Environmental Engineering Reference
In-Depth Information
Fig. 5.19 Time evolution of
the carrier density n(t)in
RR-P3HT:PCBM (50:50 w/
w) blend films measured at
a 700 nm and b 1000 nm.
The excitation intensity at
400 nm is 0.8, 1.8, and
4.7 lJcm
-2
from bottom to
top in each panel. The broken
lines represent fitting curves
with an empirical power
equation: n(t) = n
0
/(1 ? at)
-
a
. Reprinted with permission
from [
20
]. Copyright 2010
American Chemical Society
(a)
17
10
10
16
10
15
(b)
10
17
16
10
15
10
-6
-5
-4
-3
10
10
10
10
Time / s
thermal annealing, and increases to g
CD
= 93 % after the thermal annealing,
which is three times larger than that for RRa-P3HT:PCBM blend films.
Such a high dissociation efficiency is consistent with the efficient device per-
formance of RR-P3HT:PCBM solar cells. However, it cannot be rationally
explained by the classical models such as those of Onsager [
59
] and Braun [
60
].
Recent theoretical studies demonstrate that the presence of donor-acceptor phase-
separated interface increases the charge dissociation probability in comparison
with the homogeneous blend [
61
-
63
]. Recently, Durrant pointed out the impor-
tance of considering the change in entropy associated with changing from a single
exciton to two separated charges, by which the effective Coulomb capture radius is
estimated to be *4 nm at a typical donor-acceptor heterojunction [
64
]. This is
much shorter than the Onsager radius (r
C
= 14-19 nm) in organic materials at
room temperature as mentioned above. Interestingly, this effective Coulomb
capture radius is comparable to our estimations of the delocalization radius of
singlet excitons: singlet excitons with a radius of *4.3-6.7 nm in RR-P3HT
pristine films can be effectively dissociated into free polarons, while singlet ex-
citons with a radius of *3.2 nm in RRa-P3HT pristine films form bound radical
pairs. This correlation suggests that the separation distance of two charges at the
interface is closely related to the delocalization radius of singlet excitons. On the
other hand, Deibel and his coworkers have demonstrated that the efficient charge
dissociation can be explained by kinetic Monte Carlo simulations considering
delocalization of charge carriers within conjugated segments in polymer
chain [
65
]. This is also consistent with our findings of the different delocalization
radius of singlet excitons, because delocalized singlet excitons would convert to
delocalized polarons. We therefore conclude that the longer separation distance of
bound radical pairs[4 nm can promote the dissociation of bound radical pairs and
the formation of free polarons effectively, whereas the shorter separation distance
of bound radical pairs \4 nm causes in a significant loss due to the geminate
recombination of bound radical pairs at the interface. In addition, desirable phase-
separated structures in RR-P3HT:PCBM blend films can also promote the disso-
ciation of bound radical pairs effectively, whereas homogeneously mixed blend
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