Revealingthe Position Effect of an Alkylthio SideChain in Phenyl-Substituted Benzodithiophene-Based Donor Polymerson the Photovoltaic Performance of Non-Fullerene Organic Solar Cells

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-b:4,5-b′]dithiophene (BDTP) and difluorobenzotriazole (FTAZ). The polymer based on the meta-position-alkylthiolated BDTP, named m-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 para-positioned structural isomeric analogue polymer (named p-PBDTPS–FTAZ), that is, m- and p-PBDTPS–FTAZ with the side chain structured as ethylhexyl- in the phenyl unit and hexyldecyl- in the FTAZ moiety. When blended with ITIC, m-PBDTPS–FTAZ showed a comparable crystallinity but more uniform morphology compared to that of p-PBDTPS–FTAZ. A high power conversion efficiency of 13.16% was achieved for m-PBDTPS–FTAZ:ITIC devices with a high open circuit voltage (VOC) of 0.95 V, which is higher than that of p-PBDTPS–FTAZ:ITIC devices (10.86%) with a VOC of 0.89 V. ...