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

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 ( V OC ) of 0.95 V, which is higher than that of p -PBDTPS–FTAZ:ITIC devices (10.86%) with a V OC of 0.89 V. Therefore, m -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.