Efficiency-Limiting
Processes in Low-Bandgap Polymer:Perylene Diimide Photovoltaic Blends
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Abstract
The charge generation and recombination
processes following photoexcitation of a low-bandgap polymer:perylene
diimide photovoltaic blend are investigated by transient absorption
pump–probe spectroscopy covering a dynamic range from femto-
to microseconds to get insight into the efficiency-limiting photophysical
processes. The photoinduced electron transfer from the polymer to
the perylene acceptor takes up to several tens of picoseconds, and
its efficiency is only half of that in a polymer:fullerene blend.
This reduces the short-circuit current. Time-delayed collection field
experiments reveal that the subsequent charge separation is strongly
field-dependent, limiting the fill factor and lowering the short-circuit
current in polymer:PDI devices. Upon excitation of the acceptor in
the low-bandgap polymer blend, the PDI exciton undergoes charge transfer
on a time scale of several tens of picoseconds. However, a significant
fraction of the charges generated at the interface are quickly lost
because of fast geminate recombination. This reduces the short-circuit
current even further, leading to a scenario in which only around 25%
of the initial photoexcitations generate free charges that can potentially
contribute to the photocurrent. In summary, the key photophysical
limitations of perylene diimide as an acceptor in low-bandgap polymer
blends appear at the interface between the materials, with the kinetics
of both charge generation and separation inhibited as compared to
that of fullerenes