ch.4: polymer solar cells: photoinduced charge transfer ▪ photoinduced electron transfer between...

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Ch.4: Polymer Solar Cells: Photoinduced Charge Transfer Photoinduced Electron Transfer between Conjugated Polymer and Fullerene N. S. Sariciftci & A. J. Heeger et al., Science 258 (1992) C.B. V.B. C 60 LUMO Level 50fs ~ ms Conjugated Polymer C 60 C 60 HOMO Level High Quantum Efficiency ~ 100% -Very efficient process faster than any other competing processes such as polaron, bipolaron, and exciton formation ( <50 fs ). -Back transfer is slowed down by forming polarons in C60. (>1 ms) 1

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Page 1: Ch.4: Polymer Solar Cells: Photoinduced Charge Transfer ▪ Photoinduced Electron Transfer between Conjugated Polymer and Fullerene N. S. Sariciftci & A

Ch.4: Polymer Solar Cells: Photoinduced Charge Transfer

▪ Photoinduced Electron Transfer between Conjugated

Polymer and Fullerene

N. S. Sariciftci & A. J. Heeger et al., Science 258 (1992)

C.B.

V.B.

C60 LUMOLevel

50fs

~ ms

Conjugated Polymer C60

C60 HOMOLevel

High Quantum Efficiency

~ 100%

-Very efficient process faster than any other competing processes such as polaron, bipolaron, and exciton formation ( <50 fs ).

-Back transfer is slowed down by forming polarons in C60. (>1 ms)

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Page 2: Ch.4: Polymer Solar Cells: Photoinduced Charge Transfer ▪ Photoinduced Electron Transfer between Conjugated Polymer and Fullerene N. S. Sariciftci & A

▪ Photoinduced Electron & Hole Transfer

Photoinduced Charge Transfer Process

(c.f. Photoinduced Energy Transfer)

▪ Experimental Evidence: LESR

2

Page 3: Ch.4: Polymer Solar Cells: Photoinduced Charge Transfer ▪ Photoinduced Electron Transfer between Conjugated Polymer and Fullerene N. S. Sariciftci & A

▪ Experimental Evidence: Luminescence Quenching

n

O

O

O

OMe

2 3 4 5 60.0

0.4

0.8

1.2

1.6

2.0

MEH-PPV PCBM

Abs

orpt

ion

(Nor

mal

ized

)

Energy (eV)

1. MEH-PPV:PCBM (95:5)

2. MEH-PPV:PCBM (90:10)

3. MEH-PPV:PCBM (80:20)

4. MEH-PPV:PCBM (50:50)

1.4 1.6 1.8 2.0 2.2 2.40

1x104

2x104

3x104

4x104

5x104

6x104

MEH-PPV PCBM 5wt.% PCBM 10wt.% PCBM 20wt.% PCBM 50wt.%

PL

In

ten

sity

(A

rb. U

nit

)

Energy (eV)

HOMO

HOMO

LUMO

LUMO

MEH-PPV

PCBM

Photoinduced Charge Transfer

2 3 4 5 60.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4 MEH-PPV MEH-PPV:PCBM ( 5wt.%) MEH-PPV:PCBM (10wt.%) MEH-PPV:PCBM (20wt.%) MEH-PPV:PCBM (50wt.%)

Ab

sorp

tion

Energy (eV)

3

Page 4: Ch.4: Polymer Solar Cells: Photoinduced Charge Transfer ▪ Photoinduced Electron Transfer between Conjugated Polymer and Fullerene N. S. Sariciftci & A

▪ Experimental Evidence: Photoconductivity Measurements

Transient photoconductivity measurement

C.H.Lee et al., Phys. Rev. B. 48, 15425 (1993).

▪ Experimental Evidence: Photoinduced Infrared Spectroscopy

Infrared photoexcitation spectroscopy

K.Lee, S. N. Saricifci, A. J. Heeger Phys. Rev. B. 49, 5781 (1994).

C60- peak at 1.15 eV

and 1.25 eV between t1u and t1g 4

Page 5: Ch.4: Polymer Solar Cells: Photoinduced Charge Transfer ▪ Photoinduced Electron Transfer between Conjugated Polymer and Fullerene N. S. Sariciftci & A

Bicontinuous donor/acceptor bulk heterojunction

- +

Al

PEDOT:PSSITO

Glass

Light

e

e-

-

-++

+

+

e-

Light

PCBM

S n

P3HT

G. Yu & A. J. Heeger et al., Science 270 (1995)

Interpenetrating Network in Polymer/C60 Composites

Advantages

• Bulk donor-acceptor

heterojunction

• Achievement of large

interfacial area

Requirements

• Formation of percolation path

• Morphology control

Photovoltaic Cells using Polymer/C60 Composites

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Page 6: Ch.4: Polymer Solar Cells: Photoinduced Charge Transfer ▪ Photoinduced Electron Transfer between Conjugated Polymer and Fullerene N. S. Sariciftci & A

Metal/Semiconductor/Metal structure

Photovoltaic Effect in Polymer Solar Cells

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

PCBM 5 wt.% 10 wt.% 20 wt.% 50 wt.%

Cur

rent

Den

sity

(m

A/c

m2 )

Voltage (V)

n

O

O

HOMO

HOMO

LUMO

LUMO

MEH-PPV PCBM

AlITO

Typical PV effect in MEH-PPV/PCBM mixtureO

OMe

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