Exciton and Charge Dynamics in Polymer Solar Cells Studied by Transient Absorption Spectroscopy - Organic Solar Cells: Materials and Device Physics - page 131

Table 5.1 Efficiency of each photovoltaic conversion event in P3HT:PCBM solar cells a . Rep-
rinted with permission from [ 19 ]. Copyright 2010 American Chemical Society
Blend films g ED g CT g CD g CC IQE/ %

5 b RRa-P3HT:PCBM 1 1 0.31 0.15

42-55 c RR-P3HT:PCBM

before annealing

0.93 1 0.80 *0.6-0.7

75-83 d

RR-P3HT:PCBM
after annealing

0.89 1 0.93 [0.9

a g ED : Exciton diffusion efficiency to the interface of P3HT/PCBM, g CT : Charge transfer effi-
ciency at the P3HT/PCBM interface, g CD : Overall charge dissociation efficiency, g CC : Charge
collection efficiency, IQE: internal quantum efficiency at 400 nm, which is calculated by
IQE = EQE/g A where EQE is the external quantum efficiency at 400 nm and g A is estimated
from twice the absorbance at 400 nm under the following assumptions: (a) 4 % incident light loss
at the air/glass interface and (b) 100 % reflection of the Al electrode.
b

Taken from Ref. 79 .

c

Taken from Refs. 57 and 78 .

d

Taken from Refs. 57 , 78 , and 80 .

RR-P3HT:PCBM blends [ 78 , 82 ]. For the charge transfer at the interface, g CT is as
high as *100 % both for RRa-P3HT:PCBM and RR-P3HT:PCBM blend films,
suggesting that it is dependent on the combination of donor and acceptor materials
rather than blend structures. For the charge dissociation, g CD is as low as *30 %
for RRa-P3HT:PCBM blend films, while it is as high as 80 % for RR-
P3HT:PCBM blend films before the thermal annealing and is improved to 93 %
after the thermal annealing. For the charge collection, g CC is as low as 15 % for
RRa-P3HT:PCBM blend films, while it is *60-70 % for RR-P3HT:PCBM blend
films before the thermal annealing and is improved up to [90 % after the thermal
annealing. The large differences in g CD and g CC are mainly ascribed to the phase-
separated networks and the crystallization of RR-P3HT, both of which result in
improved carrier mobility and larger separation of bound radical pairs. The high
charge collection efficiency is consistent with our conclusion in the previous
section. In conclusion, there is not much difference in the charge generation yield
between RRa-P3HT:PCBM and RR-P3HT:PCBM blend films. Rather, the charge
dissociation and collection have a critical impact on the device performance of
P3HT:PCBM solar cells. Further studies are required to address the origin of high
efficiency in the charge dissociation and collection in polymer solar cells.

5.7 Concluding Remarks

Polymer solar cells have made rapid progress in the device performance during the
last decade. In 2012, a PCE of 10 % has been listed in the solar cell efficiency
tables. For such remarkable progress, development of various new materials has
played a leading role. On the other hand, there still remain unsolved issues on the
underlying mechanism of photovoltaic conversion in polymer solar cells. The