Exciton and Charge Dynamics in Polymer Solar Cells Studied by Transient Absorption Spectroscopy - Organic Solar Cells: Materials and Device Physics - page 108

Figure 5.4 a shows transient absorption spectra of RRa-P3HT pristine films.
Immediately after the laser excitation, a large absorption band is observed at
around 1000 nm. This band is ascribable to singlet exciton, because the decay
constant is consistent with the lifetime of fluorescence measured by the time-
correlated single photon counting (TC-SPC) method. Note that the averaged decay
constant measured by transient absorption was typically shorter than the lifetime
measured by the TC-SPC method because singlet-singlet exciton annihilation is
involved in the transient absorption measurement with excitation intensity much
higher than that in the TC-SPC measurement. Thus, a longer decay fraction
independent of the excitation intensity should be extracted by changing the
excitation intensity. If it is in agreement with the fluorescence lifetime, the initial
product can be safely assigned to singlet exciton.
As shown in Fig. 5.4 a, the singlet exciton band at around 1000 nm disappears
at 100 ps after the laser excitation, and instead a small absorption band is observed
at around 800 nm. This is indicative of the interconversion from singlet to triplet
excitons. Figure 5.4 b shows transient absorption spectra of RRa-P3HT pristine
films on a time scale of microseconds, which is similar to the long-lived species
observed on a time scale of nanoseconds mentioned above. As shown in the inset
to the figure, the transient signal decays monoexponentially with a lifetime of 7 ls

Fig. 5.4 a Transient
absorption spectra of
RRa-P3HT films excited at
400 nm measured at 0, 1, 10,
100, and 3000 ps from top to
bottom. b Transient
absorption spectra of RRa-
P3HT films excited at
450 nm measured at 0.5, 2, 4,
6, and 10 ls from top to
bottom. The inset shows the
transient absorption decay at
850 nm under Ar and O 2
atmosphere. Adapted with
permission from [ 18 ].
Copyright 2009 American
Chemical Society

(a)

40

30

20

10

0

10 -3

(b)

Ar

10 -4

1 O 2

10 -5

0 10 20

μ s

0.5

Time /

0

500 1000 1500

Wavelength / nm