Exciton and Charge Dynamics in Polymer Solar Cells Studied by Transient Absorption Spectroscopy - Organic Solar Cells: Materials and Device Physics - page 117

Fig. 5.11 a Transient
absorption spectra of RR-
P3HT:F8BT blend films
excited at 400 nm measured
at 0, 1, 10, 100, 1000 ps from
top to bottom. b Time
evolution of F8BT singlet
excitons (open circles) and
RR-P3HT singlet excitons
(closed circles). The rise and
decay curves are fitted with
the following equation:
DOD R (t) = A[1 - exp(-t/
s R )] ? B and DOD D (t) = A
exp(-t/s D )
(s R = s D = 0.3 ps). Adapted
with permission from [ 21 ].
Copyright 2011 Society of
Photo Optical
Instrumentation Engineers

(a)

6

4

2

0

600 800 1000 1200 1400 1600

Wavelength / nm

(b)

0 0.2 0.4 0.6 0.8 1

Time / ps

singlet excitons are generated with a rise constant of *0.3 ps. The prompt gen-
eration of RR-P3HT singlet excitons is ascribed to the direct excitation of RR-
P3HT at 400 nm. The delayed generation of RR-P3HT singlet excitons is ascribed
to the energy transfer from F8BT singlet excitons, because the rise time of RR-
P3HT singlet excitons is in agreement with the decay constant of F8BT singlet
excitons. Such rapid energy transfer within a picosecond suggests that all F8BT
singlet excitons are efficiently transferred into RR-P3HT domains without exciton
diffusion in the blend film. In other words, there are no large pure domains in the
blend: F8BT is molecularly dispersed in RR-P3HT domains and/or RR-P3HT is
dispersed in F8BT rich domains. This is probably because chloroform is so rapidly
evaporated that largely phase-separated pure domains cannot be formed. On the
basis of the Förster theory assuming point dipoles, such rapid energy transfer is
possible at a separation distance of *0.9 nm between F8BT and RR-P3HT.
Consequently, at least 60 % of RR-P3HT singlet excitons are located near the
interface of RR-P3HT/F8BT because of the efficient energy transfer from F8BT
singlet excitons. This would be beneficial for the subsequent charge separation but
the charge generation efficiency is actually not as high as that in RR-P3HT:PCBM
blends. This may be due to the difference in the interfacial structures [ 21 ].