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

• Published in: Advanced energy materials Vol. 12; no. 15
• Main Authors: Kahle, Frank‐Julian, Rudnick, Alexander, Wedler, Stefan, Saxena, Rishabh, Ammenhäuser, Robin, Scherf, Ullrich, Bagnich, Sergey, Bässler, Heinz, Köhler, Anna
• Format: Journal Article
• Language: English
• Published: 01.04.2022
• Subjects: donor–acceptor blends; inhomogeneous line broadening; non‐fullerene acceptors; photoluminescence‐linewidth; spectral diffusion
• ISSN: 1614-6832; 1614-6840
• DOI: 10.1002/aenm.202103063

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. Static and dynamic disorder characterize the electronic solar cell properties of charge transfer states in organic donor acceptor blends. Time‐dependent luminescence spectroscopy allows the identification of the role of spectral diffusion in the acceptor phase that can lead to an overestimation of dynamic disorder. Thus static disorder can be a major contribution, requiring consideration when analyzing charge transfer states and their dynamics.