Impact of an electron withdrawing group on the thiophene-fused benzotriazole unit on the photovoltaic performance of the derived polymer solar cells

In this work, two new donor-acceptor (D-A) copolymers (P32 and P33) for non-fullerene polymer solar cells (PSCs) have been synthesized based on a thiophene-fused benzotriazole (BTAZT) unit and thiophene substituted benzodithiophene (BDTT) unit, in which two different electron-withdrawing groups (i.e., fluorine and ester groups) have been introduced at the side of BTAZT unit, respectively. It is found that P32 with a long alkyl chain ester group shows a more redshifted intramolecular charge transfer absorption, and also exhibits a better complementary absorption spectrum with a small molecular acceptor based on the dithienocyclopentathieno [3,2-b]thiophene core end-capped with 1,1-dicyanomethylene-3-indanone unit (IHIC) than P33. Moreover, P32:IHIC blended film shows higher and more balanced hole/electron charge mobilities as well as better morphologies with more suitable phase separation scale for efficient charge separation and transport than P33:IHIC blended film. Thus, the P32:IHIC based PSC device exhibits the higher power conversion efficiency of 8.35% with higher short-circuit current density (J(SC) = 16.77 mA cm(-2)) and fill factor (FF = 64.89%) compared with the P32:IHIC based PSC device, indicating that the different electron-withdrawing side groups at BTAZT unit exerts a significant influence on the properties of the polymers as well as the photovoltaic performances of the PSCs.