Advanced Electron Transport Materials for Application in Organic

Photovoltaics (OPV)

Alan Sellinger

GCEP Research Symposium 2011

October 5, 2011

1

Eight19 Limited

Why Solar?

1 % of land with solar cells could meet our electricity needs

6 Boxes at 3.3 TW Each

• Amount of solar energy in hours, received each day on an optimally tilted surface during the worst month of the year.

3 Germany 50%

USA 9%! Total PV installation:

Types of Solar Technologies

4

Lowest cost $/W

Attractive properties:

•Abundant: ~100,000 tons/year

•Mature industry/markets

•Low materials cost: ~$1/g 15¢/m2

•Low-cost processing: printing

•Excellent stability

•Non-toxic

CuPc Copper Phthalocyanine

Why Organic Semiconductors?

5

Organic LEDs for Displays

Organic LEDs for Lighting

New Applications for Organic Solar Cells

Chemistry Nobel Prize Day! Organic Electronics: Combination of 3 Nobel Prizes!

9

S

O O

n

PEDOT

C60 C70

• 2000 Nobel Prize in Chemistry - Alan Heeger, Alan G. MacDiarmid, Hideki Shirakawa for their discovery and development of conductive polymers

• 2010 Nobel Prize in Chemistry - Richard F. Heck, Ei-ichi Negishi, Akira Suzuki for palladium-catalyzed cross couplings in organic synthesis • C-C bond formation

• 1996 Nobel Prize in Chemistry - Robert F. Curl Jr., Sir Harold Kroto, Richard E. Smalley for their discovery of fullerenes (C60)

N

O

O

NS

N

N

O

O

S

S

C6H13

S

S

C6H13

C6H13

C6H13

n/4

Motivation for Organic-Based PV

• Lower production cost

– Roll-to-roll coating

– Inexpensive active materials

– Inexpensive processing

– Lower capital cost

10

Energy Payback Time (yrs)

CO2 Emission in Production (g)

Crystalline silicon 2.3 1560

Thin film (CIGS, CdTe, etc.) 1.2-3.2 900-2560

Polymer PV (Glass substrate) 1.3 820

Polymer PV (Plastic substrate) 0.2 130

Inhabitat.com

Roes et al., Prog. Photovoltaics. 2009 (17) 372-393

• Lower environmental impact - $/W

Cost Analysis for Solar Cell Technologies

Ingot Paint

Crystalline Si

Thin Film

Organic/Hybrid

Material and Manufacturing Cost

today

near term/today

future

Eff

icie

ncy

•$/W basis competitive

Recombination

Charge Separation

+

-

++

+

+ + ++

+ +-

-

--

-

- - -

Generation

-

Diffusion

Drift/Diffusion

Donor Material Acceptor Material

Photon

Exciton

HoleElectron

Photon

An

od

eC

ath

od

eSimplified Working Principle

Kietzke, T., Adv. OptoElect., 2007, Article ID 40285.

What is an Organic Photovoltaic?

50-200 nm

Progress in Organic Solar Cells

Solution processes (Polymer, PCBM) Vacuum processed

2005 2010

•Similar development for organic solar cells as for amorphous silicon 20 years ago

•Both approaches are reaching nearly similar efficiencies

•Improvements are largely materials discovery based

10%? Mitsubishi 2011

?

8.3% Heliatek 2010

9% Solarmer 2011

Estimated lifetimes of OPVs

Burn-in loss Burn-in time

Lifetime in linear regime*

P3HT:PCBM 16% 55 days 3.5 years

PCDTBT:PC70BM 27% 38 days 6.7 years

*Lifetime assumes 5.5 hrs/day of one-sun intensity1

1. C. Peters, M. McGehee, Adv. Energy Mater. in press, 2011

• Heliatek (BASF and Bosch) has achieved 30 year lifetimes with tandem OPVs2

2. www.heliatek.com

Bulk heterojunction (BHJ) Polymer Solar Cell

• Advantages:

– Solution processing (roll-to-roll), increased contact area between active phases,

• Disadvantages:

– Difficult to achieve optimum morphology, polymers generally less pure than molecules, solvent dependence

Current world record: • 8-10%

• Extrapolated lifetimes >6 years

http://www.solarmer.com/

16

High-efficiency BHJ solar cells

• Record OPV cells ALL use fullerene derivatives

• Recent development based on donor materials discovery

PBT7:PC70BM

7.4% PCE

Liang et al., Adv. Mat. 2010 (22) E135-E138

PCDTBT:PC70BM

6.1% PCE

Park et al., Nat. Phot. 2009 (3) 297-303

P3HT:ICBA*

6.5% PCE

Zhao et al., Adv. Mat. 2010 (22) 4355-4358

* ICBA: Indene-C60 Bisadduct

PC60BM

O

O

PC70BM

O

O

PC70BM

17

Drawbacks for fullerenes

Reduced solar spectrum

absorption

300 400 500 600 700 8000

0.5

1

1.5

2

2.5x 10

5

Wavelength (nm)

Ab

so

rpti

on

Co

eff

icie

nt

(M-1

cm

-1)

New Acceptor

PC60

BM

PC70

BM

Lower VOC

-3.1 eV

-5.1 eV

P3HT

PCBM

-4.3 eV

-3.7 eV

-6.1 eV

-5.6 eV

-3.1 eV

-5.1 eV

P3HT -3.9 eV

-6.1 eV

New

Acceptor

-3.4 eV

-5.9 eV

Difficult synthesis and purification = Higher cost

PC60BM: $50/g

CuPc: $1-5/g

Fullerene acceptor – up to 10% of total PV system cost

Roes et al., Prog. Photovoltaics. 2009 (17) 372-393

Effect of Production Steps on Cost

• Significant increases (2X) for higher purification

• PC71BM approximately 3.4X more expensive than PC61BM

18

Anctil et al., Environ. Sci. Technol. 45, (2011) 2353-9

O

OO

O

PC60BM PC70BM

19

Non-fullerene acceptors

• Lower efficiencies due to lower JSC and lower FF

POPT:CN-PPV

2% PCE

Holcombe et al., J. Am. Chem. Soc. 2009 131 (40)

14160-14161

P3HT:99’BF

~2% PCE

Brunetti et al., Ang. Chem. Int. Ed. 2010 49 (3) 532-536

Verreet et al., Adv. Ener. Mat. . 2011 (1) 565-568

POPT:EV-BT

1.4% PCE

Woo et al., Chem. Mater. 2010 22 (5) 1673-1679

SubPc:FSubPc dimer

2.5% PCE (vacuum)

OO

O

O

NC

CN

O

O

n

NS

N

N

NNC

NCN

NCN

CN

N N

N

N

N

B

N

N

N

B

N

NN

N

F

F

F

F

F

F

F

F

F

F

F

F

F

F

F

F

Cl

Cl

New Acceptors based on Vinazene

• Original applications as high nitrogen containing materials for reduced flammability. Without vinyl groups has been used in the agriculture sector (fertilizers)

• Their highly electron deficient properties make them candidates as acceptor materials in organic electronic applications

Vinazene 2-vinyl-4,5-dicyanoimidazole 4,5-dicyanoimidazole

HN N

NC CN

HN N

NC CN

R

HN N

NC CN

R

Commercially available

Alkylation of 2-Vinyl-4,5-dicyanoimidazole

NHN

NC CN

Vinazene

Acetone, K2CO3

Acetone, K2CO3

I

NN

NC CN

1-Butylvinazene (90%)

NN

NC CN

1-Hexylvinazene (90%)

NN

NC CN

1-Ethylhexylvinazene (56%)

DMF, K2CO3, 70oC

Reflux

Reflux

I

Br

Tunable Synthesis: Heck Chemistry

Shin R.Y.C., Sonar, P., Siew, P.S., Chen, Z.C., Sellinger, A., J. Org. Chem., 2009, 74 (9), 3293–3298.

Optical properties of selected vinazene derivatives

300 350 400 450 500 550 600 650

0.0

0.2

0.4

0.6

0.8

1.0

No

rma

lis

ed

In

ten

sit

y (

a.u

.)

Wavelength (nm)

1

4

5

10

13

350 400 450 500 550 600 650 700 750

0.0

0.2

0.4

0.6

0.8

1.0

No

rma

lis

ed

In

ten

sit

y (

a.u

.)Wavelength (nm)

1

4

5

10

13

UV spectra of the molecules in toluene

PL spectra of the molecules in toluene

Significant absorption in visible spectrum

(1)

NS

N

(4)

NS

N

S

S

(5)

(10) (13)

Vinazenes used for OPVs

HV-BT

EV-BT Better solubility

N

N

CN

CN

N

N

NC

NC

NS

N

Solubilizing groups for

solution processing

Electron accepting sites

Conjugated

chemical links

•Can be thermally sublimed as well

Preparation of OPV Devices

25

New Electron Acceptor

POPT donor polymer

• Solution processed active layer

POPT with HV-BT

• Kietzke, T., Shin R.Y.C., Egbe, D.A.M., Chen, Z.K., and Sellinger, A., Macromolecules, 2007, 40, 4424-4428. • Shin R.Y.C., Kietzke, T., Sudhakar, S., Dodabalapur A., Chen, Z.K., and Sellinger, A., Chem. Mater., 2007, 19(8), 1892-

1894. • Woo, C.W., Holcombe, T.W., Tam, T.L., Sellinger, A., Frechet, J.M.J., Chem. Mater. 2010, 22 (5), 1673–1679

Voc = 0.62 V, FF = 0.40, PCE = 1.41%

POPT

Can we make better acceptors?

N

N

CN

CN

N

N

NC

NC

NS

N

Solubilizing groups for

solution processing

Electron accepting sites

Conjugated

chemical links

•Corresponding electron donor has charge mobility (cm2/V sec) in 10-4 range. Is charge transport mis-match a problem?

Improving Organic Acceptor Materials

Improved Acceptor Materials?

NS

N

N

O

O

N

O

O

Acceptor Materials: Computational Studies

Ground-state geometries

LUMO

HOMO

31

Synthetic Scheme for PI-BT/NI-BT

PI-BT

PI-BT

NI-BT

NI-BT

32

New Acceptor Properties

• Larger ELUMO,Acc – EHOMO,Don than for P3HT:PC60BM

200 300 400 500 600 700 800 9000

0.2

0.4

0.6

0.8

1

Wavelength (nm)

No

rmal

ized

Ab

sorp

tio

n (

arb

. u

nit

s.)

PI-BT

NI-BT

Thin Film

-3.69 eV

-6.05 eV

-3.1 eV

-5.1 eV

-3.84 eV

-5.99 eV

-3.1 eV

-5.1 eV

P3HT

PI-BT NI-BT

P3HT

ELUMO ~ 0.6 eV

EHOMO ~ 0.9 eV

ELUMO ~ 0.7 eV

EHOMO ~ 0.9 eV

• Peak acceptor absorption is in visible spectrum

• Tunable LUMO – expected higher Voc than fullerenes

33

J-V Curve – Optimal Devices

• Believed highest efficiency BHJ with non-fullerene acceptor and commercially available P3HT donor

P3HT:PI-BT P3HT:NI-BT

Solvent Chlorobenzene Chloroform

Thickness (nm) ~ 90 ~ 120

Cathode LiF(1nm)/Al Ca(7nm)/Al

Anneal Pre/Post Post Pre

Ann. Temp/Time 110 C/3min 110 C/10min

Jsc (mA/cm2) 4.7 1.2

Voc (V) 0.96 0.51

FF 0.56 0.35

PCE (%) 2.54 0.22

-1 -0.5 0 0.5 1 1.5-8

-6

-4

-2

0

2

4

6

8

Voltage (V)

Cu

rre

nt D

en

sity (

mA

cm

-2)

PI-BT

NI-BT

• High voltage as expected with higher-lying LUMO

• Why is the efficiency for NI-BT 10X lower?

34

EQE Spectra of P3HT:PI-BT Device

• Significant photocurrent generation from acceptor absorption

300 400 500 600 700 800 9000

0.1

0.2

0.3

0.4

0.5

Wavelength (nm)

EQ

E (

%)

or

Ab

so

rptio

n (

arb

. u

nits) P3HT:PI-BT EQE

P3HT Absorption

PI-BT Absorption

35

Molecular Simulation – Ground State Geometry

• Twisting of NI-BT molecule due to steric interactions may prevent crystallization in films

PI-BT NI-BT

2.06

2.06 27.3

36

GIXS – Acceptor Crystallization

• More acceptor peaks in PI-BT sample than NI-BT

P3HT:PI-BT (CB) P3HT:NI-BT (CF)

P3HT Only - CB P3HT Only - CF

OPV Conclusions

• Very promising new acceptor materials as potential replacement for fullerene derivatives

– Synthesized in minimal step/moderate yield reactions

– Tunable energy levels

– Anticipated to be much less expensive than fullerenes

• Initial device performance very promising

– From initial PCE of 0.1% >2.54% PCEmax, 5 years of R&D, (5-7 total researchers)

– Close interaction between chemists and device engineers/physicists

38

Acknowledgements

• Co-PI - Prof. Michael McGehee

• Synthesis – Dr. Xu Han (now at DuPont R&D Shanghai), Dr. Andrew Higgs Device Fabrication/Characterization – Jason Bloking, Jack Kastrop

• Quantum Calculations – Dr. Chad Risko, Dr. Joe Norton, Laxman Pandey, Dr. Jean-Luc Bredas (Georgia Tech)

• Funding Sources: