Exciton and Charge Dynamics in Polymer Solar Cells Studied by Transient Absorption Spectroscopy - Organic Solar Cells: Materials and Device Physics - page 112

Fig. 5.7 a Transient
absorption spectra of RR-
P3HT:PCBM (50:50 w/w)
blend films after thermal
annealing (solid lines)
measured at 0, 1, 10, 100, and
3000 ps (from top to bottom).
The broken line represents
transient absorption spectrum
of an RR-P3HT pristine film
(broken line) measured at
0 ps. The transient absorption
is corrected for variation in
the absorption at an excitation
wavelength of 400 nm.
b Transient absorption
spectra of RR-P3HT:PCBM
(50:50 w/w) blend films after
thermal annealing excited at
400 nm measured at 0.5, 1, 2,
5, 10, 20 and 100 ls (from
top to bottom). The inset
shows transient absorption
decays at 1000 (upper) and
700 nm (lower). Adapted
with permission from [ 19 ,
20 ]. Copyright 2010
American Chemical Society

(a)

40

30

20

10

0

10 -4

(b)

0.5

10 -5 0.4

10 -6

0.3

10 -5 10 -4

0.2 Time / s

0.1

0

600 800 1000 1200 1400 1600

Wavelength / nm

In the two cases described above, polarons are efficiently generated from singlet
excitons and therefore can be separately observed on a time scale of microseconds.
In some cases, however, a triplet exciton band overlaps with a polaron band. Here,
we show an example of simultaneous observation of triplet excitons and polarons.
Figure 5.8 a shows transient absorption spectra of a blend film of poly[(4,4 0 -dide-
cyl[2,2 0 -bithiophene]-5,5 0 -diyl)-1,4-phenylene] (PT 10 PhT 10 ) and PCBM. A large
absorption band is observed at 700 nm with a shoulder at around 850 nm at 10 ls
after the laser excitation. The absorption spectrum varies with time, with the
absorption peak shifting from 700 at 10 lsto*900 nm for time delays longer than
100 ls. This spectral change suggests that there are two different transient species in
the blend film. Figure 5.8 b shows transient absorption decays of PT 10 PhT 10 :PCBM
blend films monitored at 700 nm. This decay dynamics can be fitted with the sum of
a single exponential function and a power-law equation. As shown in the figure, the
exponential decay component is quenched under oxygen atmosphere, and therefore
ascribed to triplet exciton. On the other hand, the exponent of the power-law decay
remains the same even under oxygen atmosphere, and therefore is ascribed to
polymer polarons. In conclusion, for the assignment of polarons, it is essential to
observe polarons separately from singlet and triplet excitons. Singlet excitons can be
excluded by transient absorption measurements on a time scale of microseconds. In
order to distinguish between triplet excitons and polarons, it is useful to analyze the