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Fig. 5.15 Intensity
dependence of the simulated
transient absorption decays
for a the monomolecular
recombination (the first order
reaction) and b the
bimolecular recombination
(the second order reaction). In
the first-order reaction, the
lifetime is independent of the
initial concentration n
0
. In the
second-order reaction, the
half-life is dependent on n
0
:it
is half at twice the initial
concentration
10
2
(a)
n
0
10
1
10
0
10
-1
10
-2
10
2
n
0
(b)
10
1
10
0
0
1
2
3
4
5
Time (a.u.)
microseconds as shown in Fig.
5.6
b. In other words, the charge dissociation effi-
ciency is as low as g
CD
= 30 % in RRa-P3HT:PCBM blend films.
In RR-P3HT:PCBM blend films, there are two pathways for polaron generation:
one is prompt generation (\100 fs) at the interface and the other is delayed
generation (*10 ps) after the exciton migration to the interface. Here, we focus on
the nanosecond dynamics of two polarons bands: the delocalized polarons at
700 nm and localized polarons at 1000 nm. This assignment is consistent with
previous studies on P3HT pristine films where the 700 and 1000 nm bands are
ascribed to interchain delocalized polarons and intrachain localized polarons,
respectively [
13
,
29
,
30
]. As shown in Fig.
5.18
, the decay dynamics of the two
bands is dependent on the excitation intensity at higher excitation intensities,
indicating the bimolecular recombination of free polarons. We therefore conclude
that all of the polarons at 700 and 1000 nm are ascribed to dissociated free po-
larons on a time scale of nanoseconds. In contrast to RRa-P3HT:PCBM blends, no
decay is observed for RR-P3HT:PCBM blends even at lower excitation intensities.
In other words, the charge dissociation efficiency is estimated to be as high as
*100 % for these two polarons. In addition, without going into detail, the other
polaron in amorphous domains exhibits a charge dissociation efficiency of 38 %
before the thermal annealing, which is similar to that observed for
RRa-P3HT:PCBM blends, and 69 % after the thermal annealing. Consequently,
the overall charge dissociation efficiency is as high as g
CD
= 80 % before the
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