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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
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