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.
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)
1
▪ Photoinduced Electron & Hole Transfer
Photoinduced Charge Transfer Process
(c.f. Photoinduced Energy Transfer)
▪ Experimental Evidence: LESR
2
▪ 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
▪ 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
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
5
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
6