D. Gehrig, S. Roland, I.A. Howard, V. Kamm, H. Mangold, D. Neher, F. Laquai,
J. Phys. Chem. C, 118 (35), 20077-20085, (2014)
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.