Revealing the Position Effect of an Alkylthio Side Chain in Phenyl-Substituted Benzodithiophene-Based Donor Polymers on the Photovoltaic Performance of Non-Fullerene Organic Solar Cells

Abstract

In this work, position effects of an alkylthio side chain were investigated by designing and synthesizing two copolymers based on a phenyl-substituted benzo[1,2-<i>b</i>:4,5-<i>b</i>']dithiophene (BDTP) and difluorobenzotriazole (FTAZ). The polymer based on the <i>meta</i>-position-alkylthiolated BDTP, named <i>m</i>-PBDTPS-FTAZ, showed a relatively broader bandgap (2.00 vs 1.96 eV) and lower highest occupied molecular orbital (HOMO) energy level (-5.40 vs -5.32 eV) than its <i>para</i>-positioned structural isomeric analogue polymer (named <i>p</i>-PBDTPS-FTAZ), that is, <i>m</i>- and <i>p</i>-PBDTPS-FTAZ with the side chain structured as ethylhexyl- in the phenyl unit and hexyldecyl- in the FTAZ moiety. When blended with ITIC, <i>m</i>-PBDTPS-FTAZ showed a comparable crystallinity but more uniform morphology compared to that of <i>p</i>-PBDTPS-FTAZ. A high power conversion efficiency of 13.16% was achieved for <i>m</i>-PBDTPS-FTAZ:ITIC devices with a high open circuit voltage (<i>V</i>_OC) of 0.95 V, which is higher than that of <i>p</i>-PBDTPS-FTAZ:ITIC devices (10.86%) with a <i>V</i>_OC of 0.89 V. Therefore, <i>m</i>-BDTPS could be an effective donor unit to construct high-efficiency polymers due to its effectively decreased HOMO energy level of polymers while still maintaining good molecular stacking.