Synthesis and solar cell applications of semiconducting polymers based on vinylene-bridged 5-alkoxy-6-fluorobenzo[c][1,2,5] thiadiazole (FOBTzE)

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 (J(sc)) and PCE.

Automated summary

A novel electron acceptor unit, FOBTzE, was designed and synthesized to improve the aggregation behavior and molecular orientation of polymer PFE4T. The introduction of alkoxy side chains and FOBTzE resulted in higher HOMO energy level and changed molecular orientation, allowing for the formation of a uniform blended film with PC61BM and enhanced carrier transport. However, the limited solubility of PFOE4T led to phase separation and restricted the short-circuit current density and power conversion efficiency.