Exciton and Charge Dynamics in Polymer Solar Cells Studied by Transient Absorption Spectroscopy - Organic Solar Cells: Materials and Device Physics - page 114

Fig. 5.9 Transient
absorption spectra of a
polystyrene film doped with
TMPD(20 wt %) and
PCBM(30 wt %) at 1, 2, and
10 ps after the laser
excitation at 400 nm. The
inset shows the transient
decays at 600 nm (solid line)
and 1050 nm (broken line).
Reprinted with permission
from [ 17 ]. Copyright 2008
Wiley-VCH Verlag GmbH &
Co. KGaA

15

10 -2

10 -3

10

10 -4

1 10 100

Time / ps

5

0

600 800 1000 1200

Wavelength / nm

reported for the oxidation product of TMPD called Wurster's Blue [ 32 ], and is
therefore safely assigned to the TMPD radical cation. As shown in the inset to the
figure, both bands exhibit the power-law decay dynamics with the same exponent on
a longer time scale ([10 ps), indicating the bimolecular recombination without
other decay pathways. In other words, no other transient species such as singlet and
triplet excitons contributes to the transient absorption spectra. Thus, the absorption
band at 1020 nm is assigned to the PCBM radical anion, which is consistent with that
of radical anions of other fullerene derivatives: a C 60 radical anion (1,080 nm), a
methanofullerene radical anion (1040 nm), and a fulleropyrrolidine radical anion
(1010 nm) [ 33 ]. Furthermore, the molar absorption coefficient of the PCBM radical
anion can be evaluated to be e = 6000 M -1 cm -1 at 1020 nm on the basis of that of
the TMPD radical cation (e = 12000 M -1 cm -1 )[ 32 , 34 ]. In conclusion, it is useful
to employ known donor or acceptor materials for the assignment of unknown charge
carriers. Note that it is important to analyze the spectrum and the dynamics carefully
to confirm that there is no contribution of other species.

5.5 Exciton Dynamics

Here, we focus on the dynamics of singlet and triplet excitons mainly in pristine
polymer films [ 18 ] and also describe the energy transfer in a polymer/polymer
blend [ 21 ].