Bin Hu

Department of Materials Science and Engineering

University of Tennessee, Knoxville

Magneto-Optical Studies

on Internal Photovoltaic Processes

in Organic Solar Cells

Wu Han National Laboratory for Optoelectronics

Huazhong University of Science and Technology

I. Overview

II. Recent progress

III. Perspective

• Excited states

• Inter-molecular interface

• Electrode interface

Content:

Topics:

Interface issues in organic solar cells

Polymer chain

Exciton

OCH3O

PCBM

ITO

Meta

l

Inter-molecular interface Device interface

Binding energy

at D:A interface

Charge collection

at electrode interface

Polymer chain

Exciton D:A interfacedissociation

C60

Inter-molecular interface: Overview

Two necessary conditions: 1. Interfacial electrical polarization to break excitons

2. Interfacial energy offsets to facilitate exciton dissociation

Two key questions: 1. At D:A interface, are electrons and holes bound?

2. What control the binding energy of e-h pairs at D:A interface?

Polymer chain

Exciton Charge-transfercomplex

C60 Lig

ht

abso

rpti

on

LUMO

HOMO

Donor Acceptor

LUMO

HOMO

CT

com

ple

x

Experimental tools to study D:A interface

PA PL EL MFEPC Ab

O. Inganäs. JACS. 131, 11819, 2009 B. Hu, Adv. Func. Mater. 18, 2611, 2008

Bin Hu, Adv. Mater. 21, 1500, 2009 Z. Xu & B. Hu, Adv. Func. Mater. 18, 2611, 2008

Magnetic field effects of photocurrent to show internal OPV processes

N SPV filmITO

Al

Light

Solar cell

Dissociation

Inside polymer D:A interface

Low field

(< 200 mT)

High field

(> 200 mT)

B Increase Singlet ratio Increase Jsc（Dissociation： S e + h）

Decrease triplet ratio Decrease Jsc （Charge reaction：T + C e + h）

0 40 80 120 160-1.0

-0.5

0.0

0.5

1.0

1.5

Singlet dissociation

Triplet charge reaction

ITO/polymer/Al

P3HT

MEHPPV

Photo

curr

ent change (

%)

Magnetic field (mT)

Experimental evidence:

Singlet MEHPPV:

Only increasing component

Triplet P3HT：

Both increasing and decreasing components

Setup

Our experimental tool Magnetic field effects of photocurrent: Jsc changes with B.

Polymer chain

PCBM

CT

Exciton

Our experimental tool

0 200 400 600 800 1000

0.0

0.1

0.2

0.3

0.4

0.5

0.6P3HT:PCBM

Photo

curr

ent change (

%)

Magnetic field (mT)

High PCBM doping-real solar cell

Signature of CT complexes

Low PCBM doping

0 40 80 120 160-3

-2

-1

0

1

ITO/PEDOT/P3HT+(x%)PCBM/Al

>5%

1%

0%

Isc c

hange (

%)

Magnetic field (mT)

P3HT: Triplet = high

0 40 80 120 160-1.0

-0.5

0.0

0.5

1.0

1.5

Singlet dissociation

Triplet charge reaction

ITO/polymer/Al

P3HT

MEHPPV

Photo

curr

ent change (

%)

Magnetic field (mT)

MEHPPV: Triplet 1.2%

H. D. Burrows, JACS, 2003

Magnetic field effects of photocurrent: Jsc changes with B.

Isc

B150 mT

P3HT

P3HT:PCBMDissociation in polymer and D:A interface

Inside polymer At D:A interface

0 40 80 120 160

-0.5

0.0

0.5

1.0

PTB

P3HT

Isc c

ha

ng

e (

%)

New polymer versus P3HT

Magnetic field (mT)0 40 80 120 160

-0.8

-0.4

0.0

0.4

0.8

1.2

P3HT:PCBM

Isc c

han

ge

(%

)

Meganetic field (mT)

x=0%

x=5%

5% PCBM doping in P3HT is equivalent to pure PTB.

S

SS

S

R1OOCR2

R2

n

S n

n-C6H13

P3HT Y. Liang, et.al. JACS. 131, 56, 2009

Low-field (< 200 mT): dissociation within PV polymer

0 50 100 150

-0.5

0.0

0.5

1.0

Isc c

ha

ng

e (

%)

Magnetic field (mT)

ITO/PEDOT/P3HT:ICBA(x)/Ca/Al

5% ICBA

P3HT

0 50 100 150

-0.5

0.0

0.5

1.0

5% PCBM

P3HT

Isc c

ha

ng

e (

%)

ITO/PEDOT/P3HT:PCBM(x)/Ca/Al

Magnetic field (mT)

Dissociation with new acceptor ICBA

-0.8 -0.4 0.0 0.4 0.8-12

-6

0

6

FF=63.9%

=5.1%

Ph

oto

cu

rre

nt

(mA

/cm

2)

Voltage (V)

ITO/PEDOT/P3HT:x/Ca/Al

ICBA

PCBMFF=62.4%

=3.2%

Information

from IV curves

• High Voc

• Large Isc ICBA

Our information：dissociation with ICBA can be further improved by 50%

ICBA：partial dissociation PCBM：complete dissociation

Collaboration with Prof. Yongfang Li

Polymer chain

PCBM

CT

Exciton

PCBM doping effects on dissociation at D:A interface

0 200 400 600 800 1000

0.0

0.1

0.2

0.3

0.4

0.5

0.6P3HT:PCBM

Ph

oto

curr

en

t ch

an

ge

(%

)

Magnetic field (mT)

High PCBM doping: real solar cell

Signature of CT complexes

Low PCBM doping

0 40 80 120 160-3

-2

-1

0

1

ITO/PEDOT/P3HT+(x%)PCBM/Al

>5%

1%

0%

Isc c

ha

ng

e (

%)

Magnetic field (mT)

Polymer chain

PCBM

CT

Exciton

Binding energy at D-A interfaces Magnetic field + electric field

- +

Magnetic field effects of photocurrent detect existence of CT states

Electric field dissociates CT states at D-A interfaces. +

Binding energy of CT states

Isc

B150 mT

P3HT

P3HT:PCBM

Low field: dissociation in P3HT

High field: dissociation at D:A interface

0 300 600 900

0.0

0.5

1.0

1.5

2.0

-2V

PTB2:PCBM=1:1

Isc c

ha

ng

e (

%)

Magnetic field (mT)

0V

0 300 600 900

0.0

0.1

0.2

0.3

0.4

Annealed

-2V

0V

P3HT:PCBM=1:0.8

Isc c

ha

ng

e (

%)

Magnatic field (mT)

Binding energy at D:A interface in organic solar cells

0.0 0.4 0.8

ITO/PEDOT/Polymer:PCBM/Ca/Al

Isc (

mA

/cm

2)

Voltage (V)

P3HT:PCBM

PTB:PCBM

-20

-10

0

10

Low binding energy High binding energy

S

SS

S

R1OOCR2

R2

n

S n

n-C6H13

Huidong Zang, et.al., Adv. Energy. Mater. 1, 923, 2011

Perspective: D:A binding energy

Donor AcceptorFAFD FD

AnodeCathode

WC

Vacuum

WA

Built-in electric field

TK

er

B

2

e-h capture radius

Two forces:

Coulomb attraction + Drifting

222

2

1

2

1

4

1hhee vmvm

r

eU

Energy

Columb interaction Kinetic Energy

Mobilities r

Interface-enhanced charge collection

-1.0 -0.5 0.0 0.5 1.0

FF

Isc (

mA

/cm

2)

Voltage (V)

1 Sun

68717265 62

ITO/PEDOT/P3HT:PCBM/Ca/Al

0.1Sun

-16

-808

162432

0.0 0.2 0.4 0.6 0.8-20

-15

-10

-5

0

5

Curr

en

t (m

A/c

m2)

Bias (V)

(a)

Inverted cell(PCE:7.8%)

Normal cell(PCE:6.0%)

ITO-Glass

PTB7:PC70BM

Gold

TiOx

PTB7:PC70BM

MoO3

ITO-Glass

PTB7:PC70BM

Aluminum

PEDOT:PSS

PTB7:PC70BM

Calcium

Interface increases Jsc: increasing charge collection.

-1.0 -0.5 0.0 0.5 1.00

5

10

15

20

25

Ca

pa

cita

nce

(n

F)

Bias (V)

0.1sun

1 sun

Norma cell(a)0.30V

-1.0 -0.5 0.0 0.5 1.00

10

20

30

40

Ca

pa

cita

nce

(n

F)

Bias (V)

0.1 sun

1 sun

Inverted cell(b)0.06V

ITO-Glass

PTB7:PC70BM

Gold

TiOx

PTB7:PC70BM

MoO3

ITO-Glass

PTB7:PC70BM

Aluminum

PEDOT:PSS

PTB7:PC70BM

Calcium

Interface effects on charge accumulation Photoinduced impedance studies

More accumulation Less accumulation

Perspective: Electrode interface

An interfacial layer

Electrical polarization

0 20 40 60 80 100

6

7

8

9

10

11

12

Ca

pa

cita

nce

(nF

)

Light intensity (mW/cm2)

Inverted cell

Normal cell

Charge tunneling Decreasing traps

Acknowledgement

NSF-ECCS project for organic solar cells:

The research has been collaborated with

Dr. Ilia Ivanov at ORNL

Prof. Luping Yu at University of Chicago

Prof. Tzung-Fang Guo at NCKU

Prof. Yongfang Li at Institute of Chemistry (China)

Prof. Guanghua Wei at Taiwan Jiaotong University

Magneto-Optical Studies of Charge Dissociation, Transport,

and Collection in Organic Solar Cells

The research was done by Huidong Zang, Yu-Che Hsiao,

Qing Liu, Mingxing Li, and Michael Stanford.

Huidong Zang Yu-Che Hsiao

Min

gxin

g L

i

Qin

g L

iu

Mic

hae

l S

tan

ford