Static and Dynamic Disorder of Charge Transfer States Probed by Optical Spectroscopy

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Kahle, Frank-Julian ; Rudnick, Alexander ; Wedler, Stefan ; Saxena, Rishabh ; Ammenhäuser, Robin ; Scherf, Ullrich ; Bagnich, Sergey ; Bässler, Heinz ; Köhler, Anna:
Static and Dynamic Disorder of Charge Transfer States Probed by Optical Spectroscopy.
In: Advanced Energy Materials. Bd. 12 (2022) Heft 15 . - No. 2103063.
ISSN 1614-6840
DOI der Verlagsversion: https://doi.org/10.1002/aenm.202103063

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Abstract

Since the key role of charge transfers (CT) states has been identified for organic solar cells (OSCs), research into their properties is a timely topic. Conventionally, their absorption and emission spectra are described in terms of Marcus’ electron transfer theory. This is a single site approach with the essential parameter being the reorganization energy. Thus, it ignores ensemble effects, notably the role of static disorder that is inevitably present in a spin-coated OSC film. Here time dependent photoluminescence spectroscopy is applied on blends of the polymeric donor MeLPPP with either the non-fullerene acceptor SF-PDI2 or with PC61BM within a temperature range from 295 to 5 K. The authors monitor how initially excited singlet states are converted to emissive CT states. Concomitantly, emission from residual singlets on the acceptor is observed rather than hybrid CT-states. The role of spectral diffusion in this process is discussed. From the temperature and time dependent linewidths of absorption, fluorescence, and CT emission, the static and dynamic contributions to the total disorder are inferred. In both blends, at 295 K, the contribution of static disorder is comparable to the dynamic disorder.