Effect of alkylthio side chain isomerization of nonfused electron acceptors for organic solar cells

Abstract

Side chain isomerization has shown significant influence on the intermolecular interaction and aggregation morphology of organic small molecule acceptors. In this work, two isomeric nonfused ring electron acceptors (NFREAs) of 2T2STT-F and 2T2TTS-F were designed and synthesized, in which two alkylthio side chains were pended into the 6-position of two thieno[3,2-b]thiophene (TT) units closer to the core for 2T2STT-F and the 3-position of TT closer to the end group for 2T2TTS-F, respectively. The influence of pending positions on photophysical and photovoltaic properties were studied. Compared with 2T2TTS-F, 2T2STT-F possesses redshifted absorption, stronger intermolecular packing and higher molecular crystallinity, and forms large aggregates when blending with polymer PM6, leading to an inferior power conversion efficiency (PCE) of 1.31%. PM6:2T2TTS-F based devices exhibit a higher PCE of 8.81%, which is mainly attributed to the appropriate morphology with more suitable phase separation and fibril network of the PM6:2T2TTS-F blend, less charge carrier recombination and efficient charge transport.