organic semiconductor-based plastic solar cells

Organic Materials for electronics & Photonics

Organic Semiconductor-based Plastic Solar Cells


Light energy (photons) Electrical energy

When sunlight is absorbed by some materials, the solar energy knocks electrons loose from their atoms, allowing the electrons to flow through the material to produce electricity. This process of converting light (photons) to electricity (voltage) is called the photovoltaic (PV) effect.

What’s PVs


At earth’s surface average solar energy is ~ 4 x 1024 J / yearGlobal energy consumption (2001) was ~ 4 x 1020 J / year (increasing ~ 2% annually)

In US, average power requirement is 3.3 TW.With 10% efficient cells we would need 1.7% of land area devoted to PV (~ area occupied by interstate highways)

Source: DOE (U.S. Department of Energy) Source: N. Lewis (Caltech)

Solar energy


History

• 1839 : Finding of Photovoltaic effect with liquid (Edmond becquerel)• 1876 : Photovoltaic effect in a solid (Heinrich Hertz)• 1883 : Se solar cell (C. Fritts)• 1930 : Research of Cu2O/Cu solar cell • 1941 : Patent of Si solar cell (R. Ohl) • 1954 : Crystalline Si solar cell (Bell Lab.) ; 4 % efficiency • 1958 : Using as assistant power in the spaceship (Vanguard I ) ; 5 mW• 1973 : oil crisis• 1980 : solar cell using CdTe, CuInSe2 ,TiO2 etc.• 1997 : world product 100MWp • 2000 : research of an advanced materials and structures (dye sensitized solar cell, organic solar cell) cheap process , flexible substrate


p-type Semiconductor

Space charge region

n-type Semiconductor

Metal top electrode

Metal back contact

h (photon)

Photovoltaic (PV) effect

In a conventional semiconductor, light absorption generates an electric field that separates the photo-induced charges. Ec and Ev are the energies at the conduction and valence bands, respectively.


100S

mpmp

P

VI

ocsc

mpmp

VI

VIFF

100S

ocsc

P

FFVI

ISC : Short-circuit current

Current value when V = 0

VOC : Open-circuit voltage

Voltage value when I = 0

P : Power output of the cell

P = IV

F.F : Fill factor

Under AM 1.5G simulated solar illumination

)] ( )][( [

)] ( ][ 0124[

incidnet photons of no.

circuit external he through telectrons of no.

2

2

cmmWirradiancenmwavelength

cmAdensityntphotocurrenmeV

IPCE

< Power conversion efficiency (η) >

< Incident-photon-to-current conversion efficiency (IPCE) >

Cell efficiency


Air mass (AM)

θz


Sunlight spectrum

Global condition (G) : including the diffusion component (indirect component owing to scattering and reflection in the atmosphere and surrounding landscape)

Direct condition (D) : without the diffusion component


Classification

태양전지

실리콘 반도체 화합물 반도체 기타

결정계

비결정

계

다결

정

II VI• 족 CdS CdTe…

III V• 족 GaAs InP…

I•III•VI 족CuInSe2..

유기태양전

지 염료감응태양전

지

단결

정

Conventional inorganic p/n junction solar cell


Progress of cell efficiencies

Under AM 1.5G simulated solar illumination


PV systems installed in Korea


Advantages of Organic PVs (OPVs) -Processed easily over large area using -spin-coating -doctor blade techniques (wet-processing) -evaporation through a mask (dry processing) -printing -Low cost -Low weight -Mechanical flexibility and transparency -Band gap of organic materials can be easily tuned chemically by incorporation of different functional group

Why OPVs?


Why OPVs?

20

40

60

80

100

Eff

icie

ncy

(%)

Cost (US$/m2)

100 500400300200

US$ 3.50/W

US$ 1.00/W

US$ 0.50/WUS$ 0.20/WUS$ 0.10/W

12

3-1

3-2

3

Thermodynamic limit

Cost-efficiency analysis for first-, second-, and third-generation PV technologies ( labeled 1, 2, and 3, respectively). Region 3-1 depicts very-high-efficiency devices that require novel mechanisms of device operation. Region 3-2 (the region in which organicPV devices lie) depicts devices with moderate efficiencies and very low costs.

The concept of third-generation PV technologies, originally developed by Martin Green of the University of New South Wales

1: PV cell based on silicon wafers2: thin-film technology 3: high-efficiency thin-film technology using concepts such as hot carriers, multiple electron–hole pair creation, and thermophotonics


Source: Siemens AG

Why OPVs?


Why OPVs? (Application)


Requirement of OPVs

Source: Siemens AG


History of OPVs


e-

h+

D

A

ITO

metalhv

Light is absorbed in the polymer layer

Absorption creates a bound electron-hole pair (exciton)

Exciton is split into separate charges which are collected at contacts

Exciton must be seperated so that a photocurrent can be collected.

Excitons dissociated by electron transfer to an acceptor material, or hole transfer to a donor.

Simplest approach is to make a donor-acceptor heterojunction

PV effect in conjugated polymer


Excitons dissociate at interfaces between materials having different ionization energies and electron affinities

Excitons are produced in a conducting polymer. An incident photon produces bound electron–hole pairs called excitons, which transport charges in photovoltaic polymers.

Exciton : generation & separation


Photo-induced charge transfer

donor

acceptor

anodecathode

AED

CT

CC

Photon absorption (A)

Exciton generation by absorption of light

Exciton diffusion (ED)

Exciton diffusion over ~LD (~20 nm)

Charge-transfer reaction (CT)

Exciton dissociation by rapid and efficient charge transfer

Collection of the carriers (CC)

Charge extraction by the internal electric field

EQE = AIQE = AEDCTCC

EQE: external quantum efficiency

IQE: internal quantum efficiency


ITO glass

Top electrode

Active layer

Single-layer

Bilayer

Bulk heterojunction

Device architecture

Single-layer PV cell Bilayer PV cell Bulk heterojunction PV cell


I. Organic or polymer single-layer PVs

Disadvantage


Organic single crystals

Single-layer PVs


High exciton binding energy

Low bipolar mobility in one molecule

Limitation of single-layer PVs

Low efficiency


Charge transfer can occur between two semiconductors with offset energy levels. Excitons can diffuse approximately 10 nm to an interface. (less than 20 nm) A film thickness of approximately 100 nm is needed to absorb most of the light. Polymer bilayer cell showed 1.9 % energy conversion efficiency. Small molecule bilayer cell showed 3.6 % power conversion efficiency with 3 layers.

II. Organic or polymer bilayer PVs


Small molecular organic bilayer PV cell Improving molecular PV cell

Bilayer PVs


Limitation of organic or polymer bilayer PVs


III. Bulk heterojunction (BHJ) PVs


Device geometries


Working principle of BHJ device

1. Incoming photons are absorbed ⇒Creation of excitons on the Donor /Acceptor

2. Exciton is separated at the donor /acceptor interface Creation of charge carrier⇒s

3. Charge carriers within drift distance reach electrodes Creation of short circuit current I⇒ SC

1. The “photodoping” leads to splitting of Fermi levels Creation of open circuit voltage V⇒ OC

2. Charge transport properties, modulegeometry Fill factor FF⇒


Photoinduced charge generation


Charge transfer


Charge recombination


3-D percolation


3-D percolation - PL quenching


3-D percolation - morphology and transport


3-D percolation – morphology and transport


Efficiencies


Production


Film preparation


1. Conjugated polymer with low band gap

Maximum photon flux of sun = 700 nm

Eg= 1.24 / 0.7= 1.77 [eV]

Maximum absorption of photon of sun

2. Bulk heterojunction morphology

exciton diffusion length of conjugated polymer

= below 20 nm

3. High carrier mobility

electron and hole mobility of conjugated polymer

Isc: tuning of the transport property (mobility); Optimization of cell geometry in dependence of the cell thickness

Voc: tuning of the electronic energy level of the donor-acceptor system; Voc of ~2 V observed in polymeric donor- acceptor system

F.F: tuning of the contacts and morphology: lowering of serial resistance

Optimization for high efficiency

100S

ocsc

P

FFVI


donor acceptor

Polymer C60 derivative

Polymer Polymer

Polymer

CdSe nanocrystal

Polymer Metal oxide nanocrystal

Small molecule Small molecule

*

*

O

O

MDMO-PPV

OMe

O

PCBM

*S

*

P3HT

*

NS

N

*

F8BT

TiO2 or ZnO nanoparticles

CuPc

pentaceneN N **

PFB

Materials for BHJ organic solar cell

C60


*

*

O

O

OMe

O

MDMO-PPV PCBM

Donor/Acceptor composite solution

DA

Voc = 0.82 V

Jsc = 5.25 mA/cm2

FF = 0.61

AM1.5G = 2.5 % (under 80 mW/cm2)

< S. E. Shaheen, et al., Appl. Phys. Lett. 1998, 395, 257 >

glass

ITO

LiF

PEDOT:PSS

Active layer

Metal electrode

A. Polymer/PCBM interpenetrating system


Materials issue - matching the solar emission

The flexibility in chemical tailoring is necessary for matching the absorption of the PV material to the solar emission spectrum.


Bandgap engineering

< The parameters determining the bandgap of conjugated polymers >

E∆r : the energy contribution from bond length alternation RE : the resonance energyEΘ : the energy caused by the inter ring torsion angle ESUB : the influence of the substituents.

EG = E r△ + RE + Eθ + Esub

1. Aromatic form shows higher stabilization energy and therefore the higher bandgap.2. Resonance energy leads to an energy stabilization and so to an increased splitting of t

he HOMO-LUMO energy.3. Torsion between the ring plain interrupts the conjugation and therefore increases the

bandgap.4. Electron donating groups raise the HOMO level and electron withdrawing groups

lower the LUMO.5. In the solid phase, additional intermolecular effects between the chains have to be ta

ken into account, which generally leads to broader bands and a lower bandgap.


Synthetic strategy

1. Introduction of side groups:increase or decrease the electron density

2. Push-pull polymers:The bandgap of copolymers with alternating

of electron rich and electron poor compounds can decrease significantly.

(The bond length alternation is reduced and so the Peierls stabilisation.)

3. Introduction of methine groups betweenthe ring systems:The quinoid form minimize the inter annular

rotation by the double bond character of the bridge bonds as well as the bond length alternation .

(The structure becomes more flat and theresonance between the rings is increased.)

< Potential diagram vs. bond length alternation for (a) trans-polyacethylene as conjugated polymer with degenerate ground state; (b) for polyphenylene as conjugated polymer with nondegenerate ground state>


Poly-N-dodecyl-2,5-bis(2´-thienyl)pyrrole-2,1,3-benzothiadiazole: PTPTB

Voc = 720 mVISC = 3 mA/cm2

FF = 0.38η = 1 %

the push-pull concept by altering electron rich N-dodecyl-2,5-bis(2´-thienyl)pyrrole and electron deficient 2,1,3-benzothiadiazole groups

Low bandgap polymer - PTPTB

Enhancement of absorption area


Which is the Voc?

Is it in the electrodes? (MIM picture) Voc

Is it in the bulk-heterojunction? (p/n-like picture) Voc

Origin of open-circuit voltage (Voc)


TOP ELECTRODE

WORK FUNCTION [eV]

Ca 2.87

Al 4.28

Ag 4.26

Au 5.1

S1 > S2

1. Voc in plastic solar cell is directly related to the acceptor strength of the fullerene.

2. The variation of negative electrode work function influences the Voc in only a minor way.

Variation of Voc


Optimization of contacts

LiF layers forms ohmic contact for electrons at the Al electrode:Contact resistivity limits FF.


Optimization of morphology

Voc = 810 mVISC = 5.2 mA/cm2

FF = 0.62η = 3 %IPCEmax = 50 %EQE = 80~90 %

The efficiency is strongly enhanced byusing cholorobenzene in MDMO-PPV /PCBM mixture.


P3HT post production treatment: • increased absorption strength in the red• Higher conversion efficiency• Diode characteristics is improved

P3HT:PCBM “post production treatment”


Percolation problem in composites

Both donor and acceptor phases have to be percolated.


“Double Cable” polymers

Both donor and acceptor phases will be percolated at very low filling into a host polymer

Isc = 0.42 mA/cm2 Voc = 0.83 VFF = 0.29 (white light 88mW/cm2)


B. Hybrid polymer/nanoporous TiO2 system

• Almost all excitons can be split• No deadends• Polymer chains can be aligned

• Easy to model• Semiconductors can be changedwithout changing the geometry.

For efficient photoinduced charge generation

< Ideal device structure - ordered bulk heterojunctions >

TiO2 can be easily patterned into a continuous network for electron transport.


Photoinduced charge generation - infiltration

TiO2 Polymer

e-

h+

Radiative decay (recombination)

Charge geneation

Good infiltration

Effective charge generation

Low PL efficiency

Poor infiltration

charge recombination

High PL efficiency

<Infiltrating polymers into mesoporous TiO2 film>

Because excitons in conjugated polymer typically travel less than 20 nm before recombination, electron donor and acceptor must form interpenetration of 3-D network for efficient photoinduced charge generation using a infiltration step.

<PL quenching measurement>


< Melt infiltration step >

33 % of the volume of the film can be filled in several min.

< Mesoporous titania films >

P3HT/nanoporous TiO2 PV cell


Quantum dotCdSe Well defined photosensitivity large excition bohr radiu

s quantum confinement

7 nm by 7 nm 7 nm by 30 nm 7 nm by 60 nm

W.U. Huynh, J. Dittmer, A.P. Alivisatos Science, 295 (2002) 2425

C. Polymer/CdSe nanocrystal system

Power conversion efficiency: 1.7 %


D. Small molecular weight organic system< Small molecular organic semiconductor materials >


C. W. Tang’s Heterojunction Solar Cell• first heterojunction for efficient charge generation• ~0.95% conversion efficiency• nearly ideal IVs (FF~0.65)• under full solar illumination (1 sun)

• Photoluminescence (PL) probes the exciton lifetime• Exciton lifetime depends on proximity of donor acceptor interface

CuPc/PTCBI device


Double heterojunction

• cathode metal diffusion• deposition damage• exciton-plasmon interaction• vanishing optical field• electrical shorts

Introduce ‘Exciton Blocking Layer’ (EBL) to:• confine excitons to active region• act as a damage-absorber


Exciton blocking layer

Exciton Blocking Layer (EBL)Improves thin cell efficiency


E. Laminated polymeric system

DA

**S

*NC

*

O

O

POPT rich MEH-CN-PPV rich

ITO or PEDOT on gold

glassAl or Ca

laminated fabrication

anealing200 oC

Au/PEDOT/ POPT:MEH-CN-PPV (19:1) laminated at 200 C onto theMEH-CN-PPV:POPT (19:1) /Ca

Power conversion efficiency around 4.8 % at 480nm irradiation.Calculated AM1.5 efficiency around 1.9 %

Large scale large area fabrication potential

M. Granström, K. Petritsch, A. Arias, A. Lux,

M. Andersson and R. H. Friend,Nature 395, 257 (1998)


< Advantages >

Low cost

Utilization of visible range of light

Simple manufacturing process

Environmental compatibility

Transparent solar cell

- Window

Moderate efficiency ~10%

ITO TiO2 PtAdsorbed dye

electrolyte(A/A-)

(S+/S)

vb

cb

(S+/S)

e-

e-

e-

e-

e-

e-VOC

< Cell reactions >

- S(adsorbed) + h S (adsorbed)

- S (adsorbed) S +(adsorbed) + e-

(injected)

- S +(adsorbed) + A- S(adsorbed) + A

- A(cathode) + e- A-(cathode)

< Structure & Principle >

B. O’Regan and M. Grätzel, Nature, 353, 737 (1991)

E.E. Dye-Sensitized Solar Cell (DSSC)Dye-Sensitized Solar Cell (DSSC)


1. Nanocrystalline SC large surface area, high porosity, pore size distribution, l

ight scattering, electron percolation, Anatase (TiO2), ZnO, SnO2, Nb2O5

2. Sensitizers (Dye) distribution of the dyes on the semiconductor surface, spect

ral properties, redox properties in the ground and excited state, anchoring groups (carb

oxylate or phosphonate), Polypyridyl, Porphyrins, or Phthalocyanines complexes

3. Electrolyte ionic conductivity, electron barrier and hole conductor, redox pote

ntial, mechanical separator, interfacial contact for dye, TiO2 and counter electrode (I–/I

3–)

4. Extra transparent conductive oxide (conductivity, transmittance), sealing, meta

l grid, counter electrode

Key ComponentsKey Components


(1) Excitation of dye under illumination (ns)

(2) Electron injection (ps)

(3) Electron transport (ms)

(4) Regeneration of dye (10 ns)

(5) Recombination with oxidized redox (ms)

(6) Recombination with oxidized dye (s)

30 mM of I– is enough to reduce the most of dye cations

I. Montanari et al., J. Phys. Chem. B, 106, 12203 (2002)

Dynamics Dynamics


Power curve (I-V curve)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.70

1

2

3

4

5

6

7

8

9

10

J (m

A/c

m2 )

Voltage (V)

Jsc

Voc

FF

Jsc: Diffusion coefficient (Length) H+ or Li+ cation on TiO2

TiCl4 acidic sol. Treatment Increase in adsorbed dye

Voc: Electron lifetime (Recombination) TBP, Ammonia in electrolyte Secondary oxide layer

FF: Series and Shunt Resistance (Recombination) Secondary oxide layer

To increase performance of DSSC

Competition between Jsc and Voc: High carrier High probability of recombination

= Pr

Pmax =Pr

FF IscVoc

Current issuesCurrent issues


• Polymer PV devices are widely recognized to have potential to provide flexible, low cost, renewable energy for a wide range of applications• For these devices to be commercially viable, three important areas must be addressed

Source: CDT

Summary: technology challenges


Materials• Up to now, polymers for PVs have largely been taken from the LEP program• Work underway at CDT to develop new polymers optimized to absorb solar radiation• Materials optimized for electron or holetransport

Device Architecture• Morphology of polymer blend crucial to determining device performance• Morphology can be controlled through careful processing, surface treatment and materials design• Many advances in LEP architecture are applicable to PV device development

Source: CDT

Summary: optimization


Source: Linz Institute for Organic Solar Cells (LIOS)

Summary: interdisciplinary R & D

organic semiconductor-based plastic solar cells

Documents

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