Dr. John AnthonyProfessorUniversity of Kentucky

John AnthonyCenter for Applied Energy Research / Department of Chemistry University of Kentucky

Carbon-Based Materials for Solar Power Generation

OFET

OLED

OPV

Anthony AromaticsResearch Group

Crystalline silicon solar cell Power conversion efficiency = 18 - 24%

Amorphous silicon solar cellPower conversion efficiency = 6 - 9%

Carbon-based solar cellPower conversion efficiency = 3 - 5%

Solar Cells

Pow

er C

onve

rsio

n E

ffici

ency

Module C

ost

An impressive amount of energy goes into Si production

3,400 °F

Silicon

+ Carbon Dioxide

(1.6 pounds CO2 produced per pound of silicon)

arc furnace, C electrodes

Extract the starting materials needed to make carbon-based solar cells from agricultural feedstocks.

Use the resulting “active inks” to form solar cells by spray-painting or inkjet printing

(donor) (acceptor)

e-

Organic Excitonic Solar Cells

+ - -+ -+-+ -+

Donor Acceptor

Voc

charge separation LUMO

HOMO

LUMO

HOMO

Organic Solar Cells

The “easy” stuff

Voc

Jsc

FFPCE = (Voc)(Jsc)(FF)

(Plight)x100%

Advantage: Allows deposition of best-quality films of donor & acceptor

Single heterojunction solar cell

Disadvantages: • Small interface area between donor & acceptor• Layer thickness limited by exciton diffusion length (< 40 nm)NOTE: Amount of light absorbed is directly related to the thickness of the film (Beer’s law).

Tang, C. W., Two-layer organic photovoltaic cell, Applied Physics Letters (1986), 48(2), 183-5

Molecules must self-segregate during film formation

• Use one crystalline component and one amorphous component• Crystalline molecules with very different shapes - spheres vs. rods, for example

maximizes interfacial area

Bulk heterojunction solar cell

G. Yu, J. Gao, J. C. Hummelen, F. Wudl, A. J. Heeger “Polymer photovoltiac cells: enhanced efficiencies via a network of internal donor-acceptor heterojunctions” Science 1995, v. 270, 1789

J. J. M. Halls, C. A. Walsh, N. C. Greenham, E. A. Marseglia, R. H. Friend, S. C. Moratti, A. B. Holmes “Efficient photodiodes from interpenetrating polymer networks” Nature 1995, v.376, 498

+ -

In this case, crystal packing can impact:• Morphology• Phase separation• Exciton diffusion length• Charge transport

Organic Solar Cells

Bulk heterojunctionmaximizes interfacial area

Basic aromatic self-assembly

}

What is necessary for charge transport?

Induced pi-stacking

1) Disrupt edge-to-face interactions with substitution at peri positions2) Place substituent far enough from π-system to allow (or enhance) stacking. 3) Adjust size of substituent to control amount of π-overlap4) Substituent on central ring lends both solubility and stability to the material

Tuning a chromophore for a particular application takes place in three stages:

– Coarse: Induce (electronics) or eliminate (optics) π-stacking– Medium: Explore broad π-stacking motifs to optimize for desired application– Fine: Fine-tune precise nature of stacking to optimize performance

Ried, W.; Anthöfer, F. Angew. Chem. 1953, 65, 601

(> 85%)

(scale: up to 30 g)

The silylethyne-substituted pentacenes are highly soluble, stable and easy to prepare, and form large, high quality crystals from solution.

Templating acene self-assembly

X

X

y

X

X

X

1-D, long-axis slip

1-D, short-axis slip

1-D column, regular & dimerized

2-D “brickwork”

Alter π-stacking motif

J. E. Anthony, D. L. Eaton and S. R. Parkin Org. Lett. 2001, 4, 15.

Carbon-based active layer is added by spin-casting

Aluminum cathode is added by evaporation under high vacuum

ITO

ITO glass (used in cell phones)

Bulk-heterojunction solar cells from a highly crystalline donor and much less crystalline acceptor

PCBM (acceptor)

ADT(donor)

We can engineer semiconductors with various π-stacking motifs, which in turn impacts charge transport properties and crystal growth

S. Subramanian, S. R. Parkin, S. Park, T. N. Jackson, J. E. Anthony J. Mater. Chem. 19 7984 - 7989 (2009).

Intrastack contact: 3.5 ÅInterstack contact: 3.58 Å

µFET = 0.5 cm2 / Vs

“Intrastack” contact: 3.48 Å“Interstack” contact: 3.23 Å

µFET = 1.0 cm2 / Vs

Intrastack contact: 3.49 ÅInterstack contact: 3.84 Å

µFET = 10-4 cm2 / Vs

2-D π-stack

1-D π-stack

Intrastack contact: 3.83 Å µFET = 10-3 cm2 / Vs

2-D π-stack

Approximate 2-D π-stack

Crystal packing

PV efficiency

Excellent2-D

No PV response

Excellent2-D

No PVresponse

Poor2-D

(> 3.8 Å)

0.24%Voc = 0.82 V

Jsc = 0.2 mA / cm2

Type of crystal packing has significant impact on PV performance

60 sODCB vapor

120 sODCB vapor

180 sODCB vapor

“as spun”

Solvent vapor annealing of Ethyl TES ADT / PCBM films

Prof. George Malliaras, Matthew Lloyd

Device 1

Device 2

Device 3

Device 5

Device 4

Device 6

r2 = 0.98

“Starburst” concentration relates directly to solar cell current

Fluorescence micrograph of actual device

Prof. George Malliaras, Matt Lloyd

Jsc = 3 mA/cm2, Voc = 0.84 V, > 1% PCE

1-D π-stack, poor overlapµFET = 10-4 cm2 / Vs

Bulk-heterojunction acene solar cells

Lloyd, Mayer, Subramanian, Mourey, Herman, Bapat, Anthony, Malliaras J. Am. Chem. Soc. 2007, 129, 9144

Small molecule bulk heterojunction solar cells

Organic active layer is added by spin-casting

Silver cathode is added by evaporation under high vacuum

ITO

Lloyd, Mayer, Subramanian, Mourey, Herman, Bapat, Anthony, Malliaras J. Am. Chem. Soc. 2007, 129, 9144

+

1-D π-stack

Device performance before annealing:Voc= 0.84VIsc= 0.971 mA/cm2

Eff = 0.25%FF = 0.372RR = -0.391

Small molecule bulk heterojunction solar cells

Dr. R. Shashidhar, Dr. Guofeng Li

Recent progress

A. B. Tamayo, B. Walker, T.-Q. Nguyen J. Phys. Chem. C. 2008, 112, 11545

Voc = 0.67 VJsc = 8.42 mA / cm2

FF = 0.45PCE = 2.33%

Voc = 0.78 VJsc = 14.4 mA / cm2

FF = 0.59PCE = 6.70%

Y. Sun, G. C. Welch, W. L. Leong, C. J. Takacs, G. C. Bazan, A. J. Heeger Nature Materials 2012, 11, 44

ITO AnodeGlass substrate

P3HT / Pentacene (1:1 wt:wt)

Pentacenes as drop-in replacements for PCBM

+ PEDOT / PSS

CsF:Al Cathode

Under these conditions, P3HT/PCBM devices yield PCE = 2.5 - 3%

Pentacene acceptor

Controls crystal packing, tunes morphology, phase separation - related to device current (Jsc)

Controls LUMO energy - charge separation efficiency, voltage (Voc)

Prof. George Malliaras, Yee-Fun Lim

P3HT donor

Voc = 0.79 V Voc = 0.59 V Voc = 0.56 V

Voc = 0.84 V Voc = 0.70 V Voc = 0.64 V

Pentacene acceptors for organic solar cellsPositional impact of substituent on open-circuit voltage

(differences in electrochemical LUMO are typically less than 0.05V)

Prof. George Malliaras, Yee-Fun Lim

Voc = 0.88 V

Voc = 0.96 V

1-D slipped stack

2-D brickwork

sandwich herringbone

double 1-D

Pentacene acceptors for organic solar cells

Impact of crystal packing on short-circuit current

Structure R Packing Voc Jsc FF PCEn-propyl double

sandwich 0.84 V 0.75 mA/cm2 0.40 0.25%

iso-propyl 1-D slipped 0.80 V 1.27 mA/cm2 0.34 0.34%

n-butyl 2-D brickwork XX XX XX no PV effect

cyclopentyl sandwich 0.84 V 4.04 mA/cm2 0.45 1.53%ethyl 1-D slipped 0.74 V 1.20 mA/cm2 0.20 0.22%

iso-propyl double sandwich 0.72 V 1.86

mA/cm2 0.31 0.41%

n-propyl 1-D cruciform 0.72 V 1.10 mA/cm2 0.27 0.18%

iso-butyl sandwich 0.90 V 3.34 mA/cm2 0.48 1.28%

cyclopentyl 1-D slipped 0.80 V 3.06 mA/cm2 0.36 0.88%

iso-propyl double sandwich 0.64 V 2.28

mA/cm2 0.33 0.48%

iso-butyl ??? 0.64 V 0.90 mA/cm2 0.28 0.14%

cyclopentyl ??? 0.70 V 2.60 mA/cm2 0.32 0.59%

iso-propyl ??? 0.60 V 0.55 mA/cm2 0.35 0.12%

n-propyl sandwich 0.76 V 1.83 mA/cm2 0.53 0.83%

cyclopentyl ??? 0.72 V 2.07 mA/cm2 0.45 0.87%

iso-propyl ??? 0.70 V 0.94 mA/cm2 0.37 0.24%

cyclopentyl 1-D Herringbone 0.96 V 2.51 mA/cm2 0.43 1.03%

Shu, Lim, Li, Purushothaman, Hallani, Kim, Parkin, Malliaras, Anthony Chemical Sciences 2, 363 – 366 (2011)

stability

contribution to photocurrent

• Correlation between single-crystal packing and device performance is only valid if that form is seen in the active layer films.

GIXD studies by both the Loo group and Amassian group support this assumption - generally, no other crystalline form observed in P3HT / small molecule acceptor blends.

• What is it about the “sandwich herringbone” packing that leads to better performance (typically resulting from higher Jsc)? Can we explain the exceptions?

What can we learn from the performance of this big group of compounds?

Prof. Aram Amassian, Dr. Ruipeng Li

a

a

a

c/a = 2.2

c/a = 2.25

c/a = 3.1

c

c

c

Packing motif

Unit cellThin film texture

<001> is dominantMosaicity of

<001> crystallitesπ-π*

in films

Low mosaicity confines transport to the plane of the substrate

Texture and mosaicity – low JSC

Prof. Aram Amassian, Dr. Ruipeng Li

Texture and mosaicity – high JSC

Motif Unit cellThin film texture<001> dominant

Mosaicity of <001> crystallites

π-π* in films

Increased mosaicity allows transport out of the plane of the substrate

<001>

<001>

<001>

a

c

a

c

a

c

c/a = 1.04

c/a = 1.15

c/a = 1.05

<101>

1D herringbone

Prof. Aram Amassian, Dr. Ruipeng Li

Mosaicity of charge transport direction

ac

ac

ac

c/a = 2.2

c/a = 2.25

c/a = 3.1

a

c

a

c

a

ac

c/a = 1.04

c/a = 1.15

c/a = 1.05

FWHM = 2.4°

FWHM = 3.4°

FWHM = 3.5°

<001>

<001>

<001>

<001>

FWHM = 7.5°

<001>

FWHM = 14.1°

<001>

FWHM = 26.2°

<101>

FWHM = 21.6°

c

c/a

Structure R Packing Voc Jsc FF PCEn-propyl double sandwich c/a = 2.05 0.84 V 0.75 mA/cm2 0.40 0.25%

iso-propyl 1-D slipped c/a = 1.85 0.80 V 1.27 mA/cm2 0.34 0.34%

n-butyl 2-D brickwork c/a = 2.8 XX XX XX no PV effect

cyclopentyl sandwich c/a = 0.96 0.84 V 4.04 mA/cm2 0.45 1.52%

ethyl 1-D slipped c/a = 3.7 0.74 V 1.20 mA/cm2 0.20 0.22%

iso-propyl double sandwich c/a = 3.77 0.72 V 1.86 mA/cm2 0.31 0.41%

n-propyl 1-D cruciform c/a = 3.1 0.72 V 1.10 mA/cm2 0.27 0.18%

iso-butylsandwich c/a =

1.050.90 V 3.34 mA/cm2 0.48 1.28%

cyclopentyl 1-D slipped c/a = 1.2 0.80 V 3.06 mA/cm2 0.36 0.88%

iso-propyl double sandwich c/a = 3.8 0.64 V 2.28

mA/cm2 0.33 0.48%

iso-butyl ??? 0.64 V 0.90 mA/cm2 0.28 0.14%

cyclopentyl ??? 0.70 V 2.60 mA/cm2 0.32 0.59%

iso-propyl ??? 0.60 V 0.55 mA/cm2 0.35 0.12%

n-propyl sandwich c/a = 2.1 0.76 V 1.83

mA/cm2 0.53 0.83%

cyclopentyl ??? 0.72 V 2.07 mA/cm2 0.45 0.87%

iso-propyl ??? 0.70 V 0.94 mA/cm2 0.37 0.24%

cyclopentyl 1-D Herringbone c/a - 0.97 0.96 V 2.51

mA/cm2 0.43 1.03%

Generality?

We have evaluated c/a for the other OPV acceptors we have investigated.

In each case, as c/a 1.0, Jsc is maximized (note - Jsc not necessarily high, just the best in the series)

sandwich herringbone for OPVs

Voc = 0.92 V, Jsc = 1.87 mA/cm2, FF=0.37, PCE=0.64%

As acceptor, 1:1 blend of TSBS FADT with P3HT

Prof. George Malliaras, Yee-Fun Lim

c/a = 1.2

One more thing . . .Charge transfer from P3HT to acceptor (vs. acceptor crystal structure)

Cyano TCPS PentaceneSandwich herringbone

TIPS pentacene2-D π-stack

Conclusion

• Fullerene replacements within 50% of current benchmark in P3HT blends

• Design rules to carry forward to next-gen materials - unit cell isotropy - appropriate tuning of LUMO energy - significant exposed π-surface in crystals

New renewable energy research center:Batteries, Biofuels and Photovoltaics

Crystallography: Dr. Sean Parkin

Alumni:David EatonDr. Zhong LiSankar SubramanianSue OdomGenay Jones

Ying ShuDr. Marcia PayneRawad HallaniMatt Bruzek

The OPV Group:

Many thanks to our collaborators!Prof. George Malliaras / Yee-Fun Lim & Matt LloydProf. Lynn Loo / Stephanie LeeProf. Aram Amassian / Ruipeng Li

Dr. John AnthonyProfessorUniversity of Kentucky