New Dithiazole Side Chain Benzodithiophene Containing D–A Copolymers for Highly Efficient Nonfullerene Solar Cells

To examine the effects of pendent alkyl chain on the benzodithiophenedione (BDD) acceptor in the donor–acceptor (D–A) conjugated copolymers, two D–A copolymers with same thiazole ring substituted benzo[1,2‐b:4,5‐b′]dithiophene (BDTTz) and different acceptors BDD with two pendent alkyl units PBDTTz‐BDD and one pendent alkyl unit in benzo[1,2‐b:4,5‐c′]dithiophene‐4,8‐dione (BDTD) PBDTTz‐BDTD are synthesized. There is small effect on the optical properties but significant effect on the intermolecular π–π stacking, charge transport, and photovoltaic performance. PBTTz‐BDTD (with one pendent alkyl chain) on the BDD exhibits smaller π–π stacking distance as compared to PBDTTz‐BDD (with two pendent alkyl chains). The balanced charge carrier mobilities for the PBDTTZ‐BDTD based blends than those for PBDTTz‐BDD based blends are beneficial for suppressing the recombination processes, resulting in higher short circuit current and fill factor for the polymer solar cells (PSCs) based on PBDTTz‐BDTD. Combined with BThIND or Y6 as acceptor, PBDTTZ‐BDTD based PSCs show power conversion efficiency of 12.85% and 14.08%, respectively, which are larger than that for PBDTTz‐BDD counterparts (10.46% and 10.90% for BThIND and Y6, respectively). Thus, the photovoltaic performance of the D–A copolymer donors can be regulated through the slight variations of the pendent alkyl chains in the BDD acceptor unit.