Design and Synthesis of Design and Synthesis of Conjugated Polymers towards Conjugated Polymers towards Organic Solar Cell ApplicationsOrganic Solar Cell Applications

Toshihiro Okamoto, Ying Jiang, Zhenan Bao

� Solar energy � plentiful and free� Organic vs inorganic

� Low cost� High-speed production (solution, roll-to-roll

processing)� Mountable on flexible substrates� Chemical flexibility for modifications via

synthetic methods� Downside: low efficiency, instability

Organic Organic PhotovoltaicsPhotovoltaics (OPV)(OPV)

Photovoltaic energy conversionPhotovoltaic energy conversion

hν

-

+

Donor Acceptor

E

Anode Cathode

LUMOLUMO

HOMOHOMO

ABS COLLDISSTOTALη = × η×ηη

1. Light absorption and exciton formation.

2. Charge dissociationat heterojunction.

3. Charge transport and collection at electrodes.

Double-layer heterojunction

Bulk heterojunction

Organic Heterojunction Solar CellsOrganic Heterojunction Solar Cells

Ordered bulk heterojunction

5.5Bulk heterojunction

15 � 20Target for OPV

> 30Inorganic solar cell

Efficiency (%)Type of Device

The idealmaterial

for asolar cell

Highabsorptioncoefficient

Solubility

Low bandgap(<1.7 eV)

High mobility

Exhibits percolating morphology

Stability

PCE (%)

FF

VOC (V)

ISC(mA/cm2)

3.0[1]

0.56

0.80

5.4

~10-7[3]

2.2[4]

0.42

0.59

8.9

~10-6 � 10-4

1.6

Fluorenecopolymers

4.6[2]

Best performance

reported 0.65

0.65

12.5

~0.01-0.1Mobility (cm2/V s)

~1.9 � 2.2[5]2.0 � 2.2HOMO-LUMO gap (eV)

Structure

P3HTsPPVsMaterial

Representative/Promising classes of materials toRepresentative/Promising classes of materials to--datedate

1. Chem. Commun., 2003, 2116�2118; 2. Solar Energy Materials & Solar Cells 91 (2007) 1187�1193; 3. J. Phys. Chem. B 2004, 108, 5235-5242; 4. Adv Mater 2006, 18, 2169-2173; 5. Adv. Funct. Mater. 2007, 17, 1071-1078

Desirable bandgap: < 1.7 eV, mobility > 0.01cm2/Vs

Our approach: Our approach: aceneacene--containing conjugated polymerscontaining conjugated polymers

Low Low bandgapbandgap conjugated conjugated pentacenepentacene derivativesderivatives

� Pentacene� Optical band gap 2.1 eV in solution, 1.77 eV in thin film

� Further reduction in band gap by extending conjugation

−1.626 eV

−7.612 eV

−1.772 eV

−7.644 eV

0.146 eV0.146 eV

0.032 eV0.032 eVHOMO

LUMO

ADTADT--fluorenefluorene copolymercopolymer

2.4

2.9

3.8

PDI

-5.23

-5.22

-5.24

HOMO (eV)

2.02

2.05

2.03

Optical Band Gap

(eV)

24×103

23×103

131×103

Mw

10×1032-Ethylhexyl

8×103Dodecyl

46×103Octyl

MnR

Copolymer (R = octyl) in toluene

Copolymer film (R = octyl)

Eg = 2.2 eV;More stable than pentacene

UVUV--VisVis stability in solutionstability in solution� Absorption measured in ODCB solution

� Decrease in absorption with t1/2 ~ 600 min

� Pentacene t1/2 (575 nm) ~ 250 min in dark1

300 400 500 600 700 8000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Abs

orba

nce

Wavelength (nm)

0 min 5 min 15 min 30 min 60 min 240 min 420 min 1500 min (25 hr)

YJ2-36a

1 Chem Mater, Vol 16, No 24, 4980

Absorbance at 587nm vs Time

00.050.1

0.150.2

0.250.3

0.35

0 500 1000 1500 2000

Time (min)

Abs

orba

nce

Copolymer 1

Thin film fieldThin film field--effect mobilityeffect mobility

-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0-100-80-60-40-200

VDS (V)

I DS ( µ

A)

Vg=20

Vg=10

Vg=00

Vg=-10

Vg=-20

Vg=-30

Vg=-40

Vg=-50

Vg=-60

Vg=-70

Vg=-80

Vg=-90

Vg=-100

1E-09

1E-08

1E-07

1E-06

1E-05

-100 -80 -60 -40 -20 0 20

VG (V)

- ID

S ( µ

Am

ps )

0.0000

0.0005

0.0010

0.0015

0.0020

0.0025

0.0030

(- I D

S )1/

2 (A

)1/2

µ=0.0090 cm2/Vs

On/Off = ~4000

Vth = -5.5 V

Preliminary solar cell resultsPreliminary solar cell results

-0.5 0.0 0.5 1.0-5

0

5VOC = 495 mV

Jsc = 4.05 mA/cm2

FF = 0.38Eff. = 0.76%

Cur

rent

Den

sity

(mA

/cm

2 )

Voltage (V)

Toluene and ODCB Light Toluene and ODCB Dark ODCB Light ODCB Dark

High seriesresistancepossibly dueto amorphousstructure.

1:1 TIPSEDTAF:PCBM C60

Higher VOC'spossible.

TIPSEPTIPSEP--fluorenefluorene copolymercopolymer

2.06

2.77

3.21

PDI

17×103

49×103

117×103

Mw

1.80

1.79

1.78

Optical Band Gap

(eV)

5.17

5.19

5.20

HOMO(eV)

8.2×1032-Ethylhexyl

36×103Octyl

18×103Dodecyl

R Mn

Eg = 1.86 eV

TIPSEPTIPSEP--dTfluorenedTfluorene copolymercopolymerMonomer synthesis:addition of thiophene linkers

Copolymerization

1.67

1.74

Eg(eV)

457745On Glass

443715In ODCB

λmax (nm)

λonset (nm)

UV-Vis absorption spectra of TIPSEP-dTfluorene copolymer

7: R = C12H25

TIPSEPTIPSEP--diethynylbenzenediethynylbenzene copolymercopolymer

Modest solubility in common organic solvents (< 5 mg/ml)

Dark Green Film(Polymer 9)

2.41

2.21

PDI

1.76

1.68

Eg (eV)

58×103

25×103

Mw

24×1032-Ethylhexyl

11×103Dodecyl

MnR Films ODCB

Ref.) S. L. Buchwald and G. C. Fu et al. Org. Lett. 2000, 2, 1729

Summary and future workSummary and future work

� Molecular design towards stable, low bandgap materials for OPV

� Synthesized copolymers with promising properties

� Continue to build and characterize a library of pentacene-derivative-containing copolymers

� Fabricate devices to assess materials

AcknowledgementsAcknowledgements

SynthesisMichelle Senatore (UG)

Fei Qu (UG)

Transistor DeviceDr. Hector Becerril

The Air Force Office of Scientific Research, AFRL

Solar Cell DeviceDr. Alex MyerJack Parmer

Professor Mike McGehee