To improve the strong aggregation behavior and molecular orientation of the previously reported polymer PFE4T with vinylene-bridged 5,6-difluorobenzo[c][1,2,5]thiadiazole (FBTzE), we designed and synthesized a vinylene-bridged 5-alkoxy-6-fluorobenzo[c][1,2,5]thiadiazole (FOBTzE) moiety as a novel electron acceptor unit and its copolymer PFOE4T. By installing a strong electron-donating alkoxy group into the FBTzE framework instead of an electron-withdrawing fluorine atom, the highest occupied molecular orbital (HOMO) energy level of the resulting polymer PFOE4T was found to be ca. 0.1 eV higher than that of the previously reported polymer PFE4T but comparable to that of typical difluorobenzothiadiazole-based polymers. On the other hand, the introduction of alkoxy side chains reduced the strong aggregation tendency and changed the molecular orientation of the polymers from edge-on to bimodal orientation, providing a uniform polymer blended film with PC61BM and enhancing carrier transport. These results indicate that the fabricated PFOE4T/PC61BM-based solar cells exhibited a power conversion efficiency of 4.52% with a high fill factor (FF) of 0.68. However, because PFOE4T still has strong aggregation and low solubility, the PFOE4T/PC61BM blended film formed a large phase separation, resulting in limited short-circuit current density (Jsc) and PCE.
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This study was supported in part by Grant-in-Aid for Young Scientists (No. 19K15650) from the Japan Society for the Promotion of Science, Okayama Prefecture Industrial Promotion Foundation, and the Yakumo Foundation for Environmental Science. The GIWAXS experiments were performed at BL13XU and BL46XU of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposals 2019A1765 and 2022A1656). We are grateful to Prof. Itaru Osaka, and Dr. Masahiko Saito (Hiroshima University), as well as Dr. Tomoyuki Koganezawa (JASRI), for measurements of GIWAXS images; Prof. Koichi Mitsudo and Prof. Seiji Suga (Okayama University) for CV measurements; Prof. Tsutomu Ono and Prof. Takaichi Watanabe (Okayama University) for DSC measurements; Prof. Naoshi Ikeda (Okayama University) for AFM images; Prof. Yoshihiro Kubozono (Okayama University) for thickness measurements; and Megumi Kosaka and Motonari Kobayashi at the Department of Instrumental Analysis, Advanced Science Research Center, Okayama University, for elemental analysis measurements. We also thank the SC-NMR Laboratory of Okayama University for the NMR spectral measurements.
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Mori, H., Asanuma, Y., Hosogi, R. et al. Synthesis and solar cell applications of semiconducting polymers based on vinylene-bridged 5-alkoxy-6-fluorobenzo[c][1,2,5]thiadiazole (FOBTzE). Polym J 55, 405–415 (2023). https://doi.org/10.1038/s41428-022-00706-z