Ultrafast Dynamics Group
Frédéric Laquai's Group

Efficiency-Limiting Processes

Efficiency-Limiting Processes in Low-Bandgap Polymer:Perylene Diimide Photovoltaic Blends

Efficiency-Limiting Processes in Low-Bandgap Polymer:Perylene Diimide Photovoltaic Blends
Dominik W. Gehrig, Steffen Roland, Ian A. Howard, Valentin Kamm, Hannah Mangold, Dieter Neher, and Frédéric Laquai
J. Phys. Chem. C2014118 (35), pp 20077–20085
DOI: 10.1021/jp503366m
Dominik W. Gehrig, Steffen Roland, Ian A. Howard, Valentin Kamm, Hannah Mangold, Dieter Neher, Frédéric Laquai
Perylene Diimide, Charge Generation, Charge Recombination, Transient Absorption Spectroscopy
2014
Abstract Image 

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.