

requiring precise knowledge of injection barrier heights. An estimates of energy levels (for

instance, from relatively straight forward electrochemical measurements such as cyclic or

differential pulse voltametry15) and the selection of electrodes that will provide a minimized

injection barrier for a single charge carrier is all that is required. The feature that measuring

charge carrier mobility can be done with an electrode that is likely to be used in an actual

photovoltaic is another notable benefit in contrast to injection limited architectures. We consider

an electron only single carrier device, but the same path of reasoning could be used to consider a

hole only single carrier device. Equation 1-4 simplifies to:

(1-9).

Now all we need to calculate the charge carrier mobility is to determine the charge carrier

density. The simplest way to achieve this is to consider a high field regime and assume space

charge limited current (SCLC) (i.e. the number of charge carriers is the most that can be

squeezed into the given volume when taking into consideration electrostatic repulsion), yielding

the Mott-Gurney Law:9

(1-10)

13

where is the vacuum permittivity (a universal constant), is the dielectric constant of the

material (easily determined from capacitance measurements), and is the thickness of the device

along the x-axis. In other words, we find that the charge carrier density is

(1-11),

so the carrier density varies with electric field, though in a simpler manner than encountered with

the injection limited case discussed previously. However, it is unlikely for an organic

semiconductor to enter the regime that corresponds to the simultaneous fulfillment of the

assumptions of a single sign of charge carrier, field independent mobility, zero trap density, and

zero injection barrier, the assumptions used to derive the Mott-Gurney Law.9 The zero trap

density in particular is challenging to achieve outside of single crystals of inorganic

semiconductors like silicon. The presence of charge carriers in traps reduces the density of

mobile charge carriers. Thus, use of the SCLC equation over-estimates the charge carrier density,

so mobility values derived from this approach will underestimate the mobility.

1.2.3.2.2 The Poole-Frenkel Effect

In most organic semiconductors, charge carriers typically spend most of their time immobilized

in trap states, with a standard Arrhenius equilibrium distribution of the population between the

immobile trap states and the mobile conduction states.16 Thus we find the population of carriers

is actually the sum of the trapped carriers and the mobile carriers:

, , (1-12)

where the ratio of trapped to mobile carriers is determined by the energetic depth of the trap

state:

,

,exp (1-13)

whereT is the energy of the trap depth).9 From this characteristic, we return to Equation 1-6

and break the carrier population into two parts, a mobile carrier concentration multiplied with the

electron mobility and a trapped carrier concentration multiplied with zero mobility. This yields

an equation that is empirically indistinguishable from Equation 1-5:

14

, , exp (1-14).

With a skillful application of temperature dependent current-voltage measurements (working

around the fact that mobility is also temperature dependent is why the data interpretation is non-

trivial) and an estimate of trap density (best controlled by introducing high trap density by way

of intentional doping), we might finally be able to ascertain the electron mobility. But nothing is

easy, because it turns out that it has been long known that the application of an electric field

causes a lowering in trap depth proportionate to the exponential of the square root of the electric

field. This is known as the Poole-Frenkel effect. Thus we are again faced with a charge carrier

density that changes as a function of electric field.

The standard approach in the literature is to ignore these vagaries, assume SCLC applies and

allow the increase in charge carriers to be captured as an electric field activated mobility,

yielding the modified Mott-Gurney Law:17

. ⁄

(1-15),

where is the zero field mobility and is the Poole-Frenkel coefficient. This is known as a

Poole-Frenkel type mobility. With this sort of fit, the mobility parameters are convoluted with

the trap density and trap energy parameters, while still suffering from the overestimation of

charge density previously mentioned as a major limitation of the SCLC approach in

semiconductors.

What we can conclude from this exploration of the ways to determine the charge carrier mobility

from steady state current-voltage measurements that steady state approaches require information

that is not accurately or precisely measureable that leads to making a series of assumptions that

greatly bias any calculated mobility. The calculations are so strongly influenced by assumptions

about trap density, trap energy distribution, and carrier density that a reasonable estimate of

mobility is impossible for new and poorly understood systems, as exemplified by the sets of

derivatives we seek to characterize.

15

1.2.4 Transient Charge Transport Measurements

There are a variety of transient techniques for measuring the charge carrier mobility of a

material. These include time of flight (TOF) and admittance spectroscopy (AS) measurements.

We will find that time of flight measurements mostly circumvent the charge carrier density

issues that hinder the steady state approaches addressed above, but the short timescales involved

in these measurements present their own challenges.

1.2.4.1 Time of Flight Measurement

Time of flight measurements18 involve applying an electrical bias to a semiconductor material

and then introducing excited states into the device by exciting either the semiconductor or a

complimentary absorbing material with a very quick laser pulse. The excited states are separated

into charge carrier pairs by the applied bias, and travel to opposite sides of the device. By

measuring the time between when the excitation even occurs and when the current generated by

the charge carriers has exited the device, and knowing the device thickness, we are able to

calculate the carrier velocity. The carrier velocity in combination with the field and device

thickness allows the calculation of the mobility. Mathematically,

(1-16)

(1-17)

where is the device thickness, is the carrier transit time, is the carrier velocity, is the

carrier mobility and is the electric field. As long as the carrier density is kept low (achieved by

control of the excitation laser intensity) no assumptions about carrier density are required. This

is a marked contrast from the steady state measurements previously described. However,

limitations related to inherent electronic noise and signal resolution mean that for materials with

mobility on the order of less than 1 cm2 V-1 s-1, where our materials are anticipated to perform,

films several micrometers thick are required to be able to detect the transients. Given typical

film forming yields on the order of 1 nm of film formed per milligram of material sublimed,

basic arithmetic indicates approximately 2 g of each new materials are required to perform a

single TOF deposition. Given new materials are typically first prepared in 100 mg batches, a 2 g

material requirement is infeasible. In addition to such masses being synthetically impractical in

16

the early stages of material development, most photovoltaic cells have tens of nanometers worth

of active material, so the properties of TOF films at least two orders of magnitude thicker may be

dominated by morphological features that are not present at photovoltaic device thicknesses.

Additionally, like most transient methods, TOF measurements struggle to identify transit times in

films with dispersive transport as transit times can be distributed across long timescales.

1.2.4.2 Admittance Spectroscopy

Admittance spectroscopy, also known as impedance spectroscopy,16 is a transient technique

involving periodic perturbation of a steady state single carrier device. It can be thought of as a

hybrid of traditional steady state and transient approaches. The technique was developed and has

been widely used in the fields of polymer electrolyte and conductive ceramic electrochemistry.

It allows the probing of charge transport processes across a wide range of frequencies to allow

the simultaneous identification of electrical processes on different timescales. For instance, in

fuel cell applications, contributions from fast electron transfer events can be observed along with

contributions due to slow ion diffusion events using the same technique.

More recently, this technique was introduced to the field of organic electronics.19 The basic

concept of admittance spectroscopy is that a direct current (i.e. constant) electrical bias is applied

to a device, and then this direct current bias is perturbed by an alternating current bias with a

magnitude less than the thermal voltage. The thermal voltage,

(1-18),

where is Boltzmann's constant, is the thermodynamic temperature, and is the fundamental

charge. The thermal voltage is 26 mV at 27 °C (roughly ambient conditions). The alternating

current is sequentially applied at a variety of frequencies. By measuring the capacitance

response at a variety of probing frequencies, minima in capacitance can be identified that

correspond to the carrier transit time. As with time of flight analysis, the combination of a

carrier transit time, a device thickness, and an applied bias allows the ability to calculate the

mobility of the carriers, independent of any charge carrier density assumptions. The detailed

mathematical justification is described elsewhere,19 and our specific experimental protocols are

described in Chapter 2. The application of admittance spectroscopy as a characterization

technique is a major focus of this thesis.

17

1.3 Boron Subphthalocyanines

Boron subphthalocyanines (BsubPcs) are a class of semiconducting organic molecules of

particular interest to the Bender Lab and the focus of considerable synthetic effort. First

synthesized in 1972,20 chloro boron subphthalocyanine (Cl-BsubPc) is a three lobed variant on

the much larger phthalocyanine (Pc) class of four lobed molecules. The first Cl-BsubPc crystal

structure was subsequently determined by single crystal X-ray diffraction two years later,21

where the unusual, for a Pc, conical/pyramidal molecular shape was confirmed. The molecule

was almost completely ignored for the next two and a half decades. Once Cl-BsubPcs began to

draw attention from synthetic chemists, the significant derivatization options available became

apparent with three main locations on the molecule being of primary interest: the axial position,

where moieties bond directly to the central boron atom, the terminal positions, where moieties

bond to carbon atoms in the peripheral phenyl ring, and the bay positions, another location where

moieties bond to carbon atoms in the peripheral phenyl ring. These locations are highlighted in

Figure 1-5. Several of my colleagues have also demonstrated an experimentally validated

molecular mechanics model that emphasizes the synthetic flexibility of the BsubPc molecule.22

Substitutions at the axial position tend to have minimal impact on the frontier orbital energies of

BsubPc derivatives, while substitutions of electron donating and electron withdrawing groups at

the terminal and bay positions influence the frontier orbital energies of these compounds. Much

like the famous 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) developed by John

Anthony (see next section),23 this means that crystal structure and electrical structure can be

manipulated almost independently through axial and peripheral synthetic handles. The

laboratory has developed a large library of BsubPc derivatives that are in need of testing and

ideally those test results can also guide future synthetic efforts.

18

Figure 1-5. The molecular structure of boron subphthalocyanine and the location and names of

the primary points for derivativization.

One of my colleagues has ably reviewed the use of BsubPcs in organic electronic devices,24 so I

will only highlight a few relevant results and note recent developments and limitations of the

work to date. A BsubPc compound (Cl-BsubPc) was first incorporated into an organic electronic

device in 2006, where it was used as a donor layer paired with fullerene (C60) as the acceptor

layer in an OPV.25 In the years since, Cl-BsubPc has been used as an acceptor layer,26 and an

ambipolar interlayer in OPVs.27 The only other BsubPc to receive a thorough testing in different

device architectures is the Bender lab developed pentafluorophenoxy BsubPc (F5-BsubPc),

which has been tested as hole and electron transport layers28 and as a dopant in an OLED,29 and

as a donor, acceptor, and interlayer in OPVs.30 Beyond these two derivatives, there remain

dozens of already synthesized BsubPc derivatives where the properties and device performance

of the material has not been explored due to the current time intensive approach to testing new

materials in the full gamut of electronic device applications. There remain few rational design

guidance criteria to facilitate the development of more effective derivatives.

1.3.1 The Precedent of TIPS-Pentacene

The molecule 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene), first reported by

Anthony et al. in 200123 is among the first examples of intentional modification of an organic

semiconductor's crystal structure through the addition of sterically bulky groups. The

19

triisopropylsilylethynyl groups disrupt the typical herringbone packing structure seen in

unmodified pentacene, instead resulting in a brick like packing structure which maximizes the

contact between the planar parts of adjacent pentacene molecules (where the delocalized electron

density is located) despite the bulky nature of the TIPS group. Measurements of single crystals

of TIPS-pentacene showed resistivity varied by up to four orders of magnitude between the

different crystallographic directions, indicating a large degree of charge transport anisotropy.

Solution cast films of TIPS-pentacene were found to have comparable resistivity to the middle of

the range of single crystal values, which in combination with X-ray diffraction data indicated

very crystalline films could be formed with similar resistivities to those observed in single

crystals. Ultimately, solution cast TIPS-pentacene films were demonstrated where the charge

carrier mobility of the films was only an order of magnitude less than the highest mobility

direction in the single crystal,31 and anisotropy in film transport was also demonstrated,32

ultimately being quantified as up to an order of magnitude difference in mobility between

perpendicular devices.33 These results emphasize that charge transport can be modified by

crystal engineering but that anisotropy is a major feature of highly crystalline films, so an

emphasis on measuring charge transport in a configuration as close to an actual device as

possible will yield the most relevant data.

1.3.2 Existing Literature Inconsistencies

In addition to the inherent limitations and assumptions related to the use of each of the

techniques for measuring charge carrier mobility, there is huge variability between reports and

between methods as to the actual value of the charge carrier mobility of the prototypical

subphthalocyanine; chloro boron subphthalocyanine (Cl-BsubPc). While we are the only ones to

report measurements for the mobility of a variety of subphthalocyanine derivatives, there are a

number of reports focused on Cl-BsubPc. As is shown in Figure 1-6, published values for the

room temperature electron and hole mobilities vary by orders of magnitude with no observable

pattern, including a report of a highly improbable negative Poole-Frenkel slope.

20

Figure 1-6. Reported mobility parameters for electrons and holes in Cl-BsubPc, presented as a

plot of zero field mobility (μ ) vs. Poole-Frenkel coefficient ( ). There is no consistency within

or between the techniques of thin film transistor measurements (TFT),34 space charge limited

current (SCLC),35-37 and admittance spectroscopy (AS).38

Since nearly the inception of the field, the challenges of differentiating a Poole-Frenkel type field

activation of the charge carrier mobility and a voltage activated injection barrier effect based

solely upon current-voltage measurements have been noted39 and alternatives sought out. In the

case of well defined band structures, such as those found in single crystal elemental and

bielemental semiconductor materials, the use of temperature dependent current voltage

measurements is sufficient to differentiate between these effects but the significant number and

width of tailing states present with organic semiconductor energy bands means that temperature

dependence is unable to separate these effects for organic semiconductors.40

21

1.4 Summary

In short, organic semiconducting materials hold considerable promise for use in electronic

devices due to the ability to synthesize custom molecular designed optimized for specific

applications, including organic photovoltaic devices, organic light emitting diodes, and organic

thin film transistors. We seek to develop quantifiable metrics to differentiate between boron

subphthalocyanine derivatives and direct the molecular design of new derivatives, as there is

currently much uncertainty as to its electronic properties of this material class and their optimal

application within various device architectures.

Because admittance spectroscopy is a relatively new and uncommon technique, in Chapter 2, we

review the experimental details of the technique, which is then employed extensively in Chapter

3 through Chapter 5. In Chapter 3, we report the mobility for a series of fluorinated phenoxy

boron subphthalocyanine derivatives and propose how these mobilities relate to their single

crystal structures. In Chapter 4, we report on the mobility and photovoltaic performance of

chloro boron subphthalocyanine and a related derivative serving as acceptors and propose

desirable characteristics for a more effective derivative. In Chapter 5, we report on difficulties

encountered with determining the mobility of additional subphthalocyanine derivatives and their

application in photovoltaic devices. In Chapter 6, we discuss device fabrication considerations

related to higher molar mass organic compounds and then in Chapter 7, we discuss the design

considerations and calculations that were used to develop the fabrication and testing system that

enabled the photovoltaic experiments reported in the subsequent chapters. Chapter 8 applies

these considerations to the testing of a high molar mass boron subphthalocyanine derivative by

assessing the material's performance in photovoltaic devices. In Chapter 9, we report on

derivative matching the design criteria articulated in Chapter 4 and articulate a vision for future

material development. Finally, in Chapter 10, we summarize our progress and propose future

directions to continue the line of inquiry into the development of new organic electronic

materials and improved device performance.

22

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26

Chapter 2 Admittance Spectroscopy Experimental Procedures

2.1 Admittance Spectroscopy Motivation

Admittance spectroscopy (AS) is a technique for measuring the admittance of an electronic

device.1 As indicated in Chapter 1, this technique is relatively new and uncommon in the field of

organic electronics, so here we will detail the technique which is then applied in Chapters 3

through 5. In particular, we will examine the literature precedent and validation with respect to

the much more common time of flight technique (TOF), then we will discuss the details of what

quantities measured and how these measured quantities are transformed into information about

charge transport. Finally, we include details of our own internal calibration standards and

experiments which validate our system relative to the published literature, before we move on to

applying the technique to new materials in the subsequent chapters.

2.2 Previous Certification in the Literature

Admittance spectroscopy has been recently applied to a variety of organic semiconductors to

determine charge carrier mobility for electron transport2 and hole transport.3-6 These results have

been verified to be in good agreement with TOF results conducted on thicker films of the same

material for electron transport in Alq3,2 and hole transport in NPD3 and MTDATA.5 Previous

certifications of the technique are shown in Table 2-1.

Table 2-1. Mobility determined by time of flight and admittance spectroscopy methods in the

literature.

Material (carrier) Zero Field Mobility, cm2 V-1 s-1

(Technique)*

Source

Alq3 (electron) 1 × 10-9 (TOF), 1 × 10-9 (AS) Ref 2

NPD (hole) 3 × 10-4 (AS), 2 × 10-4 (TOF) Ref 3, Ref 4

MTDATA (hole) 1 × 10-5 (TOF), 1.3 × 10-5 (AS) Ref 5

* TOF = Time of Flight, AS = Admittance Spectroscopy

27

2.3 Measured Quantities

To determine the mobility we measure the admittance with an admittance spectrometer while our

sample is isolated from the environment inside a variable temperature cryostat. The cryostat is

described in detail elsewhere,7 but we detail the admittance procedure here.

The admittance (Y) is defined as a complex number. The real part of the admittance is the

conductance (G) and the imaginary part of the admittance is the susceptance (B). The

susceptance is the product of the capacitance (C) and the circular frequency

2 f (2-1),

where f is the frequency. Algebraically, this is

B C (2-2).

The complete admittance is of the form

Y G C (2-3)

where is the square root of negative one. To measure the admittance, an electrical voltage

probing signal is applied to the device and then an electrical current response signal is measured

and from this information electrical parameters are extracted. The probe voltage is described by

three variables, the direct current component ( ), the alternating current amplitude ( ) and

the alternating current frequency (f). From this periodic probing voltage, a periodically varying

current is induced in the device. From this current response the conductivity and capacitance are

calculated. The conductance (G), is calculated as the time averaged current ( ) divided by the

DC voltage:

G (2-4).

The capacitance (C) is the displaced charge ( ) divided by the AC voltage:

C (2-5)

28

where the charge is the absolute value of the deviation of the current from the steady state value

during one half cycle of the AC perturbation,

Q I‐Iavg. /f

(2-6).

This analysis leads to a single data point, a triplet of the form (f, G, C) that the admittance

spectrometer outputs to a file. A representative schematic of this measurement data that the

operator does not see, but that is used to determine the admittance, is shown in Figure 2-1. We

then vary the between zero and a maximum voltage selected to ensure the total device

current remains below 1 mA, as that is the maximum current that our system accepts. In our

case, we typically select between one and two dozen voltages in this range to measure. For each

voltage we then measure the admittance at 200 logarithmically distributed frequencies between

40 Hz and 107 Hz (again, this is the frequency range of our system). This then leads to

somewhere on the order of a few thousand triplets that are carried on to the subsequent analysis.

This process is time consuming with about one triplet acquired per half second, testing across

two dozen voltages can take more than two hours. Additionally, the temperature is varied and

the entire and f varying process is repeated at a number of temperatures to illuminate the

thermal activation of the transport processes. The cryostat is left to equilibrate for half an hour at

each new temperature before measurements are taken. We typically focus on the range of

temperatures between 200 K and 350 K as this includes the entire range of terrestrial

temperatures.

29

Figure 2-1. Schematic of the probe and response signals for an admittance spectroscopy

measurement. The plots show a representative probe of VDC = 12 V, VAC = 10 mV, frequency =

105 Hz. The displaced charge, Q, and average current, Iavg, are also shown.

From the triplet of (f, G, C) the doublet of (f, ΔB) is calculated. Recall Equation 1-20 allows us

to calculate susceptance (B). At low frequency, C is typically invariant; from this we identify the

geometric capacitance (C ). Determining C allows the calculation of the negative

differential susceptance:

30

ΔB 2 f C C (2-7).

We then construct a curve from the points of the 200 measured frequency by plotting (f, ΔB).

A representative plot of ΔB vs. f at a single is shown in Figure 2-2. The peak in ΔB vs. f

occurs at a specific transit frequency, f , corresponding to the carrier transit time. From the

carrier transit time, the mobility for a given voltage ( ) can be calculated as an extension of

Equation 1-17,

(2-8).

Figure 2-2. A representative plot of negative differential susceptance vs. frequency plot. The

transit frequency, ft, is labeled.

Advantages of admittance spectroscopy include the ability to use much thinner films (hundreds

of nanometers instead of the thousands to tens of thousands of nanometers required for TOF).

Thinner films are closer to the thicknesses encountered in organic photovoltaic devices and

require much less mass of material to be synthesized for testing. However, like TOF, admittance

spectroscopy struggles to identify transit times when charge transport is dispersive (poorly

31

ordered).5 Despite these constraints, we have had some success with the characterization of

candidate acceptor materials using admittance spectroscopy.

2.4 Verification of Our System's Performance

Before starting the characterization of new materials by admittance spectroscopy, we tested our

system by calibration with capacitors of known capacitance and by measuring the well known

OLED semiconductor N,N'-bis(1-naphthyl)-N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine (NPD).

A calibration of the admittance spectroscopy system is shown in Figure 2-3. A 100 pF silver

mica capacitor was selected as this is in the typical range of capacitances for a 500 nm thick film

with dielectric constant 3 and an area of 2 mm2. This plot emphasize the importance of longer

delay times in measuring to ensure they system has time to reach its equilibrium capacitance

before measurements are taken. Also visible in this figure is the large amount of measurement

noise at low frequencies. This noise is greatly reduce by having the system average three

measurements and employing adjacent averaging (window size 5 data points, total data set size

200 frequencies).

Figure 2-3. Calibration of the admittance system against a 100 pF (+/- 1%) capacitor vs.

frequency. Inset, capacitance averaged between 102 Hz and 106 Hz with various measurement

delay times. Error bars show one standard deviation.

32

We constructed single carrier devices of the structure ITO/MoOx (5 nm)/NPD (1090 nm)/MoOx

(5 nm)/Ag (100 nm) to measure the hole mobility of NPD, mimicking the structure Nguyen et

al. employed to perform admittance measurements on NPD.3 Nguyen et al. reported a zero

field mobility (0) of 2.9 × 10-4 cm2 V-1 s-1, which is very similar to our measured value of 1 ×

10-4 cm2 V-1 s-1, and Tong et al.'s TOF derived value of 2 × 10-4 cm2 V-1 s-1.4 The structure of

NPD and our admittance derived mobility measurements are shown in Figure 2-4.

(a)

(b)

Figure 2-4. The molecular structure of NPD (a) and the admittance derived hole mobility as a

function of the square root of electric field (b).

From the measuring of a capacitor we attain high confidence in the absolute values of the

capacitances measured by our system, while from the successful replication of a mobility

measurement from literature, we attain high confidence in the frequency accuracy of our system

and our ability to convert this frequency data to physically accurate mobility values that are in

agreement with two different measuring techniques.

2.5 Thickness Dependence

When studying Cl-BsubPc (as reported in Chapter 4)8 we also measured the resistance and

charge carrier mobility for single carrier devices at a variety of thicknesses to check for the

Ohmic nature of the device structure. We fabricated devices over a range of thicknesses from

250 nm to 600 nm and measured the admittance spectrums of these devices. The resistance at 0

V extrapolated to zero thickness is statistically indistinguishable from zero at 0.05 confidence

33

level, so there is no apparent contact resistance interfering with the admittance procedure.

Though there is a slight reduction in measured zero field mobility with increasing thickness,

there was no experimentally relevant difference in the measured zero field mobilities (0) or

Poole-Frenkel Coefficients () for this range of thicknesses, which served to increase our

confidence in the measurement technique. These results are shown in Figure 2-5.

(a)

(b)

Figure 2-5. Resistance at zero voltage as a function of film thickness shows no contact resistance

(a) and mobility parameters of Cl-BsubPc from single carrier devices at several thicknesses.

Device structure: Glass/Al (50 nm)/Cl-BsubPc(Device Thickness)/TPBi (3 nm)/Al (100 nm) (b).

Error bars show the 95 % confidence interval.

34

References 1. Barsoukov, E.; Macdonald, J. R., Impedance Spectroscopy : Theory, Experiment, and

Applications. 2nd ed.; Wiley-Interscience: Hoboken, N.J., 2005; p 595.

2. Wang, Y.; Chen, J.; Dong, L.; Ma, D., Determination of Electron Mobility in Tris(8-

Hydroxy-Quinolinato) Aluminum by Admittance Spectroscopy. J. Appl. Phys. 2013, 114,

113703.

3. Nguyen, N. D.; Schmeits, M.; Loebl, H. P., Determination of Charge-Carrier Transport in

Organic Devices by Admittance Spectroscopy: Application to Hole Mobility Inα-Npd. Physical

Review B 2007, 75, 075307.

4. Tong, K. L.; Tsang, S. W.; Tsung, K. K.; Tse, S. C.; So, S. K., Hole Transport in

Molecularly Doped Naphthyl Diamine. Journal of Applied Physics 2007, 102, 093705.

5. Tsang, S. W.; So, S. K.; Xu, J. B., Application of Admittance Spectroscopy to Evaluate

Carrier Mobility in Organic Charge Transport Materials. J. Appl. Phys. 2006, 99, 013706, 1-4.

6. Hoping, M.; Schildknecht, C.; Gargouri, H.; Riedl, T.; Tilgner, M.; Johannes, H. H.;

Kowalsky, W., Transition Metal Oxides as Charge Injecting Layer for Admittance Spectroscopy.

Appl. Phys. Lett. 2008, 92, 213306.

7. Helander, M. G.; Wang, Z. B.; Greiner, M. T.; Qiu, J.; Lu, Z. H., Experimental Design

for the Determination of the Injection Barrier Height at Metal/Organic Interfaces Using

Temperature Dependent Current-Voltage Measurements. Rev. Sci. Instrum. 2009, 80, 033901.

8. Beaumont, N.; Castrucci, J. S.; Sullivan, P.; Morse, G. E.; Paton, A. S.; Lu, Z.-H.;

Bender, T. P.; Jones, T. S., Acceptor Properties of Boron Subphthalocyanines in Fullerene Free

Photovoltaics. J. Phys. Chem. C 2014, 118, 14813-14823.

35

Chapter 3 Charge Carrier Mobility in Fluorinated Phenoxy Boron

Subphthalocyanines: Role of Solid State Packing

Adapted with permission from Castrucci, J. S.; Helander, M. G.; Morse, G. E.; Lu, Z.-H.; Yip, C.

M.; Bender, T. P., Charge Carrier Mobility in Fluorinated Phenoxy Boron Subphthalocyanines:

Role of Solid State Packing. Crystal Growth & Design 2012, 12, 1095-1100. Copyright 2012

American Chemical Society. I conducted the electrical experiments with the help of MGH. I

processed and interpreted all admittance data and wrote most of the manuscript. GEM grew the

-F5-BsubPc crystal and wrote the crystal structure description. CMY taught me to operate and

aided in the collection of AFM characterization data. ZHL and TPB directed the research. All

authors approved the manuscript.

36

Chapter 3 Charge Carrier Mobility in Fluorinated Phenoxy Boron

Subphthalocyanines: Role of Solid State Packing

3.1 IntroductionThe potential to manufacture low cost organic electronic

devices from sublimable small molecules continues to attract research interest 20 years after the

initial proof of concept.1,2 One parameter important to the function of electronic devices is the

charge carrier mobility (µ), which can be measured via a number of techniques. These include

direct measurement in thin film transistors (TFT),3 and in specially constructed single carrier

devices using the time-of-flight (TOF) method4 or admittance spectroscopy (AS).5 The reported

electron mobility of small molecule based n-type (electron-transport or electron accepting)

materials ranges across many orders of magnitude and is typically field dependent. Films of the

common n-type charge conductor tris(8-hydroxyquinolinato)aluminum (Alq3) have a zero field

mobility (µ0 of~10-8 cm2 V-1 s-1 and a field sensitivity (commonly referred to as the Poole-

Frenkel coefficient, the slope of the field dependent mobility plot) of ~0.004 (V/cm)-1/2 as

determined using TOF.6 A common p-type charge conductor 4,4’-N,N’-dicarbazole-biphenyl

(CBP) has a µ0of~10-4 cm2 V-1 s-1 and a β of ~0.001 (V/cm)-1/2 also determined using TOF.7

Other common charge conductors, rubrene (~10-2 cm2 V-1 s-1),8 perfluoropentacene (~10-1 cm2 V-

1 s-1),9 and C60 (~1 cm2 V-1 s-1),10 are reported to have significantly higher mobilities measured

directly in a TFT.

The role of molecular packing and its influence on charge transport mobility has been

demonstrated with targeted synthesis11 and theoretical calculations.12 Fluorination has been

demonstrated as a viable method of guiding crystal formation (solid state arrangement)13 with the

goal of achieving molecular alignments which maximize the intermolecular overlap of frontier

orbitals,14 by directing -stacking15 while simultaneously inducing or enhancing n-type charge

transport characteristics16-19 and exciton diffusion lengths;20 all desirable characteristics for an

organic electronic material. Fluorination has the further advantages of improved thermal stability

while undergoing a physical vapor deposition process (sublimation) and once present in the final

device.

37

Boron subphthalocyanines (BsubPcs) are an emerging class of functional small molecules. Their

uncommon bowl shaped molecular geometry21 and uniqueness compared to the common

phthalocyanine have made BsubPcs the target of increasing interest and application including in

organic light emitting diodes (OLEDs) as emitters 22,23 and dopants,24 and in organic

photovoltaics (OPVs) as both donors,25 acceptors,26 and in dyad structures.27 Recently, graded

heterojunction solar cells utilizing chloro-BsubPc as a donor28 and planar heterojunction cells

with both donor and acceptor layers made from BsubPc compounds29 have been demonstrated.

Commonly open circuit voltages greater than 1 V and efficiencies of 2.7% - 4.2 % are achieved.

The basic optical and electrochemical properties of BsubPcs have been examined30 but the

discussion of the charge carrier mobility of BsubPcs has been limited to a determination in a thin

film transistor (TFT) configuration of a single BsubPc derivative (Cl-BsubPc).31 The

configuration and functioning of a TFT relies on the application of an electrical field something

not possible within an OPV device32,33 wherein zero field mobility is of importance.

In this chapter, we have examined the electron mobility of three fluorinated phenoxy boron

subphthalocyanine derivates (FnBsubPc, where n = 5, 12, or 17, Figure 2-1) using admittance

spectroscopy (AS). Using this technique we are able to extrapolate the zero field mobility of each

derivative. We discuss correlations between the measured mobility of each FnBsubPc, the field

dependence of the mobility and the crystal structure (solid-state arrangement) of each of these

compounds. The crystal structures of F12BsubPc and F17BsubPc have been previously

disclosed.23 This study however was enabled by the determination of a new crystal structure for

F5BsubPc (denoted as -F5BsuPc) obtained by diffraction of crystals obtained by train

sublimation.

38

Figure 3-1. Fluorinated boron subphthalocyanines (a) -F5BsubPc, (b) F12BsubPc, and (c)

F17BsubPc and their crystal packing. Within columns the intermolecular distances are 9.141 Å,

5.729 Å, and 5.589 Å, respectively (BsubPc fragment centroid-to-centroid distance).

39

3.2 Results and Discussion.

The synthesis of F5BsubPc, F12BsubPc, and F17BsubPc has been previously described.23 In our

original report the single crystals from which the solid state arrangement was determined were

grown from a mixture of vapor diffusion (F5BsubPc) and sublimation (F12BsubPc and

F17BsubPc). We have now successfully diffracted a single crystal of F5BsubPc grown by

sublimation. We denote this new polymorph as -F5BsubPc (original polymorph now denoted as

-F5BsubPc). Its determination on a crystal grown under sublimation conditions, conditions that

closely parallel the conditions under which organic electronic devices are fabricated, is an

enabling piece of data making this study and its conclusions possible. Selected crystallographic

data is summarized in Table 2-1 and detailed crystallographic appears in the supplementary

material accompanying this article. Briefly, -F5BsubPc (C30H12BF5N6O) formed a monoclinic

crystal (P21/C) with a, b, c = 14.2834(6) Å, 11.3575(2) Å and 115.1707(7) Å and =

99.7330(15)°. -F5BsubPc contains four molecules in each unit cell and a density of 1538 kg/m3.

Table 3-1: Selected crystallographic data for FnBsubPcs.

-F5-BsubPc23 -F5-BsubPc F12-BsubPc23 F17-BsubPc23

Chemical formula C30H12BF5N6O C30H12BF5N6O C30H5BF12N6O C30BF17N6O

Formula Mass 578.27 578.27 704.21 794.17

Method of Growth Solvent

diffusion

Sublimation Sublimation Sublimation

Crystal system Monoclinic Monoclinic Monoclinic Orthorhombic

Density (kg/m3) 1605 1538 1758 1912

Density (, mol/m3) 2775.52 2659.65 2496.41 2407.54

<d> (Å, calculated) - 8.547 8.729 8.835

a/Å 19.7225(4) 14.2834(6) 11.5399(7) 10.8303(3)

40

b/Å 10.3637(3) 11.3575(2) 10.6340(4) 15.0749(6)

c/Å 23.4591(6) 15.1707(7) 21.9972(13) 16.9006(6)

α/° 90.00 90.00 90.00 90.00

β/° 93.3870(15) 99.7330(15) 99.799(2) 90.00

γ/° 90.00 90.00 90.00 90.00

Unit cell volume/Å3 4786.6(2) 2425.62(16) 2660.0(2) 2759.29(17)

Temperature/K 150(1) 150(1) 150(1) 150(1)

Space group C2/c P21/c P21/c P212121

No. of formula units per unit

cell, Z

8 4 4 4

Absorption coefficient, μ/mm-1 0.128 0.126 0.169 0.200

No. of reflections measured 15196 19341 17217 20857

No. of independent reflections 5439 5515 6033 3539

Rint 0.0388 0.0702 0.0756 0.0541

Final R1 values (I > 2σ(I)) 0.0432 0.0540 0.0652 0.0474

Final wR(F2) values (I > 2σ(I)) 0.1048 0.1164 0.1350 0.1098

Final R1 values (all data) 0.0674 0.1134 0.1473 0.0808

Final wR(F2) values (all data) 0.1205 0.1450 0.1709 0.1274

Goodness of fit on F2 1.052 1.037 1.037 1.045

41

We have previously shown that -F5BsubPc assembles in the solid-state in a concave-concave

head-to-head arrangement similar to other phenoxy-BsubPcs34 and in contrast to the concave-to-

ligand packing observed for F12BsubPc and F17BsubPc.23 However the solid state arrangement of

-F5BsubPc resembles that F12BsubPc and F17BsubPc (Figure 2-1). We have a limited amount

of data to suggest this packing motif may be common to the general class of phenoxy-

dodecafluoro-BsubPcs (of which F12BsubPc and F17BsubPc belong).35 The crystal of -

F5BsubPc is less densely packed than -F5BsubPc (2659 mol/m3 compared to 2775 mol/m3).

Within the crystal there is a distinct interaction between one of the 6-membered rings of the

isoindoline lobe (C18/C19/C20/C21/C22/C23) of the BsubPc molecular fragments, with a

neighboring axial pentafluorophenoxy fragment (C25/C26/C27/C28/C29/C30) at a centroid-to-

centroid distance of 3.7873(14) Å (see Supplementary Information Figure S3 for atomic

numbering). The result is that each -F5BsubPc molecule within a column is spaced at 9.141 Å

(BsubPc fragment centroid-to-centroid, along the c axis, Figure 2-1a) and each column is spaced

at 8.023 Å. This arrangement is similar to what is seen in each of F12BsubPc and F17BsubPc

(Figure 2-1b-c) although sequential molecules are spaced at 5.729 Å and 5.589 Å and the

columns are spaced from one another by a distance of 11.661 Å, and 11.902 Å (BsubPc fragment

centroid-to-centroid) respectively. F12BsubPc and F17BsubPc each have a lower density than -

F5BsubPc, with F12BsubPc (2496.41 mol/m3) being more dense than F17BsubPc (2407.54

mol/m3).

For the determination of the charge carrier mobility of F5BsubPc, F12BsubPc and F17BsubPc we

considered the use of a standard TFT configuration or the use of a specialized device for either

time-of-flight (TOF) or admittance spectroscopy (AS) methods. Although the TOF technique is a

widely used method for the determination of charge carrier mobility, it requires films several

microns thick which are not only impractical to fabricate, but also may not accurately reflect the

transport characteristics of the films used in OPV and OLED devices which are an order of

magnitude thinner (~100 nm). Although using a TFT device configuration allows for the

determination of the charge carrier mobility of films ~50 nm thick, TFTs only yield the lateral

field effect mobility (charge moves laterally between the electrodes), which may also not be

applicable to OLEDs and OPVs wherein charge moves medial to the electrodes. We therefore

focused on the use of admittance spectroscopy (AS) as it allows for the determination of medial

charge carrier mobility as a function of electric field in films of a thickness of ~100 - 500 nm.

42

The theory behind the AS technique has been extensively discussed in the literature.36,37 In short,

the AS technique is based on the measurement of the frequency-dependent capacitance of an

organic thin film. The capacitance is found to exhibit a characteristic minimum as a function of

frequency, which is indicative of the carrier relaxation time (r) in the organic film. The dc

mobility (dc) for non-dispersive transport is then given by5

2

0.56dcr

t

V

(1)

where t is the device thickness and V is the applied voltage.

Single carrier electron transporting only devices of the structure Al (50 nm)/FnBsubPc (500

nm)/TPBi (3 nm)/Al (100 nm), were constructed by vacuum deposition on a 50 × 50 mm2 glass

substrate cleaned with a standard procedure of (sequentially) ALCONOX®, acetone, methanol,

and UV ozone. Individual devices were 2 × 1 mm2 in area. A TPBi layer was included for use as

an electron injection layer in an attempt to form an Ohmic contact.23 Using the same method,

single carrier hole only devices of the structure ITO/MoOx (0.7 nm)/FnBsubPc (500 nm)/Ag (100

nm) were constructed, but no consistent current voltage characteristics could be captured from

the devices. This may be indicative of instability toward oxidative processes related to hole

transport.23 Devices were fabricated in a Kurt J. Lesker LUMINOS® cluster tool with a base

pressure of ~10-8 Torr. Organic layers were deposited at a rate of 0.5 Å/s, metal oxide was

deposited at a rate of 0.3 Å/s, and the Al layers were deposited at a rate of 1.0 Å/s. Samples were

mounted in a vacuum cryostat38 and impedance measurements were collected using an Agilent

4294A as a function of temperature from 120 K up to 330 K. Electron mobilities were calculated

from the impedance data following the method of Tsang et al.5 Film thickness was determined

by spectroscopic ellipsometry39 using a Sopra GES 5E.

Figure 2-2a shows the frequency-dependent capacitance (C) of F5BsubPc single carrier devices.

At zero applied bias the capacitance is nearly constant with frequency and is representative of the

geometric capacitance (Cgeo). The increase in capacitance at high frequency is due to the parasitic

capacitance of the test fixture and does not affect the present analysis. With increasing applied

bias the characteristic minimum in the capacitance curve shifts to higher frequency, indicative of

the field dependent mobility of the molecule. Figure 2-2b shows the negative differential

43

susceptance –B = –(C–Cgeo) as a function of voltage for the same device. The characteristic

frequency fr=r-1, indicated by the arrows, shifts to higher frequency with increasing applied bias.

This trend indicates that the carrier relaxation time is shorter at higher applied bias and hence the

mobility increases with electric field. Additionally, the dielectric constant (r) can be calculated

from the geometric capacitance (Cgeo = r0A/d).

104 105 106 10710-2

10-1

100

101

102100

110

120

130

140

150

Applied Potential

0 V 9 V 12 V 15 V 18 V 21 V

-B

(S

)

Frequency (Hz)

Cap

acita

nce

(pF

)

Figure 3-2. Capacitance and negative differential susceptance as a function of frequency for

F5BsubPc at 300 K. The arrows denote the peak in negative differential susceptance used to

identify the carrier transit time.

Figure 2-3 shows the measured electron mobility as a function of the square root of electric field

strength ( F ) at 300 K for F5BsubPc, F12BsubPc and F17BsubPc. The mobilities as a function

of temperature for the three compounds are given in the Supporting Information for reference

(Figure S1). The charge carrier mobilities of this series of compounds follow a Poole-Frenkel

type field dependence,5

44

0 exp F , (2)

where is the charge carrier mobility, 0 is the zero field mobility, is the Poole-Frenkel

coefficient, and F is the electric field. F12BsubPc was found to have the highest zero field

electron mobility of the derivatives studied, though the mobilities are comparable at higher field

strengths with values in the range of ~10-4 cm2 V-1 s-1. Table 2-2 summarizes the key electrical

characteristics of all FnBsubPcs at 300 K.

0 200 400 600 800 1000 120010-7

10-6

10-5

10-4

F5BsubPc

F12

BsubPc

F17

BsubPc

Ele

ctro

n M

obili

ty µ

e,d

c (cm

2V

-1s-1

)

E1/2 [(V/cm)1/2]

Figure 3-3. A plot of field dependant mobility at 300 K. Solid lines are best fits for the data and

show the extrapolation back to obtain zero field mobility. Note the similarity in slope for the

F12BsubPc and the F17BsubPc.

45

Table 3-2. Electrical characteristics of the three fluorinated SubPcs at a temperature of 300 K

Compound Zero Field

Mobility, 0

(cm2V-1s-1)

Poole-Frenkel

Coefficient,

(V/cm)-1/2

Dielectric

constant, r

Electron

Mobility e

F= 0.01

MV/cm

Electron

Mobility, e

F=1 MV/cm

F5BsubPc 4 x 10-7 0.0076 2.7 8 x 10-7 8 x 10-4

F12BsubPc 2 x 10-5 0.0033 3.2 3 x 10-5 6 x 10-4

F17BsubPc 1 x 10-5 0.0031 3.3 1 x 10-5 2 x 10-4

It has been shown that molecules which pack in extended -stacked or -aligned arrays, and thus

form a continuous pathway for electrical conduction, have high charge carrier mobilities in

OTFTs.11 This generally results in a zero field mobility two to three orders of magnitude greater

than systems which rely upon hopping transport.40 With this in mind, the difference in zero field

mobility of the FnBsubPcs (Figure 2-2) may be explained by their differences in solid state

arrangements and, more specifically, the arrangement of their -electron systems.

Using computational methods, we can show that the distribution of the lowest occupied

molecular orbital (LUMO) density within the individual molecules is solely located on the

BsubPc molecular fragment for the LUMO through to the LUMO+4 regardless of structure (see

supporting information, Table S5). Therefore electron transport in the solid arrangement should

take place between BsubPc molecular fragments. In the case of -F5BsubPc, electron transport

might take place by hopping along one of two pathways each involving the BsubPc molecular

fragment. The first, is an intercolumn pathway whereby the electron hopping would be roughly

aligned with the plane defined by the crystallographic a- and b-axis at a distance of 8.023 Å

(Figure 2-1a). The second where electron hopping could occur along the columns of BsubPc

fragments aligned with the c axis at a distance of 9.141 Å (Figure 2-1a). Given the small

difference it is difficult to say with certainty which pathway electrons may take as they progress

through the solid. By comparison, within the crystals of F12BsubPc and F17BsubPc the distance

between BsubPc molecular fragments in the columns is 5.729 Å and 5.589 Å respectively,

whereas the intercolumn distances are 11.661 Å and 11.902 Å respectively (Figure 2-1b and 2-

46

1c). Due to this relatively large difference, one can infer that electron hopping would occur over

the shorter distance of 5.729 Å and 5.589 Å in the direction of the b- and a- crystallographic axis

for F12BsubPc and F17BsubPc respectively. These centroid to centroid distances of 5.729 Å and

5.589 Å are shorter than the 8.023 Å seen within the columns of -F5BsubPc. We must therefore

conclude that it is a combination of the distances between the LUMOs of neighboring BsubPc

fragments (as exemplified by the centroid to centroid distances) and their associated symmetry

along a crystallographic axis leading to a distinctive pathway through the solid which is most

important to achieve higher zero field charge carrier mobility –exemplified by F12BsubPc and

F17BsubPc which have the highest zero field mobility. By extension, the small difference in zero

field mobility between F12BsubPc and F17BsubPc may also be attributable to the differences in

centroid-to-centroid distances: 5.729 Å (0 = 2 x 10-5 cm2V-1sec-1) and 5.589 Å (0 = 1 x 10-5

cm2V-1sec-1) for F12BsubPc and F17BsubPc respectively.

The charge carrier mobility at 300 K of F5BsubPc is more sensitive to changes in the electric

field than F12BsubPc and F17BsubPc (see Figure 2-3). This difference can be explained with

theory.41 The slope () is an inverse measure of the required activation energy for the electron

transport process (i.e. a lower activation energy means greater sensitivity to electric field

strength, and thus a steeper slope) and is known to relate to molecular density by correlation to

molecular spacing (d),41 such that 1 2d . The average molecular spacing (<d>) can be used

to explain the difference in transport activation energy. The distance <d> is not related to any

crystallographic distances but can be estimated based on the measured molar density (ρ) of the

single crystals by <d> = (NA)-1/3, where NA is Avogadro’s number. We have estimated <d> for

F5BsubPc, F12BsubPc and F17BsubPc to be 8.547 Å, 8.729 Å and 8.835 Å respectively (Table 1).

Given the closeness of approximation for F12BsubPc and F17BsubPc similar activation energies

for charge transport can be expected and are reflected in their slopes (). Indeed this is the case

as we have measured for F12BsubPc and F17BsubPc to be 0.0033 and 0.0031 respectively. In

comparison, F5BsubPc with a significantly smaller average separation distance showed lower

activation energy for charge transport and we have measured a steeper slope (roughly twice as

large at = 0.0076).

We have demonstrated that the electrical performance of FnBsubPcs can be correlated to their

respective solid state arrangements and the associated metrics as determined from x-ray

47

diffraction of single crystals grown by sublimation. However the bulk films used for the AS

measurements, while fabricated under similar sublimation conditions, are not expected to be

single crystals and will contain grain boundaries. To examine the presence and frequency of

grain boundaries and the general film forming properties of the FnBsubPcs we grew 150 nm

thick films of each on glass substrate and examined the films using AFM (Figure 2-4). If we

consider that topology is caused by grain boundaries then less frequent changes in topology

mean fewer grain boundaries. As can been see from Figure 2-4, 150 nm thick films of FnBsubPcs

are relatively flat having no more than a 5-6 nm variation in their thickness across the ~5 m

sampling area. For charge transport, these continuous films are better than and in contrast to the

fiber-like film morphology reported for the related Cl-BsubPc on a potassium bromide

substrate42 or the pyramidal morphology found by Torres on copper substrate.43 Comparing the

three samples, films of F5BsubPc have considerably fewer grain boundaries than the films of

F12BsubPc or F17BsubPc. Since charge mobility across grain boundaries is going to be slower

than that through a crystalline domain, we can then conclude is that the mobility difference

between F5BsubPc and F12BsubPc/F17BsubPc is even larger in idealized thin films engineered to

have minimal grain boundaries. We can further conclude that the good film forming properties of

this series of FnBsubPcs does nothing to counter our conclusions made above with regards to the

correlation of the electrical performance and the metrics taken from the x-ray determined

structures of each compound.

48

(a)

(b)

(c)

Figure 3-4. AFM images of 150 nm thick films of (a) F5BsubPc, (b) F12BsubPc and (c)

F17BsubPc on glass.

49

3.3 Conclusion.

In conclusion, the electron mobilities for a series of fluorinated phenoxy boron

subphthalocyanines – F5BsubPc, F12BsubPc and F17BsubPc – were measured in single carrier

devices made from vacuum deposited thin films using AS. While F5BsubPc was found to have

the lowest zero field mobility within the set, its mobility increased much more significantly in

response to an increasing electric field. The dependence of mobility on applied field was

correlated to the average molecular spacing (<d>). All three compounds were found to have

electron mobilities of at least ~10-4 cm2 V-1 s-1 at high electric fields and the differences can be

attributed to differences in solid state arrangement of each compound. Based on these

observations, we can propose desirable solid state packing characteristics to achieve high charge

carrier mobility in a BsubPc while simultaneously having a minimal field dependence: the

BsubPc compound should have good alignment (close association) of its molecular orbitals in a

one-dimensional column (close intra-column distance) while each column should be relatively

separated from one another (larger inter-column distance). The larger inter-column distance

leads to an increase in the average molecular spacing <d> and a smaller Poole-Frenkel parameter

(). Additionally, the BsubPc should have good film forming properties (on the order of 100-500

nm) with minimized inter grain boundaries. We are currently working to design and engineer

additional BsubPc systems which have these characteristics such that we can further support this

hypothesis.

50

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53

Chapter 4 Acceptor Properties of Boron Subphthalocyanines in Fullerene

Free Photovoltaics

Adapted with permission from Beaumont, N.; Castrucci, J. S.; Sullivan, P.; Morse, G. E.; Paton,

A. S.; Lu, Z.-H.; Bender, T. P.; Jones, T. S., Acceptor Properties of Boron Subphthalocyanines in

Fullerene Free Photovoltaics. J. Phys. Chem. C 2014, 118, 14813-14823. Copyright 2012

American Chemical Society. This chapter was originally published as NB and I are co-first

authors who contributed to equally to all parts of the manuscript. I conducted and analyzed the

admittance experiments while NB conducted and analyzed the optical and AFM experiments.

NB performed the majority of the PV experiments, but NB and I contributed equally to

interpreting the PV data. I was sole originator of the singlet fission discussion as this was added

later in response to reviewer commentary. PS conducted some of the PV experiments and gave

feedback that facilitated manuscript integration. GEM and ASP grew the Cl-Cl6BsubPc crystal

and wrote the crystal structure description. ZHL, TPB, and TSJ directed the research. All

authors approved the manuscript.

54

Chapter 4 Acceptor Properties of Boron Subphthalocyanines in Fullerene

Free Photovoltaics

4.1 Introduction

Small molecule organic photovoltaics (OPVs) have received significant interest over the past few

years with power conversion efficiencies now reaching ~ 12 %.1 In order to further increase

efficiencies, new materials,2-3 electrodes4 and structures5 are being investigated in the hope of

competing with existing solar energy harvesting technologies whilst also reducing costs thus

making solar cells economical for the average consumer. A limitation of most small molecule

OPV devices is the low spectral coverage in comparison to inorganic photovoltaics. In order to

improve power conversion efficiencies there must be a low spectral overlap between paired

materials and preferably absorption into the infra-red part of the spectrum to increase solar

energy harvesting, which should manifest itself as increased current density from the OPV.

Currently, there are a limited number of electron-acceptors commonly used within the field of

small molecule OPVs, and they are generally fullerene (C60/C70) based. A few polymer based

acceptors, such as F8TBT (poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-

2,1,3-benzo-thiadiazole]-2,2diyl)) have shown reasonable performance of ~ 2 %.6-8 A number of

moieties, including benzothiadiazole and perylene diimide based small molecules have been

reported, but this work is primarily in bulk heterojunction architectures and paired with standard

polymer donors.9-10 Recent work has reported efficiencies of ~ 4 %,11-13 but these results

continue to suffer from low external quantum efficiency (EQE) relative to analogous soluble

fullerene derivative based cells, indicating that there is still much opportunity for improvement.

Although C60 is a good electron-acceptor material, it is not strongly absorbing and therefore does

not contribute largely to the photocurrent. C70 has an improved absorption range but is extremely

expensive (~ by approximately 10 times).14-15 High efficiency single junction devices also

depend on a well-tuned interface gap (Ig = Highest Occupied Molecular Orbitaldonor – Lowest

Unoccupied Molecular Orbitalacceptor) at the heterojunction. C60 based acceptors have low lying

LUMO levels which drives efficient charge separation, but reduces the maximum Voc achieved

when paired with oligoacene donors. This difference in LUMO levels (ΔLUMO) between the

donor and acceptor is extremely important when suitably matching materials. Despite having

55

similar efficiencies, the open circuit voltage (Voc) in Tc / C60 cells is almost double that of Pent /

C60 (0.74 V vs. 0.4 V) due to the larger interface gap. Through decreasing the ΔLUMO and

increasing the Ig the achievable Voc will increase, potentially resulting in a larger PCE. Long term

stability is another concern for OPVs and C60 is known to readily photo-oxidise which results in

a gradual reduction in the observed conductivity.16

Recently, interest in replacing fullerenes in OPVs has increased and steady progress is being

made to find replacements with equal or improved power conversion efficiencies (PCEs).2-3

Phthalocyanines (Pcs) are remarkably stable and strong light absorbing materails and have thus

been identified as suitable candidates for use in OPVs.17-18 Boron subphthalocyanines (BsubPc)

is a unique form of phthalocyanine and is emerging as a viable class of organic electronic

materials with preliminary research into understanding the structure-property relationships

between functionalization, solid state arrangement and electronic properties recently

published.19-21 Modified boron subphthalocyanine chlorides have been previously shown to work

well as acceptors. Sullivan et al. replaced C60 with a peripherally chlorinated boron

subphthalocyanine chloride (hexachloro boron subphthalocyanine chloride, Cl-Cl6BsubPc)

alongside a boron subphthalocyanine chloride (Cl-BsubPc) donor and reported a large increase in

Voc from 1.10 V to 1.31 V for a single junction `all-BsubPc` cell.2 A perfluorinated analogue

synthesised by Gommans et al. increased the Voc in the majority of cases when C60 was replaced

but, in contrast to the work by Sullivan et al., did not manage to retain a similar PCE.3

Subsequent work using Cl-Cl6BsubPc as an acceptor paired with a subnaphthalcyanine donor

resulted in a significantly elevated PCE.22

We have previously shown that the Cl-BsubPc23 and pentafluorophenoxy-BsubPc24 can be used

as acceptors, although Cl-BsubPc is more commonly used as a donor material paired with C60.25

By replacing C60 with Cl-BsubPc in Tc devices an increase in short circuit current (Jsc), Voc and

PCE is achieved, clearly demonstrating that Cl-BsubPc can also serve as an acceptor.

Oligoacenes, such as tetracene (Tc) and pentacene (Pent) have shown promise for use in organic

electronics such as organic thin film transistors (OTFTs) and OPVs due to their optoelectronic

properties resulting for their planar fused aromatic ring systems and crystalline bulk structures.

Their structural properties allow for high charge carrier mobilities ( > 1 cm2 V-1 s-1)26 and

relatively long exciton diffusion lengths ( ~ 60 nm)27 and have been successfully applied as

56

donor materials in OPVs. Pent and Tc when used in combination with C60 can give power

conversion efficiencies (ηp, PCE) of ~ 2%.23, 28-31

Pent and Tc are also notable for their ability to split one photoexcited singlet state into two triplet

states32-33 in a process known as singlet fission. The singlet fission process has led to quantum

efficiencies exceeding 100% when paired with C60 in highly engineered OPV devices.34 The

number of known singlet fission capable materials is limited,35 and the number of materials that

have been paired with singlet fission capable materials to yield triplet harvesting photovoltaic

devices are equally sparse,36 with but a handful of organic semiconductors reported.37 Thus the

desire to identify more pairings of singlet fission capable materials with additional triplet

dissociating organic acceptors at this point in time is clear.

In this chapter we compare the use of BsubPcs as acceptor materials to C60 with reference to the

previous published work on Tc / Cl-BsubPc devices. We show that Cl-BsubPc exhibits dual

donor / acceptor character by investigating charge mobility through admittance spectroscopy and

photoluminescence quenching. Although charge carrier mobility is identified as a key

characteristic in developing new nonfullerene acceptors,38 with a mismatch between donor and

acceptor mobilities known to detrimentally impact the fill factor,39 there are few reports on the

charge carrier mobility of BsubPcs, so we begin to address this shortfall by measuring the

electron mobility for both Cl-BsubPc and Cl-Cl6BsubPc using admittance spectroscopy. We

examine the mobility of Cl-BsubPc and Cl-Cl6BsubPc in light of the long known40 and recently

confirmed41 crystal structure of Cl-BsubPc and the crystal structure of Cl-Cl6BsubPc.42

Photovoltaic behavior is exhibited in Pent / BsubPc devices and we show a large increase in Voc

attributed to the increase in Ig of the system. However, a dramatic drop in photocurrent for this

pairing is also observed and explained by the relatively poor ability of BsubPcs to harvest triplet

energy from Pent compared to C60. That being said, we have observed for the first time the

ability of a BsubPc, Cl-Cl6BsubPc to harvest triplet energy from Pent. In Tc devices the

replacement of C60 also results in improved cell stability in air, as indicated by measured device

parameters.

57

4.2 Methodology

All OPV samples were grown on commercially available indium tin oxide (ITO) coated glass

substrates (Thin Film Devices, 145 nm thick, Rs < 15 Ω sq-1) after cleaning by sonication in

acetone, detergent, water and isopropanol, and an ultraviolet / ozone treatment decontamination

system to remove carbon residues (Novascan PSD-UVT).

Photoluminscence Spectroscopy samples were grown on quartz substrates and PL measurements

were taken using a Horiba Yvon with excitation wavelengths 465 nm for Tc samples.

All AFM images were taken using an Asylum Research MFP-3D in tapping mode. MFP-3D

software (based on Igor Pro) was used to reconstruct and analyse the images. The images were

5um x 5um and the samples were on ITO substrates (Thin film solids) with a donor layer

(pentacene or tetracene) thickness of 60 nm and an acceptor thickness of 5 nm.

Admittance spectroscopy devices were fabricated in a Kurt J. Lesker Luminos cluster tool with a

base pressure of ~10-8 Torr. Cl-BsubPc and Cl-Cl6BsubPc used for admittance spectroscopy

were synthesised by modification of a previously reported method,43 and purified twice using

train sublimation44 before deposition. The TPBi was purchased from Lumtec and used as

received. Organic materials were deposited at a rate of 0.5 Å/s and metal contacts were deposited

at a rate of 1.0 Å/s in a separate metallization chamber that the sample was transferred to in

vacuo. Metal electrodes were deposited through a shadow mask, forming devices with an area of

1 mm × 2 mm. The sample was transferred through ambient to a vacuum cryostat where

impedance spectroscopy measurements were done with a Agilent 4294A Impedance Analyzer

and custom LabView software. Film deposition rate was monitored by a quartz crystal monitor

and film thicknesses were measured by step edge method using a KLA-Tencor P16+ surface

profilometer set to an applied force of 2 mg and a scan speed of 10 um/s.

The OPVs were fabricated using a Kurt. J. Lesker Spectros vacuum evaporation system. The

organic materials Tc (Acros, 98%), C60 (Nano-C Inc., 99.5%), were purified using thermal

gradient sublimation before deposition and Cl-BsubPc (Lumtec), bathocuproine (BCP, Aldrich,

96%) and molybdenum oxide (MoOx, Aldrich, 99.99%) were used as received. The Cl-

Cl6BsubPc was prepared by a previously reported method.2 The aluminium electrodes were

deposited in situ by evaporation through a shadow mask to a thickness of 100 nm to give an

58

active pixel area of 0.16 cm2. Current density - voltage (J-V) characteristics were recorded using

a Keithley 2400 sourcemeter with simulated AM 1.5 G solar illumination at 100 mW cm-2 (1

sun) from a Newport Oriel solar simulator. External quantum efficiency (EQE) measurements

were obtained using a Sciencetech SF150 xenon arc lamp and a PTI Monochromator. The

monochromatic light intensity was calibrated with a Si photodiode (Newport 818-UV) and

chopped at 500 Hz. Signal detection was performed with a current-voltage amplifier (Femto

DHPCA-100) and lock-in amplifier (Stanford Research SR 830 DSP).

All p values were calculated using a two tailed z-test. Uncertainties are reported as ± one

standard deviation of the sample mean.

4.3 Results and Discussion

The materials used in this study, their respective reported frontier orbital energy levels2, 28, 45-46

and their respective UV-vis electronic absorption spectra are shown in Figure 4-1. First by

considering the energy levels of the Tc / C60 systems, (Figure 4-1f) and by replacing C60 with

Cl-Cl6BsubPc or Cl-BsubPc there is a corresponding increase in the interface gap from 1.2 eV to

1.7 eV and to 1.9 eV, respectively. Due to the large increase in Ig for the Tc / Cl-Cl6BsubPc

system we would expect an increase in Voc for this cell as seen previously in the Tc / Cl-BsubPc

system.23 A similar increase in Voc is also expected for the Pent / BsubPc devices. In the Tc / Cl-

BsubPc system there is reduced spectral overlap between the donor / acceptor (D/A Figure 4-1g)

and since the electronic absorption spectrum for the Cl-Cl6BsubPc is almost unchanged from the

underivatised Cl-BsubPc we would also expect to see photovoltaic behaviour resulting with

increased Jsc relative to the Tc / C60 system for these devices. In the Pent / C60 there is little

overlap but the Pent / BsubPc systems have increased spectral overlap between the donor /

acceptor (D/A) and this may result in a decrease in Jsc in the Pent / BsubPc devices relative to the

Pent / C60 devices.

59

Figure 4-1. Molecules used within this study: (a) Tetracene,23 (b) Pentacene,45-46 (c) Cl-

BsubPc,23 (d) Cl-Cl6BsubPc,2 (e) C60,23 (f) their reported energy levels, and (g) their UV – Vis

absorption spectra.

4.3.1 Photophysical and Morphological Characterization

The PL quenching of Tc emission by the BsubPcs was investigated with comparison to C60

(Figure 4-2) to indicate the acceptor character of the BsubPcs. In a vacuum evaporated bilayer

structure (60 nm of Tc and 10 nm of BsubPc) PL quenching of ~ 30 % of the peak emission at

530 nm is observed from the Tc when using Cl-BsubPc (and Cl-Cl6BsubPc) as the acceptor

whereas C60 (10 nm) quenches ~ 15 % of the peak. This effect is not attributed to optical effects,

such as a decreased light absorption of the Tc, which would also result in a decrease in emission

(discussion shown in the Supporting Information). The peak at ~ 620 nm shows a decrease in PL

intensity (~ 15%) due to the quenching effect of C60 but there is an increase with the Tc / BsubPc

bilayers. This is due to the emission of the BsubPc layer itself, confirmed in the PL spectra for a

pristine 20 nm Cl-BsubPc layer (as shown in the inset). This implies that Förster resonant energy

transfer (FRET) is also occurring from the Tc to the BsubPcs. FRET is dependent on the overlap

of the emission of the donor and the absorption of the acceptor which is quite substantial in the

Tc / BsubPc case. The resultant transfer to the BsubPcs then causes the BsubPc to emit and this

is apparent with the reasonable peak seen at 620 nm. The origin of the additional peak at 720

nm, which is only observed in the Tc / Cl-BsubPc pairing, yet not any of the other pairings, is

unclear. We have previously observed aggregate induced PL and EL emissions from

pentafluorophenoxy-BsubPc near 706 nm,47-48 but this peak appears to be sufficiently different

60

in wavelength, and conspicuously absent from the neat film PL, to preclude that explanation

here.

Figure 4-2. Photoluminescence emission spectra of Tc and Tc / (C60 / Cl-BsubPc / Cl-

Cl6BsubPc) bilayers at an excitation wavelength of 465 nm. Inset: PL emission of a 20 nm

pristine Cl-BsubPc layer at an excitation of 570 nm. Samples were illuminated from the

tetracene side.

In order to exclude morphological factors affecting the replacement of C60, bilayer structures

were imaged using atomic force microscopy (AFM). Pristine films of Tc and Pent are shown in

Figure 4-3 (T1 and P1) for comparison of the underlying morphology. After the deposition of

C60, Cl-BsubPc and Cl-Cl6BsubPc onto the acene layers small features appear on the acene

crystallites. Generally the 5 nm of electron-acceptor coats the underlying morphology with a

relatively uniform film. The rationale for using a 5nm acceptor layer was to identify any change

in crystallite shape or size of the initial layer near the donor/acceptor interface as this interface is

extremely important to the photovoltaic properties. Once bulk thicknesses >10nm are reached,

the bulk properties of the acceptor layer would dominate the morphology and it would be

difficult to differentiate between the different acceptors at the donor/acceptor interface.

As in previous examples, when a thin film of C60 is deposited on top of an organic layer small

crystallites form across the surface, rather than filling the gaps in between the crystals,29, 49 and

this is also observed when the BsubPcs are deposited on top of the acene layers.49 One difference

61

to note between the two acenes is the growth of dendritic crystals in the pentacene films. This is

common to all three layers with electron-acceptors and is not uncommon in pentacene films.

Thin films of pentacene typically reveal large crystallites when grown at 1 Ås-1 and is

particularly sensitive to growth rate (faster rates resulting in smaller crystallite size). As seen

previously, this deposition rate of growth for Pentacene causes the formation of islands with the

potential to form large dendritic grains.50 This is due to the molecules arriving at the surface with

increased opportunity for the strong Pentacene – Pentacene interactions to overcome any surface

interaction. Although this is different to the tetracene layers, the three different electron-

acceptors appear to deposit in a similar fashion across their respective acenes to each other.

Therefore due to the large influence of the underlying crystalline layers, (the morphology of the

acenes has a larger influence over device performance) any change in performance cannot be

attributed to the electron-acceptor layer morphology.

Figure 4-3. AFM images LHS: (T1) Tc (T2) Tc / C60 (T3) Tc / Cl-BsubPc and (T4) Tc / Cl-

Cl6BsubPc and (P1) Pentacene, (P2) Pent / C60 (P3) Pent / Cl-BsubPc and (P4) Pent / Cl-

Cl6BsubPc.

4.3.2 Electrical Device Characterization and Solid State Arrangements

Admittance spectroscopy was used to measure the charge transport in BsubPc films. Admittance

spectroscopy measures the complex admittance (conductance and capacitance) of a single carrier

device exposed to a small, variable frequency AC perturbation to a DC bias. The dielectric

62

relaxation frequency (fr) of the film is identified by a peak in the negative differential

susceptance (-B = 2f(C-Cgeo)), where f is the AC frequency, C is the frequency dependent

capacitance, and Cgeo is the geometric capacitance). The mobility is then calculated as =

d2fr/(0.56V), where d is the film thickness and V is the DC bias.51 The mobility was measured at

several temperatures and all measurements were performed under vacuum in a variable

temperature cryostat.52 Electron only devices with the structure ITO / Al (100 nm) / Cl-BsubPc

or Cl-Cl6BsubPc (200 to 600 nm) / TPBi (3 nm) / Al were used for the impedance spectroscopy

measurements where TPBi acts as a buffer layer for protecting BsubPcs from the deposition of

aluminium electrodes.53 The temperature and field dependence of the electron mobility of Cl-

BsubPc and Cl-Cl6BsubPc are shown in Figure 4-4, with the numerical values provided in Table

4-1 and the Supporting Information.

Figure 4-4. The temperature and field dependence of the electron mobility of (a) Cl-BsubPc and

(b) Cl-Cl6BsubPc

63

Table 4-1 Electron mobility parameters near ambient temperature (standard error from non-

linear curve fit in brackets) for electron only single carrier devices analysed by impedance

spectroscopy with a structure ITO / Al (100 nm) / Acceptor (200 nm) / TPBi (3 nm) / Al (100

nm) and some crystal structure parameters.

Acceptor Zero Field Mobility

/ cm2 V-1 s-1

Poole-Frenkel Slope

/ MV-1/2 cm1/2

Molar Density

/ kmol m-3

Motif

Cl-BsubPc 1 × 10-7(1 × 10-8) 8.8 (0.12) 3.56 Concave-

Concave

Ribbon

Cl-Cl6BsubPc 8 × 10-7

(2 × 10-7)

7.7 (0.53) 2.72 Concave-

Convex

Slipped

Column

Reports in the literature of the charge transport properties of BsubPcs are sparse and

contradictory. One paper reports mobility within a thin film transistor (TFT) for Cl-BsubPc with

the electron and hole mobility ~10-5 cm2 V-1 s-1.54 Single carrier devices are more relevant device

architecture to OPVs since charge transport is perpendicular to the substrate in a single carrier

device as opposed to parallel to the substrate, as is the case in TFTs. Further, organic

semiconductors are typically observed to demonstrate a Poole-Frenkel type (electric field

activated) mobility of the form (F) = 0exp(F1/2), where 0 is the zero-field mobility, is the

Poole-Frenkel slope and F is the electric field. Using single carrier devices, Poole-Frenkel type

hole mobilities of 0 = 4.5×10-8 cm2 V-1 s-1, MV-1/2 cm1/2 and electron mobilities of 0 =

5.2×10-10 cm2 V-1 s-1, MV-1/2 cm1/2 have been measured for BsubPcs.55 Then again, it is

notoriously difficult to extract reliable mobility measurements from the current / voltage

characteristics of single carrier devices due to the confounding of space charge limited current

(SCLC) and charge injection limitations, which are also electric field activated. SCLC analysis

of this sort can underestimate mobility by orders of magnitude in the presence of an injection

barrier.56

64

Alternative charge carrier mobility measurement techniques include time-of-flight (TOF) and

admittance spectroscopy. While TOF is a widely used approach for well established materials, it

requires at least 1 m thick films, which is a disadvantage as it both requires large quantities of

material and may not be representative of transport in more device relevant film thicknesses of

less than 100 nm. Conversely, admittance spectroscopy is able to probe much thinner films and

is less sensitive to charge injection limitations at the contacts.51 There is one study which uses

admittance spectroscopy techniques to report an electron mobility of ~10-9 cm2 V-1 s-1 for Cl-

BsubPc.57 All the reported Cl-BsubPc charge mobilities to date are at ambient temperature and

there are no known reports on charge transport in Cl-Cl6BsubPc. We have previously published

an admittance spectroscopy study of electron transport in fluorinated phenoxy boron

subphthalocyanines. In that paper we explored temperature dependence and the impact of solid

state arrangement. The results supported the hypothesis that close packing in different crystal

directions could impact zero field mobility and the Poole-Frenkel slope for phenoxy-BsubPcs.20

As with the fluorinated phenoxy BsubPcs, we also considered the temperature activation of the

charge carrier mobility. The thermal activation of Cl-BsubPc electron mobility greatly exceeds

the thermal activation of the Cl-Cl6BsubPc electron mobility with the former showing a change

in zero field mobility from 1×10-7 cm2 V-1 s-1 at 297 K to 2×10-4 cm2 V-1 s-1 at 330 K (three

orders of magnitude) while the latter only increases from 8×10-7 cm2 V-1 s-1 to 6×10-6 cm2 V-1 s-1

(one order of magnitude) over the same temperature range. Application of the Gaussian Disorder

Model58 to characterize the temperature dependence yielded a poor fit for Cl-BsubPc (R2 = 0.86)

and yielded fit parameter values for Cl-Cl6BsubPc which had no correlation or meaning in the

physical world. Given the Gaussian Disorder Model was originally developed to describe

hopping charge transport between isolated sites in amorphous glasses, the fit to the model may

indicate that charge transport in Cl-BsubPc is more similar to transport in an amorphous film,

while the lack of fit to the model for Cl-Cl6BsubPc may indicate transport is dominated by more

crystalline pathways. Attempts to quantify the charge transport with a band transport model were

equally unfruitful, which is unsurprising given band transport is typically only observed when

mobilities approach or exceed 1 cm2V-1s-1.59 Either way, the similarity in ambient temperature

charge transport values and optical properties for Cl-BsubPc and Cl-Cl6BsubPc suggests these

materials should result in solar cells with similar efficiencies (especially similar optimized device

layer thicknesses) if all other device parameters are held constant.

65

The presence or absence of crystalline transport pathways in Cl-Cl6BsubPc or Cl-BsubPc can be

further supported by considering the solid state arrangements of each compound as determined

by x-ray crystallography of single crystals of each compound (Figure 4-5). The Cl-BsubPc solid

state arrangement has been long known40 and recently confirmed by our group41 from single

crystals grown by train sublimation. Single crystals of Cl-Cl6BsubPc were grown by acetonitrile-

benzene vapour diffusion (CCDC deposition number: 963247).42 Each crystal was free of solvent

or solvates.

Solid state interactions (including interaction) between nieghbouring Cl-BsubPc molecules

are practically non-existent (Figure 4-5a). There is a minor interaction between C12 and C8 of

nieghbouring molecules at a distance of 3.379 Å (not shown in Figure 4-5a), however, it is not

of the geometry that would be indicative of a interaction. Overall the solid state arrangement

can be described as a ribbon motif permeating through the crystal.

In contrast, the solid state arrangement of Cl-Cl6BsubPc shows interactions and forms a

columnar assembly (Figure 4-5b) with concave-convex bowl interactions and interactions less

than the sum of the van der Waals radii.42 The most relevant interactions are in nature and

are between the isoindoline fragments of neighboring Cl6BsubPc molecules. Two of the three

isoindoline units of each Cl-Cl6BsubPc are associated with two isoindoline units of its neighbour

at a distance less than 4 Å (3.848 Å and 3.796 Å, Figure 4-5b) indicating interactions. The

third isoindoline unit is involved in a chlorine-π interaction between the axial chloride and the

neighboring 6-membered outer ring with an interaction distance of 3.058 Å (Figure 4-5b). These

three dominant intermolecular interactions drive the formation of what resembles a slip-stacked

columnar arrangement of columns throughout the crystal structure of Cl-Cl6BsubPc. The

presence of such strong interactions in the solid state lends itself to the idea that Cl-Cl6BsubPc

might be more crystalline in the bulk than Cl-BsubPc.

66

(a)

(b)

Figure 4-5. Solid state arrangement of (a) Cl-BsubPc40 and (b) Cl-Cl6BsubPc.42 Hydrogen atoms

have been omitted for clarity. Atomic colors: boron – yellow; carbon – grey; nitrogen – blue;

chlorine – green; In (b) alternating Cl-Cl6BsubPc molecules have been colored light blue and

orange for clarity; relevant centroids have been colored red.

4.3.3 Photovoltaic Device Characterization

In order to probe the effect of replacing C60 with the BsubPcs, we fabricated bilayer

heterojunction OPVs comprising of ITO / MoOx (5 nm) / Tc or Pent (60 nm) / Acceptor (dA nm)

/ BCP (8 nm) / Al. The insertion of the 5 nm MoOx interlayer allows for improved energy level

67

alignment at the electrode / Tc interface and was used in the Pent devices to allow for

consistency but has no effect on cell performance as shown previously.4 Current density vs.

voltage characteristics in the dark and under 1 sun illumination (AM 1.5G) for these cells are

shown in Figure 4-6 with key device parameters listed in Table 4-2.

Figure 4-6. J-V data under 1 sun illumination for (a) planar Tc heterojunctions and (b) EQE of

the Tc / Cl-Cl6BsubPc device.

68

Table 4-2. Device parameters obtained from OPV devices. Devices A - C have the structure:

ITO / MoOx (5 nm) / Tc (60 nm) / Acceptor (tA nm) / BCP (8 nm) / Al and D – F have the

structure ITO / MoOx (5 nm) / Pent (60 nm) / Acceptor (tA nm) / BCP (8 nm) / Al. The standard

deviation for each characteristic is shown in brackets.

Donor

/ tD nm

Acceptor

/ tA nm

Jsc (SD)

/ mA cm-2

Voc (SD)

/ V

FF (SD) ηp (SD)

/ %

Number of

cells tested

A: Tc

/ 60 nm

C60

/ 40 nm

2.34 (0.21) 0.67 (0.07) 0.55 (0.08) 0.89 (0.23) 13

B: Tc

/ 60 nm

Cl-BsubPc / 35 nm 2.21 (0.25) 1.18 (0.07) 0.53 (0.08) 1.36 (0.24) 11

C: Tc

/ 60 nm

Cl-Cl6BsubPc / 35

nm

2.54 (0.32) 0.89 (0.13) 0.50 (0.12) 1.12 (0.31) 17

D: Pent

/ 60 nm

C60

/ 40 nm

6.86 (0.48) 0.38 (0.07) 0.53 (0.08) 1.37 (0.35) 50

E: Pent

/ 60 nm

Cl-BsubPc / 25 nm 1.35 (0.21) 0.87 (0.10) 0.59 (0.08) 0.65 (0.22) 71

F: Pent

/ 60 nm

Cl-Cl6BsubPc / 25

nm

2.09 (0.30) 0.50 (0.02) 0.48 (0.07) 0.50 (0.10) 39

Figure 4-6a shows the device characteristics for the replacement of C60 with BsubPcs in Tc

based devices. As expected, the replacement of C60 with Cl-Cl6BsubPc yielded an increase in

both Voc to 0.89 ± 0.13 V (p = 2.8 × 10-9) and Jsc to 2.54 ± 0.32 mA cm-2 (p = 0.039) and the

resulting PCE is improved by 50 % (p = 0.019). When compared with the two reference devices

the Voc falls between the Tc / C60 and Tc / Cl-BsubPc devices (due to the Ig falling between these

two extremes) whereas the short circuit current is unchanged from the Tc / Cl-BsubPc cell (p =

0.17). The fill factors are also identical between the Cl-BsubPc and Cl-Cl6BsubPc devices (p =

0.43). Both these similarities are attributed to similar quenching abilities, electron mobilities and

efficient exciton dissociation. Cl-Cl6BsubPc also contributes to the photocurrent in the external

quantum efficiency similar to the Tc / Cl-BsubPc devices published previously, as shown in

Figure 4-6b.

Current density vs. voltage characteristics of the Pent devices are shown in Figure 4-7 with the

corresponding parameters also in Table 4-2. The optimal Pent thickness when paired with 40 nm

69

C60 is ~ 60 nm, with a Jsc of 6.86 ± 0.48 mA cm-2, a Voc of 0.38 ± 0.07 V, and a FF and np of 0.53

± 0.08 and 1.37 ± 0.35 % respectively which is similar to previous literature.29 In the previous

case of Tc / Cl-BsubPc there was no increase in Jsc due to the reduced spectral overlap in

comparison to Tc / C60 but in this case, the Pent / BsubPc pairing overlap greatly at the 595 nm

absorption peak of BsubPc and therefore a small decrease in Jsc was expected. However, there is

a loss of ~ 5 mA cm-2 for both BsubPcs, leading to Cl-BsubPc and Cl-Cl6BsubPc devices having

lower Jsc of 1.35 ± 0.21 mA cm-2 (p < 10-200) and 2.09 ± 0.30 mA cm-2 (p < 10-200). Fill factor is

largely unchanged between Cl-BsubPc and Cl-Cl6BsubPc in the Tc system with 0.53 ± 0.08 vs.

0.50 ± 0.12 (p = 0.43) while in the Pent system the fill factors differ, with 0.59 ±0.08 vs. 0.48 ±

0.07 (p = 4.86 × 10-5). The optimized acceptor layer thickness in Tc containing cells is the same

(35 nm) for both Cl-BsubPc and Cl-Cl6BsubPc, and the same holds for the optimized acceptor

layer thickness (25 nm) in Pent containing cells. This highlights how an understanding of charge

transport can be useful in device design even if it is not the only relevant material parameter, as

emphasized by the dramatic drop in Jsc for the Pent containing devices. It seems apparent that

there must be another cause for the dramatic loss of photocurrent.

Figure 4-7. J-V data under 1 sun illumination for (a) planar Pent heterojunctions and (b) EQE of

the pentacene devices Pent / C60, Pent / Cl-BsubPc, and Pent / Cl-Cl6BsubPc with the absorption

spectra of a pentacene thin film (red line).

70

In an effort to better understand the dramatic decrease in Jsc the external quantum efficiency

(EQE) of the Pent / BsubPc OPV cells are shown in Figure 4-7b. Although we would expect to

see the Pent contribution between 500 - 700 nm (as shown in the absorption spectra, Figure 4-

1g) the only dominant peak present is at 595 nm, a wavelength associated with BsubPc

absorption. This is in contrast with some other pentacene/non-fullerene acceptor cells where

pentacene has been the dominant contributor to photocurrent.60 The current would be reduced

due to the overlap in absorption spectra, but we would expect more of a contribution from the

Pent layer, particularly at wavelengths longer than 600 nm. The Pent / C60 cell is known to

benefit from enhanced photocurrent due to triplet fission,32, 36-37 so we propose the dramatic loss

of photocurrent in the Pent / BsubPc cells is a result of the interface poorly dissociating these

pentacene originating triplets. (There are few measurements of triplet energies in BsubPcs, but

an estimate of 1.45 eV for a related derivative,61 in contrast to Pent's triplet energy, recently

bounded between 0.85 eV and 1.00 eV36 suggests that triplets generated in the Pent layer would

not transfer to the BsubPc layer.) The small EQE peak at 670 nm in the Pent / Cl-Cl6BsubPc cell

that is completely absent in the Pent / Cl-BsubPc cell, yet is a dominant feature in the Pent / C60

cell. We thus interpret these results to indicate that Cl-BsubPc does not facilitate the dissociation

of Pent triplets, but Cl-Cl6BsubPc does dissociate some Pent derived triplets albeit not as

efficiently as C60. This is the first report of a BsubPc facilitating triplet harvesting from Pent.

Following the reasoning of Ehrler et al.36 we would propose that a BsubPc derivative with a

deeper LUMO energy would likely be an even more effective at harvesting Pent derived triplets,

resulting in enhanced photocurrent. Similar observations were not made in the Tc containing

OPVs as a pair of triplets in Tc are essentially degenerate with the singlet energy,62 so the triplet

generation process is orders of magnitude slower in Tc33 than in Pent.32

We previously demonstrated improved cell stability with the replacement of C60 with Cl-

Cl6BsubPc in BsubPc (as the donor) OPVs. A large improvement in cell stability towards air

(and N2) was achieved when tested under constant illumination of 100 mW cm-2 AM1.5G for 60

mins.2 To investigate the stability of the electron-accepting materials a summary of the

degradation of PV parameters over 1100 mins of constant AM 1.5G illumination in air are shown

in Figure 4-8. Twelve devices per structure were tested simultaneously and representative device

performance is shown in the figure. The cells studied have the structure: ITO / MoOx / Tc /

Acceptor / BCP / Al and were tested in air. Tc was chosen as the electron-donor material due to

71

both layers contributing to the photocurrent, although similar trends were observed in the Pent

OPVs. In all cases the cells degrade, the most dramatic of which is the Tc / C60 device. The main

decrease is due to a fast degradation in Jsc and FF presumably due to the oxidation of C60 (in air)

resulting in a decrease in conductivity. When replacing the acceptor layer with BsubPc, the cells

are more stable than their C60 counterparts in all parameters and still retain 20 % of the original

efficiency at ~ 1000 mins (compared to 0 % at 800 mins with Tc / C60). The Tc / BsubPc cell

shows no change in Voc over the measurement time and the main cause of the degradation is the

decrease in Jsc. The Tc / Cl-Cl6BsubPc device is the most stable over the time frame, losing only

40 % of its efficiency. We attribute the unusual initial rise in the Jsc of the Tc / Cl-Cl6BsubPc

cells and the FF of the Tc / Cl-BsubPc cells to be a possible thermal effect as the cells reach an

equilibrium temperature in our testing station. The FF is extremely stable, with most of the

change in PCE due to the sharp initial reduction in Voc and the steady decrease in the Jsc. The

chlorinated derivative is likely to be more stable than Cl-BsubPc due to the lower ionisation

potential and has been seen previously for halogenated derivatives.63-64 Although the degradation

of the photocurrent is slower than the other systems it still shows the poor stability of these small

molecules in air. As the decrease in Jsc occurs in all systems, it is probably partly down to the Tc

layer, with the [4+2] cycloaddition with singlet oxygen the main degradation pathway rendering

it colourless over time.65 This consistent degradation highlights the further need for good

encapsulants as although the stability can be improved with simple modifications, like the

replacement of fullerenes, O2 infiltration and moisture remains a problem.

Figure 4-8. Device parameters of Tc / C60 (black); Tc / Cl-BsubPc (purple); Tc / Cl-Cl6BsubPc

(pink) cells under constant illumination over 23.2 hours in air.

72

4.4 Conclusion

In summary, we have demonstrated that the archetypal donor material Cl-BsubPc has ambipolar

characteristics both when derivatised (Cl-Cl6BsubPc) and underivatised (Cl-BsubPc), showing

reasonable electron mobility and photoluminescence quenching of excitons from Tc films. When

incorporated in devices as the acceptor material, the voltage can be dramatically increased both

in Tc and Pent containing devices, and this large improvement in Voc was observed due to the

increase in interface gap. The similarity in room temperature electron mobility of Cl-BsubPc and

Cl-Cl6BsubPc may explain why devices with each material were found to have the same optimal

acceptor thicknesses. The device stability can also be improved on the replacement of C60 in

nearly all cases, and highlights another advantage for using BsubPcs. This dual donor / acceptor

character for BsubPc semiconductors allows for an increased assortment of materials for the

future design of high voltage and singlet fission harvesting devices. This work also highlights the

need for optimising energetics and measuring charge carrier mobility in devices to identify new

material development targets and ultimately achieve higher efficiencies.

73

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Phthalocyanines. J. Mater. Sci. 1982, 17, 2781-2791.

45. Watkins, N. J.; Gao, Y., Interface Formation and Energy Level Alignment of Pentacene

on Sio2. J. Appl. Phys. 2003, 94, 5782-5786.

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46. Schroeder, P. G.; France, C. B.; Park, J. B.; Parkinson, B. A., Energy Level Alignment

and Two-Dimensional Structure of Pentacene on Au(111) Surfaces. J. Appl. Phys. 2002, 91,

3010-3014.

47. Morse, G. E.; Helander, M. G.; Maka, J. F.; Lu, Z.-H.; Bender, T. P., Fluorinated

Phenoxy Boron Subphthalocyanines in Organic Light-Emitting Diodes. ACS Applied Materials

& Interfaces 2010, 2, 1934-1944.

48. Helander, M. G.; Morse, G. E.; Qiu, J.; Castrucci, J. S.; Bender, T. P.; Lu, Z. H.,

Pentafluorophenoxy Boron Subphthalocyanine as a Fluorescent Dopant Emitter in Organic Light

Emitting Diodes. ACS Applied Materials & Interfaces 2010, 2, 3147-52.

49. Cnops, K.; Rand, B. P.; Cheyns, D.; Heremans, P., Enhanced Photocurrent and Open-

Circuit Voltage in a 3-Layer Cascade Organic Solar Cell. Appl. Phys. Lett. 2012, 101, 143301 1-

4.

50. Potscavage, W. J.; Sharma, A.; Kippelen, B., Critical Interfaces in Organic Solar Cells

and Their Influence on the Open-Circuit Voltage. Acc. Chem. Res. 2009, 42, 1758-1767.

51. Tsang, S. W.; So, S. K.; Xu, J. B., Application of Admittance Spectroscopy to Evaluate

Carrier Mobility in Organic Charge Transport Materials. J. Appl. Phys. 2006, 99, 013706 1-7.

52. Helander, M. G.; Wang, Z. B.; Greiner, M. T.; Qiu, J.; Lu, Z. H., Experimental Design

for the Determination of the Injection Barrier Height at Metal/Organic Interfaces Using

Temperature Dependent Current-Voltage Measurements. Rev. Sci. Instrum. 2009, 80, 033901.

53. Morse, G. E.; Helander, M. G.; Maka, J. F.; Lu, Z. H.; Bender, T. P., Fluorinated

Phenoxy Boron Subphthalocyanines in Organic Light-Emitting Diodes. ACS Appl Mater

Interfaces 2010, 2, 1934-1944.

54. Yasuda, T.; Tsutsui, T., N-Channel Organic Field-Effect Transistors Based on Boron-

Subphthalocyanine. Mol. Cryst. Liq. Cryst. 2007, 462, 3-9.

55. Pandey, R.; Gunawan, A. A.; Mkhoyan, K. A.; Holmes, R. J., Efficient Organic

Photovoltaic Cells Based on Nanocrystalline Mixtures of Boron Subphthalocyanine Chloride and

C60. Adv. Funct. Mater. 2012, 22, 617-624.

56. Wang, Z. B.; Helander, M. G.; Greiner, M. T.; Qiu, J.; Lu, Z. H., Analysis of Charge-

Injection Characteristics at Electrode-Organic Interfaces: Case Study of Transition-Metal

Oxides. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 80, 235325 1-9.

57. Singh, M.; Mahajan, A.; Bedi, R. K.; Aswal, D. K., Dielectric Spectroscopic Studies of

Boron Subphthalocyanine Chloride Thin Films. Electron. Mater. Lett. 2013, 9, 101-106.

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58. Bassler, H., Charge Transport in Disordered Photoconductors. Phys. Stat. Sol. B 1993,

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60. Pandey, A. K.; Dabos-Seignon, S.; Nunzi, J.-M., Pentacene: Ptcdi-C13h27 Molecular

Blends Efficiently Harvest Light for Solar Cell Applications. Appl. Phys. Lett. 2006, 89, 113506

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61. Gonzalez-Rodriguez, D.; Torres, T.; Guldi, D. M.; Rivera, J.; Herranz, M. A.;

Echegoyen, L., Subphthalocyanines: Tuneable Molecular Scaffolds for Intramolecular Electron

and Energy Transfer Processes. J. Am. Chem. Soc. 2004, 126, 6301-6313.

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Temperature-Independent Singlet Exciton Fission in Tetracene. Journal of the American

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Semiconductors: Molecular Architecture, Electronic, and Crystal Structure Tuning of Arene-

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1993, 97, 10489-10497.

79

Chapter 5 Challenges of Mobility Determination and The Case for TPBi

5.1 Admittance Spectroscopy of Boron Subphthalocyanines – the ones that did not work

We applied admittance spectroscopy to measure the electron mobility for a large number of

subphthalocyanine derivatives and met with varying degrees of success. A theoretical

background on admittance spectroscopy is included in Chapter 1. As mentioned in Chapters 2

and 3, we successfully measured and published data on five derivatives1-2 but faced significant

difficulties with five other derivatives investigated. Figure 5-1 shows the boron

subphthalocyanine derivatives where we successfully measured the electron mobility and also

shows an example of what the data from the successful experiments looks like. Minima in

capacitance are readily identifiable and clearly migrate toward higher frequencies with

increasing voltage. Conversely, in Figure 5-2 we show a number of subphthalocyanine

derivatives where the capacitance was essentially unchanged and peaks in negative differential

susceptance are unchanging in frequency or magnitude with increasing voltage. The peaks in

susceptance stay at the same frequency, indicating that the carrier transit time is unchanged with

increasing voltage. The peaks do not change in magnitude, which indicates the number of

charge carriers involved in responding to the voltage is also unchanged, which makes little sense

given the increasing current density that should be present in a device driven at higher voltage.

If we refer back to Equation 1-17, ( ), is mobility, is device thickness, is carrier

transit time, and is electric field) if carrier transit time is constant while field increases, then

the mobility must decrease. In other words, this result suggests that with increasing driving force

the charge carriers are moving slower, resulting in a highly unlikely negative Poole-Frenkel

slope. When we then extrapolate back to the zero field mobility, we find 0 > 10-2 cm2 V-1 s-1,

which would imply our material is more conductive than C60. The current density and

photovoltaic cell results obtained thus far are inconsistent with this high of a mobility.

Additionally, the Poole-Frenkel slopes calculated are an order of magnitude larger than should be

possible for the dielectric constant of these materials.3 In all, these admittance spectroscopy

results are orders of magnitude and sign convention direction incompatible with any other way of

80

describing these organic semiconductors. This is a strong indicator that these admittance

spectroscopy fits are not a reliable description of our materials' properties.

(a) (b)

Figure 5-1. Boron subphthalocyanine derivatives where the electron mobility was successfully

measured using admittance spectroscopy (a), and a typical result (b)1.

104 105 106 10710-2

10-1

100

101

102100

110

120

130

140

150

Applied Potential

0 V 9 V 12 V 15 V 18 V 21 V

-B

(S

)

Frequency (Hz)

Ca

pa

cita

nce

(p

F)

81

(a)

(b)

(c)

Figure 5-2. Boron subphthalocyanine derivates were electron mobility was not able to be

measured using admittance spectroscopy (a), and a typical result (b). The peaks bunch together,

leading to an improbable negative Poole-Frenkel effect (c).

82

5.2 Alternate Admittance Spectroscopy Data Analysis Approaches

The inconsistency of our admittance spectroscopy studies was a cause for concern, so I explored

alternative ways of processing the admittance spectroscopy data for the compounds shown in

Figure 5-2a to try to fit the data to more complex equivalent circuit representations, shown in

Figure 5-3a. These approaches centered around the idea of plotting the complex impedance (the

reciprocals of the admittance) with frequency as a parameter. The plotted curves should show

circular or elliptical curves that can be used to determine which circuit element combination best

describes the frequency response of the device, with the appropriate curve fit allowing the

determination of the values for the individual component values.4 Figure 5-3a shows equivalent

circuit model we initially used as well as two more complex alternate models that we thought

might better described our single carrier devices. However, as can be seen in Figure 5-3b, our

impedance data did not yield the hoped for circular or elliptical curves that would have

facilitated using these alternate models for admittance spectroscopy analysis.

(a)

(b)

Figure 5-3. Equivalent circuit diagrams used for admittance spectroscopy analysis (a), and a

representative plot of the complex impedance, specifically for F5-BsubPc (b).

5.3 Further Analysis of the Admittance Data

While analysis of the admittance data in terms of extracted charge carrier mobility did not yield

additional insights, by stepping back from the assumption that the peaks correspond to carrier

Simple RC Circuit (Model We Use Currently)

Debye Equivalent Circuit (Simplest description of barrier)

Injection Barrier Model Circuit

Capacitor

Resisitor

83

transit times and simply considering the negative differential susceptance (-B) peaks without

additional mathematical transformation, additional insight was gained. The data sets for MsO-

BsubPc and F-BsubPc, where admittance measurements were done on devices with at least four

thicknesses and at least four temperature, were examined in much greater detail. For the

materials showing negative charge carrier mobility, the peak in -B does not change with

applied voltage, so we can calculate an average peak position for each device thickness. We

designate this averaged frequency as the critical frequency for the device (fcrit). If the process

described by the peak is a bulk device transport property (i.e. related to charge carrier drift or

diffusion) fcrit should vary roughly linearly with device thickness. If the process is not a bulk

device transport property, fcrit should be independent of thickness. By taking the ambient

temperature data for MsO-BsubPc and F-BsubPc and conducting a linear least squares

minimization fit to the data, we find that neither slope is significantly different from zero at the

95% confidence level. The plots of fcrit vs. device thickness are shown in Figure 5-4. This

supports the contention that the immobile -B peaks are thickness independent and thus do not

correspond to a bulk transport process.

250 300 350 400 450 500 550 600 6500.0

2.0x106

4.0x106

Slope = -10120 Hz/nm 95% CI (-16225 to -3986)

ANOVA analysis indicates slope is not significantly different from zeroat the 0.05 confidence level.

f crit

(Hz)

Thickness (nm)

a)

250 300 350 400 450 500 550 600 6500.0

2.0x106

4.0x106

f crit

(Hz)

Thickness (nm)

Slope = -1362 Hz/nm 95% CI (-3418 to 694)

ANOVA analysis indicates slope is not significantly different from zero at the 0.05 confidence level.

b)

Figure 5-4. Critical frequency as a function of device thickness for MsO-BsubPc (a) and F-

BsubPc (b). Error bars show the 95% confidence interval.

Temperature dependence of the peaks was considered next. If the -B peaks corresponded to an

injection process, we would expect them to be thickness independent with an Arrhenius style

thermal activation of the from,

84

fcrit = fcrit,0 × exp (Ea/kBT) (1)

where fcrit,0 is the pre-exponential factor, Ea is the activation energy, kB is Boltzmann's constant,

and T is the absolute temperature. If this was an injection process, we would expect an Ea of

between 300 meV and a few thousand meV. Anything less than 300 meV should not produce a

detectable energy barrier at ambient temperature, while an injection barrier of greater than a few

eV would be impossible based upon the known frontier orbital energies. Non-linear least

squares minimization fits of the data yielded an Ea of -91 meV (95% CI (-112 to -69) meV) for

MsO-BsubPc and an Ea of 43 meV (95% CI (27 to 59) meV) for F-BsubPc. The critical

frequency is, at most, weakly influenced by thermal effects. The negative Ea for MsO-BsubPc

does not correspond to any known electrical process, and the small activation energy for F-

BsubPc both suggest an injection barrier related process is not responsible for the fcrit. Equally, a

trap related process would not yield a negative activation energy, and such a shallow trap depth

of less than a few kBT (i.e. less than roughly 75 to 100 meV) would not show any meaningful

contribution to charge transport dynamics.

0.0030 0.0033 0.0036 0.0039

106

2x106

3x106

f crit (

Hz)

1/T (K-1)

Ea = -91 meV

95% CI (-112 to -69) meV

a) 320 300 280 260T (K)

0.0030 0.0033 0.0036 0.0039

106

2x106

3x106

f crit (

Hz)

1/T (K-1)

Ea = 43 meV

95% CI (27 to 59) meV

b) 320 300 280 260T (K)

Figure 5-5. Critical frequency as a function of device temperature for MsO-BsubPc (a) and F-

BsubPc (b). Error bars show the 95% confidence interval.

We further note that fcrit varies little between materials. At ambient temperature, fcrit ranges from

1 MHz to 5 MHz in MsO-BsubPc devices and fcrit ranges from 1 MHz to 2 MHz in F-BsubPc, so

we propose that the phenomena may also be material independent. Having cast doubt on

hypotheses proposing that fcrit arises from a thickness dependent transport processes, thermally

85

activated injection processes, or trapping processes, and that it may even be material

independent, we propose that the -B peak is likely an experimental artifact. With this in mind,

a re-examination of the previously measured materials where mobility was determined (F5-

BsubPc, F12-BsubPc, F17-BsubPc, Cl-BsubPc, Cl-Cl6BsubPc) led to the discovery of small but

detectable -B peaks near 1 MHz in F12-BsubPc and F17-BsubPc. As such we feel confident

that the low MHz range negative differential susceptance anomaly can be safely ignored in the

analysis of susceptance data to extract charge carrier mobility values. This allows a re-analysis

of the -B spectrums where we exclude the immobile peaks near 1 MHz from any subsequent

calculations. When re-considered in light of this insight, additional peak series were identified

for Phth-BsubPc and MsO-BsubPc. Zero field mobility and Poole-Frenkel coefficients are

shown in Table 5-1.

Table 5-1. Charge carrier mobility for additional BsubPc compounds determined by excluding

immobile peaks near 1 MHz. The standard error of the mean for the point estimators are shown

in brackets.

Compound Zero field mobility

(cm2 V-1 s-1)

Poole-Frenkel coefficient

(MV-1/2 cm1/2)

Phth-BsubPc 1 × 10-5 (3 × 10-6) 4.8 (0.31)

MsO-BsubPc 3 × 10-8 (4 × 10-9) 9.6 (0.22)

5.3.1 Low Frequency Susceptance Peaks

For Phth-BsubPc and MsO-BsubPc, an additional series of peaks at a lower frequency range was

also detected. While we had successfully determined electron mobility from peak series in the

frequency range of roughly 10 kHz to 10 MHz, we actually measured admittance at frequencies

in the range 100 Hz to 10 MHz. For only these two compounds, we detected a second series of

peaks in the roughly 100 Hz to 10 kHz range. A process at these lower frequencies would have

to correspond to a dramatically slower process than the charge carrier mobilities calculated in

Table 5-1. These two sets of peak series for Phth-BsubPc as shown in Figure 5-6. The data for

MsO-BsubPc is similar in appearance.

86

Figure 5-6. Negative differential susceptance vs. frequency for Phth-BsubPc. There is a series

of peaks at low frequency (a) and a second set of peaks at high frequency become clear and

usable to determine mobility once the immobile peaks near 1 MHz are excluded from the

calculation (b).

The origin of the low frequency peak series present in Phth-BsubPc and MsO-BsubPc is

unknown, but intriguing. As admittance spectroscopy has been employed to great effect to

simultaneously observe multiple transport processes in solid state electrolytes, such as fuel cells.

In that application, the processes of electron conduction and injection can be measured as high

frequency peak series while slower ion conduction processes manifest as lower frequency peak

series. As Cl-BsubPc films are known to be nanocrystalline,5 it is possible that films formed

from other BsubPc derivatives could greater or lesser crystalline fractions. In a film with a lower

crystalline fraction, significant charge might be transported through poorly ordered amorphous

regions. Charge transport through amorphous regions would be much slower than through a well

ordered crystalline region both due to poorer inter-molecular electronic coupling resulting from

geometric disorder and due to greater trap density in disordered regions. Taken together, the low

frequency peak series could originate from charge transport in amorphous regions of the film.

The reason these slower processes were observed for only two of the derivatives might be that

these films have larger volume fractions of amorphous regions. A greater volume fraction of

amorphous region would have greater amount of charge present within it, creating a detectable

signal. Conversely, other materials may have a lower amorphous volume fraction and a

87

correspondingly lower amount of charge, making detection impossible with our admittance

spectroscopy system. This interpretation would suggest that the presence of a low frequency

peak train would be indicative of an undesirable high volume fraction amorphous film.

Determining the crystalline fraction of a number of BsubPc films would quickly resolve this

hypothesis. However, obtaining X-ray diffraction data of subphthalocyanine thin films is

particularly challenging as there are no heavy atoms present with large X-ray cross sections to

facilitate scattering. By considering the scattering cross section of the standard bench top source

of 8267 eV for boron (1.806 cm2 g-1)6 and assuming the films have similar density to the

diffracted single crystals (0.0385 g B cm-3),7 and comparing these values to copper

phthalocyanine (46.32 cm2 g-1,6 0.179 g Cu cm-3,8 respectively) we find we would need a film

(0.179 g Cu cm-3 x 46.32 cm2 g-1)/(0.0385 g B cm-3 x 1.806 cm2 g-1) = 119 times greater in

thickness to gain a similar scattering effect. So while a 50 nm CuPc film is readily diffracted, a

Cl-BsubPc film of 6 um is required for the same effect. It is our belief that a film of that

thickness will be dominated by bulk film character, while the thinner, device relevant films, are

so strongly influenced by the surface they are deposited upon that diffraction of a thick film will

not yield relevant crystallographic data.

Another possibility is that the lower frequency process is a distorted image of the hole transport.

If the contacts selected are not completely effect at hole blocking, but still have a large barrier to

hole injection, the hole transport process could appear to be orders of magnitude slower than it

would be in a single carrier with an ohmic injecting contact. This might indicate that hole

transport in these two materials is notably faster than in other films tested, because no signal is

observed in other materials. Testing this hypothesis would require hole only single carrier

devices, which is beyond the scope of this work.

5.3.2 The 1 MHz Susceptance Artifact

In an attempt to further isolate the nature of the 1 MHz, voltage independent, peaks in some of

the negative differential susceptance spectra, a new MsO-BsubPc single carrier device was

fabricated using almost entirely different device fabrication facilities. An alternate batch of

MsO-BsubPc was prepared using the same method9 by a different synthetic chemist. The ITO

coated glass substrate was cleaned using air plasma instead of a UV-ozone generating lamp. The

88

device was fabricated in a different physical vapour deposition chamber on a different vacuum

system from all previous single carrier studies. For the device made on the alternate system, the

voltage independent peak is of comparable magnitude but has shifted to a lower frequency. The

voltage dependent peak train which was used to determine electron mobility in the original

device is completely absent, possibly obstructed by the frequency shifted artifact. The reasons

for this change are unclear, but the continued presence of a voltage independent peak series lends

further credit to the contention that the 1 MHz feature is an experimental artifact as opposed to a

material property.

Figure 5-7. The original MsO-BsubPc single carrier device admittance spectrum with mid-

frequency peak series and ~1 MHz feature (a). The MsO-BsubPc single carrier fabricated with an

alternate batch of materials on an alternate vacuum system. Only the ~1 MHz feature is visible

(b). Both devices are of the structure Glass/Al (50 nm)/MsO-BsubPc (500 nm)/TPBi (3 nm)/Al

(100 nm).

Further, we made atomic force microscope measurements of the MsO-BsubPc film used in the

new single carrier device. Images were captured using tapping mode on the same instrument as

we had previously imaged F5-BsubPc, F12-BsubPc, and F17-BsubPc films.1 The films were

generally smooth and featureless, consistent with both our previous report and others in the

literature.1, 5 We thus conclude that there are no readily obvious difference in film morphology

that account for the presence or absence of the 1 MHz admittance feature.

89

Figure 5-8. Atomic force microscope image of the MsO-BsubPc film used in a single carrier

device.

5.4 Summary of Observed Admittance Phenomena

We have now identified three sets of admittance phenomena: First, peak series roughly in the

frequency range 100 Hz to 10 kHz which increase in frequency with voltage. Second, peak

series roughly in the frequency range 10 kHz to 10 MHz which increase in frequency with

voltage. Third, peak series near 1 MHz which do not change with voltage. The third

phenomena, constant frequency peaks near 1 MHz, are observed to sometimes obstruct the

observation of phenomena tow, making the determination of mobility more difficult. The third

phenomena is largely independent of device thickness, temperature, and even material, so it is

tempting to attribute it to experimental artifact, yet phenomena three is absent in three of the

materials measured, making this identification provisional as opposed to a high confidence

statement.

Table 5-2 and Figure 5-9 both summarize which phenomena are observed with each of the

materials for which electron only single carriers have been fabricated. Of these phenomena, the

90

first, low frequency peak series, if used to calculate charge carrier mobilities yield values that are

so unusually (typically 0 < 10-8 cm2V-1s-1, > 10 MV-1/2cm1/2) as to strain the bounds of

believability for materials that have already been shown to function effectively in semiconductor

devices. It is thus unclear exactly what sort of phenomena these low frequency peaks arise from.

The second phenomena, moderate frequency peak series, seem to correspond to charge carrier

mobility values. Theses peak series show consistent values between devices of the same

material at different thicknesses, and the zero field mobility values, ranging from 10-8 to 10-4

cm2V-1s-1, are in a credible range to describe semiconductor materials which have been shown to

function in organic electronic devices. The third phenomena, constant frequency peaks near 1

MHz, are observed to sometime obstruct observation of phenomena two, making determination

of mobility more difficult. The third phenomena is largely independent of device thickness,

temperature, and even material, so it is tempting to attribute it to an experimental artifact, yet

phenomena three is absent in three of the materials measured, making this identification

provisional as opposed to a high confidence statement.

Figure 5-9. A diagram showing the presence of the three sorts of admittance phenomena

observed among the various BsubPc derivatives measured.

91

Table 5-2. Summary of admittance phenomena for various subphthalocyanine compounds.

Material Admittance Spectrum Feature

0.1 kHz - 10 kHz -B

peak series

10 kHz - 10 MHz -B

peak series

~1 MHz -B peak

F5-BsubPc Present

F12-BsubPc Present Present

F17-BsubPc Present Present

Cl-BsubPc Present

Cl-Cl6BsubPc Present

F-BsubPc Present

MsO-BsubPc Present Present Present

Phth-BsubPc Present Present Present

Cl4Phth-BsubPc Present

-Naphthoxy-BsubPc Present

92

5.5 Can Admittance Results Predict Device Performance?

Numerical characterization of carrier mobility by admittance spectroscopy is ultimately driven

by a desire to improve organic photovoltaic device efficiency. Having now determined the

electron mobility for seven BsubPc derivatives, plotted in Figure 5-10(a), can we predict trends

in photovoltaic device performance for these materials? For the sake of completeness, we also

show the more questionable mobility numbers derived from the low frequency phenomena only

observed for MsO-BsubPc and Phth-BsubPc. All error bars in this chapter show the 95 %

confidence interval for the data sets.

Figure 5-10. Electron mobility plotted as zero field mobility vs. Poole-Frenkel coefficient on a

semi-log plot for all BsubPc derivatives that have been measured by admittance spectroscopy (a).

The lower frequency peak series observed in Phth-BsubPc and MsO-BsubPc, converted into

mobility values (b). Each point is labeled to shown the corresponding compound and device

thickness. All error bars shown 95% confidence intervals. Due to the ordinate axis using a log

scale, zero field mobility intervals which contain zero are shown to extend beyond the bottom of

the plot.

Since energy level offsets between donor and acceptor are another major factor in device

performance, it would be ideal to eliminate this effect by only comparing compounds with near

identical HOMO energies and optical absorbances. Accordingly, we exclude F12-BsubPc, F17-

BsubPc, and Cl-Cl6BsubPc from further consideration as substitution of peripheral hydrogen

atoms on the BsubPc moiety shifts their HOMO energy level to a higher energy than Cl-

BsubPc10-11 and the remaining derivatives. We would also exclude Cl-Cl6BsubPc from further

93

consideration because of the shift in its solid state absorption peak relative to Cl-BsubPc11 and

the remaining derivatives.

An effective electron accepting material in a photovoltaic device should have a high electron

mobility that is weakly dependent on field, as the electric field in a photovoltaic device is

typically small. Higher carrier mobility should result in a lower density of charges within the

device, which minimizes the rate of carrier recombination and charge trapping, thus maximizing

the extracted current. When considering the presentation of mobility parameters by the plot in

Figure 5-10, these criteria would suggest a material that is higher and further left would be

preferable. As such, we would predict that when used in organic photovoltaic devices as

acceptors performance for the remaining compounds should show a trend of performance in the

order Phth-BsubPc > F5-BsubPc > Cl-BsubPc > MsO-BsubPc.

We fabricated organic photovoltaic devices of the standardized12 structure ITO/PEDOT:PSS/

6T (55 nm)/BsubPc (20 nm)/BCP (10 nm)/Ag (80 nm), where BsubPc was one of Phth-

BsubPc, F5-BsubPc, Cl-BsubPc, MsO-BsubPc. The Cl-BsubPc containing version of this cell

with slightly different layer thicknesses has been reported previously, with an efficiency of 4.7

%.13 Our VOC and FF are comparable to this report with our lower efficiency resulting from

lower JSC. The JV curves and external quantum efficiency for these cells are shown in Figure 5-

11 and described numerically in Table 5-3.

0.0 0.5 1.0

-6

-4

-2

0

2

Acceptor Phth-BsubPc F5-BsubPc Cl-BsubPc MsO-BsubPc

Cur

rent

Den

sity

(m

A c

m-2)

Voltage (V)

a)

300 400 500 600 7000

20

40

60

Ext

erna

l Qua

ntum

Effi

cien

cy (

%)

Wavelength (nm)

b)

Figure 5-11. (a) JV curve comparing four BsubPcs employed as electron acceptors. (b) External

quantum efficiency spectra. Error bars show the 95% confidence interval.

94

Table 5-3. Mean device parameter comparison of four BsubPcs employed as electron acceptors.

The standard deviation (SD) of each value is shown in parentheses. Device structure is

ITO/PEDOT:PSS/a6T (55 nm)/BsubPc (20 nm)/BCP (10 nm)/Ag (80 nm).

BsubPc JSC

(SD)/mA

cm-2

VOC

(SD)/V

FF

(SD)

P

(SD)/%

Rshunt(SD)

/cm2

Rseries(SD)

/cm2

No. of cells

tested

Phth-

BsubPc

-2.1

(0.16)

0.92

(0.07)

0.40

(0.03)

0.76

(0.08)

230 (41) 27 (5.1) 15

F5-BsubPc -4.6

(0.47)

1.16

(0.01)

0.44

(0.01)

2.4

(0.24)

140 (25) 6.9 (0.65) 12

Cl-BsubPc -6.1

(0.48)

1.09

(0.01)

0.56

(0.02)

3.7

(0.22)

270 (120) 5.5 (0.38) 15

MsO-

BsubPc

-5.3

(0.19)

1.04

(0.01)

0.55

(0.01)

3.0

(0.09)

310 (88) 5.9 (0.45) 8

Unexpectedly, we find that the performance is almost the opposite of what we predicted based

upon mobility data. The photovoltiac performance parameters of short circuit current density

(JSC) and power conversion efficiency (P) both show the trend of Cl-BsubPc > MsO-BsubPc >

F5-BsubPc > Phth-BsubPc. For the parameter of open circuit voltage (VOC), MsO-BsubPc and

F5-BsubPc materials differ by less than 0.1 V from Cl-BsubPc, while Phth-BsubPc is

significantly lower at 0.92 (0.88-0.96, 95% confidence interval) V relative to Cl-BsubPc at 1.09

(1.087-1.093) V. For the parameter of fill factor (FF), the value is typically maximized when

charge transport of electrons in the acceptor layer is within an order of magnitude of the charge

transport of holes in the donor layer,14-15 and we find statistically indistinguishable FF for Cl-

BsubPc and MsO-BsubPc, suggesting these two materials are very similar in their charge

transport capabilities. When the JV curves are fit using the equivalent circuit model featuring a

series and a shunt resistance, a comparison of series resistance (Rseries) is another way to isolate

the impact of electrical effects on photovoltaic performance. A low series resistance should be

correlated with a high carrier mobility, yet the series resistance is of the trend Cl-BsubPc ~ MsO-

BsubPc < F5-BsubPc < Phth-BsubPc, again the opposite of the trend predicted by mobility

95

measurements. In this case, the series resistance of the Cl-BsubPc and MsO-BsubPc cells are not

statistically significant in their difference (p = 0.12).

MsO-BsubPc's lower current density suggests its excitonic properties may be less favourable

than Cl-BsubPc's, perhaps due to MsO-BsubPc's reduced choromophore density relative to Cl-

BsubPc. When considering the external quantum efficiency, the peak near 600 nm corresponds

to the strongest absorption for BsubPc, while the peak near 400 nm corresponds to a peak in the

6T absorption spectrum. In all four cases, both peaks are present, indicating excitons from both

layers are harvested at the donor/acceptor interface. From these results, we conclude that the use

of admittance spectroscopy to determine electron mobility trends in BsubPcs does not translate

into an ability to predict photovoltaic device performance trends when they are used as

acceptors.

5.3.1 Potential Property Correlations

The single crystal structures of MsO-BsubPc,9 Cl-BsubPc,16-17 F5-BsubPc,1 and Phth-BsubPc18

have been previously determined by x-ray diffraction, and from these the chromophore density19

and mass density of the thin films can be estimated based upon the single crystal values. The

chromophore densities are 2.95 kmol m-3, 3.56 kmol m-3, 2.74 kmol m-3, 2.67 kmol m-3,

respectively, and the mass densities are 1.57 Mg m-3, 1.41 Mg m-3, 1.08 Mg m-3, 1.05 Mg m-3,

respectively. In a further effort to propose molecular design parameters, we plotted the device

properties of JSC, VOC, Rshunt, and Rseries against chromophore density and mass density (Figure 5-

12). The device parameters of FF is not considered as these are not optimized device

architectures, and FF is very sensitive to device layer thickness. Since PCE is derived from the

product of FF with other terms, it is also not considered. While a data set of four points is of

limited statistical use, least squares minimization regressions were applied to the data. Linear

regression yields no notable linear correlations (R2 < 0.7 in all cases), though an exponential fit

non-linear regression relating zero field mobility to either density parameter does have a high R2

value ( > 0.999 in both cases) and a negative exponential coefficient in both cases. Desirable

lower P-F coeffiecients appear to occur at lower densities as well Based upon this limited data

set, a hypothesis of lower crystal density corresponding to higher zero field mobility and lower

P-F coefficients is worth further exploration. Possible phenomena responsible for this trend

might be comparable to the trend observed by Holmes et al. where lower BsubPc concentration

96

in a mixture resulted in longer exciton diffusion lengths.20 Of the device parameters considered,

none show any strong trends with either type of density. We further considered the relationship

between film property of electron mobility (described by zero field mobility (0) and Poole-

Frenkel Coefficient ()) and two types of crystal densities along with the material property of

LUMO energy, again seeing no correlations (Figure 5-13). This was advantageous as it allowed

us to again consider the three materials where we had electron mobilities but had not

incorporated them into PV devices. The lack of correlation between the measured mobilities and

the LUMO energies increases our confidence that the measurement technique is valid and being

unduly impacted by the difference in frontier orbital energies for these materials.

97

Figure 5-12. Device parameters plotted against chromophore density and mass density

determined by X-ray diffraction of single crystals.

98

Figure 5-13. Transport parameters plotted against chromophore density, mass density, and

LUMO energy.

99

5.6 Current Density vs. Voltage Approaches

The simplest of current density to voltage comparisons is just that, a direct comparison of current

density at a given voltage for single carrier devices of the same thickness. Since this is not

typically possible to achieve practically we introduce the assumptions of a negligible injection

barrier (which is how the electrodes were selected) and a uniform voltage drop across the device.

We thus can divide the voltage by the device active layer thickness to calculate the electric field,

in other words normalizing the voltage to facilitate comparison across different single carrier

device thicknesses.

The results of this simple comparison for the four roughly constant HOMO derivatives explored

in Section 5.5 are shown in Figure 5-14. A higher current density is assumed to be better, just as

a higher mobility should theoretically correspond to better photovoltaic performance. From this

set of assumptions, we would predict a trend of F5-BsubPc > Phth-BsubPc > Cl-BsubPc > MsO-

BsubPc, which once again does not track well with the observed photovoltaic performance of Cl-

BsubPc > MsO-BsubPc > F5-BsubPc > Phth-BsubPc. In a similarly unhelpful manner, while our

mobility measurement for previously published materials1-2 would predict a trend of F12-BsubPc

~ F17-BsubPc > Cl-BsubPc ~ F5-BsubPc ~ Cl-Cl6BsubPc for greatest to least conductivity, the

current density vs. electric field curves (Figure 5-14(b)) suggest a trend of F5-BsubPc ~ F12-

BsubPc > Cl-Cl6BsubPc > Cl-BsubPc ~ F17-BsubPc. There is no consistency between the

admittance derived mobility trends and the current density vs. electric field trends. Figure 5-

14(c) is included to show the current density vs. electric field characteristics of F-BsubPc,

Cl4Phth-BsubPc and -Naphthoxy-BsubPc, for which no corresponding mobility data was able

to be determined by admittance spectroscopy (see discussion in Section 5.3). Relative to Cl-

BsubPc, this suggests that Cl4Phth-BsubPc and -Naphthoxy-BsubPc are worth further study

since their curves are so similar to Cl-BsubPc. Conversely, F-BsubPc shows such low current

density in comparison to Cl-BsubPc that it would be anticipated not to make a good acceptor.

100

0.0 0.2 0.4 0.6 0.8 1.0 1.210-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

Cur

rent

Den

sity

(A

cm

-2)

Electric Field (MV cm-1)

F5-BsubPc Phth-BsubPc Cl-BsubPc MsO-BsubPc

a)

0.0 0.5 1.010-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

Cur

rent

Den

sity

(A

cm

-2)

Electric Field (MV cm-1)

Cl-BsubPc Cl-Cl

6BsubPc

F5-BsubPc F12-BsubPc F17-BsubPc

b)

0.0 0.5 1.010-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

Cur

rent

Den

sity

(A

cm

-2)

Electric Field (MV cm-1)

Cl-BsubPc F-BsubPc Cl

4Phth-BsubPc

-naphthoxy-BsubPc

c)

Figure 5-14. Current density vs. electric field comparisons of electron only single carrier devices

for materials previously explored in Section 5.5 (a), for the materials previously published (b),

and for the materials which we were unable to extract a charge carrier mobility (Cl-BsubPc

included on this plot as a point of comparison) (c).

5.3.2 Space Charge Limited Current

Since the absolute minimal number of assumptions required to interpret the current density vs.

voltage data yielded predictions at odds with the admittance measurements, we turned to more

theoretical treatments of the data, which require a greater number of assumptions, but could yield

more deeply meaningful material properties (most desirably, determination of electron mobility

from a current voltage measurement). If we could identify a relation between our materials for

101

where mobility determined by admittance spectroscopy matched mobility from current vs.

voltage measurements, we could expand our data set to include materials where only the

admittance spectroscopy failed but current density vs. voltage had been measured. We began by

reanalyzing existing data for the compounds shown in Figure 1a using space charge limited

current (SCLC) assumptions and attempting a correlation between the two methods to see if each

gave similar or corelatable results. I was able to fit a field activated mobility (with a positive

Poole-Frenkel slope) and compare to the values already determined by admittance spectroscopy.

The results are summarized in Figure 5-15 and Table 5-4. The values determined by SCLC

differ both numerically (all underestimated the mobility compared to admittance spectroscopy)

and also did not preserve the trends seen in the admittance data. As described in Chapter 1, the

admittance determined mobility values require fewer assumptions, so we attribute these

differences to the additional assumptions inherent in SCLC calculations. Since not even the

trends in mobility are preserved, we are thus extremely hesitant to extrapolate an SCLC analysis

to the materials where admittance data did not yield mobility numbers.

0 400 800 1200

10-10

10-9

10-8

10-7

10-6

10-5

10-4

F5 Cf F12 Cf F17 Cf F5 JV F12 JV F17 JV

Ele

ctro

n M

obili

ty µ

e,dc

(cm

2 V-1s-1

)

F1/2 [(V/cm)1/2]

0.0 4.0x102 8.0x102 1.2x103

10-10

10-9

10-8

10-7

10-6

10-5

10-4

Ele

ctro

n M

obi

lity

(cm

2 (V

s)-1

)

Sqrt(Field) (V/cm)1/2

Cl Cf Cl7 Cf Cl JV Cl7 JV

Figure 5-15. Comparison of electron mobility determined by Admittance Spectroscopy (empty

boxes, Cf) vs. mobility determined by SCLC method modified for field dependent mobility(filled

boxes, JV). Mobility is underestimated to varying degrees by SCLC compared to AS.

102

Table 5-4. Comparison of calculated mobility parameters for the seven materials using

admittance spectroscopy (AS) and space charge limited current (SCLC).

Compound

and method

0

(cm2 V-1 s-1)

(MV/cm)-1/2

F5 AS 5E-7 7.3

F5 SCLC 2E-6 8.7

F12 AS 2E-5 3.4

F12 SCLC 5E-8 12

F17 AS 8E-6 3.3

F17 SCLC 3E-13 15

Compound

and method

0

(cm2 V-1 s-1)

(MV/cm)-1/2

Cl AS 1E-7 8.8

Cl SCLC 3E-9 12

Cl7 AS 8E-7 7.7

Cl7 SCLC 3E-13 21

MsO AS 3E-8 9.6

MsO SCLC 1E-13 24

Phth AS 1E-5 4.8

Phth SCLC 5E-12 24

From this, it is clear that the mobility parameters determined by SCLC are poorly correlated with

those determined by admittance spectroscopy. When these SCLC values are compared with the

photovoltaic device parameters reported in Section 5.5 and compared in the same manner as in

Section 5.5.1 there is little evidence of predictive power in this case either. Relevant plots are

shown in Figure 5-16.

103

Figure 5-16. Comparison of admittance and device parameters to space charge limited current

determined zero field mobilities.

104

5.3.3 Injection Limited Current

Finally, an attempt was made to analyze the data in terms of the injection limited current model

(ILC). Using the assumption that the density of states was equal to the molecular density, still

leaves the analyst with a single equation (Equation 5 in Chapter 1) and two unknowns (mobility

and barrier height). As described in Chapter 1, with no way to deconvolute these two values, an

infinite number of solutions exist which match the current density vs. voltage data. By

comparing the mobility measurements to the ILC curve fits for a material where we have both

data sets, such as Cl-BsubPc, as shown in Figure 5-17, we can choose a suitable barrier height to

yield matching mobilities. This would seem to be about a 0.5 eV barrier for Cl-BsubPc, which is

suspect given the high open circuit voltages achieved with these materials in photovoltaic cells.2

Even then, we are still faced with the question of how to treat the data sets where admittance

derived mobility data is unavailable, and thus conclude that ILC analysis is an equally unfruitful

approach to determining the charge carrier mobility of materials where admittance data is

suspect.

0 400 80010-16

10-13

10-10

10-7

10-4

10-1

102

105

108 B = 0 eV

B = 0.2 eV

B = 0.4 eV

B = 0.6 eV

B = 0.8 eV Cf mobility fit

Mob

ility

(cm

2 V-1 s

-1)

sqrt (Field) (V/cm)1/2

Figure 5-17. A plot of mobility vs. square root of electric field for Cl-BsubP comparing a range

of injection barrier heights to the mobility as measured by admittance spectroscopy (Cf).

105

5.7 Identifying an Electron Selective Electrode Contact

Others have also reported similar challenges when trying to measure the charge carrier mobility

of organic semiconductors, sometimes being unable to determine the mobility of seemingly well

ordered materials without the intentional doping of the material.21 We hypothesized that the

inability to resolve these peaks could be attributed to insufficiently ohmic contact at the

electrode. This reason would also help to explain the huge variability between reported

mobilities between different measurement techniques seen in the literature (see Chapter 1).

Therefore, in an attempt to understand the problems, we set out to screen a variety of electrode

contacts with a series of BsubPc derivatives in an attempt to find a universally ohmic

injection/extraction electron selective contact compatible with a range of BsubPcs. We selected

four BsubPc derivatives and five electron selective contacts, the molecular structures and

electron energy levels of which are shown schematically in Figure 5-18. The derivative Cl-

BsubPc, F5-BsubPc, and Cl-Cl6BsubPc were chosen because we had already successfully

measured their electron mobilities, so if we could be confident any change from that baseline

would be attributed to the electrical contact and Phth-BsubPc was chosen because it was a

BsubPc that we felt should be similarly characterizable due to its similar energy levels, yet we

had been unsuccessful in finding peaks to extract a charge carrier mobility. For the electrical

contacts, we chose BCP and BPhen because they were commonly used contacts in photovoltaic

cells. We selected LiF because of its long history in organic light emitting diodes and its

common use in bulk heterojunction photovoltaics. We selected TPBi due to its use in organic

photovoltaics and our previous success when using it as an electron transport layer in BsubPc

containing organic light emitting diodes. We also included a direct deposition of the aluminium

contact onto the BsubPc as a control, though there is much work in the field indicating that many

organic semiconductors are damaged and form poor contacts when aluminium is deposited by

physical vapour deposition on top of an organic layer.

106

a)

Figure 5-18. Boron subphthalocyanine compounds and electrode contacts used in this study and

their corresponding HOMO and LUMO energy levels in electron Volts (or work function of

aluminium and lithium fluoride layer followed by aluminium) (a), and single carrier device

structure for testing electron selective contacts (b).

Using the same technique as was earlier used to construct the single carrier devices for

admittance spectroscopy, we fabricated a series of BsubPc based single carrier devices with the

structure Al (50 nm)/BsubPc (200 nm)/Buffer Layer/ Al (100 nm), as shown in Figure 5-18.

Results are shown in Figure 5-19. The desirable characteristics are a high current density around

a field of 0.5 MV cm-1, as this is about the maximum field we would expect to be present in a

photovoltaic device, corresponding to a low resistance contact, and a low current density at zero

field, as this would indicate a low recombination current, corresponding to a good electron

selectivity for the contact.

107

Based upon these criteria, the TPBi and LiF performed similarly well for all four BsubPcs, with

essentially identically low zero field current densities across all four materials and TPBi having a

slightly higher current density for Cl-BsubPc and the pair being essentially indistinguishable for

the other three materials. In contrast, BCP, while appearing to be a more selective contact for

F5-BsubPc was otherwise of significantly lower current density than the TPBi and LiF. The

other contacts (ie. no buffer and BPhen) were sufficiently inferior when paired with F5-BsubPc

and Cl-Cl6BsubPc that we did not test them with Cl-BsubPc and Phth-BsubPc. We found TPBi

to yield the clearest admittance spectroscopy results and used it for subsequent single carrier

devices of materials that we had previously been unable to get meaningful measurement from

(ie. the materials in Figure 5-2). The results were similar to our previous attempt, which leads us

to hypothesize that the inability to obtain meaningful admittance signals from these compounds

is material specific as opposed to deriving from poor electrode choice. In summary, the

experiments indicated that TPBi and LiF form more ohmic electrical contacts than BCP, but this

better electrical contact did not lead to clearer or more sensible admittance spectroscopy data.

108

0.0 0.5 1.0 1.510-1210-1110-1010-910-810-710-610-510-410-310-210-1100101

C

urr

ent

Den

sity

(A

cm

-2)

Electric Field (MV cm-1)

BCP 10 nm LiF 1 nm TPBi 3 nm

(a)

0.0 0.5 1.0 1.510-1210-1110-1010-910-810-710-610-510-410-310-210-1100101

Cu

rre

nt D

ensi

ty (

A c

m-2)

Electric Field (MV cm-1)

BCP 10 nm LiF 1 nm TPBi 3 nm

(b)

0.0 0.5 1.0 1.510-1210-1110-1010-910-810-710-610-510-410-310-210-1100101

Cu

rre

nt D

ensi

ty (

A c

m-2)

Electric Field (MV cm-1)

No Buffer TPBi 10 nm BPhen 10 nm BCP 10 nm LiF 1 nm TPBi 3 nm

(c)

0.0 0.5 1.0 1.510-1210-1110-1010-910-810-710-610-510-410-310-210-1100101

Cu

rre

nt D

ensi

ty (

A c

m-2)

Electric Field (MV cm-1)

No Buffer TPBi 10 nm BPhen 10 nm BCP 10 nm LiF 1 nm TPBi 3 nm

(d)

Figure 5-19. Current vs. electrical field of electron only single carrier devices for several

electrode mediating contacts. Electron transport materials are Cl-BsubPc (a), Phth-BsubPc (b),

F5-BsubPc (c), and Cl-Cl6BsubPc (d).

109

5.8 Electron Selective Contacts in Photovoltaic Applications

Based on the promising electrical contact measurements discussed in Identifying an Electron

Selective Electrode Contact, we then took TPBi and LiF and fabricated photovoltaic cells and

contrasted these results with a baseline BCP cell. We selected a device configuration of

ITO/PEDOT:PSS/Pentacene (60 nm)/ BsubPc (25 nm)/ Electron Selective Layer/ Aluminium

(100 nm) (Figure 5-20) as we had previously published on Cl-BsubPc, Cl-Cl6BsubPc, and F5-

BsubPc with this cell configuration and BCP as an electron selective layer. For this study, we

used BCP (8 nm), TPBi (3 nm), TPBi (8 nm), and LiF (1 nm) as electron selective layers. We

selected 3 nm TPBi because it was a thickness that had previously yielded good performance in

F5-BsubPc OLEDs. We selected 8 nm TPBi so that we could have a near direct comparison of

the optical impact of TPBi compared to BCP, as the same layer thicknesses would lead to very

similar optical density distributions within the cells. LiF layers of 1 nm are selected as this is the

thickness commonly used in organic electronic devices, and larger thicknesses are not

conductive.

Figure 5-20. Photovoltaic device structure for testing electron selective contacts when BsubPcs

are used as acceptors.

When considering photovoltaic device, a number of different parameters are available to

describe the performance of the device. First, when looking at the current density vs. voltage

(JV) plots (Figure 5-22, Figure 5-24, Figure 5-26) a good photocurrent characteristic will start

our nearly horizontal with a near constant negative value with increasing voltage, until it reaches

a sharp upturn at a positive voltage where the current will then reach a positive value and

continue to increase sharply. This diode-like JV curve shape is indicative of good optoelectronic

110

performance. The photovoltaic performance is typically quantified by three parameters. The

first parameter is the short circuit current density (Jsc), which is the value of the current density at

zero applied bias (the y-axis intercept on the JV plot). The Jsc is primarily a function of the

number of excitons dissociated at the donor acceptor interface, and so is influenced by the

locations where photons are absorbed within the cell and the exciton diffusion length. The

second parameter is the open circuit voltage (Voc), which is the value of the voltage at zero

current. The Voc is primarily a function of the energy level offset between the electron donor

HOMO energy level and the electron acceptor LUMO energy level, and is also reduced by

interfacial electrical resistance. The third parameter is the fill factor (FF), is the ratio of

maximum power point to the product of the Jsc and Voc. The FF is primarily a function of the

charge carrier balance.15, 22 In other words the FF is maximized when the charge transport and

extraction rate of electrons in the electron accepting layer is matched to the charge transport and

extraction rate of holes in the electron donor layer. The product of these three parameters is the

maximum electrical power that can be extracted from the photovoltaic. The ratio of the

maximum electrical power to the incident photo flux power is the power conversion efficiency of

the device. While the ultimate concern of a commercial photovoltaic device is a high power

conversion efficiency, the prior three parameters are helpful for gaining insight into which

processes in influencing device performance.

For Cl-BsubPc containing devices, we found that BCP and LiF contacts yielded comparable

device efficiency. Devices with 3 nm of TPBi had slightly lower Jsc, which at first suggests a

slight drop in the exciton harvesting efficiency due to the change in optical field distribution

resulting from a thinner contact layer than the baseline 8 nm BCP. However a change in optics

seems a less convincing argument given the same was not observed for the even thinner LiF

layer. Nonetheless, unchanged Voc and FF imply minimal disruption to device electronic

properties. Those devices with 8 nm TPBi had dramatically lower Jsc, Voc, and FF, which

suggests major detrimental changes in both the excitonic and electronic properties of the device.

Device parameters are shown in Figure 5-21 while the JV curves are shown in Figure 5-22.

111

Figure 5-21. Device parameters for Pentacene/Cl-BsubPc photovoltaic cells. Error bars show

the 95 % confidence interval. Red corresponds to BCP (8 nm), green corresponds to TPBi (3

nm), blue corresponds to TPBi (8 nm) and teal corresponds to LiF (1 nm).

112

Figure 5-22. Current density-voltage plots for pentacene/Cl-BsubPc photovoltaic cells. Error

bars show the 95 % confidence interval. Red corresponds to BCP (8 nm), green corresponds to

TPBi (3 nm), blue corresponds to TPBi (8 nm) and teal corresponds to LiF (1 nm).

For Cl-Cl6BsubPc containing devices, we found that BCP and 8 nm TPBi showed comparable

device efficiency. Both 3 nm TPBi devices and LiF devices showed lower Jsc and FF. Similar

drops in Jsc suggest both buffer layers negatively impacted the optics and the drops in FF suggest

a negative impact on charge balance. Device parameters are shown in Figure 5-23 while the JV

curves are shown in Figure 5-24.

113

Figure 5-23. Device parameters for Pentacene/Cl-Cl6BsubPc photovoltaic cells. Error Error bars

show the 95 % confidence interval. Red corresponds to BCP (8 nm), green corresponds to TPBi

(3 nm), blue corresponds to TPBi (8 nm) and teal corresponds to LiF (1 nm).

114

Figure 5-24. Current density-voltage plots for pentacene/Cl-Cl6BsubPc photovoltaic cells. Error

bars show the 95 % confidence interval. Red corresponds to BCP (8 nm), green corresponds to

TPBi (3 nm), blue corresponds to TPBi (8 nm) and teal corresponds to LiF (1 nm).

For F5-BsubPc containing devices, we found that BCP and 3 nm TPBi showed comparable

device performance. The LiF devices had a slightly lower open circuit voltage suggesting an

increased contact resistance at the metal electrode. The 8 nm TPBi containing devices showed

very low short circuit current density suggesting a loss in exciton harvesting efficiency.

However, as with the Cl-BsubPc containing devices changes in the optical field distribution seem

unlikely for a change from 8 nm BCP to 8 nm TPBi. Device parameters are shown in Figure 5-

25 while the JV curves are shown in Figure 5-26.

115

Figure 5-25. Device parameters for Pentacene/F5-BsubPc photovoltaic cells. Error bars show

the 95 % confidence interval. Red corresponds to BCP (8 nm), green corresponds to TPBi (3

nm), blue corresponds to TPBi (8 nm) and teal corresponds to LiF (1 nm).

116

Figure 5-26. Current density-voltage plots for pentacene/F5-BsubPc photovoltaic cells. Error

bars show the 95 % confidence interval. Red corresponds to BCP (8 nm), green corresponds to

TPBi (3 nm), blue corresponds to TPBi (8 nm) and teal corresponds to LiF (1 nm).

In summary, for Cl-BsubPc as the electron acceptor a layer of 1 nm LiF is comparable to the

baseline BCP layer. For Cl-Cl6BsubPc as the electron acceptor a layer of 8 nm of TPBi is

comparable to the baseline BCP layer. For F5-BsubPc as the electron acceptor a layer of 3 nm of

TPBi is comparable to the baseline BCP layer. The other combinations sometimes showed

detrimental changes to device excitonics, other times showed detrimental changes to device

electronics, and sometime detrimental changes to both excitonics and electronics. The

inconsistency as to which electrical contact yields devices most comparable to the standard BCP

contact when paired with different BsubPc acceptors is especially perplexing given the

similarities in LUMO energies for Cl-BsubPc and F5-BsubPc. Further, if interfacial electrical

117

resistance were the limiting factor, then the extremely low work function of the LiF contact

should have been best with all three materials. In all, no universal electron selective contact

alternative to BCP was conclusively identified.

5.9 Conclusion

In summary, we sought to identify an analytical method that would allow the determination of

the charge carrier mobility of newly synthesized candidate semiconductor materials for organic

photovoltaic applications. We found the method of admittance spectroscopy to be desirable as it

required the fewest assumptions about the material to yield estimates of the mobility. While five

materials were readily characterized, five closely related materials yielded results inconsistent

with the observed electrical performance of the materials. Further investigation led to the

identification of an admittance artifact that obstructed mobility measurements in some materials.

From the electron mobility measurements we were able to extract, we predicted the photovoltaic

performance for when these materials were used as acceptors in photovoltaic cells. The trends

predicted from admittance spectroscopy did not hold when the materials were used in

photovoltaic devices.

We explored a variety of alternative techniques to characterize the materials, initially using more

complex admittance spectroscopy models but the data did not even fit the expected form of these

models. We also attempted current density vs. voltage models for space charge and injection

limited currents. The space charge limited models yielded different trends compared to the

admittance data that was available, thus extrapolation to data where admittance was unavailable

was deemed unsuitable. Injection limited models lacked crucial data about the injection barriers

between the electrode and the subphthalocyanine and so could yield almost any conceivable

mobility dependent upon the assumed barrier energy, thus leading to another dead end for

determining mobility.

Over the course of this investigation we also examined a number of possible electron selective

contacts and electrical measurements suggested that two of these materials (TPBi and LiF) could

be strong contenders to replace BCP as an electron selective contact in BsubPc acceptor

photovoltaic devices. Subsequent tests of these materials in photovoltaic cells showed that for

some material combinations TPBi and LiF yielded similarly efficient cells to the baseline BCP

containing cells, but no universally effective replacement for BCP was identified.

118

References 1. Castrucci, J. S.; Helander, M. G.; Morse, G. E.; Lu, Z.-H.; Yip, C. M.; Bender, T. P.,

Charge Carrier Mobility in Fluorinated Phenoxy Boron Subphthalocyanines: Role of Solid State

Packing. Cryst. Growth Des. 2012, 12, 1095-1100.

2. Beaumont, N.; Castrucci, J. S.; Sullivan, P.; Morse, G. E.; Paton, A. S.; Lu, Z.-H.;

Bender, T. P.; Jones, T. S., Acceptor Properties of Boron Subphthalocyanines in Fullerene Free

Photovoltaics. J. Phys. Chem. C 2014, 118, 14813-14823.

3. Mark, P.; Hartman, T. E., On Distinguishing between the Schottky and Poole-Frenkel

Effects in Insulators J. Appl. Phys. 1968, 39, 2163-2164.

4. Barsoukov, E.; Macdonald, J. R., Impedance Spectroscopy : Theory, Experiment, and

Applications. 2nd ed.; Wiley-Interscience: Hoboken, N.J., 2005; p 595.

5. Pandey, R.; Gunawan, A. A.; Mkhoyan, K. A.; Holmes, R. J., Efficient Organic

Photovoltaic Cells Based on Nanocrystalline Mixtures of Boron Subphthalocyanine Chloride and

C60. Adv. Funct. Mater. 2012, 22, 617-624.

6. BerkleyLabs X-Ray Properties of the Elements.

http://henke.lbl.gov/optical_constants/pert_form.html (accessed April 3, 2014).

7. Meller, A.; Ossko, A., Phthalocyaninartige Bor-Komplexe : 15c-Halogeno-

Triisoindolo[1,2,3-Cd:1',2',3'-Gh:1",2",3"-Kl]-[2,3a,5,6a,8,9a,9b]-Hexaazaboraphenalene.

Monatsh. Chem. 1972, 103, 150-155.

8. Schuster, C.; Kraus, M.; Opitz, A.; Brutting, W.; Eckern, U., Transport Properties of

Copper Phthalocyanine Based Organic Electronic Devices. Eur. Phys. J. Special Topics 2010,

180, 117-134.

9. Paton, A. S.; Morse, G. E.; Castelino, D.; Bender, T. P., Pseudohalides of Boron

Subphthalocyanine. J. Org. Chem. 2012, 77, 2531-2536.

10. Morse, G. E.; Helander, M. G.; Maka, J. F.; Lu, Z.-H.; Bender, T. P., Fluorinated

Phenoxy Boron Subphthalocyanines in Organic Light-Emitting Diodes. ACS Appl. Mater.

Interfaces 2010, 2, 1934-1944.

11. Sullivan, P.; Duraud, A.; Hancox, l.; Beaumont, N.; Mirri, G.; Tucker, J. H. R.; Hatton,

R. A.; Shipman, M.; Jones, T. S., Halogenated Boron Subphthalocyanines as Light Harvesting

Electron Acceptors in Organic Photovoltaics. Adv. Energy Mater. 2011, 1, 352-355.

119

12. Josey, D. S.; Castrucci, J. S.; Dang, J. D.; Lessard, B. H.; Bender, T. P., Evaluating

Thiophene Electron Donor Layers for the Rapid Assessment of Boron Subphthalocyanines as

Electron Acceptors in Organic Photovoltaics; Solution or Vacuum Deposition? ChemPhysChem

2015, 16, 1245-1250.

13. Cnops, K.; Rand, B. P.; Cheyns, D.; Verreet, B.; Empl, M. A.; Heremans, P., 8.4%

Efficient Fullerene-Free Organic Solar Cells Exploiting Long-Range Exciton Energy Transfer.

Nat. Comm. 2014, 5, 3406.

14. Tress, W.; Corvers, S.; Leo, K.; Riede, M., Investigation of Driving Forces for Charge

Extraction in Organic Solar Cells: Transient Photocurrent Measurements on Solar Cells Showing

S-Shaped Current–Voltage Characteristics. Adv. Energy Mater. 2013, 3, 873-880.

15. Tress, W.; Petrich, A.; Hummert, M.; Hein, M.; Leo, K.; Riede, M., Imbalanced

Mobilities Causing S-Shaped Iv Curves in Planar Heterjunction Organic Solar Cells. Appl. Phys.

Lett. 2011, 98, 063301-1 - 063301-3.

16. Kietaibl, H., Die Kristall-Und Molekiilstruktur Eines Neuartigen Phthalocyaniniihnliehen

Borkomplexes. Monatshefte fur Chemie 1974, 105, 405-418.

17. Fulford, M. V.; Jaidka, D.; Paton, A. S.; Morse, G. E.; Brisson, E. R. L.; Lough, A. J.;

Bender, T. P., Crystal Structures, Reaction Rates, and Selected Physical Properties of Halo-

Boronsubphthalocyanines (Halo = Fluoride, Chloride, and Bromide). J. Chem. Eng. Data 2012,

57, 2756-2765.

18. Morse, G. E.; Castrucci, J. S.; Helander, M. G.; Lu, Z. H.; Bender, T. P., Phthalimido-

Boronsubphthalocyanines: New Derivatives of Boronsubphthalocyanine with Bipolar

Electrochemistry and Functionality in Oleds. ACS Appl. Mater. Interfaces 2011, 3, 3538-44.

19. Fulford, M. V.; Lough, A. J.; Bender, T. P., The First Report of the Crystal Structure of

Non-Solvated -Oxo Boron Subphthalocyanine and the Crystal Structures of Two Solvated

Forms. Acta Cryst. B 2012, 68, 636-645.

20. Menke, S. M.; Luhman, W. A.; Holmes, R. J., Tailored Exciton Diffusion in Organic

Photovoltaic Cells for Enhanced Power Conversion Efficiency Nat. Mater. 2013, 12, 152-157.

21. Gommans, H. H. P.; Kemerink, M.; Andersson, G. G.; Pijper, R. M. R., Charge Transport

and Trapping in Cs-Doped Poly(Dialkoxy-P-Phenylene Vinylene) Light-Emitting Diodes. Phys.

Rev. B 2004, 69, 155216, 1-4.

120

22. Tress, W.; Leo, K.; Riede, M., Influence of Hole-Transport Layers and Donor Materials

on Open-Circuit Voltage and Shape of I-V Curves of Organic Solar Cells. Adv. Funct. Mater.

2011, 21, 2140-2149.

121

Chapter 6 Considerations for the physical vapor deposition of high molar

mass organic compounds

Adapted with permission from Castrucci, J. S.; Dang, J. D.; Kamino, B. A.; Campbell, A.; Pitts,

D.; Lu, Z.-H.; Bender, T. P., Considerations for the Physical Vapor Deposition of High Molar

Mass Organic Compounds. Vacuum 2014, 109, 26-33. Copyright 2014 Elsevier Ltd. This chapter

was originally published as I led the execution of the experiments, interpreted the data, and

wrote most of the manuscript. JDD and BAK aided with the execution of the experiments and

synthesized the materials. AC, DP, ZHL, and TPB directed the research. All authors approved

the manuscript.

122

Chapter 6 Considerations for the Physical Vapor Deposition of High Molar

Mass Organic Compounds

6.1 Introduction

The last two decades have seen significant increases in the interest in organic semiconductors

and a corresponding increase in the variety of novel organic semiconductor compounds that have

been synthesized. New candidate organic semiconducting materials are regularly designed and

synthesized with targeted applications in organic field effect transistors (OFETs),1 organic light

emitting diodes (OLEDs),2 and organic photovoltaic (OPV)3 devices. When small organic

molecules are targeted (for example < 700 g mol-1) a common technique for the preparation of

the resulting devices is the sublimation of the material from a resistively heated crucible at

reduced pressures (< 10-6 Torr, < 10-4 Pa) to form nanometer scale thin films on a targeted

substrate located above and some distance away from the crucible, a process referred to as

physical vapor deposition (PVD).4

The successful use of PVD for organic compounds is predicated upon heating an organic

electronic material to a point where it is volatile enough to undergo sublimation and be

subsequently deposited at an appreciable rate (usually in the range 0.05-0.2 nm s-1) onto the

target substrate without thermal degradation either in the crucible or in the gas phase while in

transition between the crucible and the target substrate. Thus the vapor pressure of a compound

can have a major impact on its suitability as a PVD depositable organic electronic material. All

other things being equal, increasing the molar mass of a material will decrease its vapor pressure,

suggesting high molar mass semiconductors may pose a challenge to deposit via traditional PVD

techniques.

Chemical engineers have a long history of working with the inorganic semiconductor industry in

the development of industrial PVD processes,5-7 and more recent reports on film uniformity exist

in the literature,8 but discussion on the unique challenges of organic semiconductors in contrast

to existing process knowledge accumulated for inorganic processes is sparse.9 Such information

would be valuable especially during initial screening of new candidate compounds when often

only a small amount of the new material will be available for testing due to synthetic limitations

123

in early stage development. While such information may be available to those who have

practiced in the field, whether in industry or academia, we were unable to find publications in the

literature on to how to deal with these challenges, especially once the additional complications of

a rotating substrate holder that is not coaxial with the source and non-point sublimation sources

are considered.

Additionally, newly conceived and synthesized organic semiconductors are very valuable.

Before making devices each new material requires a test deposition to determine the ratio

between test material reading on a quartz crystal microbalance (QCM) and the actual film

thickness to establish a tooling factor is required. The establishment of a tooling factor consumes

material without actually producing a functional organic electronic device. If the tooling factor

for a new material could be predicted, then the test deposition and the associated use of the

valuable organic material could be avoided. Thus there is a desire to identify a universal tooling

factor for a given system that would help with the rapid testing of new materials and the

maximization of new material usage.

In this paper, we have used a variety of semiconductor materials of proportionally increasing

molar mass that can only be made in relatively small quantities to identify how increasing molar

mass impacts the physical vapor deposition of films. We have performed the depositions using a

small PVD chamber with a rotating substrate holder and four resistive heating sources. We have

explored how changes in the source to substrate positioning and the type of thermal source type

(including a non-line of sight source, a single aperture source, and a multi-aperture source)

influence film yield, mass utilization and film uniformity. We also report helpful relationships

for estimating the tooling factor of a new material. Additionally, as an aside and important to

our laboratory and others, we also report that axial fluorination of a subphthalocyanine10,11 can

offset an increase in molar mass and increase the resulting vapor pressure.

6.2 Experimental

The structure of the six compounds explored in this study and their corresponding molecular

weights are shown in Figure 6-1. Cl-BsubPc,12 F5-BsubPc,10 -oxo-(BsubPc)2,13 and F5-

GBsubPc14 were synthesized and purified by train sublimation as previously reported.

Pentafluorophenoxy boron subnaphthalocyanine (F5-BsubNc) was prepared in a two-step

124

process starting with the cyclotrimerization of 2,3-dicyanonaphthalene with boron trichloride to

give chloro boron subnaphthaloycanine (Cl-BsubNc), followed by an axial displacement reaction

with pentafluorophenol. Alq3 was purchased from Sigma-Aldrich and used as received.

Figure 6-1. Compound structures, corresponding abbreviations, and corresponding molecular

weights of the compounds used in this study.

All films were formed by vacuum depositions conducted in an Angstrom Engineering Covap II

vacuum system with a base pressure < 10-7 Torr (< 10-5 Pa) and at operating pressure < 5 × 10-6

Torr (< 7 × 10-4 Pa). Material deposition was monitored by a quartz crystal microbalance

(QCM). Samples were mounted on a circular substrate holder which revolved at 40 rotations per

minute. Several types of thermal sources were used to sublime the organic materials, in

particular the ME2-.005Ta boat with either the ME2A-.005Ta (single aperture) top baffle or

ME2B-.005Ta (multiple aperture) top baffle and the SB-6 with SB-6A top baffle. All thermal

sources were purchased from The R.D. Mathis Company and were thoroughly degassed under

vacuum while empty before being used for depositions. The width of the thermal source holders

125

on the Covap II mean that the ME2 could be placed at either the inner edge of the holder or the

outer edge, as shown in Figure 6-2. The SB-6 is wider than the ME2 sources, so it filled the

entire thermal source holder and thus no inner edge vs. outer edge alignment distinction was

relevant for the SB-6.

Figure 6-2. (next page) The internal layout of the vacuum system used, stripped down to

highlight key components, including the rotating substrate holder (the axis of substrate holder

rotation is the black line), copper posts for thermal boat mounting, and quartz crystal

microbalance (QCM) (a). A top-down view of the system base highlighting the difference

between possible inner edge and outer edge alignments of the ME2 thermal boats (b). A bottom-

up view of the system substrate holder showing the radial positioning of substrates (green

rectangles) upon which films are deposited in this study (c). The ME2A single aperture top

thermal boat (d) and the ME2B multiple aperture top thermal boat (e). The SB-6A thermal boat

(f). All thermal boat images are from the supplier's website.23

126

127

Organic films were deposited on two types of substrates which were subjected to two different

surface treatments. Clear glass microscope slides were purchased from VWR. ITO patterned

glass slide were purchased from Kintec (sheet resistance of 15 Ω/). Both types of substrates

were rinsed with methanol before use. For the hydrophobic treatment, the slides were rinsed

with methanol then set in a freshly prepared solution of 1 mL of trichlorododecylsilane in 40 mL

of toluene at 60 °C for 20 minutes, then rinsed again with methanol. A defined area on each

substrate was masked with Kapton tape before being put into the Covap II for deposition. After

the film was deposited and the sample removed from the Covap II, and the Kapton tape was

immediately removed to create a step edge for determining film thickness. Immediate removal

of the Kapton tape was important to ensure no adhesive residue stuck to the substrate which

might have distorted the subsequent thickness measurements. A target film thickness for all

materials of about 100 nm was selected as this is on the order of thicknesses used in OPVs and

OLEDs.

Film thicknesses were measured using 2D surface profilometry of the step edges. All

thicknesses are reported as average ± one standard deviation of at least three step edge

measurements separated by 100 m along the step edge. Further, each film was measured at

three points along the radius of the circular substrate holder: near the centre of the substrate

holder, near the outer edge of the substrate holder, and at the midpoint of the radius. Surface

profilometry was performed using a KLA-Tencor P16+ with gentle scan parameters so as to not

damage the organic films: scan speed 20 m/s, sampling rate 20 Hz, applied force 2 mg. A scan

length of at least 1000 m was used for each step edge to ensure distortions near the step edge

did not influence the thickness measurements.

Yield of film formed was calculated by taking the average film thickness and dividing by the

difference between the mass of the thermal boat and deposition material post-deposition vs. the

mass of the thermal boat and deposition material pre-deposition. Film uniformity was evaluated

by comparing the measured thicknesses at the previously described three points along the

substrate.

To determine the tooling factor for each material at each source position, the QCM was set at a

tooling factor of 100 % with a density of 1.000 g cm-3 and a Z-Ratio of 1.000 and the film was

deposited. The ratio of the measured film thickness to the film thickness reported on the QCM,

128

expressed as a percentage, was then taken as the tooling factor for that combination of material

and source geometry.

Absorbance spectrometry was performed with a Perkin Elmer Lambda 25 UV/Vis Spectrometer.

6.3 Results and Discussion

For this study we selected six compounds that are of interest to our laboratory each with

proportionally increasing molar mass (Figure 6-1): Cl-BsubPc12,15 was selected as a widely used

organic photovoltaic material; F5-BsubPc is a subphthalocyanine derivative that we have used in

a variety of light emitting devices;10,11 Alq3 was chosen as a reference point because it is widely

used in organic light emitting diodes (OLEDs) with a long history in the field;16,17 -oxo-

(BsubPc)2,13 F5-BsubNc, and F5-GBsubPc14 were selected because we have synthesized these

compounds, each are of increasing molar mass and each has been synthesized in our laboratory

but each has yet to be applied in an organic electronic device. While most materials currently

incorporated into organic electronic devices by PVD have molar masses less than 700 g mol-1,

for this study we have selected a series of compounds with molar masses spread out

proportionally over the range of ~400 g mol-1 to over 1200 g mol-1.

For this study we used the Angstrom Engineering CoVap II as a representative PVD system due

to its configurational versatility including having multiple distances for sources to be offset from

the axis of rotation of the substrate. As of note, is the relatively short throw distance of the

configuration (distance between the thermal source and the substrate). A short throw distance is

expected to be desirable for improved yield of film thickness per mass of material sublimed. A 3-

D schematic view of the arrangement of components with notable distance for the CoVap II is

shown in Figure 6-2a.

Nanometer thick films of the compounds illustrated in Figure 6-1 were formed by sublimation in

vacuum from an electrically resistive heat source (Figure 6-2b) onto an unheated glass substrate

target (Figure 6-2c) that was rotated above the sample with an axis of rotation that was not co-

axial with the electrically resistive heat source and the resulting plume of material. Experimental

variables explored included resistive source location relative to the conductive posts (Figure 6-

2b) and resistive source type. A source with a single aperture for sublimate egress (ME2A,

129

Figure 6-2d) was compared with a source with multiple apertures (ME2B, Figure 6-2e). ME2

sources where chosen as they require/consume only small amounts of material which is in line

with our desire to study situation whereby mass utilization is maximized when only small

amounts of material is available (an ME2 sources takes ~100 mg of material). Due to their small

size ME2 sources could also be tested on the inner and outer edge of the conductive posts

(Figure 6-2b). A third non-line-of-sight source (SB-6A, Figure 6-2f) as also evaluated. A non-

line-of-sight source is one where the organic material is placed in left and right compartment of

the boat (Figure 6-2f). On heating and when sublimation occurs the vapor moves from the left

and right compartment to the middle and egresses via the apertures present only above the

middle compartment. A SB-6 source takes ~400 mg of material. These three classes of sources

broadly cover the primary differences found between resistively heated thermal sources. It is

expected, and has been recently modeled, that a source with multiple apertures will provide a

flux pattern reminiscent of the superposition of several point sources, thus resulting in a more

uniform flux pattern and film.18 Each source was constructed from tantalum.

Actual film thickness were determined by partially masking the substrate with a piece of Kapton

tape creating intentional step edges which were then measured by contact profilometry. Initial

experiments showed little difference in measured film thicknesses between ITO patterned glass

and plain glass microscope slide substrates, so subsequent experiments were done exclusively

with glass microscope slides. Additional preliminary experiments showed little difference in

measured film thicknesses between methanol cleaned and hydrophobicly treated substrates, so all

subsequent data reported here is for methanol cleaned glass substrates only.

Each thin film was also analyzed using absorbance spectrometry to confirm that the films of each

compound had comparable absorption profiles to what was measured using solution absorption

spectroscopy. Comparable absorption profiles were taken to indicate no decomposition of the

molecules had occurred during deposition.

Beginning with Cl-BsubPc (Figure 6-1), film yields when using ME2A, ME2B and SB-6A

sources were calculated (Figure 6-3a). It was found that the yield was significantly lower for the

SB-6A compared to the ME2A and ME2B sources (about 50% less). Also, deposition from the

SB-6A source required ~ 30% higher heating power to be applied to the crucible to achieve a

comparable deposition rate compared to the ME2A and ME2B sources (14% of total heater

130

power compared to 10.5% of total heater power). While non-line-of-sight sources are

advantageous for the deposition of many elemental inorganic materials, in our case, significantly

reduced yields for organic depositions and the need to load more materials and use more power

lead us to omit the SB-6A source from the remainder of our study.

Figure 6-3. Yield of film formed (nm film formed per mg of material consumed) as a function of

material deposition source and position for Cl-BsubPc (a), F5-BsubPc (b), Alq3 (c), and -oxo-

(BsubPc)2 (d). Error bars show calculated measurement uncertainty.

In terms of film yield, the use of the ME2B source resulted in higher film yields than the ME2A

sources for all four compounds tested (Figure 6-3a-d, Cl-BsubPc, F5-BsubPc, Alq3 and -oxo-

(BsubPc)2 respectively). Typical yields for the ME2A source were between 0.5 to 2.0 nm/mg

while the ME2B source yields were 2.5 to 3.5 nm/mg. Yield only changed within experimental

error when changing the ME2 source from the inner to the outer edge of the source holder. Film

ME2A, inner edge

ME2A, outer edge

ME2B, inner edge

ME2B, outer edge

SB6, only location

0

1

2

3

4

Yie

ld (

nm/m

g)

Boat type and location

a)

ME2A, inner edge

ME2A, outer edge

ME2B, inner edge

ME2B, outer edge

0

1

2

3

4b)

Yie

ld (

nm/m

g)Boat type and location

ME2A, inner edge

ME2A, outer edge

ME2B, inner edge

ME2B, outer edge

0

1

2

3

4

Yie

ld (

nm/m

g)

Boat type and location

c)

ME2A, inner edge

ME2B, inner edge

ME2B, outer edge

0

1

2

3

4

Yie

ld (

nm/m

g)

Boat type and location

d)

131

yield showed no strong trends related to molar mass. The larger uncertainty in the yields for -

oxo-(BsubPc)2 are attributable to subliming smaller amounts of material per run, due to the

relatively small amounts of this material we had on hand, thus introducing greater relative error

in the mass consumption measurement.

To achieve a deposition rate of ~1 Å/s required different power settings for each material but was

consistent between the ME2A and ME2B sources. In order of increasing power required for

deposition: F5-BsubPc < Cl-BsubPc ~ Alq3 < -oxo-(BsubPc)2 < F5-GBsubPc < F5-BsubNc.

When comparing the deposition power of Cl-BsubPc to F5-BsubPc, it is notable that while F5-

BsubPc has a higher molar mass (578.26 g mol-1 vs. 430.66 g mol-1), Cl-BsubPc requires a

higher power to sublime and therefore a lower vapor pressure. This is in contrast to the trend

observed with phthalocyanine compounds, where fluorination tends to decrease vapor pressure,

even for relatively non-planar phthalocyanines such as Cl-AlPc.19-21 While the correlation

between vapor pressure and chemical composition of organic electronics materials is complex,22

in the case of BsubPcs, the decrease in vapor pressure assumed to coincide with an increase in

mass appears to be offset by the increase in mass being complemented by axial fluorination.

It is unsurprising that higher power (and correspondingly higher temperatures) are required for

physical vapor deposition of higher molar mass materials. However, during our experiments, the

higher temperatures did not result in degradation of the organic material (as confirmed by

absorbance spectroscopy). For example, depositions of -oxo-(BsubPc)2 (806.41 g mol-1)

occurred without issue throughout the study. Additionally, thin films (~15 nm) of F5-BsubNc

(728.43 g mol-1) and F5-GBsubPc (1214.73 g mol-1) were also deposited without issue from an

ME2B source. In the case of F5-BsubPcNc and F5-GBsubPc, only limited quantities of material

were available therefore these two compounds were not subjected to the range of yield and

uniformity tests that the other compounds in this study. The solid state absorbance spectra for

thin films of -oxo-(BsubPc)2, F5-BsubNc and F5-GBsubPc are reported for the first time in the

Electronic Supporting Information Figure S1 and Table S1. The ease to which F5-BsubNc and

especially F5-GBsubPc deposited is further evidence that the addition of fluorinated moieties

lowers the sublimation temperature (increases the vapor pressure) of subphthalocyanine

compounds. This suggests that in the event that continued synthetic efforts identify even higher

molar mass BsubPcs that do degrade during physical vapor deposition attempts, axial

132

fluorination may be a useful approach to lowering the sublimation temperature and preventing

degradation.

In addition film yield and compound volatility/vapor pressure, film uniformity is an important

descriptor of the PVD process. The rotating substrate holder results in a radially uniform film

profile, so film thickness is reported relative to distance from the axis of rotation (the center) of

the substrate holder. A simplified schematic of the position of heater and substrate holder is

shown in Figure 6-4a. The distance from the axis of rotation to a point on the substrate holder is

denoted as r, and the image also illustrates how the angle from the normal of the heater source to

the point on the rotating holder will vary between an angle and an angle ' as the substrate

holder rotates. Rotation of the substrate typically results in less thickness variation due to

averaging each point through a variety of positions in the sublimation flux profile in contrast to a

fixed substrate position. The film thicknesses are normalized to the maximum observed

thickness to facilitate comparison of film uniformity between films of different total thickness.

Uniformity for each of the four compounds deposited from the different ME2 source tops and

source positions are shown in Figure 6-4b-e. On the whole, ME2B sources result in more

uniform films than ME2A sources and of ME2B source positions, alignment with the outer edge

of the heater mount resulted in greater radial film uniformity. This is in line with the recent

observations of Kamimura et al who modeled and executed experiments showing enhanced film

uniformity was produced by an array of small apertures covering a resistive heating source in

contrast to the film uniformity of a single aperture, even at very close source to substrate

distances.18 There also appears to be a weak trend that film uniformity decreases as molar mass

increases, but further study of the sublimation flux profiles would be required to draw concrete

numerical conclusions validating or not this trend.

133

Figure 6-4. A simplified diagram of the rotating substrate relative to the thermal boat heater.

The distance from the axis of substrate holder rotation is marked as r. Note how the substrate

rotation causes the point to move from angle to angle ' (a).Plots of normalized film thickness

versus distance from axis of rotation as a function of thermal boat type and position for Cl-

0.2

0.4

0.6

0.8

1.0

0 10 20 30 40 500.2

0.4

0.6

0.8

1.0

ME2A Inner Edge ME2A Outer Edge

Nor

mal

ized

Film

Thi

ckne

ss

ME2B Inner Edge ME2B Outer Edge

Distance from Axis of Substrate Holder Rotation (mm)

b)

0.2

0.4

0.6

0.8

1.0

0 10 20 30 40 500.2

0.4

0.6

0.8

1.0

ME2A Inner Edge ME2A Outer Edge

No

rmal

ize

d F

ilm T

hick

ness

ME2B Inner Edge ME2B Outer Edge

Distance from Axis of Substrate Holder Rotation (mm)

c)

0.2

0.4

0.6

0.8

1.0

0 10 20 30 40 500.2

0.4

0.6

0.8

1.0

ME2A Inner Edge ME2A Outer Edge

Nor

mal

ized

Film

Thi

ckne

ss

ME2B Inner Edge ME2B Outer Edge

Distance from Axis of Substrate Holder Rotation (mm)

d)

0.2

0.4

0.6

0.8

1.0

0 10 20 30 40 500.2

0.4

0.6

0.8

1.0

ME2A Inner Edge

Nor

mal

ize

d F

ilm T

hick

ness

ME2B Inner Edge ME2B Outer Edge

Distance from Axis of Substrate Holder Rotation (mm)

e)

134

BsubPc (b), F5-BsubPc (c), Alq3 (d), and -oxo-(BsubPc)2 (e). Error bars are one standard

deviation calculated from three measurements at each location.

The ability to predict tooling factors when introducing new materials into a system would be a

boon to the testing of new materials as separate deposition runs and sensitive thickness

determination equipment could be eliminated and device fabrication could begin directly without

utilization of precious material. Tooling factors for this study are reported in Figure 6-5 when

calculated using an assumed film density of 1.000 g cm-3. There are no clear trends in the

experimentally determined tooling factors as a function of molecular composition, molar mass,

source type or source position. Figure 6-S2 in the Electronic Supporting Information shows the

same tooling factors calculated assuming a film density equal to the single crystal density based

upon previously reported single crystal X-ray diffraction data. The conclusion is the same as for

when a film density of 1.000 g cm-3 is assumed, there is no indication of a universal tooling

factor.

135

Figure 6-5. Calculated tooling factor as a function of material deposited from various boat

configurations including ME2A inner edge (a), ME2A outer edge (b), ME2B inner edge (c), and

ME2B outer edge (d). Calculation assumes film density of 1 g cm-3. Error bars show

measurement precision.

6.4 Conclusions

In summary, the desire of our group and others is to maximize the utilization of highly valuable

organic semiconducting molecules newly designed and synthesized. To that end, we have

presented a study whereby factors such as selection of the type of resistive heating sources and

their location within a deposition system can lead to maximum mass utilization and film

uniformity. The configuration that maximizes both these objectives within the predetermined

geometry of the CoVap II system is the use of ME2B style boats align with the outer edge of

1Cl-BsubPc2F5-BsubPc

3Alq3 4mu-oxo-BsubPc

0

20

40

60

80

100

120

140

160

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(%)

Material

a)

1Cl-BsubPc2F5-BsubPc

3Alq30

20

40

60

80

100

120

140

160b)

To

olin

g (

%)

Material

1Cl-BsubPc2F5-BsubPc

3Alq3 4mu-oxo-BsubPc

0

20

40

60

80

100

120

140

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Material

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3Alq3 4mu-oxo-BsubPc

0

20

40

60

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Material

136

heating posts. Some general conclusions can also be drawn from this study. For example, when

selecting or designing a new research physical vapor deposition (PVD) system, the use of

multiple aperture topped line of sight sources result in superior film yield and uniformity

compared to single aperture and non-line of sight thermal sources. Small differences in the

distance from the axis of rotation to the thermal source is another important variable.

We have also demonstrated that high molar mass materials can be deposited from thermal

sources in the same manner as lower molar mass materials and that partial and/or axial

fluorination of subphthalocyanine compound lowers the required deposition power (and

temperature). Other factors yet to be explored which might also contribute to high mass use of

high molar mass compounds include exploring other thermal sources, such as open topped

ceramic crucibles and Knudsen cell type furnaces and exploration of further film properties, such

as the impact upon film roughness.

137

References

1. Klauk H. Organic thin-film transistors. Chem. Soc. Rev. Jul 2010;39(7):2643-2666.

2. Walzer K, Maennig B, Pfeiffer M, Leo K. Highly Efficient Organic Devices Based on

Electrically Doped Transport Layers. Chem. Rev. 2007;107:1233-1271.

3. Hains AW, Liang Z, Woodhouse MA, Gregg BA. Molecular semiconductors in organic

photovoltaic cells. Chem. Rev. Nov 10 2010;110(11):6689-6735.

4. Ohring M. Materials Science of Thin Films, 2nd Edition. San Diego: Academic Press;

2002.

5. Russell TWF. Semiconductor Chemical Reactor Engineering and Photovoltaic Unit

Operations. Chem. Eng. Educ. 1985;1985(2):72-77,106-108.

6. Jackson SC, Baron BN, Rocheleau RE, Russell TWF. A Chemical Reaction Model for

Physical Vapor Deposition of Compound Semiconductor Films. AIChE J.

1987;33(5):711-721.

7. Junker ST, Birkmire RW, Doyle FJ. Mass and heat transfer modeling of a physical vapor

deposition effusion source. AIChE J. 2005;51(3):878-894.

8. Zhong ZW, Wang JH. Uniformity and characterisation of PVD aluminium films. Surf.

Eng. 2005;21(2):119-124.

9. Morgan NT, Zhang Y, Molitor EJ, Bell BM, Holmes RJ, Cussler EL. Understanding rate-

limiting processes for the sublimation of small molecule organic semiconductors. AIChE

J. 2014;DOI: 10.1002/aic.14357.

10. Morse GE, Helander MG, Maka JF, Lu Z-H, Bender TP. Fluorinated Phenoxy Boron

Subphthalocyanines in Organic Light-Emitting Diodes. ACS Appl Mater Interfaces.

2010;2(7):1934-1944.

11. Helander MG, Morse GE, Qiu J, Castrucci JS, Bender TP, Lu ZH. Pentafluorophenoxy

boron subphthalocyanine as a fluorescent dopant emitter in organic light emitting diodes.

ACS Appl Mater Interfaces. Nov 2010;2(11):3147-3152.

12. Morse GE, Paton AS, Lough A, Bender TP. Chloro boron subphthalocyanine and its

derivatives: dyes, pigments or somewhere in between? Dalton Trans. Apr 28

2010;39(16):3915-3922.

138

13. Fulford MV, Lough AJ, Bender TP. The first report of the crystal structure of non-

solvated mu-oxo boron subphthalocyanine and the crystal structures of two solvated

forms. Acta Crystallogr., Sect. B: Struct. Sci. Dec 2012;68(Pt 6):636-645.

14. Kamino BA, Bender TP. Modified boron subphthalocyanines with stable

electrochemistry and tuneable bandgaps. Dalton Trans. Sep 28 2013;42(36):13145-

13150.

15. Mutolo KL, Mayo EI, Rand BP, Forrest SR, Thompson ME. Enhanced Open-Circuit

Voltage in Subphthalocyanine/C60 Organic Photovoltaic Cells. J. Am. Chem. Soc.

2006;128:8108-8109.

16. Tang CW, VanSlyke SA. Organic electroluminescent diodes. Appl. Phys. Lett.

1987;51(12):913-915.

17. Deshpande RS, Bulović V, Forrest SR. White-light-emitting organic electroluminescent

devices based on interlayer sequential energy transfer. Appl. Phys. Lett. 1999;75(7):888-

890.

18. Kamimura Y, Okada H, Naka S. Evaluation of uniformity for organic film evaporation

using two dimensional different apertures. Vacuum. 2013;92:26-31.

19. Basova TV, Kiselev VG, Sheludyakova LA, Yushina IV. Molecular organization in the

thin films of chloroaluminium hexadecafluorophthalocyanine revealed by polarized

Raman spectroscopy. Thin Solid Films. 2013;548:650-656.

20. Semyannikov PP, Basova TV, Grankin VM, Igumenov IK. Vapour pressure of some

phthalocyanines. J. Porphyrins Phthalocyanines. 2000;4:271-277.

21. Bonderman D, Carter ED, Bennett WE. Vapor Pressures, Mass Spectra, Magnetic

Susceptibilities, and Thermodynamics of Some Phthalocyanine Compounds. J. Chem.

Eng. Data. 1970;15(3):396-400.

22. Shalev O, Shtein M. Effect of crystal density on sublimation properties of molecular

organic semiconductors. Org. Electron. 2013;14:94-99.

23. https://www.rdmathis.com/storev2/default.asp

139

Chapter 7 Construction of a Vacuum Deposition System

This chapter was written entirely by me. In Section 2, N. Beaumont (my co-first author from

Chapter 2) aided with the experiments conducted on the OLGA system. In Section 3, I

conceived of the experiments and process limitations and directed A. Balawi in the development

of the finite element model and experiments used to address the thermal distribution questions.

140

Chapter 7 Construction of a Vacuum Deposition System

To facilitate the high throughput testing of new materials developed in the Bender laboratory, we

designed and constructed, primarily from existing spare parts and salvage, a physical vapour

deposition (PVD) system vacuum integrated with an inert atmosphere processing system to

fabricate and characterize organic containing photovoltaic devices. We drew upon my and my

lab mates’ experiences using a number of commercial PVD systems during our design process

including the Angstrom CoVap System (Chapter 6), Professor Timothy S. Jones' Kurt J. Lesker

Super Spectros, Professor Neal Armstrong's custom linear deposition system, and the Lu

laboratory's Kurt J. Lesker Luminos Cluster Tool. Of particular concern in the design of our

system was system flexibility, flexibility of fabrication methods and the maximization of film

yields (i.e. maximizing the thickness of film formed per unit mass of organic semiconductor

sublimed). We will first discuss the film forming calculations used to develop our apparatus, and

second will discuss benchmarking the performance of photovoltaic devices fabricated by our

system. Finally, we discuss the thermal design considerations of the system. Appendix D

contains the schematics of the MARI-KATE base plate, piping and instrumentation diagrams,

and the electronics of the heater system, all designed by me and used in the commissioning of the

system.

7.1 Material-to-Film Yield Calculations

It is highly desirable to maximize film yield when testing new materials, as new materials are

typically first synthesized and purified at around the 100 mg scale, meaning they are initially

available in very limited quantities and took many personnel hours to synthesize. When the

material is placed in a crucible and sublimed in a vacuum chamber, the sublimed material will

fly in linear translation as the mean free path of the molecules greatly exceeds the distance

between the crucible and the substrate. To predict the distribution of sublimed material emitted

upon heating, the flux distribution of this linear motion must be described. From this distribution

and the location of the substrate relative to the source, the film thickness variability can be

predicted by comparing the maximum and minimum flux intensities incident on the substrate.

141

Further the film yield can be predicted by using fraction of the total flux that is incident upon the

substrate and the material density.

Typically crucibles are covered with a perforated cap to contain and improve the uniformity of

the heat distribution in the crucible and minimize the unwanted ejection of unsublimed material

into the rest of the chamber. As such, most crucibles are best approximated as a point source or

set of point sources.

First, we will define our co-ordinate system, then we will work through these calculation for the

simplest case of a substrate located coaxially with a point source in Example 1. Then we will

explore the more relevant cases for practical vacuum systems, where the sources are not coaxial

with the rotation of the substrate in Example 2. Finally, we will discuss the design of the MARI-

KATE system, which uses elements of both Example 1 and Example 2.

7.1.1 Terminology and Definitions

To facilitate the flux field exploration examples to follow, we first establish a series of

definitions of key points, dimensions, and angles, which are labeled in Figure 7-1 and Figure 7-2,

corresponding to Example 1 and Example 2, respectively. By setting the point source as the

origin of a co-ordinate system, we then define the xy plane upon which the point is located (i.e.

the plane of the crucible lid), with the z-axis as the normal to the xy plane passing through the

centre of the substrate. This then locates the Origin, O, as the point where the z-axis intersects

the xy-plane. Set the x-axis as the direction from the origin to the centre of the boat, or located it

arbitrarily if the centre of the boat and the axis of rotation lie on the same line. We take that the

substrate plane is parallel to the plane of the boat. We then define an angle from the source

normal to any point in the system (as in a spherical co-ordinate system).

The flux distribution is then typically described by an equation of the form

cos (7-1),

where t is the intensity (corresponding to the thickness) at a given angle and is the

maximum flux intensity of the distribution. For a typical source, n = 4. Note the radial

symmetry of this distribution. This distribution is illustrated in Figure 7-1. Additionally,

142

h - point located at the centre of the top plane of the thermal boat

s - any arbitrary point on the substrate, s moves as the substrate rotates.

r - minimum distance from the z-axis to point s

R - distance from the z-axis to the boat

H - distance from the xy-plane to the substrate plane

B - distance from the boat centre (h) to the perpendicular projection of s onto the xy-plane

L - the side length of the square substrate

- angle between the xy-plane normal at h and the line segment hs (the polar angle in a spherical

co-ordinate system)

- angle of rotation of point s from the positive x-axis along the xy-plane (the equatorial angle in

a spherical co-ordinate system)

7.1.2 Example 1 - Coaxial Source and Substrate

The first arrangement of interest is one where the axis of rotation of the substrate passes through

the point source. Note the radial symmetry induced by a rotating substrate in the exact centre of

the flux field. In theory, this should yield the same film as a non-rotating substrate, but in

practicality, and to facilitate subsequent calculations, rotation is used. The geometry and

variables for Example 1 are shown in Figure 7-1. We assume a square substrate and solve for

the generic flux field, as opposed to the specific case of n = 4.

143

Figure 7-1. The flux distribution for an ideal point source (a), and the location of a substrate that

is co-axial with a point source (b). Key dimensions and points are labeled.

To calculate the difference between the thinnest and thickest points on the substrate, we must

identify the maximum and minimum fluxes incident upon the substrate. The point of maximum

flux will be in the centre of the substrate (i.e. the maximum of occurs at max = 0 radians)

and the point of minimum flux will be the point with furthest radial displacement from the

centre. In this case, the four vertexes of a square substrate all fulfill the minimum flux criteria

144

identically. To find min, we construct a right triangle with base B and height H. B is half the

length of the diagonal formed by the square substrate, so

1 2⁄ √2 √2 2⁄ (7-2).

The arctangent of the ratio of B to H, then yields the angle (min = arctan √2 2⁄ / ).

Then the ratio of thinnest point to thickest point is then:

⁄cos arctan √2 2⁄ (7-3)

If we then substitute the values for a typical system, n = 4, L = 50 mm, and H = 460 mm, this

gives a ratio of 0.988, which is less than 5 % difference in thickness, and thus very acceptable.

Most on optical coating applications tolerate thickness ratios of greater than or equal to 0.95.

To predict the yield is a more involved calculation, which we now embark upon. The fraction of

the total flux field that is incident upon the substrate will first be calculated. The flux fraction is

equivalent to a mass deposited per mass sublimed, which can then be converted to a film

thickness per mass sublimed, a more useful operational metric.

The ratio of the area of an inscribed square to the corresponding circle is 2/. The circle

circumscribed by the vertex of the square as it is rotated during deposition can then be used to set

the limit of the integration for the polar angle during the flux field summation. Conveniently,

this sub is the same as the previously calculated min. For the equatorial angle , an integration

between the limits of 0 radians and 2 radians are selected to circumscribe the entire substrate.

Mathematically, the fraction of the flux field that falls on the substrate is then 2/ multiplied with

the ratio of the substrate flux field to the whole flux field:

~

√ ⁄

(7-4)

145

The generic solution for any value of n is quite cumbersome, so we again evoke the common n =

4 case. The solution for n = 4 then simplifies

(7-5)

again substituting with L = 50 mm, and H = 460 mm, yields fsubstrate = 0.0039. This makes it

clear that we pay for the high uniformity of the film by way of wasted material as less than half

of a percent of what is sublimed is deposited on the substrate in this configuration.

If we further specify that the material being sublimed has a density of g cm-3, (a typical number

for an organic semiconductors if 1 g cm-3) we can then take the equivalent mass fraction and

convert it to a film yield per unit mass, as long as we watch our units:

g ilmgsublimed

cmρg ilm

1L mm

10 mmcm

10 nmcm

10 gsublimedmgsublimed

ilmyieldin nm mg⁄ (6-6)

This gives a predicted film forming yield of 1.5 nm/mg, very much in line with the results from

Chapter 6.

The key trend to observe from these equations is that as the substrate moves closer to the source,

the uniformity will decrease but the film yield will increase. This trade off is inescapable in this

geometry, but from a practical perspective, the greater limitation is that this configuration limits

the vacuum chamber to a single source and substrate. In practice, even basic solar cells are many

layered devices. A typical number of layers would be five (hole extraction layer, hole transport

layer, electron transport layer, electron extraction layer, metal electrode) so five sources must be

able to access the substrate. To achieve this, the sources are typically moved off the axis of

rotation of the substrate. We will examine that condition next.

6.1.1.1 Aside On The More General Solution

For those who would prefer to maintain generality as long as possible in their derivations, the

indefinite integral for the flux distribution function when n can be any value follows, and the

146

more general solution using the indefinite integral and an arbitrarily shaped substrate is left as an

exercise for the dedicated reader (who possibly has nothing better to do with their time, and quite

a lot of time besides).

cos √ , ; ;

(7-7)

Where 2F1(a,b;c;z) is Gauss's hypergeometric function, an infinite series of the form:

, ; ; 1! !

⋯ (7-8)

7.1.3 Example 2 - Non-coaxial Source and Substrate

Determining the flux pattern for a rotating substrate that is not co-axial with the thermal source is

a surprisingly under-described problem in PVD textbooks. Especially given this is the

configuration of every research apparatus I investigated with the exception of the Armstrong

lab's spherically arrayed thermal sources (which will also be discussed below). The co-axial

problem is practically trivial in comparison, but I feel my work through in the previous section is

more explicit than the existing sources. There is an obscure reference in Ohring's text (Materials

Science of Thin Films pp 110-112)1 to the discussion of this problem in Physical Vapour

Deposition,2 but Hill's text does not work through the calculation, it only provides reference

handbook style plotted figures of the family of solutions as a reference chart for the reader.

Since the precision of these figures is inadequate for design calculations, and is defined in very

unhelpful length ratios, I will derived the relevant equations from first principles in this example.

Figure 7-2 shows the geometry of the non-coaxial case.

147

Figure 7-2. System geometry from overhead and side view for Example 2, non-coaxial source

and substrate. Key dimensions and points are labeled.

6.1.1.2 Derivation

For any point, s, on the substrate, tan = B/H, and from Pythagorus cos

sin . Application of the Pythagorean trigonometric identity then leads to:

arctan (7-9)

We observe that offset is now a function of so when we substitute into the flux field equation

(old stand by cos ) we must take into account the substrate rotation, so offset is

constantly changing over the course of a substrate holder rotation through 2 radians of . Thus

if the number of rotations is large over the course of the deposition and at a constant angular

speed, then any point s will have a thickness averaged over all the positions on the circumference

of the inscribed circle generated by rotation of point s around the axis of rotation (the z-axis).

Then our flux field equation becomes:

148

(7-10)

Where any point s is described in all relevant detail by r, its distance from the z-axis. The astute

reader will recognize how we have again generated radial symmetry to keep the math clean and

will note that this as an even more involved form of the generalized integral discussed in Section

6.1.1.1. However, at this point the read has all the necessary equations to solve for thickness on

a non-coaxially located thermal source and substrate arrangement.

The equation should be solved for at a variety of r values to ensure the film meets the desired

uniformity specification, but a few general trends shall be noted. By offsetting the source from

the axis of rotation, the point of maximum thickness will now depend strongly on the relative

value of the offset. We will examine three cases, rmax > 2R, 2R ≥ rmax ≥ R, rmax < R.

Case 1: rmax > 2R

The substrate extends significantly further than the distance at which the source is offset from the

axis of rotation. The location for maximum film thickness will be at r ≤ R, and the location of

minimum film thickness will be at r = rmax, with a local minimum at r = 0. Of the offset

geometries, this case is most desirable for maximizing film yield as the largest fraction of the

flux field is incident upon the rotating substrate at all times. This setup has practical limitations,

particularly for small substrates as it is difficult to fit multiple sources this close to the axis of

rotation. Where the substrate is large enough to accommodate this arrangement, for instance in a

production setting, this provides both high uniformity and high yield while granting multiple

sources line of sight to the substrate.

Case 2: 2R > rmax > R

The substrate extends beyond the source's offset of rotation. The location for maximum film

thickness will be at r ≤ R and the location of minimum film thickness is at r = 0. Film yield will

be lower than Case 1.

149

Case 3: rmax < R

The geometry most commonly seen in commercially available fabrication chambers. The

location of maximum film thickness and the location of minimum film thickness are highly

dependent upon the exact chamber geometry and plume shape. This geometry is the worst for

film yield as the maximum of the flux field is not incident upon the substrate.

As an aside on chamber geometry, the Armstrong Lab's custom deposition chambers use an

interesting take upon Case 3 to improve film yield. Instead of locating the thermal sources on a

plane at the base of the chamber, a spherical chamber is used and the substrate is placed at the

centre of the sphere. By then having each thermal source normal to the face of the sphere, the

source normals all pass through the centre of the chamber. Since the source normals all pass

through the substrate located at the centre of the chamber, the maximum of each flux field is

incident upon the substrate, improving film forming yields. The drawback is that spherical

chambers are significantly more expensive to manufacture than cylindrical or box chambers. An

additional penalty is paid in film uniformity on that system as the substrate did not rotate during

deposition, so while the same solid angle worth of flux field was incident from each source, the

location of maximum thickness was the substrate edge closest to the thermal source. The loss of

substrate rotation was deemed an acceptable trade-off for the advantage of leaving the substrate

on a transfer arm, allowing rapid transfer between any of three spherical chambers in series,

minimizing the time interfaces are exposed before the next layer of material is deposited from a

preheated source in an adjacent chamber.

To calculate the fractional flux incident upon the substrate, a few approaches are possible,

integrating across the flux field with offset from 0 to max in ratio with integrating offset from 0 to

is one option, this is the same approach as the solution presented in Example 1. Alternatively,

since we have defined toffset as a function of r,

(7-11)

yields the same result without the trigonometric exercise to convert r back to a function of offset.

We have again converted from the circular spinning substrate holder to the circumscribed square

150

by way of a factor of 2/. Once fsubstrate is calculated, we follow the steps as in Example 1, so

that fsubstrate can be converted to film yield. However, in the offset case, the general solutions are

best left to a symbolic solver program. I recommend Mathematica.

The advantage of offsetting thermal sources from the axis of rotation of the substrate is that more

materials can be deposited upon the substrate within the same vacuum chamber without a need to

reposition the substrate or vent the chamber to change the material in the thermal source. This

arrangement also allows co-deposition, so multiple materials can be deposited simultaneously

upon the substrate (as is common in doped emitter OLEDs). The additional advantage is

improved film uniformity, especially in the cases of rmax ≥ R. The disadvantage of this

arrangement is a reduction in film yield as less of the flux field is incident upon the substrate

compared to the coaxial case. Further, for small substrates, the optimal case of rmax = 2R is

physically impractical due to the size of the thermal sources.

7.1.4 The MARI-KATE Design

A major design requirement for the MARI-KATE system (Most Awesome Restored Instrument

- "Kontraption" for the Assembly of phThalocyanine Electronics), is flexibility. We constantly

test new materials in the chamber and the system is able to make organic electronic devices of a

variety of configurations and applications including organic photovoltaic devices, organic light

emitting diodes, and organic thin film transistors. We have seen how positioning the thermal

source coaxially in the centre of the substrate is best for film yields, but limits the chamber to a

single thermal source. Conversely, offsetting sources improves uniformity while imposing a

penalty upon film yield. We decided to employ an alternate design which attempts to realize the

advantages of both geometries. For the MARI-KATE system, Figure 7-3, the sources are placed

offset from the centre of the chamber to allow multiple sources to access the substrate. However,

instead of locating the substrate at the centerline of the chamber, the substrate is also offset from

the centerline, so the substrate can be positioned individually over any one of the thermal

sources. We thus gain the yield advantage of the Example 1 geometry, but achieve this for

multiple thermal sources, as in the Example 2 geometry. The film uniformity penalty of the

Example 1 geometry is taken as an acceptable penalty for improved yield. Further, the MARI-

151

KATE system is intended for rapid screening of new materials instead of detailed device

optimization engineering, so a lower film uniformity can be tolerated.

(a)

(b)

Figure 7-3. The arrangement of the thermal sources and substrate in the MARI-KATE system.

An overhead view of the base plate, which contains 6 sources offset from the centerline (a). An

overhead schematic illustrating the positioning of the substrate (square with cross) offset from

the chamber centre and overhead of a thermal source (b).

Additionally, instead of depositing upon the substrate at a fixed distance between source and

substrate (constant H), the MARI-KATE system introduces a variable throw distance. For high

value materials where film yield is the priority (such as a small batch of a new material, or a very

thick single layer device for precise electrical characterization), throw distance (H) can be

reduced to increase efficiency of material use. Conversely, for circumstances where film

uniformity is the priority, throw distance can be increased to increase uniformity at a yield

penalty.

152

7.2 Process Conditions

When commissioning the MARI-KATE system (Most Awesome Restored Instrument -

"Kontraption" for the Assembly of phThalocyanine Electronics), an area of primary concern is

the process variability and thermal effects related to the operation of the system. We established

a baseline device architecture and compared it to the results from a leading lab in our field to

ensure reproducibility and statistical validity of the results generated by our system. We also

modeled and measured the temperature distribution in our system to identify concerns related to

the heating of chamber components during operation.

7.1.5 Process Variability

In the process of commissioning our new coupled vacuum and inert atmosphere system (MARI-

KATE) we established the performance of an important baseline photovoltaic devices, a Cl-

BsubPc/C60 photovoltaic device where the subphthalocyanine acts as an electron donor. This

device serves as the baseline devices to test new subphthalocyanine materials as donors. The

new materials can be substituted into the architecture locations currently occupied by Cl-BsubPc

and then the new devices' performances can be compared to this baselines. This architecture was

chosen because it has been used extensively as a baseline by other labs3-7 that we view as reliable

device engineers. One of these groups, operated by Prof. Timothy S. Jones at the University of

Warwick, was kind enough to let me use their system (OLGA, Organic Layer Growth

Apparatus) to prepare a baseline device for comparison. OLGA is a fully automated Kurt J.

Lesker Super Spectros Single Chamber Physical Vapour Deposition system integrated with a

nitrogen glove box. Using a single batch of material, I fabricated 24 devices using the OLGA

system, and then developed an optimized device for our system and fabricated 20 devices for

comparison using the same material batches. The optimized device architectures are shown in

Figure 7-4, along with the JV curves. The performance of the two baseline devices are

comparable, with the numerical values summarized in Table 7-1 and compared graphically in

Figure 7-4 and Figure 7-5. The most notable difference is in the standard deviation of device

parameters. MARI-KATE shows greater standard deviation than OLGA, but this is not

surprising given MARI-KATE was essentially built from spare parts, salvage, and is essentially a

one-off prototype, while OLGA is a commercial system of an established line from a mature

vacuum company.

153

Figure 7-4. Baseline device structures for testing subphthalocyanine derivatives as a donor (a)

and the performance of the Bender Lab's MARI-KATE system compared against the Jones

Group's OLGA turnkey system by way of current density vs. voltage (b). Error bars shown the

95 % confidence interval.

154

The device design settled upon for the MARI-KATE baseline varies slightly from the

OLGA baseline device. First, though we use the same ITO supplier as the Jones Group (Thin

Film Devices, Inc.), and same cleaning procedure (5 minute sonic agitation baths in deionized

water with detergent, followed by 5 minute sonication in deionized water, then acetone, then

methanol) we found an unreasonably high device short circuit rate (> 90 % short circuits, < 10 %

viable devices) when we deposited molybdenum oxide (MoOx) directly on our transparent

conductive metal oxide electrode (ITO, indium doped tin oxide). In contrast, the devices I

fabricated with OLGA showed a viable device yield of > 83 % (though the loss of two of those

devices was due to operator error, so a device yield in the low 90 %'s is likely more

representative). We were unable to pinpoint the cause of the short circuit issue in our system,

but found this short circuit problem to be eliminated when a layer of PEDOT:PSS was deposited

by spin casting before the deposition of the molybdenum oxide. Using PEDOT:PSS a viable

device yield of 95 % was achieved.

Additionally, we substituted silver (Ag) electrodes in place of the aluminium (Al)

electrodes used for devices prepared on OLGA. The change in electrode has been shown to have

a negligible impact on optimized device performance in the literature.3, 8 Further, from an

operational perspective, silver is much easier to use. It is well known that when aluminium is

heated to evaporation temperature, the surface tension effects of the liquid cause it to crawl up

the sides of the ceramic crucible, leading to spills. The spilled hot aluminium damages the

resistive heating elements holding the crucible, reducing heater lifetime. Further, the crucibles

often end up welded to the heaters when the spillover metal cools, and the infiltration of metal

into the heater well where the crucible sits tends to cause the crucibles to fracture at the edge

between the face and cylindrical wall of the crucibles. Conversely, the surface tension of liquid

silver results in the silver withdrawing into a spherical nodule that sits in the bottom of the

crucible, thus causing no undesirable spillover effects. There is the additional advantage the

higher vapour pressure of silver means that the electrode deposition happens at a lower

temperature (and applied power), which increases component lifetimes and minimizes secondary

heating of other surfaces within the chamber. Secondary heating of other surfaces can cause

degassing, which raises operating pressure, or even resublimation of other materials previously

deposited on these surfaces. Both of these processes are undesirable results addressed in the next

section.

155

Figure 7-5. Average device performance parameters for Cl-BsubPc/C60 devices fabricated with

OLGA (red) and MARI-KATE (green) fabrication systems. Error bars enclose the 95 %

confidence interval.

Table 7-1. Average photovoltaic device performance for optimized device created using the

OLGA and MARI-KATE systems. Standard deviation (SD) for the sets are shown in brackets.

Fabrication

System

VOC (SD)

/V

JSC (SD)

/mA cm‐2

FF (SD) PCE (SD)

/%

Device Yield

OLGA 1.11 (0.006) ‐4.4 (0.08) 0.56 (0.01) 2.8 (0.1) 20/24

MARI‐KATE 1.10 (0.01) ‐4.5 (0.3) 0.53 (0.02) 2.6 (0.3) 19/20

156

7.1.6 Thermal Effects

The physical vapour deposition of organic, metal, and metal oxide layers in a vacuum system is

typically achieved by placing the desired materials in ceramic crucibles that are heated by

electrically resistive heaters. While exact values vary from system to system and are highly

dependent upon system geometry and base pressure, in general most organics will be deposited

at temperatures of a few hundred degrees Celsius (an upper bound is typically fullerene

derivatives in the 500 °C to 600 °C range), metal oxides will be at temperatures in the high

hundreds of degrees to perhaps one thousand degrees, and metals will be deposited at

temperatures exceeding 1000 °C (as evidenced by the melting of silver during deposition, which

has a known melting point of 961 °C). Due to the nature of the resistive heaters used to deposit

metals, we do not have thermocouples able to directly contact and measure the temperature in the

crucible during deposition, but on our system silver is typically deposited at a heater power of

about 130 W while aluminium is typically deposited at 160 W. We conducted an extensive

study of the temperature profile inside our system, which was compiled in an Masters of

Engineering Project report,9 but I will summarize the key findings here as I conceived of and

directed the project.

The most heat intensive process to occur in our vacuum system is the physical vapour

deposition of aluminium, so the most intense scenario of the heater running at 200 W for 20

minutes to deposit aluminium was chosen as the case for all simulations and experiments.

Beyond the temperature of the thermal source, there are three key areas of the system where the

temperature is of concern: the sample substrate temperature, the temperature of the other

thermal sources, and the temperature of the system walls. For the substrate temperature, it must

not rise above 62 °C as this is the temperature at which amorphously deposited bathocuproine

(BCP) layers are known to crystallize, which causes a dramatic and undesirable shift in the

material's properties. There are two types of thermal sources, low temperature sources for

subliming organic materials and high temperature sources for subliming metal oxides and metals.

The low temperature thermal sources should not rise to a temperature above 100 °C, as we this

might cause additional material to sublime from these crucibles, contaminating the electrode.

The inactive high temperature thermal source should not rise above 500 °C as that might cause

the other material to sublime, again contaminating the metal deposition. Finally, the chamber

157

walls should not rise above 100 °C as the organic material deposited on the walls could then

resublime and contaminate the metal deposition. These key areas and temperatures are

summarized in Table 7-2. A finite element model of the vacuum chamber was constructed using

the modeling software SolidWorks 2014. When the chamber is evacuated the pressure is ~ 1 x

10-7 Torr (about one 10 billionth of an atmosphere), so no convective heat transfer is present in

this situation. The model included heat transfer by radiation, conduction, and phase change (i.e.

the enthalpy of evaporation as the aluminium leaves the crucible and the enthalpy of sublimation

as the aluminium condenses on the substrate). Table 7-2 also shows the maximum temperature

calculated from the simulations conducted and Figure 7-6 illustrates the temperature

distributions. In all cases we found that our heat shielding design meets our specifications,

keeping heating localized to the high temperature heater during metal deposition. The results of

these simulations were confirmed with experiments where arrays of thermally sensitive tapes

with different colour changing temperatures were placed at key chamber locations and the metal

depositions equivalent to the simulations were conducted.

(a)

(b)

Figure 7-6. The temperature distribution on the base plate after a metal deposition (a). The

temperature distribution on the substrate and holder after a metal deposition (b). Both images

were generated through finite element analysis simulations.

25 °C

26 °C

25 °C

200 °C

450 °C

600 °C

850 °C

158

There are two additional notable results. First, the heat shielding reaches temperatures of

up to 250 °C, so keeping the heat shields clean from unanticipated depositions is critical to

maintaining deposition purity. To achieve this, aluminium foil is placed on the heat shielding

fins and replaced every time the material in the adjacent organic source is changed. This ensures

the heat shield fins only have a single material deposited upon them, eliminating the potential for

cross-contamination. Second, the inactive high temperature source adjacent to the metal source,

reaches temperatures of up to 200 °C, so only materials with high deposition temperatures should

ever be placed in the high temperature sources. Organic materials should never be placed in the

high temperature source, as the temperature during heating (caused by thermal cross talk in the

design) might cause organic material located there to sublime during a metal deposition.

Table 7-2. Temperature limits and simulated temperatures after 20 minutes of subliming

aluminium with a 200 W heater inside the MARI-KATE system

Chamber Location Acceptable Temperature

Range

Maximum Temperature

Observed In Simulation

Meets

Specification?

Substrate < 62 °C 26 °C Yes

Low Temperature

Thermal Source

< 100 °C 45 °C Yes

Inactive High

Temperature Thermal

Source

< 500 °C 200 °C Yes

Chamber Wall < 100 °C 25 °C Yes

Heat Shielding Not Specified 250 °C N/A

159

References 1. Ohring, M., Materials Science of Thin Films : Deposition and Structure; Academic Press:

San Diego, 2002.

2. Hill, R. J., Physical Vapour Deposition. BOC Group: Temescal, 1986.

3. Mutolo, K. L.; Mayo, E. I.; Rand, B. P.; Forrest, S. R.; Thompson, M. E., Enhanced

Open-Circuit Voltage in Subphthalocyanine/C60 Organic Photovoltaic Cells. J. Am. Chem. Soc.

2006, 128, 8108-8109.

4. Yang, J. L.; Schumann, S.; Hatton, R. A.; Jones, T. S., Copper

Hexadecafluorophthalocyanine (F16cupc) as an Electron Accepting Material in Bilayer Small

Molecule Organic Photovoltaic Cells. Organic Electronics 2010, 11, 1399-1402.

5. Hancox, I.; Chauhan, K. V.; Sullivan, P.; Hatton, R. A.; Moshar, A.; Mulcahy, C. P. A.;

Jones, T. S., Increased Efficiency of Small Molecule Photovoltaic Cells by Insertion of a Moo3

Hole-Extracting Layer. Energy Environ. Sci. 2010, 3, 107-110.

6. Sullivan, P.; Duraud, A.; Hancox, l.; Beaumont, N.; Mirri, G.; Tucker, J. H. R.; Hatton,

R. A.; Shipman, M.; Jones, T. S., Halogenated Boron Subphthalocyanines as Light Harvesting

Electron Acceptors in Organic Photovoltaics. Adv. Energy Mater. 2011, 1, 352-355.

7. Morse, G. E.; Gantz, J. L.; Steirer, K. X.; Armstrong, N. R.; Bender, T. P.,

Pentafluorophenoxy Boron Subphthalocyanine (F5bsubpc) as a Multifunctional Material for

Organic Photovoltaics. ACS Appl. Mater. Interfaces 2014, 6, 1515-1524.

8. Cnops, K.; Rand, B. P.; Cheyns, D.; Heremans, P., Enhanced Photocurrent and Open-

Circuit Voltage in a 3-Layer Cascade Organic Solar Cell. Appl. Phys. Lett. 2012, 101, 143301.

9. Balawi, A. Understanding the Thermal Distribution within a Process Chamber Used for

the Physical Vapor Deposition of Thin Films for Organic Electronic Devices. University of

Toronto M. Eng. Project, Toronto, 2014.

160

Chapter 8 Characterization of -oxo-(BsubPc)2 in Organic Planar

Heterojunction Photovoltaic Devices

This chapter will be submitted as Castrucci, J. S.; Dang, J. D.; Thibau, E.; Lu, Z.-H.; Bender, T.

P., Characterization of -oxo-(BsubPc)2 in Organic Planar Heterojunction Photovoltaic Devices.

I executed the experiments (with the exception of the UPS which was done by ET), interpreted

the data, and wrote most of the manuscript. JDD synthesized the materials and wrote the

introduction section focused on the chemistry of -oxo-(BsubPc)2. ZHL and TPB directed the

research. All authors approved the manuscript.

161

Chapter 8 Characterization of -oxo-(BsubPc)2 in Organic Planar

Heterjunction Photovoltaic Devices

8.1 Introduction

The field of organic photovoltaic (OPV) devices is an area of active research for sustainable

energy alternatives, and boron subphthalocyanine chloride (Cl-BsubPc)1-3 (Figure 8-1) is a

material of particular interest. In the configuration of planar heterojunction (PHJ) OPVs, Cl-

BsubPc has been demonstrated as an electron donor,4-5 electron acceptor,5-7 ambipolar

interlayer,8 and was recently included in a multilayer PHJ that achieved 8.4 % power conversion

efficiency.9 In addition to the use of Cl-BsubPc in OPVs, a number of BsubPc derivatives have

also been demonstrated in OPV10-14 and organic light emitting diode (OLED)15-17 applications.

When used as electron donors, BsubPcs are typically paired with C60 fullerene,4-5, 18-20 or its C70

derivative,21 though the complimentary absorption of F16CuPc (Figure 8-1) has also been used.22

When used as electron acceptors, BsubPcs have been paired with other BsubPc derivatives,5, 14

acenes,6 and thiophenes9, 23 and have been shown to successfully harvest triplet excitons7

produced from singlet fission processes in complimentary layers.24-26

The oxygen-bridged dimer of BsubPc, μ-oxo-(BsubPc)2 (Figure 8-1), was serendipitously

discovered when Cl-BsubPc was treated with NaOH in the presence of a phase transfer catalyst

while attempting to synthesize HO-BsubPc.27 It was immediately realized that this dimeric

molecule had unique spectroscopic properties. For example, its solution optical absorption

spectrum shows a significant hypsochromic/blue shift (i.e. shift to higher energy) in the Q band

compared to typical BsubPcs.27 Moreover the Q and B bands are similar in intensity, whereas

normal BsubPcs have Q bands that are more intensive than the corresponding B band. To further

illustrate its unusual spectroscopic behavior, the 1H NMR spectrum of μ-oxo-(BsubPc)2 shows

an up-field shift in the resonance signals for both terminal (2,3-position) and bay (1,4-position)

hydrogens, where the effect is more pronounced on the latter. This effect is attributed to the close

proximity of these hydrogen atoms to the π-electron cloud of the second BsubPc macrocycle.27

Although μ-oxo-(BsubPc)2 has been known for nearly twenty years now,27 its physical properties

have been largely unexplored until the recent contributions made by our group. We attribute the

162

lack of study to the low yielding synthetic processes. We initially focused our efforts to prepare

μ-oxo-(BsubPc)2 in a reasonable yield and with sufficiently high purity such that it would be

suitable for detailed physical characterization and incorporation into organic electronic

devices.28-29 It was found that reacting equimolar amounts of HO-BsubPc with Br-BsubPc in the

presence of tripotassium phosphate (K3PO4) base in 1,2-dichlorobenzene at 180 °C for 1 hour

followed by the sequential purification steps of a Soxhlet extraction, Kaufmann column

chromatography, and two rounds of train sublimation was our best process that afforded highly

pure μ-oxo-(BsubPc)2 in a 27-30% yield.29 Single crystals of non-solvated μ-oxo-(BsubPc)2,

acquired from the sublimation step, exhibited a solid state arrangement with high symmetry,

close intermolecular - interactions, and high chromophore density (Figure 8-1). These features

were also present in hydrated and 1,2-dichlorobenzene-solvated crystals of μ-oxo-(BsubPc)2.28

The optical absorption spectrum (max = 532 nm) was considerably blue-shifted by comparison

to typical BsubPcs (max > 560 nm), consistent with the earlier findings (Figure 8-2).27 Evidence

of solvatochromism (i.e. solvent-dependent absorption) was found.29 Cyclic voltammetry

measurements (in degassed dichloromethane solution relative to Ag/AgCl) show a single

oxidation (+0.95 V) and reduction (-1.03 V) with potentials that are in line with Cl-BsubPc

(+1.04 and -1.05 V).29-30 Despite its high molar mass, we have previously shown that it can be

easily sublimed under vacuum deposition conditions.31 The resulting film has a solid absorption

profile31 with a peak at max = 578 nm and a shoulder at = 555 nm. In contrast, the dilute

solution in dichloromethane shows a shoulder at = 545 nm and a peak at max = 532 nm, thus

indicating a reversal in band intensity ratio (Figure 8-1).

Considering that Cl-BsubPc has already been well-established as an active material in organic

photovoltaics (OPVs)4, 8-9, 21, 23, 32 and the fact that μ-oxo-(BsubPc)2 shares similar

electrochemical behavior with Cl-BsubPc,29 we were motivated to characterize μ-oxo-

(BsubPc)2's performance in an OPV device. Furthermore as mentioned above, μ-oxo-(BsubPc)2

possesses some unique traits within the family of BsubPcs that could potentially serve as

enhancing features in this application including its solid state arrangement28 and its

hypsochromic-shifted absorption spectrum,27, 29 which aligns well with the solar irradiance

spectrum (Supporting Information, Figure S1).

163

In this chapter we explore the use of films of -oxo-(BsubPc)2 in photovoltaic cells. First, we

report the frontier energy levels and propose promising photovoltaic material pairings. Second,

we demonstrate the performance of -oxo-(BsubPc)2 as an electron donor material and as an

electron acceptor material in planar heterojunction OPVs. Finding higher efficiency with a

donor configuration, we then explore pairings with several other acceptor materials. We

demonstrate that -oxo-(BsubPc)2, as a donor, produces devices of comparable efficiency to the

widely explored Cl-BsubPc, thus opening a new class of BsubPc derived materials for future use

in organic electronic devices.

164

Figure 8-1. (a) Molecular structures and names of compounds used in this study, including the

crystal structure of -oxo-(BsubPc)2. (b) Schematic of the device structure for testing -oxo-

(BsubPc)2 as an electron donor. (c) Schematic of the device structure for testing -oxo-(BsubPc)2

as an electron acceptor. (d) Absorbance spectrum of -oxo-(BsubPc)2 in solution (purple) and in

a solid film (blue).

165

8.2 Results and Discussion

8.2.1 Opto-electronic Characterization of the Solid Film

We have previously described the synthesis and basic physical characterization of -oxo-

(BsubPc)2, the details of which are outlined in the introduction of this paper. Building on our

previous studies, we began by using ultraviolet photoelectron spectroscopy (UPS) to determine

the highest occupied molecular orbital (HOMO) energy and the Fermi energy of -oxo-

(BsubPc)2, the results of which are shown in

(a) and (b). The HOMO of -oxo-(BsubPc)2 was measured to be 5.9 eV and the Fermi level to

be 4.5 eV, respectively, with both values reported relative to a free electron in vacuum. This

compares with the measured oxidation potential of +0.95 V which by using the Thompson-

Forrest equation33 yields 5.9 eV. There are only three BsubPc derivatives in the literature where

the combination of a reversible oxidation and a UPS derived HOMO energy are reported, 10, 30, 34

so we are unable to apply the Thompson-Forrest approach to a BsubPc specific training set to see

if an alternate fit to the equation parameters would yield alternate predictions for BsubPcs.

We then measured the solid state absorption of vacuum deposited films of -oxo-(BsubPc)2

using the onset of absorption to calculate the optical band gap. Assuming an exciton binding

energy of 0.3 eV the lowest unoccupied molecular orbital (LUMO) energy was calculated using

the HOMO energy less the sum of the optical band gap and the exciton binding energy and found

to be 3.7 eV. The transport gap of 2.2 eV is compared with the electrochemical gap of 1.98 V.

Having determined the frontier orbital energy levels and their position relative to the Fermi

energy, we then constructed band diagrams under the assumption of vacuum energy alignment

for -oxo-(BsubPc)2 when paired with a number of acceptors and a donor (Figure 8-2 (c) and

(d)). We also included band diagrams for Cl-BsubPc as a baseline for comparison. The rational

for selection of these materials was based upon their previous use in Cl-BsubPc containing

devices, as previously described in the introduction. In all material pairings identified, the

HOMO and LUMO energies of the acceptor exceed those of the donor, forming a difference in

HOMO energies (HOMO) and a difference in LUMO energies (LUMO), believed to be a

perquisite for driving exciton dissociation at the donor to acceptor interface. Additionally, when

166

positioned as a donor, -oxo-(BsubPc)2's interfacial gap (Igap = HOMOD-LUMOA) than for Cl-

BsubPc upon account of the higher HOMO energy (5.9 eV vs. 5.6 eV). The Igap is positively

correlated with open circuit voltage in organic photovoltaics.35 All the compounds energy levels

are shown in numeric detail in the Supporting Information, Table S1.

Figure 8-2. Ultraviolet photoelectron spectroscopy of -oxo-(BsubPc)2. (a) Identification of

work function by way of secondary electron cut-off. (b) Identification of the HOMO to Fermi

gap. (c) Schematic of energy levels for testing -oxo-(BsubPc)2 as an electron acceptor. (d)

Schematic of energy levels for testing -oxo-(BsubPc)2 as an electron donor.a

167

a HOMO and Fermi energies other than -oxo-(BsubPc)2 are from References,10, 36-38 LUMO

energies are calculated by offsetting the HOMO by the optical gap plus an exciton binding

energy of 0.3 eV. See Supporting Information Tables S1 for more information on energy levels.

8.2.2 Testing of -oxo-(BsubPc)2 as an Electron Donor and an Electron Acceptor

To test the performance of -oxo-(BsubPc)2 as an electron donor, devices of the structure

ITO/PEDOT:PSS/MoOx(5 nm)/Donor/C60/TPBi(3 nm)/Ag(80 nm), where the donor was Cl-

BsubPc or -oxo-(BsubPc)2, were fabricated. The Cl-BsubPc/C60 baseline device is based upon

a long literature history4-5, 10 and our replication of it here serves as a baseline point of

comparison between the existing literature and our laboratory's process and fabrication system.

To test the performance of -oxo-(BsubPc)2 as an electron acceptor, devices of the structure

ITO/PEDOT:PSS/6T/Acceptor/TPBi(3 nm)/Ag(80 nm), where the acceptor was Cl-BsubPc or

-oxo-(BsubPc)2, were fabricated. We selected 6T/Cl-BsubPc baseline due to its previous use

in the BsubPc literature9 and a desire to avoid of the complication associated with singlet fission

processes.23

The -oxo-(BsubPc)2/C60 cell we report was the pinnacle of a moderate intensity optimization

program on our fabrication system. First, the donor to acceptor layer thickness ratio was

adjusted to maximize efficiency while keeping total donor plus acceptor thickness constant

(further detail in Supporting Information Table S2 and Figure S2). A poor fill factor (FF) was

present even in the optimized ratio, so a hole extraction layer was introduced by way of a 5 nm

molybdenum oxide (MoOx) layer, which, as had been previously seen in the literature for Cl-

BsubPc and related phthalocyanines.18-20, 39 The insertion of this layer greatly improved the FF

and open circuit voltage (VOC). Next, the donor to acceptor thickness ratio was held constant and

total donor plus acceptor thickness was increased, resulting in the optimal device shown in Table

8-1.Error! Reference source not found. (further detail in Supporting Information Table S3 and

Figure S3).

By comparing -oxo-(BsubPc)2 to Cl-BsubPc when both are used as a donor with C60, we find

the -oxo-(BsubPc)2 devices have a VOC of 0.90 (0.898−0.902, 95% confidence interval) V, a JSC

168

of -3.9 (-4.0−-3.8) mA cm-2, and a FF of 0.59 (0.58−0.60) for a total power conversion efficiency

(P) of 2.1 (2.0−2.1) %. Of these metrics, the VOC, JSC, and P are less than the baseline Cl-

BsubPc containing device (p = 8 × 10-15, p = 0.003, p = 9 × 10-5, respectively), while the FF

exceeds that of the baseline device (p = 5 × 10-4). The solid state absorption of -oxo-(BsubPc)2

has peak near 570 nm and C60 has an absorption peak near 350 nm. Corresponding peaks in the

external quantum efficiency spectrum for this device indicate excitons are successfully harvested

from both the donor and acceptor layers, which each contribute to the total JSC, in a manner

comparable to the baseline device.

When using -oxo-(BsubPc)2 as an acceptor instead of Cl-BsubPc, the substitution resulted in

approximately an 80 % reduction in JSC (p = 2 × 10-17), and a moderate reduction in VOC and FF

(p = 1 × 10-4, p = 4 × 10-9, respectively). This results in a final device efficiency of less than 0.5

%, which is dramatically less than the Cl-BsubPc containing device efficiency of 3.2 (3.0−3.4)

%. When considering the EQE spectrum for the-oxo-(BsubPc)2in comparison to the Cl-

BsubPc we note that the peaks near 350 nm and 600 nm, corresponding to BsubPc adsorption,

are only one third to one quarter as intense, indicating a drop in photo extraction efficiency from

the acceptor layer. Even more dramatic is the complete absence of a peak near 450 nm,

indicating there are no excitons generated in the donor layer that are extracted as current.

Exciton extraction only from the acceptor layer would suggest a poor LUMOD to LUMOA

alignment preventing the transfer of donor generated electron carriers to the acceptor, but a

reasonable HOMO facilitating the extraction of acceptor generated holes to the donor layer.

We thus conclude that -oxo-(BsubPc)2 shows more promise in use as an electron donor than as

an electron acceptor. This agrees with our previous cyclic voltametry observation of a reversible

oxidation process but an irreversible reduction process.29 The baseline results for both donor and

acceptor screening, as well as relevant solid film absorbances and EQE spectra, are shown in

Table 8-1 and Figure 8-3.

169

Table 8-1. Mean device parameter comparison of Cl-BsubPc to -oxo-(BsubPc)2 employed as

an electron acceptor.a

Donor/dD

nm

Acceptor/dA

nm

JSC (SD)/mA

cm‐2

VOC (SD)/V FF (SD) P (SD)/% No. of cells

tested

6T/55 Cl‐

BsubPc/20

‐5.5 (0.54) 1.08 (0.03) 0.55 (0.06) 3.2 (0.44) 17

6T/30 ‐oxo‐

(BsubPc)2/10

‐1.0 (0.08) 0.96 (0.06) 0.38 (0.04) 0.37 (0.04) 9

Cl‐

BsubPc/10

C60/30 ‐4.4 (0.49) 1.06 (0.01) 0.52 (0.05) 2.4 (0.21) 12

‐oxo‐

(BsubPc)2/15

C60/45 ‐3.9 (0.34) 0.90 (0.01) 0.59 (0.01) 2.1 (0.19) 7

a The standard deviation (SD) of each value is shown in parentheses. Device structure is

ITO/PEDOT:PSS/ Donor/Acceptor/TPBi(3 nm)/Ag(80 nm).

170

0.0 0.5 1.0

-6

-4

-2

0

2

C

urre

nt D

ensi

ty (

mA

cm

-2)

Voltage (V)

a)

300 400 500 600 700 8000

20

40

0.0

0.2

0.4

0.6

0.8

1.0

Ext

erna

l Qua

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E

ffic

ienc

y (%

)

Wavelength (nm)

Abs

orba

nce

(arb

.uni

ts)

b)

Figure 8-3. (a) JV curves comparing Cl-BsubPc containing devices to analogous -oxo-

(BsubPc)2 containing devices. Filled magenta squares denote 6T/Cl-BsubPc devices, filled

blue circles denote 6T/-oxo-(BsubPc)2 devices, unfilled magenta squares denote Cl-

BsubPc/C60 devices, unfilled blue circles denote -oxo-(BsubPc)2/C60 devices. (b) Solid state

film absorbances for Cl-BsubPc (magenta), -oxo-(BsubPc)2 (blue), 6T (orange), and C60 (dark

grey) and external quantum efficiency spectra using the key from (a).a

a Error bars show the 95% confidence interval.

8.2.3 Application with Other Electron Acceptors in Photovoltaic Devices

Having identified application as an electron donor as showing the greatest promise, we next

tested m-oxo-(BsubPc)2 when paired with two additional electron acceptors: C70 and F16CuPc.

Taking a cue from previously reported techniques relevant to photovoltaic devices containing Cl-

BsubPc as a donor, the substitution of C70 for C60 as the acceptor21 also improved performance

for -oxo-(BsubPc)2. The JSC was increased (p = 4 × 10-8) by nearly 50 % to -5.9 (-5.6−-6.2) mA

cm-2, while VOC remained similar at 0.89 (0.886−0.894) V and FF fell slightly (p = 2 × 10-9) to

0.50 (0.49−0.51). This resulted in an overall increase in P (p = 2 × 10-5) to 2.7 (2.5−2.9) %.

171

Based upon the quantum efficiency spectra, the increase in current density is attributed primarily

to increase quantum efficiency between 450 nm and 550 nm, and between 600 nm and 700 nm,

both areas of the spectrum where C70 absorbs strongly but -oxo-(BsubPc)2 and C60 do not.

Another area of interest is the use of non-fullerene based acceptors.5-6 We have previously

explored BsubPc derivatives as acceptors7 but figured the similarity in absorption spectrum to -

oxo-(BsubPc)2 would be detrimental to photon absobance and the small HOMO and LUMO would

be detrimental to exciton dissociation, as seen in Gommans et al.'s11 and Sullivan et al.'s5

experiments where BsubPcs were used as both donor and acceptor layers in the same device.

Alternately, we selected F16CuPc, which had also been previously paired with Cl-BsubPc,22 and

a few other donors.40-41 It was expected that the complimentary absorbance spectrum would

greatly enhance the current, while the larger Igap would result in an increase of VOC. While there

are peaks in the quantum efficiency spectrum corresponding to the peaks in the solid state

absorption of both the donor and acceptor, at no wavelength does the quantum efficiency exceed

20 %. The -oxo-(BsubPc)2 related EQE peak near 570 nm is only one third as intense as when

paired with C60. The VOC is extraordinarily low, while the JSC and FF are both roughly half of

the value found in the -oxo-(BsubPc)2/C60 device. The low current density and voltage are the

same loss in performance parameters that were found in a Cl-BsubPc/F16CuPc device.22 To allay

concerns of material or device quality with regard to the use of F16CuPc, we also fabricated a

copy of the previously reported22 Cl-BsubPc/F16CuPc device and achieved comparable

performance (Supporting Information, Table S3 and Figure S3).

We conclude that when pairing -oxo-(BsubPc)2 as a donor with C70 and F16CuPc as acceptors,

the same trends in device performance are observed as when these acceptors are paired with Cl-

BsubPc. Substitution of C70 for C60 results in increased JSC and P, while substitution of F16CuPc

results in drops in all device performance parameters, despite a much broader range of photon

wavelengths being absorbed.

172

Table 8-2. Mean device parameter comparison of Cl-BsubPc to -oxo-(BsubPc)2 employed as

an electron donor and -oxo-(BsubPc)2 paired with other acceptor materials.a

Donor/dD

nm

Acceptor/dA

nm

JSC (SD)/mA

cm‐2

VOC (SD)/V FF (SD) P (SD)/% No. of cells

tested

Cl‐

BsubPc/10

C60/30 ‐4.4 (0.49) 1.06 (0.01) 0.52 (0.05) 2.4 (0.21) 12

‐oxo‐

(BsubPc)2/15

C60/45 ‐3.9 (0.34) 0.90 (0.01) 0.59 (0.01) 2.1 (0.19) 7

‐oxo‐

(BsubPc)2/15

C70/45 ‐5.9 (0.62) 0.89 (0.01) 0.50 (0.02) 2.7 (0.34) 12

‐oxo‐

(BsubPc)2/15

F16CuPc/30 ‐1.7 (0.23) 0.06 (0.004) 0.30 (0.02) 0.03 (0.07) 6

a The standard deviation (SD) of each value is shown in parentheses. Device structure is

ITO/PEDOT:PSS/MoOx(5 nm)/Donor/Acceptor/TPBi(3 nm)/Ag(80 nm).

173

0.0 0.5 1.0

-6

-4

-2

0

2

C

urre

nt D

ensi

ty (

mA

cm

-2)

Voltage (V)

a)

300 400 500 600 700 8000

20

40

0.0

0.2

0.4

0.6

0.8

1.0

Ext

erna

l Qua

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E

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)

Wavelength (nm)

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orba

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(arb

.uni

ts)

b)

Figure 8-4. (a) JV curves comparing various acceptors for pairing with -oxo-(BsubPc)2.

Unfilled magenta squares denote Cl-BsubPc/C60 devices, unfilled blue circles denote -oxo-

(BsubPc)2/C60 devices, unfilled black diamonds denote -oxo-(BsubPc)2/C70, unfilled cyan

triangles denote -oxo-(BsubPc)2/F16CuPc devices. (b) Solid state film absorbances for Cl-

BsubPc (magenta), -oxo-(BsubPc)2 (blue), C60 (dark grey), C70 (black), and F16CuPc (cyan) and

external quantum efficiency spectra using the key from (a).a

a Error bars show the 95% confidence interval.

8.3 Conclusions

In summary, we have conducted a preliminary exploration of the use of the BsubPc derivative -

oxo-(BsubPc)2 in photovoltaic devices. We demonstrated is performance as a donor with two

fullerene derivatives, where it is shown to produce comparable to Cl-BsubPc, in particular

responding in a similar way to high work function electrode selection and lower band gap

acceptor pairing selection. When -oxo-(BsubPc)2 is used as a donor with the non-fullerene

F16CuPc serves as an acceptor, EQE data shows light is harvested from both layers but overall

efficiency suffers due to a loss of photocurrent and photovoltage. When -oxo-(BsubPc)2 is used

as an acceptor with 6T, the photovoltaic performance is poor compared to the analogous Cl-

174

BsubPc device. We conclude that -oxo-(BsubPc)2 is worthy of significantly more study as a

donor layer in photovoltaic devices but that more study is needed to put in context the poor

performance as an acceptor layer.

8.4 Experimental Section

Molybdenum(VI) oxide (Sigma-Aldrich, 99.98% trace metals basis), TPBi (Lumtec), F16CuPc

(Lumtec) and silver paint (PELCO, Conductive Silver 187) were purchased and used as

received. α6T (Sigma-Aldrich) and C60 and C70 (SES Research, 99.5%) were purchased and

purified once by train sublimation before use. Cl-BsubPc and -oxo-(BsubPc)2 were synthesized

as previously reported,29 and purified once by train sublimation before use.

For the UPS measurements, -oxo-(BsubPc)2 films were evaporated from an alumina crucible

using a transfer arm evaporator (TAE) described by Greiner et al.42 The films were deposited on

a highly oriented pyrolitic graphite (HOPG) substrate to a thickness of approximately 12 nm,

using typical small organic molecular film density, as measured by a calibrated quartz crystal

microbalance (QCM). This thickness was chosen so as to avoid charging effects, as well as any

substrate-induced interaction. The base pressure in the chamber was approximately 1x10-8 Torr.

The films were then in-situ transferred to a PHI5500 Multi-Technique System to perform the

photoemission using a non-monochromated He Iα (hν=21.22eV) source. All measurements were

done at a take-off angle of 88 degrees and the sample was held at a bias of -15 V with respect to

the spectrometer. The pressure in the analysis chamber was approximately 1x10-9 Torr.

The Work Function was calculated according to: = 21.22 - SEC, where SEC is the secondary

electron cut-off. The HOMO-Fermi energy difference was taken as the intersection of the

HOMO edge with the background noise, with the Fermi energy being calibrated to 0 eV binding

energy. The ionization energy (IE) is then simply the sum of the work function and HOMO-

Fermi energy difference.

OPV devices were fabricated on 25 mm by 25 mm glass substrates coated with indium-tin oxide

(ITO) having a sheet resistance of 15 Ω per square (Thin Film Devices, Inc.). The ITO was pre-

patterned, leaving 8 mm from one side as uncoated glass. Substrates were cleaned by successive

sonications in detergent and solvents, followed by 5 minutes of atmospheric plasma treatment.

175

PEDOT:PSS was spin-coated onto the substrates, 500 rpm, 10 s; 4000 rpm, 30 s. Substrates were

baked on a hot plate at 110 °C for 10 minutes, and then transferred into a nitrogen atmosphere

glove box (O2 < 10 ppm, H2O < 10 ppm). Substrates were transferred to a custom-built thermal

evaporation system attached to the nitrogen glove box without exposure to ambient conditions.

All subsequent device layers were thermally evaporated at ~1.0 A/s and a working pressure of ~1

x 10-7 Torr for organic layers and ~1 x 10-6 Torr for silver. Silver electrodes were evaporated

through a shadow mask, defining 0.2 cm2 as the active area for each device. A transfer back to

the glove box was required between the TPBi and silver layers to change the shadow masks.

Layer thickness and deposition rates of evaporated films were monitored using a quartz crystal

microbalance calibrated against films deposited on glass where film thickness was measured

with a KLA-Tencor P16+ surface profilometer. Solid film absorbances were measured using a

Perkin-Elmer Lambda 1050 spectrometer with the films deposited at device relevant

thicknesses on glass slides. To enhance the electrical contact during testing, silver paint was

applied to the ITO and metal electrode contact points and left to dry for 20 minutes. Devices

were kept in the nitrogen-filled glove box throughout testing. Voltage sweeps of the devices

were performed under full illumination by a 300W Xe arc lamp (Oriel) with an AM 1.5G filter,

and the corresponding currents were measured with a Keithley 2401 Low Voltage SourceMeter.

Light intensity was calibrated to 100 mW/cm2 with reference to a calibrated silicon

photodetector. Wavelengths scans at 10 nm intervals were performed using an in-line

CornerstoneTM 260 1/4 m Monochromator and the corresponding currents were measured using a

Newport Optical Power Meter 2936-R and converted to external quantum efficiencies using a

reference wavelength scan of a calibrated silicon photodetector.

All p-values reported are the results of single sided t-tests comparing the parameters with a null

hypothesis of the parameter with the larger average value exceeding the parameter with the

smaller average value. When an average value is reported followed by a range of values, that

range of values denotes the 95% confidence interval.

176

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181

Chapter 9 Boron Subphthalocyanines as Singlet Fission Harvesting

Materials within Organic Photovoltaics

This chapter was the foundational document for Castrucci, J. S.; Josey, D. S.; Thibau, E.; Lu, Z.-

H.; Bender, T. P., Boron Subphthalocyanines as Singlet Fission Harvesting Materials within

Organic Photovoltaics, J. Phys. Chem. Lett.. I executed the experiments (with the exception of

the UPS which was done by ET and the 6T/Cl-Cl12BsubPc device which was done by DSJ),

interpreted the data, and wrote most of the manuscript. ZHL and TPB directed the research. All

authors approved the manuscript.

182

Chapter 9 Boron Subphthalocyanines as Singlet Fission Harvesting

Materials within Organic Photovoltaics

Singlet fission, the process where one singlet excited state is converted into two triplet excited

states,1 is an area of active research in the field of photovoltaics.2-8 The advantages of generating

two charge carriers from a single absorbed photon in a photovoltaic device include achieving

higher current densities and a higher thermodynamic limit to the power conversion efficiency9

compared to the single junction Shockley-Queisser limit.10 There are both a limited number of

materials for which the singlet fission process has been shown to occur,11 and a limited number

of material pairings capable of converting these triplet excited states are to charge carriers in

photovoltaic devices.12-15 External quantum efficiencies (i.e. number of charge carriers extracted

per number of incident photons of a given energy) greater than 100 % has been achieved in a

highly optimized triplet-harvesting system.15 Developments in this area are focused on

fundamental questions of excited state energy and kinetics, areas that are difficult to explore

experimentally and are heavily reliant upon computational models and detailed species balances.

Consequently, most singlet fission materials explored thus far are linear acenes and related

derivatives, which typically serve as electron donors in traditional donor/acceptor configurations

of photovoltaic devices. A limited number of electron accepting materials have been

demonstrated to harvest the triplets from the acenes, which include fullerene derivatives5-6, 14-15

and quantum dots.3-4, 8, 12-13 There is generally a need for the development of molecular design

rules to guide the synthesis of new singlet fission materials and new material pairing which can

successfully dissociate triplets into charge carriers in photovoltaic devices.

Our laboratory has recently demonstrated that two classes of electron acceptor materials, silicon

phthalocyanines (SiPcs)16 and boron subphthalocyanines (BsubPcs),17 are also able to dissociate

triplet excited states generated in pentacene (Pent) singlet fission electron donor layers. In

particular, BsubPcs are believed to hold significant promise as organic electronic materials,18

having been demonstrate as both electron donors,19 electron acceptors,20-21 and ambipolar

interlayers22 in singlet fission free photovoltaic cells. When BsubPc was used with the

complimentary absorbers -sexithophene and subnaphthalcyanine chloride (Cl-BsubNc) to

construct a photovoltaic cell that absorbed a broad part of the spectrum and also featured an

183

energy cascade (transferring excitons from Cl-BsubPc to Cl-BsubNc within the cell) a field

leading power conversion efficiency of up to ~8 % was achieved for a BsubPc based small

molecule photovoltaic.23

In our work with BsubPc derivatives as electron acceptors paired with pentacene electron

donors,17 we noted that the amount of pentacene triplets converted to charge carriers could be

observed by proxy through examination of the external quantum efficiency spectrum, as the

pentacene absorbance peak near 670 nm is an area of the spectrum where BsubPcs do not absorb

and the exciton kinetics of Pent result in quantitative conversion of singlets to triplets.24 From

the results of that study, we proposed "that a BsubPc derivative with a deeper LUMO energy

[than Cl-Cl6BsubPc] would likely be an even more effective at harvesting Pent-derived triplets,

resulting in enhanced photocurrent."17 We have now synthesized a new BsubPc derivative, Cl-

Cl12BsubPc (Figure 9-1),25 that meets this criteria. In this paper, we report on the application of

this new triplet harvesting BsubPc in photovoltaic devices and contrast the results with the

results of more traditional complimentary light absorption and energy level alignment

approaches to device performance enhancement.

Figure 9-1. (next page) (a) The electron donor and electron acceptor compounds involved in

this study. (b) Thickness adjusted absorbance of the absorbing compounds in this study. (c)

Current density vs. voltage curves for three BsubPc derivative containing photovoltaic devices

with varying degrees of triplet harvesting, compared against a C60 containing device. (d) The

external quantum efficiency as a function of wavelength for the photovoltaic devices. (e) The

singlet and triplet energy levels of Pent relative to the HOMO and LUMO energies for the three

BsubPc derivatives. Shaded regions show the 95 % confidence interval. The Cl-BsubPc and Cl-

Cl6BsubPc cells were originally reported in Reference 17 and C60 cell was originally reported in

Ref 16. Energy levels are from References 12, 19, and 20.

184

(a)

(b)

(c)

(d)

(e)

185

For this study, we selected two electron donor materials to pair with Cl-Cl12BsubPc, pentacene

(Pent) for its singlet fission properties, and -sexithophene (6T) for its complimentary

absorbance and lack of singlet fission processes. We considered these results in light of our

previously reported Pent/BsubPc cells.17 The compound structures and abbreviations are shown

in Figure 9-1(a). Photovoltaic devices were fabricated in the manner previously described,26 and

included in the Supporting Information. Plots of current density vs. voltage for the photovoltaic

devices are shown in Figure 9-1(c). For the set Cl-BsubPc, Cl-Cl6BsubPc, Cl-Cl12BsubPc, we

note that by optical absorbance measurements of their solid state films the optical band gaps are

identical at 2.1 eV with nearly indistinguishably shaped absorbance profiles. The optical band

gap for pentacene was measured to be 1.9 eV. When the absorbance profiles are normalized per

unit thickness of film, as in Figure 9-1(b), we find that the Cl-Cl12BsubPc film absorbs more

strongly than the Cl-BsubPc and Cl-Cl6BsubPc films. This is surprising as, when one considers

the single crystal x-ray diffraction determined crystal structures of these materials, the trend in

chromophore densities (Cl-BsubPc 3.62 kmol m-3,27-28 Cl-Cl6BsubPc 2.72 kmol m-3,17 Cl-

Cl12BsubPc 2.25 kmol m-3),25 is the reverse of the absorbance trend. The crystals for all three

materials were grown by solvent vapor diffusion and are solvent free. Crystals with identical

structure to the solvent grown crystals have been grown for Cl-BsubPc and Cl-Cl12BsubPc by

train sublimation under low pressure (~ 20 mTorr) with inert carrier gas.

The highest occupied molecular orbitals (HOMOs) for Cl-BsubPc and Cl-Cl6BsubPc have

previously been determined to be at energies of 5.7 eV,29 6.0 eV.20 For this study, we

characterized Cl-Cl12BsubPc by ultraviolet photoelectron spectroscopy (UPS) and the results are

shown in the Supporting Information Figure S1. We measured the work function to be 4.5 eV,

the HOMO-Fermi gap to be 1.8 eV, and thus the HOMO energy to be 6.3 eV. To calculate the

lowest unoccupied molecular orbital (LUMO) we then subtract the band gap and the exciton

binding energy (~ 0.3 eV) to yield LUMO energies of 3.3 eV, 3.6 eV, and 3.9 eV, respectively.

When paired with Pent's HOMO of 4.9 eV,30 under the assumption of vacuum energy alignment,

this yields interfacial gap energies (Igap = HOMODonor - LUMOAcceptor) of 1.6 eV, 1.3 eV and 1.0

eV, respectively. While difficult to measure due to its non-emissive nature, the triplet energy of

Pent is estimated to be between 0.85 eV and 1.0 eV,12 and though not the standard

186

representation, there is evidence that our simultaneous depiction of excitonic and transport

energy levels in Figure 9-1(e) is physically accurate.31

Given the similar absorbances and progressively reducing interfacial gaps formed by this BsubPc

set, when using them as electron acceptors of identical layer thickness in photovoltaic devices we

would expect the short circuit current (JSC) to remain constant and the open circuit voltage (VOC)

to get progressively lower. Instead, we observe a rising JSC and a large reduction in VOC (Figure

1(c), Table 1). In fact, we see an almost tripling of the JSC between the Cl-Cl6BsubPc and the

Cl-Cl12BsubPc containing devices. We would note that the presented Pent/Cl-Cl12BsubPc device

is not as optimized as the Cl-BsubPc and Cl-Cl6BsubPc devices. However, previous optimization

studies for BsubPcs indicate that layer thickness variation primarily impacts fill factor instead of

JSC and VOC.29 The JSC of the Pent/Cl-Cl12BsubPc device is comparable to our system's

previously reported Pent/C60 device, a configuration that greatly benefits from the harvest of Pent

derived triplets.16

Table 9-1. Mean device parameter comparison of for BsubPc containing photovoltaic devices.a

Donor/dD nm

Acceptor/dA nm

JSC (SD)/mA cm-2

VOC (SD)/V

FF (SD) P (SD)/% No. of cells tested

Source

Pent/60 Cl-BsubPc/25

1.3

(0.21)

0.87

(0.10)

0.59

(0.08)

0.65

(0.22)

71 Ref 17

Pent/60 Cl-Cl6 BsubPc/25

2.1

(0.30)

0.50

(0.02)

0.48

(0.07)

0.50

(0.10)

39 Ref 17

Pent/60 Cl-Cl12 BsubPc/25

6.0

(0.34)

0.41

(0.003)

0.49

(0.02)

1.2

(0.05)

15 This work

Pent/40 C60/40 6.8

(0.35)

0.30

(0.005)

0.48

(0.02)

0.97

(0.08)

10 Ref 16

6T/55 Cl-Cl12 BsubPc/25

3.4

(0.17)

0.35

(0.01)

0.49

(0.02)

0.58

(0.05)

15 This work

a Device structure is ITO/MoOx(5 nm)/Donor/Acceptor/BCP(10 nm)/Al(100 nm) for first three

devices and ITO/PEDOT:PSS/Donor/Acceptor/BCP(10 nm)/Ag(100 nm) for final three devices.

187

Turning to the measurements of the external quantum efficiency for these devices (Figure 1(d)),

we see the characteristic Pent absorbance at 670 nm is completely absent for Cl-BsubPc, is small

but noticeable with a peak of ~10 % for Cl-Cl6BsubPc, and is dramatically increased to ~40 %

for Cl-Cl12BsubPc. This value exceeds the EQE at those wavelengths in Pent/C60 devices.

Further, for the shorter wavelengths in the range 500 nm to 650 nm, where we see almost no

change in EQE between the Cl-BsubPc and Cl-Cl6BsubPc, there is an increase in EQE from the

range of 20 % to 25 % almost 40 % EQE in the Cl-Cl12BsubPc device. Given that wavelength

range also has Pent absorbance, this rise in EQE is a result of either the higher efficiency of

triplet dissociation at the Pent/Cl-Cl12BsubPc interface (established by the increased EQE near

670 nm) or the stronger absorbance of the Cl-Cl12BsubPc. Detailed optical and excited state

modeling would resolve the contribution of each factor to the increase in EQE between 500 nm

to 650 nm, but is beyond the scope of this paper.

These photovoltaic results are more evidence that the energy of the Igap relative to the triplet

energy can be a reasonable molecular design rule for predicting whether a particular

organic/organic heterojunction will dissociate triplets generated within a singlet fission material.

The triplet energy in pentacene is estimated to be between 0.85 eV and 1.0 eV,12 and we see a

trend of increasingly efficient dissociation of triplets in the solar cells with Igap approaching that

range and meeting 1.0 eV (Figure 9-1(c)). Thus, as guidance for future synthetic work, and in

agreement with findings for quantum dot based triplet harvesting photovoltaics,4, 12, 14 we suggest

that organic donor and organic acceptor material based photovoltaic cells leveraging singlet

fission for increased photocurrent are hampered by a limited VOC, as an Igap near the triplet

energy is required to efficiently dissociate triplet excitons into charge carriers.

When looking at Figure 1(d) it is notable that the Pent/BsubPc pairing suffers from a drop in the

EQE from 400 nm to 500 nm, likely corresponding to a lack of light absorbance in this same

wavelength range. The design principle of complimentary absorbance suggests greater current

could be achieved if the photons in this wavelength range could also be absorbed and dissociated

into charge carriers. From Figure 2 it is clear that the light absorbance of 6T compliments that

of BsubPcs. It has in fact been previously paired with Cl-BsubPc and Cl-BsubNc,23 and we have

suggested it as a good material for pairing with BsubPc derivatives as a rapid screening

technique.26 In contrast to the Pent/Cl-Cl12BsubPc cell, the 6T/Cl-Cl12BsubPc cell (Table 1),

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shows a slightly lower VOC and a slightly reduced JSC. The overall power conversion efficiency

of the 6T/Cl-Cl12BsubPc cells was 0.58 ± 0.05 %, while the overall efficiency of the Pent/Cl-

Cl12BsubPc cells were 1.2 ± 0.03 %. While this difference is statistically significant, from an

absolute values perspective, a 0.5% difference in P for a single device pair of device structures

is insufficient to draw any wide-ranging conclusions about the preferable method for achieving

maximum efficiency from a deep HOMO energy BsubPc serving as an acceptor in a photovoltaic

device. As with the Pent containing cell, the 6T/Cl-Cl12BsubPc cell is an unoptimized cell,

with a fill factor less than we would expect from a fully optimized device. Consulting the EQE

plot (Figure 2), we see a major gain in absorbance in the 400 nm to 500 nm wavelength region

where 6T absorbs, but a loss in the longer wavelength region beyond 650 nm where the

Pent/Cl-Cl12BsubPc device previously benefitted from Pent absorbance and the singlet fission

process.

(a)

(b)

(c)

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Figure 9-2. (previous page) (a) Current density vs. voltage curves for pentacene and a6T donors

paired with Cl-Cl12BsubPc acceptor photovoltaic devices. (b) Normalized absorbance of the

materials. (c) External quantum efficiency as a function of wavelength for the photovoltaic

devices. The shaded regions show the 95 % confidence interval.

Given the comparison point between the Pent/Cl-Cl12BsubPc and 6T/Cl-Cl12BsubPc device, it

is unclear whether the device design route of improved triplet harvesting or complimentary

absorbance is preferable for maximizing energy conversion for BsubPc based photovoltaic

devices. Given the data presented herein, the data of Cnops et al.23 and the unique absorption

profile of BsubPcs it is clear that either material design approach of designing for triplet

dissociation or designing for complimentary absorbance can be used to achieve power

conversion efficiency gains. The two approaches might be combined in an energy cascade type

device to achieve even greater improvements.

In conclusion, we have incorporated the deep HOMO/LUMO energy BsubPc derivative Cl-

Cl12BsubPc into photovoltaic devices with the singlet fission material Pent where it is shown to

be the most efficient triplet harvesting BsubPc reported to date with an almost tripling in JSC

resulting in performance comparable with C60. When Cl-Cl12BsubPc is paired with the singlet

fission free material 6T, it is found to yield photovoltaic devices of similar efficiency. This

suggests that future BsubPc synthetic targets for singlet fission devices can use the criteria of

interfacial gap energy near the triplet energy to ensure efficient triplet dissociation, but that the

concepts of complimentary absorption engineering should not be ignored as an additional avenue

to boost photovoltaic cell efficiency.

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Chapter 10 Future Work

In the preceding chapters we reported on the electron mobility of a number of boron

subphthalocyanine derivatives and their relationships with single crystal structures and

photovoltaic performance. From these results we proposed some crystal engineering and device

design criteria to guide the synthesis of new materials including materials able to dissociate

triplet excitons. We demonstrated the vacuum deposition of very high molar mass organic

semiconductors, which greatly widens the design space for new derivative exploration. We

discussed vacuum equipment design considerations and calculations which can improve film

yield for high value materials thus facilitating the testing of small quantities of new material. To

build on these results, I propose a number of extensions in the realm of crystal engineering,

material description, device design, non-absorber material selection, and vacuum system design

to facilitate continued progress in the development of boron subphthalocyanine derivatives and

other organic semiconductor materials.

10.1 Crystal Engineering

Shortly after I began my doctoral studies, Anthony published an excellent review article on non-

fullerene acceptors where he concluded that “The next key step in the development [of organic

photovoltaics] will likely involve developing structure-property relationships for transport and

morphology that are as accurate and broadly applicable as the relationships already developed

for tuning the absorption and electronic properties.”1 The field as a whole has not yet reached

this point, nor have we specifically with subphthalocyanines, but my work has at least helped to

clarify the path forward. While we have shown some relationships between measured charge

carrier mobilities and the single crystal X-ray diffracted crystal structures of some BsubPcs

(Chapters 2 and 3).2-3 As guidance for future crystal engineering activities (i.e. what to look for

crystallographically to imply high charge carrier mobility), we concluded that for BsubPcs

stacked in a columnar motif, short intra-columnar distances corresponded with higher zero field

mobility and higher molar densities corresponded with shallower Poole-Frenkel coefficients.2

We also observed that different crystal motifs could correspond to similar room temperature

mobilities but result in different degrees of thermal activation of the mobility.3 We have also

identified a gap in taking this approach: the translation of crystal structure and to device film

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structure. Additionally, I feel there is a challenge in relating crystal structure to a single

electronic property. I will describe what I think is the best approach to overcome these issues.

10.1.1 Film Characterization

First, as discussed in Chapter 5, the X-ray characterization of BsubPc thin films is

experimentally challenging due to the low scattering cross section of the component atoms in

BsubPc molecules, a challenge not faced by higher mass centre element phthalocyanines (Pcs).

Currently, this leaves us unable to specifically correlate the crystal structures determined by

single crystal diffraction to the actual structures of the films at thicknesses and on substrates

relevant to organic electronics using techniques readily available and common in the field. I

think three approaches might yield better understanding of the films which I will now discuss in

order from most direct to least direct. The first would be to collaborate with a facility possessing

a high intensity X-ray source to allow more in depth probing of device relevant thickness films

containing only light atoms. The Canadian Light Source synchrotron in Saskatchewan and the

TRIUMF cyclotron in British Columbia are the two Canadian options for such a collaboration.

Both facilities have soft material characterization programs. With the greater varieties of

energies and intensities of beams available from particle accelerators, compared to the bench top

models available to us at the University of Toronto, more might be achievable to correlate single

crystal structures to actual films. For instance, the grain size distribution, percentage crystallinity

and grain orientation could play key roles in the mobility of BsubPc films, yet to date we have

been unable to characterize these properties in our films. It is expected that larger grain size

should be more facilitative of charge transport (as this is where the single crystal arrangement is

relevant), yet we are unable to say whether the grain sizes and the ratio between crystalline and

amorphous domains in different BsubPc derivative films are comparable or radically different.

Another characterization option involves radiochemistry. The isotope boron-10 (20% natural

abundance) has a high neutron absorbance cross-section and thus finds application in nuclear

fission reactor control rods and certain cancer treatments. Neutron diffraction of boron rich

species including single crystals of decaborane (B10H14) has been used to provide much greater

detail about their crystal structures than corresponding X-ray or electron diffraction studies.4

Neutron diffraction has also been employed to study long range order in boron containing

amorphous glass films,5 so this technique is has been demonstrated to be a powerful compliment

to standard X-ray crystallographic techniques for characterizing crystallinity in boron containing

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materials. Thus neutron diffraction characterization of either naturally abundant or isotopically

pure/enriched BsubPc films might be another approach to film characterization. Neutron

diffraction however requires access to a nuclear reactor, while not beyond the realm of

possibility this does generate additional challenges.

A second, less quantitative approach to film crystallinity, if sophisticated X-ray and neutron

techniques prove unviable, could be developed relating to opto-electronic properties. Our group

has demonstrated that the aggregation of BsubPc dissolved in poor solvents results in lower

energy emissions in the photoluminescence and electroluminescence emission spectrums6-7 and it

is well known that crystallization induced broadening and red-shifting of absorbance occurs for

other large aromatic electron systems used as organic semiconductors. These effects on

absorbance and emission spectra are absent in very dilute solutions in good solvents, where each

BsubPc molecule is effectively only interacting with solvent molecules, but become detectable

when aggregates start to form, so a metric which compares BsubPc derivatives between dilute

solution state and solid state spectra might be a good indirect metric. If a standardized test were

developed, where the absorbance and emission spectra of each BsubPc was measured in a dilute

solution with a non-aromatic solvent with dielectric constant of about 4 (near that of a BsubPc

film), and these spectra were compared with the spectra of device relevant thicknesses films (25

nm films on glass, for instance), commentary on relative film crystallinity within the set might be

made and could guide future synthetic work. The decrease in absorbance onset, peak, and

shoulder energies, the increase in full width at half maximum for peaks, and the increase in

intensity of red and near-IR emissions for a film relative to the solution could provide at least a

qualitative measure film crystallinity and the extent of BsubPc to BsubPc interactions. Films

formed at different deposition rates are known to show different morphologies,8 degrees of

crystallinity, and grain sizes,9 so those formed at particularly high rates might serve as

amorphous reference points.

Third, and least direct, the characterization of analogous compounds could be used to generate

design rules that might apply to BsubPcs. While not perfect structural analogues, aluminium

phthalocyanines (AlPcs) and titanium phthalocyanines (TiPcs) both form conical molecular

structures similar to BsubPcs, except with four diiminoisoindoline lobes instead of the three seen

with BsubPc. Using a conventional bench top X-ray source, one of my colleagues recently

demonstrated successful X-ray diffraction characterization of device relevant thickness films of

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silicon phthalocyanines (SiPcs), where silicon has scattering cross-section of 0.097 cm2 g-1 at

8267 eV,10 so I believe there is a reasonable expectation that similar success can be achieved

with aluminium (0.095 cm2 g-1)10 and titanium (0.083 cm2 g-1)10 based Pcs. Crystal engineering

results and design guidance more easily established with AlPc and TiPc derivatives might be fed

back into the design of BsubPc derivatives.

The second area of challenge is that we have thus far sought to relate crystal structures to charge

carrier mobility and then to go from charge carrier mobility to device performance. Given the

challenges of reliably measuring mobility and how poorly predictive our results so far have been

(Chapter 4), it raises the question of which process truly is the limiting process for device

performance in a given cell configuration or material pairing. The motion of dissociated charge

carriers is by no means the only process occurring in a these photovoltaic devices (Chapter 1).

There are excitonic diffusion processes, CT state dissociation and relaxation processes, and

excitonic and charge carrier relaxation and recombination processes in the bulk, at trap sites, and

at interfaces. Perhaps more fruitful lines of correlative testing and predictive design criteria

development would be to relate crystal structure to excitonic properties such as exciton diffusion

length or other kinetic properties such as carrier recombination rates.

10.2 Device Processes

Organic containing photovoltaic devices are opto-electronic devices, meaning that both optical

and electrical processes are fundamental to the device function. In addition, excited state

processes intervene between the optical and electrical process, but opto-excito-electronic devices

is a rather cumbersome expression, so they are not referred to by that name. We have focused

almost exclusively on electronic properties so far, but there is potentially a much larger property

space to consider. Thus three sets of processes need to be considered when moving forward to

improved devices. In fact, I argue that the development of an open source (or at cost provided)

computer program which does a detailed species balance for device configurations is the best

way forward for the field. By detailed species balance, I mean a comprehensive accounting of

the energy of a photon from the point of entry into the device to the point of exit as an electron

hole pair to the electrodes through all the intervening steps of excited state diffusion, charge

transfer state formation and dissociation, and charge carrier transport and extraction. I have

illustrated a conceptual flow chart of the computational processes in Figure 10-1. The Forrest

group described an limited integrated opto-excitonic model years ago,11 but no such program or

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calculation framework is openly available nor widely used and a three part opto-excito-electronic

model is completely absent.

A detailed species balance is a common approach to reactor engineering and in this case is

described mathematically as a set of monodirectionally coupled differential equations. The

assumptions of steady state and infinitely large device areas (to exclude the complication of

calculating edge effects) leave this as a single dimensional differential equation that a third year

engineering undergraduate would be familiar with solving. The computational intensity of these

models is not outside the range of existing desktop computers, and is thus very accessible at low

cost to a broad spectrum of researchers. In the same way as computational chemistry has

improved pre-synthesis screening of candidate compounds,12 in silico device modeling and

optimization is undoubtedly less expensive than device optimization through iterative device

fabrication. What follows is a more detailed discussion of each of the three types of processes

that would need to be included in an integrated model.

Figure 10-1. Flow chart outlining the calculations of a detailed species balance. The symbol

is the concentration of species i at spatial dimension x, while , is a generation or

destruction rate of species i at spatial dimension x. For subscripts, h/ is a photon of wavelength

, s is a singlet exciton, e is an electron charge carrier, h is a hole charge carrier.

10.1.2 Optics

Highly effective optical models for balancing charge generation in multiple sub-cells within

tandem photovoltaic devices have been demonstrated and can be based upon fundamental first

principles optical transfer function matrix methods.13-15 All that is needed to deploy this sort of

model is the complex index of refraction for a material, which can be measured by spectroscopic

ellipsometry. However, existing models are only the first part of the story, as they only consider

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spatial and spectral distribution of light absorbance, assuming the subsequent processes are near

unity efficiency. In actuality, the conversion of a photon to a molecular excited state is but the

first step. I believe that the spatial distribution of light absorbance needs to the then be used to

determine the spatial distribution of the exciton generation function.

10.1.3 Excitonics

Once the photon absorbance function is converted into an exciton generation function, then the

addition of boundary conditions, migration rates, conversion rates, and relaxation rates allows a

detailed species balance of the excited states. Those excited states that migrate to the donor

acceptor interface can form charge transfer states. These charge transfer states then relax at a

certain rate and dissociate at a certain rate, where the dissociation rate corresponds to the rate of

hole and electron charge carrier generated at the interface, which then moves us to the realm of

electrical species balances. Since many excitonic processes are concentration dependent, ways

to minimize exciton concentration while maximizing flux to dissociating interfaces, for instance

by the introduction of multiple dissociating interfaces as seen in Cnops et al.'s 2012 three layer

pentacene/BsubPc/C60 device,16 can reduce excited state loss mechanisms.

There is precedent for dramatic improvement from innovative exciton management both in terms

of energy cascades,17 and triplet management.18 The improvements to date lead me to ask if

exciton management through excited state funneling is a more efficient approach than diffusive

transport to ensure excited states reach the appropriate interface. Detailed mathematical

modeling is an effective way to approach an answer to this question.

10.1.4 Electronics

The detailed excited species balance ultimately leads to the rate of charge carrier generation at

the donor-acceptor interface. These charge carriers then migrate through the donor and acceptor

layers, some becoming trapped and recombining with opposite charge carriers through geminate

and nongeminate recombination. This is the part of device operation where charge carrier

mobility comes into play as higher mobilities will result in lower concentrations of carrier for a

constant current. As with excited states, lower carrier concentrations help to suppress loss

mechanism rates. As in the prior two sections, it should be noted that there exist detailed

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electrical species balance models that can reproduce many photovoltaic device current-voltage

characteristic shapes, including the infamous current vs. voltage s-kink,19 but these models are

not integrated with optical or excitonic considerations.

Requirements for this part of the detailed species balance emphasize the importance of proper

quantification of trap density and energy depth for both species of charge carriers in a material,

as these have a major impact upon carrier recombination rates. Quantification of trap density

and depths would also be an additional useful tool to help understand recent results our lab has

observed related to the presence of low concentrations of partially chlorinated AlPc and boron

subnaphthalcyanine (BsubNc) materials performing better than high purity batches in organic

electronic devices.

Even if trap quantification proves difficult, it should be noted that a material with a high ratio of

hole to electron mobilities will have a low concentration of electrons in the donor layer to

facilitate recombination, but a material with more evenly matched mobilities will have a higher

concentration of minority carriers, facilitating increased recombination. Given our previous

demonstration of Phth-BsubPc as an equally effective hole or electron transport layer in an

OLED,20 one wonders if the reason it performs so poorly as an acceptor in a OPV device is

because of a high minority carrier concentration resulting in a high recombination rate, thus

negatively impacting current density. I propose that regardless of a material's suitability as a

donor or acceptor, both the hole and electron mobilities should always be quantified so that the

ratio of the two can be used to judge suitability for either application in a OPV device. If the

ratio is near unity, the material may be more suited to use as a thin ambipolar layer where the

thinness minimizes recombination but takes advantage of the matched mobilities to move

carriers of both polarities from separate dissociating interfaces.

As a final comment on detailed species balances, our results so far have shown electron mobility

as a poor predictor of material performance in OPV devices. This might be because the

electrical processes are not the limiting processes in device performance, and a detailed species

balance will help to identify where the rate limiting step in the process is so that priority can be

directed improving the material properties related to that limitation. I would be unsurprised if a

detailed species balance like the one I have outlined would be able to predict all aspects of

device performance up to the level of predicting the key parameters of (or at worst, the upper

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limits to) JSC, VOC, and FF (and if you have those three, then you have power conversion

efficiency) before any devices are ever fabricated.

10.3 Holistic Approaches to BsubPc Incorporation into Electronic Devices

Most work focusing on BsubPc incorporation into electronic devices looks at how to substitute a

single BsubPc derivative as a single layer into an already established device structure. While

sometimes effects like complimentary absorbance are leveraged, on the whole, device

improvement and material testing is done on a one step at a time basis. While control of the

variable space and ensuring only a single variable is changed at a time are fundamental to

rigorous scientific inquiry, there is much to be said for the large scale combinatorial approaches

taken by our colleagues in the biological sciences. I think that the field should focus more on

design of a whole device worth of materials than the individual layers. To start with, instead of

trying to develop a good donor material or a good acceptor material, the focus should be on

metrics to guide design of good donor-acceptor pairings. There is already indication that

molecular orientation greatly impacts the charge transfer (CT) state energies at the interface

between pentacene and C60.18 Can metrics be developed to predict or improve dissociation rates

at interfaces? For instance, might higher Förster resonance energy transfer (FRET) cross-

sections between a donor-acceptor paring be more favourable for generating and dissociating

interfacial CT states? Differences in exciton energies between adjacent acceptor layers,21

adjacent donor layers,17 and between emitters in OLEDs,22 can be used to funnel excited states to

dissociating interfaces or molecules with favourable light emission characteristics. Why are

these sorts of design criteria not pursued rabidly in the development of higher efficiency OPV

cells, nor considered when developing material pairings or new cascade material sets? We could

be coming at the problem of higher efficiency solar cells from a device down side instead of a

materials up approach if it were possible to have one laboratory with both the physics and

synthetic expertise and the means to test their designs.

Ten years ago, Forrest wrote an excellent review considering the ultimate limits to organic

photovoltaic device efficiency where he suggested that the metric of the ratio of exciton diffusion

length (LD, average exciton diffusion distance) to light absorbance length (d, 90 % incident light

201

absorbed) as useful for identifying if a material will perform well in a solar cell.23 Most organic

materials have LD/d less than unity. A value near unity would indicate the same amount of

material would be able to absorb most of the incident light and those excited states would be able

to diffuse to the dissociating interface, while a value much less than unity would mean the

thickness of film required to absorb most of the light would be too thick for many of the excited

states to diffuse to the interface. This trade-off between diffusion length and absorbance length

encapsulates the primary challenge of organic photovoltaics that motivated the development of

bulk heterojunction photovoltaics, yet I have never seen this ratio reported for any material.

Clearly this is a simplification of the complex optical dynamics in a thin film device, but it does

point toward something the field lacks, design criteria which take into account multiple relevant

material properties simultaneously. Just as Ashby totally revolutionized the rational selection

and development of materials for mechanical applications by proposing material performance

functions that were ratios of material properties relevant to the limiting design case, those

developing new organic electronic materials need to take a wider view of material properties to

guide future design. One immediately sees that the diffusion length (15 nm) to absorbance

length (90 nm at 570 nm) ratio for Cl-BsubPc is around 0.17, a clear mismatch in the optical and

excitonic properties, so it should have come as no surprise Cnops et al.16 managed to improve

device performance by functionally extending the diffusion length through favourable excitonic

energy transfer. Schlenker et al.'s17 results with a pentacene derivative cascade benefit from the

same principle. Similarly, Menke, Luhman, and Holmes24 showed that dilution of Cl-BsubPc

into a wider gap semiconductor increases exciton diffusion length. It should be immediately

obvious that this dilution will also increase absorbance length, but if these properties change at

different rates, than perhaps an optimal dilution can be identified which maximizes the diffusion

to absorbance ratio, leading to better device performance. Frankly, even Sullivan's use of

BsubPc as donor and acceptor layers in the same cell can be viewed as functionally doubling the

diffusion length (i.e. excitons on two sides of a dissociating interface) while keeping the

absorbance length unchanged, again improving this key ratio. The use of Cl-BsubPc16 as an

interlayer to create two exciton dissociating interfaces can again be viewed as a way to improve

the diffusion length by harvesting from three diffusion lengths worth of material between two

dissociating interfaces instead of the two lengths enabled by a single donor-acceptor interface,

again improving the diffusion to absorbance ratio. I extend this line of thinking to propose that

electron and hole diffusion lengths to exciton diffusion length would be two more material

202

property ratios that would desirable to equal or exceed unity so that once the excitons are

dissociated at the interface we can be confident that the generated charge carriers will reach the

corresponding electrodes before recombination.

While we are on the topic of property ratios, the ratio of electron mobility to hole mobility, e/h,

is a valuable ratio to demonstrate whether a material is a more desirable donor, acceptor, or

interlayer. While I expressed reservations about the technique employed by Pandey et al.25 in

Chapter 5, based upon their data Cl-BsubPc's e/h is 0.01. A sub-unity value suggests Cl-

BsubPc is better employed as an electron donor layer than as an electron acceptor layer, yet this

ratio has never been reported for any BsubPc derivative and it contradicts the proof that in OPVs

the pairing of a BsubPc as a donor (with C60 for example) compared to paring it as an acceptor

(for example with 6T) gives comparable device performance.

With this sort of whole device approach, future device fabricators might reach new heights of

solar performance by looking at material group metrics. For instance, imagine a situation where

we have identified a dozen new materials and there are metrics for these individual materials.

Next we could take a combinatorial approach and consider the 132 possible pairings of these

materials and generate material pairing metrics. We could then use these pairing metrics to

select promising device configurations for testing. Material pairing metrics could be developed

to quantify spectral overlap (a property to be minimized), organic to organic charge injection (a

property to be maximized), and absorbance efficiency based upon the major wavelength of

absorbance relative to that wavelength's location of maximum intensity in the cell (another

property to be maximized). One can see this sort of material combination metrics is a reasonable

outgrowth of our attempt to find suitable donors and acceptors to pair with -oxo-(BsubPc)2 in

Chapter 7, where we found that simply considering frontier orbitals was a poor predictor of

device performance. Our device performance predictions could be more effective if we could

consider exciton diffusion lengths, trapping kinetics, and CT dissociation efficiencies. As

another example, imagine we have a dozen new acceptor materials with metrics for the

individual materials. None of them have particularly promising ratios of exciton diffusion length

to absorbance length. Which pair, triple, or quadruple of these materials will provide the set of

metrics which gives the best energy cascade and electron transport when they are deposited

sequentially in a cell? Get the energy level alignments and excited state energies ordered

203

effectively and the distant layers funnel excitons to the dissociating interface but don't disrupt

charge transport or increase series resistance due to interfacial injection barriers. In this case,

metrics describing exciton transfer coupling efficiency (such as FRET radius) might allow one to

predict what fraction of photons absorbed in a layer far from the donor-acceptor interface would

be effectively transferred through the intervening layers to reach the dissociating interface before

relaxation.

As mentioned in Section 10.2, a whole device, kinetics based, detailed species balance type

predictive model sounds more and more appealing to navigate the design space for organic

electronic devices. As a final summary, Figure 10-2 illustrates the inclusion of energy cascades

for exciton management in both donors and acceptors. plus multiple dissociating interfaces to

yield a multilayer cell architecture which might improve device performance by leveraging many

of the ideas that have been covered to this point.

Figure 10-2. The use of material metrics, exciton management through energy cascades, and

multiple dissociating interfaces for a whole device design approach to photovoltaics.

10.4 Leveraging Singlet Fission in Device Designs

An additional useful consideration with regards to device design is how to best use the current

density increases achieved with singlet fission derived triplet harvesting materials. While our

204

initial foray into this arena (Chapter 8) resulted in planar bilayer devices with increased

photocurrent at the expense of reduced voltage (leading to almost no increase in total power

conversion efficiency) this does not mean the results are valueless. Our work thus far considered

a single planar heterojunction photovoltaic cell, but think of the possibilities for a solar cell

where multiple subcells are assembled and connected electrically in series with each other. A

key challenge with these tandem cell architectures is achieving equal current generation in each

subcell, as this then results in the voltage from each subcell being added together for a high

voltage, low current device. Current balancing can be achieved with detailed optical models,14

but there are only so many photons to work with in a device. Consider, for instance, the subcell

closest to the reflective electrode in a tandem architecture. This cell will generally have a lower

optical field density due to the intensity node (zero optical field intensity) formed at the

reflective surface. If the designer situates a subcell that leverages singlet fission in this low field

intensity part of the cell, the subcell's current density can be increased despite the lack of optical

field density. A higher current density achieved with a lower photon density in this part of the

cell would then mean greater ability to take advantage of higher current densities in other

subcells where higher optical field densities are present, increasing overall cell performance.

10.5 An Alternative to Bathocuproine (BCP)

The genesis of Chapter 5 was a study to identify a universal material to obtain Ohmic contact for

electron injection into BsubPc derivatives used as acceptors. The current standard material is

bathocuproine (BCP), which is carried over from early OPV work in the field where C60 was

used as the acceptor. A major challenge with BCP is its low glass transition temperature. While

BCP is vacuum deposited as a transparent amorphous layer, it readily crystallizes on the

timescale of days, even in tightly controlled conditions, causing significant device performance

degradation as a result of layer delamination, damage to the above metal electrode, and a hazy

optical character that changes the optical field intensity distribution within the cell. The

crystallization process can be forestalled by co-deposition of other materials to inhibit the

process, but this added complexity is an undesirable addition to the process. Further, charge

transport through the layer happens through defect states formed by degradation induced by the

deposition of the metal contact on top of the BCP layer, so the layer is highly sensitive to metal

205

deposition process conditions and the defect states are also expected to be unstable over the long

timescales required for viable commercial devices. These challenges are ably demonstrated in

References 26 and 27. Much like ITO, BCP is used because it has acceptable properties in the

short term and is readily available, not because it is the optimal conceivable material to fill the

design space.

Since there are so many known concerns with BCP, it would still be desirable to identify a new

single molecule alternative to serve its multiple jobs as buffer layer, wide band gap optical

spacer, and electron transporter from an acceptor. Further, the BsubPc acceptors and other

phthalocyanines and analogues that the Bender Lab continues to explore have a wide span of

LUMO energies which differ from C60's LUMO by up to 1 eV. As such, Chapter 5 could serve

as the foundation of a study to explore BCP alternatives in photovoltaic devices. Ideally, it could

lead to the identification of a universal buffer layer (or set of buffer layers best for certain spans

of LUMO energies).

Chapter 5 was focused on primarily on identification of an Ohmic contact for use in admittance

experiments, but given the challenges encountered translating admittance data into photovoltaic

performance improvements, a refocusing on PV device metrics is suggested. Starting from the

results of the single carrier devices, BCP, TPBi, and BPhen should be the focus of the study.

Given recent demonstrations of low work function metal oxides, such as zinc oxide, metal oxides

might also be worthy of consideration. The standard acceptor screening device structure we

identified28 would be the device foundation I would recommend to continue this study on. By

keeping device layer thicknesses constant, the impact of the buffer layer can be isolated. In

terms of device metrics, I would suggest using the equivalent circuit model for a solar cell (see

Chapter 5) and identifying the contributions to series resistance and parallel resistance that can

be directly attributed to the contact layer. Specifically, a direct comparison of the series

resistance set of cells6T (55 nm)/Cl-BsubPc (20 nm)/test layer (x nm)/Ag, x = 5, 10, 15 nm,

would allow extrapolation of the series resistance back to zero thickness to identify contact

resistance at the Cl-BsubPc/test layer and test layer/Ag interfaces and the slope would

correspond to the resistance induced by transport through the test layer. By including multiple

materials and using a standard statistical ANOVA analysis, contributions from material, varied

thickness, and the remainder of the cell can be differentiated. The best material would have a

206

low series resistance, indicating good electron transfer, and a high parallel resistance, indicating

good hole blocking.

As a first dimension of testing, I would recommend identifying optimal layer thicknesses, a

comparison of 5, 10, 15 nm BCP with 5, 10, 15 nm of TPBi and 5, 10, 15 nm of BPhen for a

device of the structure 6T/Cl-BsubPc/test layer/Ag. Having identified an optimal thickness for

each of the three candidate materials, a second dimension of testing would be to select BsubPc

derivatives with a wider variety of LUMO energies, from the shallow LUMO energy of Cl-

BsubPc, to the deeper LUMO energy of Cl-Cl6BsubPc, to the near C60 LUMO energy of Cl-

Cl12BsubPc. If a BsubPc or close analogue with an even shallower or even deeper LUMO

energy was available, that would be an additional strong candidate for inclusion in the study.

Next, a test using other acceptors with similar LUMO energies to the initial screening materials

would lend credence to a search for a universal BCP replacement. As the final stage of testing,

lifetime testing of the BCP alternative should be pursued. While there is already one report of an

improved lifetime for TPBi compared to BCP in a C60 containing system,29 testing with a wider

variety of acceptors would again be more convincing and beneficial for identifying a universal

replacement. While metal oxides seem to show the most promise for many transparent electrode

applications, this same structure of screening with could also be used to test for widely

applicable alternatives low work function electrodes in inverted cell structures or high work

function electrodes in standard and inverted architectures, as this sort of systematic study is

rarely seen when a new electrode material is proposed in the literature.

10.6 Vacuum System Improvements

It is my belief that the reason much of organic photovoltaic research is done on solution cast

polymers is because spin coaters are relatively inexpensive pieces of experimental equipment for

the deposition of soluble conductive polymers. Additionally, soluble polymers are easier to

synthesize than insoluble materials. If the price and operational complexity of a vacuum system

can be brought down to be comparable to the costs of spin coating, this could foster more

research into insoluble and vacuum depositable organic semiconductors. I believe the key

features to such a low cost system are minimization of cost through minimizing the number of

207

thermal sources and system size. In terms of thermal sources, I would recommend resistive

boats with multiple holes on the boat top, such as those demonstrated in Chapter 5. The sources

can be used for metal or organic depositions, giving the system inherent flexibility. Thermal

boats are also significantly lower cost than crucible heater furnaces, and multiple hole tops retain

the open crucible's advantages in flux uniformity while minimizing sensitivity of the flux field to

the amount of material in the boat. A minimum of four thermal sources would be required for

the ability to deposit a hole extraction layer, donor layer, an acceptor layer, and a electron

extraction/buffer layer. For simplicity, an in situ mask change can be avoided by interfacing the

chamber with a glove box, just the way the Angstrom CoVap systems are currently designed.

Vacuum seal can then be broken to change from an organic mask to a metals mask under inert

atmosphere, which the MARI-KATE system has also demonstrated has no ill effect compared to

in situ mask change to an electrode mask. While the chamber is open for the mask change, one

of the organic depositing boats can be swapped for a metal depositing boat to be attached at the

same location. Two uses for a single deposition source minimizes chamber size. The chamber

must be as small as possible to keep down materials costs and vacuum pump requirements.

Based upon the uniformity discussion in Chapter 8, the use of a substrate offset from the axis of

rotation, allowing direct placement over each thermal source would be best. The system could

be much smaller than either 200 mm throw distance in MARI-KATE (Chapter 8) or the short

throw distance of 260 mm for the Angstrom CoVap (Chapter 6), as we've demonstrated more

than adequate uniformity and no heat transfer concerns even at a 4 inch, non-rotating, throw

distance.

I believe there is a developable market niche in small scale easy to operate vacuum systems that

would enable more synthesis based laboratories to become involved in the testing of their

materials in functional electronic devices. As the field currently lacks overarching molecular

design rules to guide the development of new organic semiconductors, a shorter cycle time

between material synthesis and device performance will enhance the rate of the current iterative

process of materials development. As more data becomes available through this iterative

process, there is a higher likelihood that general design rules will become apparent.

208

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Charge Carrier Mobility in Fluorinated Phenoxy Boron Subphthalocyanines: Role of Solid State

Packing. Cryst. Growth Des. 2012, 12, 1095-1100.

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Bender, T. P.; Jones, T. S., Acceptor Properties of Boron Subphthalocyanines in Fullerene Free

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(-Oxo-(Bsubpc)2)S. Dalton Trans. 2015, 4280-4288.

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Ito/Tpd/Alq3/Al Organic Luminescent Diodes. Solid-State Electron. 2004, 48, 2085–2088.

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C-1

Appendix A:

Supplementary Information for Chapter 3

Plots of mobility as a function of temperature and electric field are shown in Figure S1, and the

Poole-Frenkel Parameters are shown in Tables S1, S2, and S3. The Gaussian Disorder Model

(GDM) is used to analyze the form of the Poole-Frenkel mobility description to gain additional

insight into a charge hopping transport model. Temperature dependence is modeled according

to,1

2

2exp exp

3F

kT

.

(S1)

In this model, ∞ is the high temperature limit to the mobility, is the width of the density of

states (DOS) for the transport states. The energy distribution width () is extracted from the

slope of a plot of ln (0) vs. (1/T)2. Table S4 shows the fitted parameters for the FnBsubPc

compounds. Since the dominant transport mechanism is believed to change at lower

temperatures (as evidenced by the change in slope for F12BsubPc in Figure S2), curve fits for the

GDM were constructed only using data gathered at 240K and above.

C-2

FIGURES:

0 200 400 600 800 1000

10-6

10-5

10-4

240 K 250 K 260 K 270 K 280 K 290 K 300 K 310 K 320 K 330 K

Ele

ctro

n M

ob

ility

µe

,dc (

cm2 V

-1s-1

)

F1/2 [(V/cm)1/2]

a)

0 200 400 600 800 100010-8

10-7

10-6

10-5

10-4

120 K 150 K 210 K 240 K 270 K 300 K

Ele

ctro

n M

ob

ility

µe

,dc (

cm2 V

-1s-1

)

F1/2 [(V/cm)1/2]

b)

0 200 400 600 800 1000 120010-4

2x10-4

3x10-4

4x10-4

5x10-46x10-47x10-48x10-4

290 K 300 K 310 K 320 K 330 K

Ele

ctro

n M

ob

ility

µe

,dc (

cm2 V

-1s-1

)

F1/2 [(V/cm)1/2]

c)

Figure S1. Plots of electron mobility vs. the square root of electric field for (a) F5BsubPc, (b)

F12BsubPc and (c) F17BsubPc at varied temperatures.

C-3

10 20 30 40 50 60 7010-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

F5BsubPc

F12

BsubPc

F17

BsubPc

Zer

o F

ield

Mob

ility

µ0

,T (

cm2 V

-1s-1

)

(1000/T)2 (K-2)

Figure S2. Gaussian disorder model parameter extraction plot for the determination of the high

temperature mobility and the width of density of transport states.

C-4

Figure S3. Thermal ellipsoid plot showing 35% probability for -F5BsubPc. Hydrogen atoms

omitted for clarity.

Figure S4. Populated unit cell of -F5BsubPc. Hydrogen atoms omitted for clarity. Thermal

ellipsoids at a 35% probability level. Color scheme: grey – carbon; blue – nitrogen; red –

oxygen; magenta – fluorine; pink – boron.

C-5

TABLES:

Table S1. Electrical characteristics of the F5BsubPc at various temperatures

Temperature

(K)

Zero Field

Mobility, 0

(cm2V-1s-1)

Standard Error,

(cm2V-1s-1)

Poole-Frenkel

Coefficient,

(V/cm)-1/2

Standard Error,

(V/cm)-1/2

Adjusted R2

240 6.70E-09 7.37E-10 0.00927 1.38E-04 0.99779

250 1.57E-08 1.93E-09 0.00851 1.64E-04 0.99702

260 3.15E-08 6.20E-09 0.00835 2.87E-04 0.99413

270 6.33E-08 9.95E-09 0.00812 2.23E-04 0.99548

280 1.30E-07 2.57E-08 0.00779 2.88E-04 0.9932

290 2.05E-07 2.60E-08 0.00785 1.90E-04 0.99706

300 3.78E-07 6.85E-08 0.0076 2.79E-04 0.99464

310 5.42E-07 9.67E-08 0.00774 2.89E-04 0.99581

320 9.99E-07 2.11E-07 0.00735 3.37E-04 0.99371

330 1.30E-06 5.81E-08 0.0076 8.18E-05 0.99977

C-6

Table S2. Electrical characteristics of the F12BsubPc at various temperatures

Temperature

(K)

Zero Field

Mobility, 0

(cm2V-1s-1)

Standard Error,

(cm2V-1s-1)

Poole-Frenkel

Coefficient,

(V/cm)-1/2

Standard Error,

(V/cm)-1/2

Adjusted R2

120 1.38E-12 2.07E-12 0.01408 0.00103 0.97379

150 5.58E-11 1.57E-11 0.01207 2.99E-04 0.99634

210 3.02E-09 7.98E-10 0.00905 2.76E-04 0.99354

240 3.56E-07 4.96E-08 0.00604 1.82E-04 0.9955

270 5.09E-06 1.51E-06 0.00399 4.42E-04 0.97572

300 2.30E-05 3.36E-06 0.00329 2.45E-04 0.98346

C-7

Table S3. Electrical characteristics of the F17BsubPc at various temperatures

Temperature

(K)

Zero Field

Mobility, 0

(cm2V-1s-1)

Standard Error,

(cm2V-1s-1)

Poole-Frenkel

Coefficient,

(V/cm)-1/2

Standard Error,

(V/cm)-1/2

Adjusted R2

290 2.30E-06 9.52E-07 0.0042 3.25E-04 0.98806

300 9.74E-06 6.01E-06 0.0031 4.51E-04 0.9587

310 2.45E-05 1.08E-05 0.00247 3.42E-04 0.96221

320 2.36E-05 1.26E-05 0.00265 4.01E-04 0.95522

330 1.11E-05 6.36E-06 0.00352 4.24E-04 0.97145

Table S4. Calculated Gaussian disorder model parameters

Compound High

temperature

mobility limit,

inf (cm2V-1s-1)

Energetic

disorder,

(eV)

F5BsubPc 5.1 × 10-4 0.104

F12BsubPc 4.3 × 10-2 0.106

F17BsubPc 8.2 × 10-3 0.103

C-8

Table S5: Graphical illustration of the frontier molecular orbitals from LUMO+6 to HOMO-1

for compounds F5BsubPc, F12BsubPc and F17BsubPc. The orbital were calculated from their

geometrically optimized structure by the DFT method GGA-B3YLP in SPARTAN ‘06.

F5BsubPc F12BsubPc F17BsubPc

LUMO

+6

LUMO

+5

LUMO

+4

C-9

LUMO

+3

LUMO

+2

LUMO

+1

LUMO

C-10

HOMO

HOMO

-1

C-11

Table S6. Crystal data and structure refinement for -F5BsubPc.

Identification code -F5BsubPc

Empirical formula C30 H12 B F5 N6 O

Formula weight 578.27

Temperature 150(1) K

Wavelength 0.71073 Å

Crystal system Monoclinic

Space group P 21/c

Unit cell dimensions a = 14.2834(6) Å a= 90°.

b = 11.3575(2) Å b= 99.7330(15)°.

c = 15.1707(7) Å g = 90°.

Volume 2425.62(16) Å3

Z 4

Density (calculated) 1.583 Mg/m3

Absorption coefficient 0.126 mm-1

F(000) 1168

Crystal size 0.25 x 0.25 x 0.20 mm3

Theta range for data collection 2.55 to 27.51°.

Index ranges -18<=h<=18, -11<=k<=14, -19<=l<=19

Reflections collected 19341

Independent reflections 5515 [R(int) = 0.0702]

Completeness to theta = 27.51° 98.8 %

Absorption correction Semi-empirical from equivalents

Max. and min. transmission 0.981 and 0.758

Refinement method Full-matrix least-squares on F2

Data / restraints / parameters 5515 / 0 / 388

Goodness-of-fit on F2 1.037

Final R indices [I>2sigma(I)] R1 = 0.0540, wR2 = 0.1164

R indices (all data) R1 = 0.1134, wR2 = 0.1450

Largest diff. peak and hole 0.271 and -0.330 e.Å-3

C-12

Table S7. Atomic coordinates (× 104) and equivalent isotropic displacement parameters (Å2 ×

103)

for -F5BsubPc. U(eq) is defined as one third of the trace of the orthogonalized Uij tensor.

______________________________________________________________________________

__

x y z U(eq)

______________________________________________________________________________

__

F(1) 1184(1) 2101(1) 6344(1) 40(1)

F(2) 1232(1) 4368(1) 6880(1) 56(1)

F(3) 2793(1) 5718(1) 6798(1) 53(1)

F(4) 4305(1) 4761(1) 6171(1) 41(1)

F(5) 4277(1) 2495(1) 5676(1) 34(1)

O(1) 2720(1) 1062(1) 5796(1) 26(1)

N(1) 2466(1) -586(2) 4768(1) 22(1)

N(2) 781(1) -688(2) 4415(1) 26(1)

N(3) 1575(1) 1144(2) 4358(1) 23(1)

N(4) 2258(1) 2663(2) 3590(1) 24(1)

N(5) 3200(1) 1097(2) 4294(1) 22(1)

N(6) 3996(1) -741(2) 4345(1) 26(1)

C(1) 3226(2) -1235(2) 4587(2) 25(1)

C(2) 2890(2) -2450(2) 4520(2) 27(1)

C(3) 3344(2) -3500(2) 4367(2) 32(1)

C(4) 2805(2) -4516(2) 4268(2) 38(1)

C(5) 1839(2) -4500(2) 4312(2) 40(1)

C(6) 1369(2) -3467(2) 4446(2) 34(1)

C(7) 1903(2) -2432(2) 4556(2) 27(1)

C(8) 1632(2) -1208(2) 4637(2) 25(1)

C(9) 769(2) 469(2) 4230(2) 24(1)

C(10) 41(2) 1191(2) 3698(2) 25(1)

C(11) -917(2) 993(2) 3381(2) 30(1)

C(12) -1420(2) 1862(2) 2865(2) 34(1)

C-13

C(13) -979(2) 2896(2) 2648(2) 33(1)

C(14) -22(2) 3091(2) 2927(2) 30(1)

C(15) 492(2) 2236(2) 3460(2) 26(1)

C(16) 1497(2) 2133(2) 3829(2) 25(1)

C(17) 3097(2) 2104(2) 3791(2) 24(1)

C(18) 3971(2) 2205(2) 3422(2) 24(1)

C(19) 4311(2) 3070(2) 2912(2) 29(1)

C(20) 5173(2) 2883(2) 2638(2) 35(1)

C(21) 5685(2) 1849(2) 2851(2) 37(1)

C(22) 5361(2) 968(2) 3354(2) 31(1)

C(23) 4498(2) 1153(2) 3649(2) 25(1)

C(24) 3954(2) 421(2) 4163(2) 23(1)

C(25) 2727(2) 2232(2) 5996(1) 22(1)

C(26) 1965(2) 2742(2) 6305(2) 27(1)

C(27) 1982(2) 3903(2) 6573(2) 34(1)

C(28) 2770(2) 4585(2) 6537(2) 34(1)

C(29) 3533(2) 4106(2) 6219(2) 28(1)

C(30) 3505(2) 2943(2) 5957(2) 26(1)

B(1) 2497(2) 705(2) 4850(2) 23(1)

______________________________________________________________________________

__

C-14

Table S8. Bond lengths [Å] and angles [°]

for -F5BsubPc.

____________________________________

_________________

F(1)-C(26) 1.342(3)

F(2)-C(27) 1.346(3)

F(3)-C(28) 1.345(3)

F(4)-C(29) 1.342(3)

F(5)-C(30) 1.348(3)

O(1)-C(25) 1.363(3)

O(1)-B(1) 1.472(3)

N(1)-C(8) 1.371(3)

N(1)-C(1) 1.378(3)

N(1)-B(1) 1.472(3)

N(2)-C(8) 1.342(3)

N(2)-C(9) 1.343(3)

N(3)-C(9) 1.370(3)

N(3)-C(16) 1.374(3)

N(3)-B(1) 1.485(3)

N(4)-C(17) 1.344(3)

N(4)-C(16) 1.346(3)

N(5)-C(24) 1.365(3)

N(5)-C(17) 1.368(3)

N(5)-B(1) 1.485(3)

N(6)-C(1) 1.340(3)

N(6)-C(24) 1.347(3)

C(1)-C(2) 1.459(3)

C(2)-C(3) 1.395(3)

C(2)-C(7) 1.420(3)

C(3)-C(4) 1.380(3)

C(3)-H(3A) 0.9500

C(4)-C(5) 1.393(4)

C(4)-H(4A) 0.9500

C(5)-C(6) 1.383(4)

C(5)-H(5A) 0.9500

C(6)-C(7) 1.396(3)

C(6)-H(6A) 0.9500

C(7)-C(8) 1.453(3)

C(9)-C(10) 1.458(3)

C(10)-C(11) 1.390(3)

C(10)-C(15) 1.426(3)

C(11)-C(12) 1.383(3)

C(11)-H(11A) 0.9500

C(12)-C(13) 1.399(3)

C(12)-H(12A) 0.9500

C(13)-C(14) 1.379(3)

C(13)-H(13A) 0.9500

C(14)-C(15) 1.392(3)

C(14)-H(14A) 0.9500

C(15)-C(16) 1.454(3)

C(17)-C(18) 1.457(3)

C(18)-C(19) 1.389(3)

C(18)-C(23) 1.423(3)

C(19)-C(20) 1.380(3)

C(19)-H(19A) 0.9500

C(20)-C(21) 1.392(4)

C(20)-H(20A) 0.9500

C(21)-C(22) 1.385(3)

C(21)-H(21A) 0.9500

C(22)-C(23) 1.396(3)

C(22)-H(22A) 0.9500

C(23)-C(24) 1.451(3)

C(25)-C(30) 1.383(3)

C(25)-C(26) 1.383(3)

C-15

C(26)-C(27) 1.379(3)

C(27)-C(28) 1.375(3)

C(28)-C(29) 1.377(3)

C(29)-C(30) 1.378(3)

C(25)-O(1)-B(1) 118.51(17)

C(8)-N(1)-C(1) 112.94(18)

C(8)-N(1)-B(1) 122.62(19)

C(1)-N(1)-B(1) 122.47(19)

C(8)-N(2)-C(9) 117.52(19)

C(9)-N(3)-C(16) 113.01(19)

C(9)-N(3)-B(1) 122.38(18)

C(16)-N(3)-B(1) 123.08(19)

C(17)-N(4)-C(16) 117.19(19)

C(24)-N(5)-C(17) 113.71(18)

C(24)-N(5)-B(1) 122.72(18)

C(17)-N(5)-B(1) 123.22(18)

C(1)-N(6)-C(24) 117.19(19)

N(6)-C(1)-N(1) 122.7(2)

N(6)-C(1)-C(2) 130.8(2)

N(1)-C(1)-C(2) 105.00(19)

C(3)-C(2)-C(7) 120.7(2)

C(3)-C(2)-C(1) 131.5(2)

C(7)-C(2)-C(1) 107.53(18)

C(4)-C(3)-C(2) 117.8(2)

C(4)-C(3)-H(3A) 121.1

C(2)-C(3)-H(3A) 121.1

C(3)-C(4)-C(5) 121.5(2)

C(3)-C(4)-H(4A) 119.2

C(5)-C(4)-H(4A) 119.2

C(6)-C(5)-C(4) 121.8(2)

C(6)-C(5)-H(5A) 119.1

C(4)-C(5)-H(5A) 119.1

C(5)-C(6)-C(7) 117.7(2)

C(5)-C(6)-H(6A) 121.2

C(7)-C(6)-H(6A) 121.2

C(6)-C(7)-C(2) 120.6(2)

C(6)-C(7)-C(8) 131.9(2)

C(2)-C(7)-C(8) 107.29(19)

N(2)-C(8)-N(1) 122.50(19)

N(2)-C(8)-C(7) 130.0(2)

N(1)-C(8)-C(7) 105.55(19)

N(2)-C(9)-N(3) 122.5(2)

N(2)-C(9)-C(10) 130.5(2)

N(3)-C(9)-C(10) 105.51(18)

C(11)-C(10)-C(15) 120.6(2)

C(11)-C(10)-C(9) 132.0(2)

C(15)-C(10)-C(9) 107.33(19)

C(12)-C(11)-C(10) 117.9(2)

C(12)-C(11)-H(11A) 121.1

C(10)-C(11)-H(11A) 121.1

C(11)-C(12)-C(13) 121.4(2)

C(11)-C(12)-H(12A) 119.3

C(13)-C(12)-H(12A) 119.3

C(14)-C(13)-C(12) 121.7(2)

C(14)-C(13)-H(13A) 119.1

C(12)-C(13)-H(13A) 119.1

C(13)-C(14)-C(15) 117.7(2)

C(13)-C(14)-H(14A) 121.2

C(15)-C(14)-H(14A) 121.2

C(14)-C(15)-C(10) 120.8(2)

C(14)-C(15)-C(16) 132.0(2)

C(10)-C(15)-C(16) 107.13(19)

N(4)-C(16)-N(3) 122.2(2)

C-16

N(4)-C(16)-C(15) 130.6(2)

N(3)-C(16)-C(15) 105.59(19)

N(4)-C(17)-N(5) 122.0(2)

N(4)-C(17)-C(18) 131.6(2)

N(5)-C(17)-C(18) 105.16(18)

C(19)-C(18)-C(23) 120.6(2)

C(19)-C(18)-C(17) 132.3(2)

C(23)-C(18)-C(17) 107.08(19)

C(20)-C(19)-C(18) 118.2(2)

C(20)-C(19)-H(19A) 120.9

C(18)-C(19)-H(19A) 120.9

C(19)-C(20)-C(21) 121.3(2)

C(19)-C(20)-H(20A) 119.3

C(21)-C(20)-H(20A) 119.3

C(22)-C(21)-C(20) 121.7(2)

C(22)-C(21)-H(21A) 119.1

C(20)-C(21)-H(21A) 119.1

C(21)-C(22)-C(23) 117.6(2)

C(21)-C(22)-H(22A) 121.2

C(23)-C(22)-H(22A) 121.2

C(22)-C(23)-C(18) 120.5(2)

C(22)-C(23)-C(24) 131.9(2)

C(18)-C(23)-C(24) 107.55(19)

N(6)-C(24)-N(5) 122.0(2)

N(6)-C(24)-C(23) 131.5(2)

N(5)-C(24)-C(23) 105.27(18)

O(1)-C(25)-C(30) 122.4(2)

O(1)-C(25)-C(26) 120.52(19)

C(30)-C(25)-C(26) 117.0(2)

F(1)-C(26)-C(27) 118.7(2)

F(1)-C(26)-C(25) 119.66(19)

C(27)-C(26)-C(25) 121.7(2)

F(2)-C(27)-C(28) 119.9(2)

F(2)-C(27)-C(26) 120.1(2)

C(28)-C(27)-C(26) 120.1(2)

F(3)-C(28)-C(27) 120.5(2)

F(3)-C(28)-C(29) 120.0(2)

C(27)-C(28)-C(29) 119.6(2)

F(4)-C(29)-C(28) 120.4(2)

F(4)-C(29)-C(30) 120.0(2)

C(28)-C(29)-C(30) 119.5(2)

F(5)-C(30)-C(29) 117.8(2)

F(5)-C(30)-C(25) 119.96(19)

C(29)-C(30)-C(25) 122.2(2)

N(1)-B(1)-O(1) 110.79(19)

N(1)-B(1)-N(5) 105.31(18)

O(1)-B(1)-N(5) 114.55(19)

N(1)-B(1)-N(3) 106.30(19)

O(1)-B(1)-N(3) 115.11(19)

N(5)-B(1)-N(3) 103.93(19)

____________________________________

_________________________

Symmetry transformations used to generate

equivalent atoms:

C-17

Table S9. Anisotropic displacement parameters (Å2 × 103) for -F5BsubPc. The anisotropic

displacement factor exponent takes the form: -2p2[ h2 a*2U11 + ... + 2 h k a* b* U12 ]

______________________________________________________________________________

U11 U22 U33 U23 U13 U12

______________________________________________________________________________

F(1) 35(1) 34(1) 57(1) -8(1) 21(1) -7(1)

F(2) 46(1) 36(1) 92(1) -18(1) 36(1) 3(1)

F(3) 54(1) 23(1) 85(1) -18(1) 21(1) -4(1)

F(4) 34(1) 35(1) 54(1) -5(1) 8(1) -12(1)

F(5) 26(1) 38(1) 40(1) -10(1) 9(1) 0(1)

O(1) 34(1) 19(1) 24(1) -1(1) 6(1) 2(1)

N(1) 24(1) 17(1) 27(1) 0(1) 5(1) 1(1)

N(2) 29(1) 24(1) 27(1) 0(1) 10(1) -1(1)

N(3) 24(1) 19(1) 26(1) 0(1) 8(1) 1(1)

N(4) 24(1) 22(1) 27(1) 0(1) 5(1) 0(1)

N(5) 21(1) 20(1) 25(1) 0(1) 5(1) 0(1)

N(6) 29(1) 22(1) 27(1) -2(1) 6(1) 2(1)

C(1) 29(1) 22(1) 24(1) -1(1) 3(1) 6(1)

C(2) 35(2) 19(1) 26(1) 2(1) 7(1) 2(1)

C(3) 40(2) 25(1) 31(1) 1(1) 6(1) 6(1)

C(4) 51(2) 21(1) 41(2) 0(1) 7(1) 4(1)

C(5) 56(2) 20(1) 43(2) 0(1) 6(1) -5(1)

C(6) 40(2) 27(1) 36(2) 3(1) 6(1) -4(1)

C(7) 38(2) 20(1) 24(1) 2(1) 5(1) 0(1)

C(8) 27(1) 24(1) 23(1) 1(1) 6(1) -2(1)

C(9) 26(1) 24(1) 25(1) -3(1) 9(1) -1(1)

C(10) 25(1) 25(1) 26(1) -5(1) 9(1) 0(1)

C(11) 28(1) 34(1) 29(1) -3(1) 7(1) -2(1)

C(12) 26(1) 42(2) 34(1) -4(1) 6(1) 2(1)

C(13) 34(2) 35(1) 30(1) -1(1) 4(1) 10(1)

C(14) 31(2) 30(1) 28(1) 2(1) 8(1) 6(1)

C(15) 28(1) 24(1) 26(1) -2(1) 8(1) 3(1)

C-18

C(16) 29(1) 20(1) 29(1) -1(1) 8(1) 3(1)

C(17) 26(1) 21(1) 24(1) -3(1) 4(1) -3(1)

C(18) 24(1) 24(1) 23(1) -4(1) 4(1) -3(1)

C(19) 29(1) 30(1) 28(1) -1(1) 4(1) -7(1)

C(20) 34(2) 38(2) 33(1) 0(1) 10(1) -11(1)

C(21) 27(2) 50(2) 35(1) -6(1) 11(1) -6(1)

C(22) 24(1) 38(1) 31(1) -3(1) 4(1) 0(1)

C(23) 23(1) 30(1) 23(1) -5(1) 4(1) -2(1)

C(24) 22(1) 25(1) 23(1) -3(1) 3(1) 2(1)

C(25) 26(1) 19(1) 22(1) -2(1) 4(1) 0(1)

C(26) 26(1) 25(1) 32(1) -1(1) 8(1) -5(1)

C(27) 32(2) 28(1) 44(2) -6(1) 15(1) 5(1)

C(28) 39(2) 20(1) 43(2) -7(1) 8(1) -1(1)

C(29) 27(1) 26(1) 31(1) -1(1) 3(1) -6(1)

C(30) 23(1) 30(1) 24(1) -2(1) 3(1) 4(1)

B(1) 25(2) 22(1) 24(1) -2(1) 6(1) 0(1)

______________________________________________________________________________

C-19

Table S10. Hydrogen coordinates ( x 104) and isotropic displacement parameters (Å2 × 103)

for -F5BsubPc.

______________________________________________________________________________

__

x y z U(eq)

______________________________________________________________________________

__

H(3A) 4001 -3515 4332 38

H(4A) 3100 -5242 4168 45

H(5A) 1493 -5218 4249 48

H(6A) 707 -3463 4461 41

H(11A) -1216 283 3514 36

H(12A) -2080 1754 2655 40

H(13A) -1349 3480 2300 40

H(14A) 276 3786 2760 36

H(19A) 3960 3772 2755 35

H(20A) 5422 3472 2298 42

H(21A) 6272 1745 2645 44

H(22A) 5714 264 3494 37

______________________________________________________________________________

__

REFERENCES

(1) Bässler, H. Phys. Status Solidi B 1993, 175, 15.

B-1

Appendix B:

Supplementary Information for Chapter 4

Figure S1. Absorptance profile vs Wavelength for the pristine thin films and bilayers of Tc

and Cl-BsubPc calculated using a transfer matrix model.

The plot of the absorptance profile vs wavelength for the studied pristine thin films and the

bilayer of Tc (60 nm) and Cl-BsubPc (10 nm) have been calculated using a typical transfer

matrix model, accounting for the complex refractive index of each material.1 The n and k data

for the pristine Tc and Cl-BsubPc was sourced from literature.2-3 The pristine Tc absorptance

peaks at ~ 0.38 (525 nm) and is marginally reduced on the addition of 10 nm of Cl-BsubPc.

This indicates that the photoluminescence decrease we observe is not due to a reduction in

the initial light absorption due to the addition of another layer.

It has been long established that interfacial gap (Igap) is related to open circuit voltage

(Voc) and is often viewed as an upper bound (ie. Voc < Igap). While the difference between

the interfacial gaps and open circuit voltages for the cells we report (Igap - Voc) are on the

order of 0.5, these results are consistent with previously reported acene containing cells and

our own previous acene / subphthalocyanine work, as shown in Table S2.

B-2

Table S2. Comparison of interfacial gap and open circuit voltage for a number of acene and

subphthalocyanine containing cells.

Interface

(donor/acceptor)

Igap (eV) qVoc (eV) Igap ‐ qVoc Source

Tc/C60 1.2 0.7 0.5 Ref 4

Tc/C60 1.2 0.7 0.5 Ref 5

Tc/C60 1.2 0.7 0.5 This work

Tc/Cl‐BsubPc 2.0 1.2 0.8 Ref 5

Tc/Cl‐BsubPc 2.0 1.2 0.8 This work

Tc/Cl‐Cl6BsubPc 1.7 0.9 0.8 This work

Pent/C60 0.7 0.4 0.3 Ref 6

Pent/C60 0.7 0.4 0.3 Ref 7

Pent/C60 0.7 0.4 0.3 Ref 8

Pent/C60 0.7 0.4 0.3 This work

Pent/Cl‐BsubPc 1.5 0.9 0.6 This work

Pent/Cl‐Cl6BsubPc 1.2 0.5 0.7 This work

B-3

Table S2. Temperature dependent, field dependent electron mobility parameters for Cl-

BsubPc.

Temperature

(K)

Zero Field

Mobility

(cm2 V-1 s-1)

Standard

Error

(cm2 V-1 s-1)

P-F

Coefficient

((MV/cm)-

1/2)

Standard

Error

((MV/cm)-

1/2)

297 1.26E-07 1.06E-08 8.75 0.117

310 4.21E-07 4.11E-08 7.91 0.146

320 6.44E-06 6.65E-07 8.75 0.280

330 1.77E-04 1.48E-05 5.73 0.240

B-4

Table S3. Temperature dependent, field dependent electron mobility parameters for Cl-

Cl6BsubPc.

Temperature

(K)

Zero Field Mobility

(cm2 V-1 s-1)

Standard

Error (cm2 V-1

s-1)

P-F Coefficient

((MV/cm)-1/2)

Standard

Error

((MV/cm)-1/2)

260 4.00E-07 1.87E-07 6.96 0.528

280 2.90E-06 1.29E-06 5.31 0.518

298 7.64E-07 2.66E-07 7.70 0.428

320 4.35E-06 8.62E-07 5.81 0.271

330 2.49E-06 5.49E-07 7.04 0.301

340 6.20E-06 1.26E-06 6.12 0.292

Table S4. Pair-wise two tailed Z-test results for comparison of device parameters from Table

2 in the main article.

Z-test - Tc vs

Pent Jsc Voc FF np

C60 0 2.10241E-40 0.372308713 3E-09

Cl-BsubPc 2.41603E-27 1.58329E-37 0.01342862 3E-20

Cl-Cl6BsubPc 8.21759E-07 8.40898E-35 0.49419977 7E-16

B-5

Z-test - Cl-

BsubPc vs Cl-

Cl6BsubPc Jsc Voc FF np

Tc 0.00228724 2.11713E-14 0.427404738 0.021

Pent 1.45827E-42 4.8366E-199 6.97533E-14 1E-06

Z-test - C60 vs

Cl-BsubPc Jsc Voc FF np

Tc 0.172343644 9.36774E-71 0.541701221 1E-06

Pent 0.00 1.2948E-220 4.85706E-05 7E-38

Z-test - C60 vs

Cl-Cl6BsubPc Jsc Voc FF np

Tc 0.039292838 2.82393E-09 0.17187027 0.02

Pent 0 8.96171E-31 0.001692374 9E-63

B-6

Figure S2. Thermal ellipsoid plot of Cl-Cl6BsubPc (including atomic numbering ellipsoids

show n at the 50% probability level: hydrogen atoms omitted for clarity).

B-7

Table S5. Crystal data and structure refinement for Cl-Cl6BsubPc.

Identification code Cl-Cl6BsubPc

Empirical formula C24 H6 B Cl7 N6

Formula weight 637.31

Temperature 150(1) K

Wavelength 0.71073 Å

Crystal system Monoclinic

Space group P 21/n

Unit cell dimensions a = 13.1035(3) Å α= 90°.

b = 7.4123(2) Å β= 99.5430(14)°.

c = 25.5026(6) Å γ = 90°.

Volume 2442.71(10) Å3

Z 4

Density (calculated) 1.733 Mg/m3

Absorption coefficient 0.843 mm-1

F(000) 1264

Crystal size 0.36 x 0.20 x 0.06 mm3

Theta range for data collection 2.67 to 27.51°.

Index ranges -17<=h<=16, -8<=k<=9, -33<=l<=33

Reflections collected 16092

Independent reflections 5584 [R(int) = 0.0446]

Completeness to theta = 27.51° 99.3 %

Absorption correction Semi-empirical from equivalents

Max. and min. transmission 0.954 and 0.721

Refinement method Full-matrix least-squares on F2

Data / restraints / parameters 5584 / 0 / 343

Goodness-of-fit on F2 1.042

Final R indices [I>2sigma(I)] R1 = 0.0444, wR2 = 0.1065

R indices (all data) R1 = 0.0727, wR2 = 0.1226

Largest diff. peak and hole 0.346 and -0.416 e.Å-3

B-8

Table S6. Atomic coordinates ( x 104) and equivalent isotropic displacement parameters

(Å2x 103) for Cl-Cl6BsubPc. U(eq) is defined as one third of the trace of the orthogonalized

Uij tensor.

___________________________________________________________________________

x y z U(eq)

___________________________________________________________________________

Cl(1) 6127(1) -2522(1) 3389(1) 33(1)

Cl(2) 3569(1) 7807(1) 4092(1) 55(1)

Cl(3) 5626(1) 7814(1) 4921(1) 39(1)

Cl(4) 12060(1) 2784(1) 4099(1) 42(1)

Cl(5) 12096(1) 1658(1) 2915(1) 46(1)

Cl(6) 6008(1) 1850(1) 242(1) 45(1)

Cl(7) 3776(1) 2924(1) 437(1) 40(1)

N(1) 6030(2) 1210(3) 3479(1) 25(1)

N(2) 7622(2) 1841(3) 4052(1) 27(1)

N(3) 7597(2) 98(3) 3265(1) 24(1)

N(4) 7738(2) -283(3) 2353(1) 25(1)

N(5) 6088(2) 136(3) 2616(1) 25(1)

N(6) 4661(2) 1916(3) 2773(1) 26(1)

C(1) 5130(2) 2094(3) 3283(1) 26(1)

C(2) 5005(2) 3455(4) 3677(1) 27(1)

C(3) 4261(2) 4789(4) 3694(1) 32(1)

C(4) 4451(2) 6081(4) 4087(1) 32(1)

C(5) 5369(2) 6072(4) 4465(1) 32(1)

C(6) 6104(2) 4748(3) 4459(1) 28(1)

C(7) 5920(2) 3438(3) 4068(1) 27(1)

C(8) 6591(2) 2044(3) 3911(1) 26(1)

C(9) 8112(2) 966(3) 3705(1) 25(1)

C(10) 9182(2) 1117(3) 3618(1) 25(1)

C(11) 10055(2) 1844(3) 3928(1) 28(1)

C(12) 10951(2) 1961(4) 3711(1) 31(1)

C(13) 10972(2) 1439(4) 3182(1) 30(1)

C(14) 10104(2) 728(3) 2868(1) 28(1)

B-9

C(15) 9209(2) 528(3) 3090(1) 25(1)

C(16) 8167(2) -49(3) 2864(1) 24(1)

C(17) 6708(2) -47(3) 2238(1) 25(1)

C(18) 6081(2) 525(3) 1745(1) 25(1)

C(19) 6351(2) 740(4) 1245(1) 28(1)

C(20) 5639(2) 1498(4) 855(1) 29(1)

C(21) 4654(2) 2032(3) 954(1) 29(1)

C(22) 4391(2) 1889(3) 1454(1) 28(1)

C(23) 5121(2) 1151(3) 1862(1) 26(1)

C(24) 5178(2) 1017(3) 2439(1) 27(1)

B(1) 6466(2) -245(4) 3187(1) 26(1)

___________________________________________________________________________

B-10

Table S7. Bond lengths [Å] and angles [°]

for Cl-Cl6BsubPc.

__________________________________

Cl(1)-B(1) 1.840(3)

Cl(2)-C(4) 1.725(3)

Cl(3)-C(5) 1.732(3)

Cl(4)-C(12) 1.728(3)

Cl(5)-C(13) 1.731(3)

Cl(6)-C(20) 1.730(3)

Cl(7)-C(21) 1.731(2)

N(1)-C(8) 1.368(3)

N(1)-C(1) 1.369(3)

N(1)-B(1) 1.478(4)

N(2)-C(9) 1.343(3)

N(2)-C(8) 1.347(3)

N(3)-C(16) 1.369(3)

N(3)-C(9) 1.369(3)

N(3)-B(1) 1.485(3)

N(4)-C(16) 1.341(3)

N(4)-C(17) 1.343(3)

N(5)-C(17) 1.367(3)

N(5)-C(24) 1.368(3)

N(5)-B(1) 1.486(3)

N(6)-C(24) 1.350(3)

N(6)-C(1) 1.350(3)

C(1)-C(2) 1.452(4)

C(2)-C(3) 1.394(4)

C(2)-C(7) 1.426(4)

C(3)-C(4) 1.380(4)

C(4)-C(5) 1.411(4)

C(5)-C(6) 1.377(4)

C(6)-C(7) 1.384(4)

C(7)-C(8) 1.455(4)

C(9)-C(10) 1.460(4)

C(10)-C(11) 1.387(4)

C(10)-C(15) 1.420(3)

C(11)-C(12) 1.382(4)

C(12)-C(13) 1.407(4)

C(13)-C(14) 1.382(4)

C(14)-C(15) 1.393(4)

C(15)-C(16) 1.455(3)

C(17)-C(18) 1.446(3)

C(18)-C(19) 1.388(3)

C(18)-C(23) 1.418(3)

C(19)-C(20) 1.367(4)

C(20)-C(21) 1.412(4)

C(21)-C(22) 1.381(4)

C(22)-C(23) 1.401(3)

C(23)-C(24) 1.464(3)

C(8)-N(1)-C(1) 113.2(2)

C(8)-N(1)-B(1) 122.3(2)

C(1)-N(1)-B(1) 123.1(2)

C(9)-N(2)-C(8) 116.5(2)

C(16)-N(3)-C(9) 113.2(2)

C(16)-N(3)-B(1) 122.9(2)

C(9)-N(3)-B(1) 122.4(2)

C(16)-N(4)-C(17) 116.1(2)

C(17)-N(5)-C(24) 113.7(2)

C(17)-N(5)-B(1) 122.1(2)

C(24)-N(5)-B(1) 123.0(2)

C(24)-N(6)-C(1) 117.5(2)

N(6)-C(1)-N(1) 122.1(2)

N(6)-C(1)-C(2) 130.5(2)

N(1)-C(1)-C(2) 105.6(2)

C(3)-C(2)-C(7) 119.7(2)

C(3)-C(2)-C(1) 132.3(2)

C(7)-C(2)-C(1) 107.5(2)

C(4)-C(3)-C(2) 118.2(2)

B-11

C(3)-C(4)-C(5) 121.6(2)

C(3)-C(4)-Cl(2) 118.7(2)

C(5)-C(4)-Cl(2) 119.7(2)

C(6)-C(5)-C(4) 120.9(2)

C(6)-C(5)-Cl(3) 118.7(2)

C(4)-C(5)-Cl(3) 120.2(2)

C(5)-C(6)-C(7) 118.0(2)

C(6)-C(7)-C(2) 121.5(2)

C(6)-C(7)-C(8) 130.9(2)

C(2)-C(7)-C(8) 106.9(2)

N(2)-C(8)-N(1) 123.0(2)

N(2)-C(8)-C(7) 129.2(2)

N(1)-C(8)-C(7) 105.8(2)

N(2)-C(9)-N(3) 122.8(2)

N(2)-C(9)-C(10) 129.7(2)

N(3)-C(9)-C(10) 105.5(2)

C(11)-C(10)-C(15) 120.8(2)

C(11)-C(10)-C(9) 131.7(2)

C(15)-C(10)-C(9) 107.2(2)

C(12)-C(11)-C(10) 118.1(2)

C(11)-C(12)-C(13) 121.3(2)

C(11)-C(12)-Cl(4) 118.9(2)

C(13)-C(12)-Cl(4) 119.9(2)

C(14)-C(13)-C(12) 121.0(2)

C(14)-C(13)-Cl(5) 118.7(2)

C(12)-C(13)-Cl(5) 120.3(2)

C(13)-C(14)-C(15) 118.2(2)

C(14)-C(15)-C(10) 120.5(2)

C(14)-C(15)-C(16) 131.9(2)

C(10)-C(15)-C(16) 107.3(2)

N(4)-C(16)-N(3) 122.9(2)

N(4)-C(16)-C(15) 129.7(2)

N(3)-C(16)-C(15) 105.8(2)

N(4)-C(17)-N(5) 123.5(2)

N(4)-C(17)-C(18) 129.3(2)

N(5)-C(17)-C(18) 105.3(2)

C(19)-C(18)-C(23) 121.7(2)

C(19)-C(18)-C(17) 129.6(2)

C(23)-C(18)-C(17) 108.1(2)

C(20)-C(19)-C(18) 118.0(2)

C(19)-C(20)-C(21) 121.3(2)

C(19)-C(20)-Cl(6) 117.4(2)

C(21)-C(20)-Cl(6) 121.27(19)

C(22)-C(21)-C(20) 121.3(2)

C(22)-C(21)-Cl(7) 119.8(2)

C(20)-C(21)-Cl(7) 118.9(2)

C(21)-C(22)-C(23) 118.1(2)

C(22)-C(23)-C(18) 119.5(2)

C(22)-C(23)-C(24) 133.4(2)

C(18)-C(23)-C(24) 106.8(2)

N(6)-C(24)-N(5) 121.9(2)

N(6)-C(24)-C(23) 130.8(2)

N(5)-C(24)-C(23) 105.1(2)

N(1)-B(1)-N(3) 105.5(2)

N(1)-B(1)-N(5) 105.2(2)

N(3)-B(1)-N(5) 105.2(2)

N(1)-B(1)-Cl(1) 113.41(19)

N(3)-B(1)-Cl(1) 113.67(19)

N(5)-B(1)-Cl(1) 113.07(18)

__________________________________

___________________________

Symmetry transformations used to

generate equivalent atoms:

B-12

Table S8. Anisotropic displacement parameters (Å2x 103) for Cl-Cl6BsubPc. The anisotropic

displacement factor exponent takes the form: -22[ h2 a*2U11 + ... + 2 h k a* b* U12 ]

___________________________________________________________________________

U11 U22 U33 U23 U13 U12

___________________________________________________________________________

Cl(1) 37(1) 29(1) 33(1) 5(1) 9(1) -3(1)

Cl(2) 34(1) 58(1) 72(1) -25(1) 4(1) 14(1)

Cl(3) 40(1) 39(1) 39(1) -12(1) 11(1) -2(1)

Cl(4) 25(1) 65(1) 35(1) -9(1) 0(1) -7(1)

Cl(5) 28(1) 70(1) 42(1) -12(1) 12(1) -10(1)

Cl(6) 39(1) 71(1) 25(1) 9(1) 6(1) 5(1)

Cl(7) 32(1) 53(1) 30(1) 4(1) -4(1) 4(1)

N(1) 24(1) 28(1) 24(1) 1(1) 6(1) -1(1)

N(2) 26(1) 32(1) 22(1) 1(1) 4(1) 2(1)

N(3) 24(1) 26(1) 23(1) 3(1) 4(1) 2(1)

N(4) 23(1) 27(1) 24(1) 0(1) 3(1) 0(1)

N(5) 24(1) 26(1) 25(1) -1(1) 4(1) -2(1)

N(6) 20(1) 30(1) 27(1) 0(1) 4(1) -4(1)

C(1) 22(1) 28(1) 28(1) 2(1) 8(1) -4(1)

C(2) 25(1) 32(1) 28(1) 2(1) 11(1) -5(1)

C(3) 22(1) 37(2) 36(1) 0(1) 8(1) -4(1)

C(4) 26(1) 36(2) 39(2) -2(1) 14(1) 1(1)

C(5) 34(2) 33(2) 32(1) -3(1) 15(1) -6(1)

C(6) 30(1) 31(1) 26(1) 0(1) 10(1) -2(1)

C(7) 25(1) 29(1) 27(1) 1(1) 9(1) -2(1)

C(8) 27(1) 30(1) 20(1) 2(1) 6(1) -3(1)

C(9) 27(1) 25(1) 22(1) 4(1) 1(1) 2(1)

C(10) 25(1) 27(1) 24(1) 3(1) 2(1) 6(1)

C(11) 29(1) 32(1) 23(1) -1(1) 2(1) 1(1)

C(12) 27(1) 34(2) 31(1) -1(1) 0(1) -1(1)

C(13) 26(1) 32(1) 33(1) -1(1) 8(1) 0(1)

B-13

C(14) 27(1) 30(1) 27(1) 2(1) 6(1) 7(1)

C(15) 24(1) 24(1) 25(1) 2(1) 1(1) 3(1)

C(16) 25(1) 22(1) 26(1) 1(1) 7(1) 3(1)

C(17) 27(1) 23(1) 25(1) -2(1) 5(1) -3(1)

C(18) 26(1) 25(1) 23(1) -1(1) 2(1) -3(1)

C(19) 26(1) 32(1) 26(1) -4(1) 3(1) -3(1)

C(20) 31(1) 34(1) 22(1) -2(1) 2(1) -3(1)

C(21) 26(1) 31(1) 28(1) 1(1) -4(1) -3(1)

C(22) 25(1) 28(1) 30(1) -4(1) 1(1) -2(1)

C(23) 22(1) 26(1) 28(1) -4(1) 3(1) -2(1)

C(24) 21(1) 28(1) 31(1) 0(1) 5(1) -3(1)

B(1) 27(2) 28(2) 25(1) 2(1) 4(1) -4(1)

___________________________________________________________________________

Table S9. Hydrogen coordinates ( x 104) and isotropic displacement parameters (Å2x 10 3)

for Cl-Cl6BsubPc.

___________________________________________________________________________

x y z U(eq)

___________________________________________________________________________

H(3A) 3641 4809 3441 38

H(6A) 6719 4734 4715 34

H(11A) 10036 2250 4279 34

H(14A) 10118 386 2510 34

H(19A) 7011 372 1176 34

H(22A) 3733 2279 1521 34

________________________________________________________________________________

B-14

References

1. Pettersson, L. A. A.; Roman, L. S.; Inganas, O., Modeling Photocurrent Action Spectra of

Photovoltaic Devices Based on Organic Thin Ffilms. J. Appl. Phys. 1999, 86, 487-496.

2. Tavazzi, S.; Raimondo, L.; Silvestri, L.; Spearman, P.; Camposeo, A.; Polo, M.;

Pisignano, D., Dielectric Tensor of Tetracene Single Crystals: The Effect of Anisotropy on

Polarized Absorption and Emission Spectra. J. Chem. Phys. 2008, 128, 154709.

3. Gommans, H. H. P.; Cheyns, D.; Aernouts, T.; Porrtmans, J.; Heremans, P., Electro-

Optical Study of Subphthalocyanine in a Bilayer Organic Solar Cell. Adv. Funct. Mater. 2007,

17, 2653-2658.

4. Chu, C. W.; Shao, Y.; Shrotriya, V.; Yang, Y., Efficient Photovoltaic Energy Conversion

in Tetracene-C-60 Based Heterojunctions. Appl. Phys. Lett. 2005, 86, 243506.

5. Beaumont, N.; Cho, S. W.; Sullivan, P.; Newby, D.; Smith, K. E.; Jones, T. S., Boron

Subphthalocyanine Chloride as an Electron Acceptor for High-Voltage Fullerene-Free Organic

Photovoltaics. Adv. Funct. Mater. 2012, 22, 561-566.

6. Yoo, S.; Domercq, B.; Kippelen, B., Efficient Thin-Film Organic Solar Cells Based on

Pentacene/C-60 Heterojunctions. Appl. Phys. Lett. 2004, 85, 5427-5429.

7. Yoo, S., et al., Analysis of Improved Photovoltaic Properties of Pentacene/C-60 Organic

Solar Cells: Effects of Exciton Blocking Layer Thickness and Thermal Annealing. Solid-State

Electron. 2007, 51, 1367-1375.

8. Sullivan, P.; Jones, T. S., Pentacene/Fullerene (C-60) Heterojunction Solar Cells: Device

Performance and Degradation Mechanisms. Org. Electron. 2008, 9, 656-660.

C-1

Appendix C: Supplementary Information for Chapter 6

Illustrative Calculation of Achieving Minimal Material Use Through Maximizing Thin Film to Mass Consumption Yield

A quick screen of a candidate organic semiconductor compound might involve 6 depositions:

1. Tooling Factor determination (~100 nm)

2. Incorporation into an Organic Light Emitting Diode (OLED) as a hole transport layer

3. Incorporation into an Organic Light Emitting Diode (OLED) as a electron transport layer

4. Incorporation into an Organic Photovoltaic Device (OPV) as an electron donor layer

5. Incorporation into an Organic Photovoltaic Device (OPV) as an electron acceptor layer

6. Incorporation into an Organic Field Effect Transistor (OFET) as the active semiconductor layer (~50 nm).

If we further assume that each deposition will have 5 nm of wasted material deposited during the heat up and cool down stages of the deposition, this leads us to believe we will require at least 380 nm of film for an initial screening of a candidate material. For our system configuration, based on the use of an ME2B (multiaperture) source, resulting in a typical yield of 3 nm/mg, we would thus require at least ~130 mg of candidate material. Conversely, the ME2A (single aperture) source, with a typical yield of 1 nm/mg would require three times as much material (~400 mg) and a SB6 source (0.5 nm/mg) would require twice as much again (~800 mg). Given most new materials are first synthesized at ~100 mg reaction scales, being able to use just over 100 mg of material to do a broad survey of material performance in a variety of electronic device configurations is a huge saving in time, effort, and materials.

C-2

Table S1. Vacuum deposited thin film optical characteristics. See main paper for compound structures.

Compound Peak Absorbance

Wavelength (nm)

FWHM of Peak

Absorbance (nm)

Absorbance

Onset (nm)

Optical Gap (eV)

‐oxo‐(BsubPc)2 578 96 636 1.9

F5‐BsubNc 677 60 711 1.7

F5‐GBsubPc 684 58 716 1.7

400 500 600 700 8000.0

0.2

0.4

0.6

0.8

1.0

1.2 -oxo-(BsubPc)2

F5-BsubNc F5-GBsubPc

Nor

mal

ized

Sol

id F

ilm

Abo

srba

nce

(arb

. uni

ts)

Wavelength (nm)

578 nm

677 nm 684 nm

Figure S1. Normalized absorbance for ~10 nm thick vacuum sublimed films of several compounds in this study where sublimed film absorbance has not been previously reported. Absorbance peak wavelengths are labeled. See main paper for compound structures.

C-3

Alq3 Cl-BsubPcF5-BsubPc

mu-oxo-BsubPc

0

20

40

60

80

100

Too

ling

(%)

Material

a)

Alq3 Cl-BsubPcF5-BsubPc

0

20

40

60

80

100

Too

ling

(%)

Material

b)

Alq3 Cl-BsubPcF5-BsubPc

mu-oxo-BsubPc

0

20

40

60

80

100

Too

ling

(%)

Material

c)

Alq3 Cl-BsubPcF5-BsubPc

mu-oxo-BsubPc

0

20

40

60

80

100

Too

ling

(%)

Material

d)

Figure S2. Calculated tooling factor as a function of material deposited from various boat

configurations including ME2A inner edge (a), ME2A outer edge (b), ME2B inner edge (c), and

ME2B outer edge (d). Calculation uses the single crystal density of the material. Error bars

show measurement precision.

D-1

Appendix D: Supplementary Information for Chapter 6

This appendix contains technical drawings developed in the course of the construction of the

MARI-KATE system. The custom baseplate diagrams prepared by our supplier, Kurt J. Lesker

Company, were developed as a 3D shop drawing based upon my 2D sketches.

D-2

US

B

D-3

Bottom Perspective

Non-base plate connections on DWG 1

Kontraption for the Assembly of phThalocyanine Electronics (KATE)

Process and Instrumentation Diagram – As Built

Prepared by Jeffrey S. Castrucci

February 2, 2015

SIZE FSCM NO DWG NO REV

- 2 of 2 1

SCALE Schematic LOCATION WB332 SHEET 1 OF 2

T- Thermocouple Indicator

LEGEND

Electrical, AC power – insulated copper wire

Water – ¼ inch polypropylene tube

Electrical, signal – K Type Thermocouple Chromel/Alumel insulated wire pair

VG-2

CP-1

ABBREVIATIONS

VG- Gate ValveCP- Cryogenic Pump

A – Supply from and return to QCM Cooling Lines from

DWG 1

Water Chiller and Circulation

To Control Rack

High Current Electrical Feedthrough (two cooling water connections, one electrical power connection)

Low Current Electrical Feedthrough (one thermocouple connection, one electrical power connection)

T-1

To Control Rack

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To 120 V, 1 phase outlet

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E-1

Appendix E:

Supplementary Information for Chapter 8

Figure S1. Absorption spectrum (pink) of μ-oxo-(BsubPc)2 overlaid with the solar irradiance

(gray = orbit, black = sea level) spectrum.

0

0.2

0.4

0.6

0.8

1

0

500

1000

1500

2000

2500

200 400 600 800 1000 1200 1400 1600 1800 2000

No

rmal

ized

Ab

sorp

tio

n In

ten

sity

(a.

u.)

Sp

ectr

al Ir

rad

ian

ce (

W m

-2u

m-1

)

Wavelength (nm)

E-2

Table S1. Compound Energy Levels used to construct Figure 2(c) and 2(d). Work function, and

HOMO and LUMO energy levels are reported relative to the vacuum level.

Compound Work

Function

(eV)

EHOMO

(eV)

Optical

Gap (eV)

ELUMO

(eV)

EHOMO -

Work

Function

(eV)

Work

Function

- ELUMO

(eV)

Source

-oxo-

(BsubPc)2

4.5 5.9 1.9 3.7 1.4 0.8 This

work

Cl-BsubPc 5.0 5.6 2.1 3.2 0.6 1.8 1

6T 3.9 4.9 2.2 2.4 1.0 1.5 2

C60 4.6 6.3 1.9 4.1 1.7 0.5 1

C70 4.9 6.4 1.7 4.4 1.5 0.5 3

F16CuPc 5.0 6.3 1.5 4.5 1.3 0.5 4

E-3

Table S2. Mean device parameter comparison for optimization of -oxo as an electron donor.

The standard deviation (SD) of each value is shown in parentheses. Device structure is

ITO/PEDOT:PSS/ Donor/Acceptor/TPBi(3 nm)/Ag(80 nm).

Donor/dD

nm

Acceptor/dA

nm

JSC

(SD)/mA

cm-2

VOC (SD)/V FF (SD) P (SD)/% No. of cells

tested

-oxo/10 C60/30 -3.6 (0.88) 0.80 (0.48) 0.36 (0.11) 1.0 (0.74) 12

-oxo/20 C60/20 -2.3 (0.75) 0.70 (0.41) 0.27 (0.08) 0.43 (0.24) 12

-oxo/30 C60/10 -1.2 (0.58) 0.57 (0.62) 0.25 (0.09) 0.17 (0.15) 12

E-4

Figure S2. JV curves for optimization of -oxo as an electron donor. Error bars show the 95%

confidence interval.

0.0 0.5 1.0-5

-4

-3

-2

-1

0

1

2

Thickness(-oxo:C

60, nm)

10:30 20:20 30:10

C

urre

nt D

ensi

ty (

mA

cm

-2)

Applied Potential (V)

Table S3. Mean device parameter comparison for optimization of -oxo as an electron donor

with MoOx hole extraction layer. The standard deviation (SD) of each value is shown in

parentheses. Device structure is ITO/PEDOT:PSS/ Donor/Acceptor/TPBi(3 nm)/Ag(80 nm).

Donor/dD

nm

Acceptor/dA

nm

JSC

(SD)/mA

cm-2

VOC (SD)/V FF (SD) P (SD)/% No. of cells

tested

-oxo/10 C60/30 -4.0 (0.40) 0.84 (0.06) 0.56 (0.16) 1.9 (0.71) 7

-oxo/15 C60/45 -3.9 (0.34) 0.90 (0.01) 0.59 (0.03) 2.0 (0.19) 7

-oxo/20 C60/60 -3.1 (0.38) 0.88 (0.20) 0.48 (0.11) 1.3 (0.59) 9

E-5

Figure S3. JV curves for optimization of -oxo as an electron donor with MoOx hole extraction

layer. Error bars show the 95% confidence interval.

0.0 0.5 1.0-5

-4

-3

-2

-1

0

1

2

Thickness(-oxo:C

60, nm)

10:30 15:45 20:60

Cur

rent

Den

sity

(m

A c

m-2)

Applied Potential (V)

Table S4. Mean device parameter comparison for F16CuPc reference device and -oxo device

from main text. The standard deviation (SD) of each value is shown in parentheses. Device

structure is ITO/PEDOT:PSS/MoOx(5 nm)/Donor/Acceptor/TPBi(3 nm)/Ag(80 nm).

Donor/dD

nm

Acceptor/dA

nm

JSC

(SD)/mA

cm-2

VOC (SD)/V FF (SD) P (SD)/% No. of cells

tested

Cl-

BsubPc/15

F16CuPc/30 -2.5 (0.18) 0.41 (0.005) 0.57 (0.02) 0.58 (0.05) 18

-oxo/15 F16CuPc/30 -1.7 (0.23) 0.06 (0.004) 0.30 (0.02) 0.03 (0.07) 6

E-6

Figure S4. JV curves for F16CuPc reference device and -oxo device. Error bars show the 95%

confidence interval.

0.0 0.2 0.4-4

-2

0

2

Donor/Acceptor Cl-BsubPc/F

16CuPc

-oxo/F16

CuPc

Cur

rent

Den

sity

(m

A c

m-2)

Voltage (V)

References 1. Morse, G. E.; Gantz, J. L.; Steirer, K. X.; Armstrong, N. R.; Bender, T. P.,

Pentafluorophenoxy Boron Subphthalocyanine (F5bsubpc) as a Multifunctional Material for

Organic Photovoltaics. ACS Appl. Mater. Interfaces 2014, 6, 1515-1524.

2. Ge, Y.; Whitten, J. E., Energy Level Alignment between Sexithiophene and

Buckminsterfullerene Films. Chem. Phys. Lett. 2007, 448, 65-69.

3. Benning, P. J.; Poirier, D. M.; Ohno, T. R.; Chen, Y.; Jost, M. B.; Stepniak, F.; Kroll, G.

H.; Weaver, J. H.; Fure, J.; Smalley, R. E., C60 and C70 Fullerenes and Potassium Fullerides.

Phys. Rev. B 1992, 45, 6899-6913.

4. Shen, C.; Kahn, A., Electronic Structure, Diffusion, and P-Doping at the Au/F16cupc

Interface J. Appl. Phys. 2001, 90, 4549-4554.

F-1

Appendix F:

Supplementary Information for Chapter 9

1 Experimental Methods Molybdenum(VI) oxide (Sigma-Aldrich, 99.98% trace metals basis), Poly(3,4-

ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) (Heraeus, ClevoisTM P VP AI

4083), Pentacene (Lumtec), bathocuproine (BCP) (Sigma-Aldrich, Sublimed Grade) , Silver

(Ag) (R.D. Mathis, 99.999%), and silver paint (PELCO, Conductive Silver 187) were

purchased and used as received. α6T (Sigma-Aldrich) and was purchased and purified once by

train sublimation before use. Cl-Cl12BsubPc was synthesized as previously reported,1 and

purified once by train sublimation before use.

For the UPS measurements, Cl-Cl12BsubPc films were evaporated from an alumina crucible

using a transfer arm evaporator (TAE) described by Greiner et al.2 The films were deposited on a

highly oriented pyrolitic graphite (HOPG) substrate to a thickness of approximately 12 nm, using

typical small organic molecular film density, as measured by a calibrated quartz crystal

microbalance (QCM). This thickness was chosen so as to avoid charging effects, as well as any

substrate-induced interaction. The base pressure in the chamber was approximately 1x10-8 Torr.

The films were then in-situ transferred to a PHI5500 Multi-Technique System to perform the

photoemission using a non-monochromated He I (h= 21.22eV) source. All measurements

were done at a take-off angle of 88 degrees and the sample was held at a bias of -15 V with

respect to the spectrometer. The pressure in the analysis chamber was approximately 1x10-9

Torr.

The Work Function was calculated according to: = 21.22 - SEC, where SEC is the secondary

electron cut-off. The HOMO-Fermi energy difference was taken as the intersection of the

HOMO edge with the background noise, with the Fermi energy being calibrated to 0 eV binding

energy. The ionization energy (IE) is then simply the sum of the work function and HOMO-

Fermi energy difference.

OPV devices were fabricated on 25 mm by 25 mm glass substrates coated with indium-tin oxide

(ITO) having a sheet resistance of 15 Ω per square (Thin Film Devices, Inc.). The ITO was pre-

patterned, leaving 8 mm from one side as uncoated glass. Substrates were cleaned by successive

F-2

sonications in detergent and solvents, followed by 5 minutes of atmospheric plasma treatment.

PEDOT:PSS was spin-coated onto the substrates, 500 rpm, 10 s; 4000 rpm, 30 s. Substrates were

baked on a hot plate at 110 °C for 10 minutes, and then transferred into a nitrogen atmosphere

glove box (O2 < 10 ppm, H2O < 10 ppm). Substrates were transferred to a custom-built thermal

evaporation system attached to the nitrogen glove box without exposure to ambient conditions.

All subsequent device layers were thermally evaporated at ~1.0 A/s and a working pressure of ~1

x 10-7 Torr for organic layers and ~1 x 10-6 Torr for Ag. Ag electrodes were evaporated through

a shadow mask, defining 0.2 cm2 as the active area for each device. A transfer back to the glove

box was required between the BCP and Ag layers to change the shadow masks.

Layer thickness and deposition rates of evaporated films were monitored using a quartz crystal

microbalance calibrated against films deposited on glass where film thickness was measured

with a KLA-Tencor P16+ surface profilometer. Solid film absorbances were measured using a

Perkin-Elmer Lambda 1050 spectrometer with the films deposited at device relevant thicknesses

on glass slides. To enhance the electrical contact during testing, silver paint was applied to the

ITO and metal electrode contact points and left to dry for 20 minutes. Devices were kept in the

nitrogen-filled glove box throughout testing. Voltage sweeps of the devices were performed

under full illumination by a 300W Xe arc lamp (Oriel) with an AM 1.5G filter, and the

corresponding currents were measured with a Keithley 2401 Low Voltage SourceMeter. Light

intensity was calibrated to 100 mW/cm2 with reference to a calibrated silicon photodetector.

Wavelengths scans at 10 nm intervals were performed using an in-line CornerstoneTM 260 1/4 m

Monochromator and the corresponding currents were measured using a Newport Optical Power

Meter 2936-R and converted to external quantum efficiencies using a reference wavelength scan

of a calibrated silicon photodetector.

F-3

1.1 UPS RESULTS

(a)

(b)

Figure S1. The identification of work function by way of secondary electron cut-off (a) and the

HOMO to Fermi gap (b) for a ~ 12 nm film of Cl-Cl12BsubPc vacuum deposited on highly

oriented pyrolitic graphite (HOPG) and measured by ultraviolet photoelectron spectroscopy

(UPS).

References 1. Morse, G. E.; Gong, I.; Kawar, Y.; Lough, A. J.; Bender, T. P., Crystal and Solid-State

Arrangement Trends of Halogenated Boron Subphthalocyanines. Cryst. Growth Des. 2014, 14,

2138-2147.

2. Greiner, M. T.; Helander, M. G.; Wang, Z. B.; Lu, Z. H., Transfer-Arm Evaporator Cell

for Rapid Loading and Deposition of Organic Thin Films. Rev. Sci. Instrum. 2009, 80, 125101-1

- 125101-4.