Exciton and Charge Dynamics in Polymer Solar Cells Studied by Transient Absorption Spectroscopy - Organic Solar Cells: Materials and Device Physics - page 110

Fig. 5.5 Transient
absorption spectra of RR-
P3HT films measured at 0, 1,
10, 100, and 3000 ps from
top to bottom in each panel.
The excitation intensity is as
follows: a 15, b 30, c 60, and
d 120 lJcm -2 . The
excitation wavelength is
400 nm. Adapted with
permission from [ 18 ].
Copyright 2009 American
Chemical Society

(a)

0.04

0.02

(b)

0

0.05

(c)

0

0.1

(d)

0

0.2

0.1

0

500 1000 1500

Wavelength / nm

be described in detail Sect. 5.6.1 , there are spectral overlaps among P3HT singlet
excitons, P3HT polarons, and PCBM radical anions at an early time stage. On the
other hand, as shown in Fig. 5.6 b, no change in transient absorption spectra is
observed for RRa-P3HT:PCBM blend films on a time scale of microseconds,
suggesting that transient species remains the same. The large absorption band at
900 nm and the small and sharp absorption band at 1030 nm are safely ascribed to
P3HT hole polaron and to PCBM radical anion, respectively. Both bands exhibit
power-law decay dynamics with an exponent of 0.35 over a long time range up to
milliseconds. The power-law decay is characteristic of the bimolecular recombi-
nation of long lived, dissociated charge carriers. The subunit value for the expo-
nent suggests trap-limited bimolecular recombination of polarons in an
energetically disordered matrix while the exponent is unity for trap-free bimo-
lecular recombination [ 31 ].