Development of new nonacyclic small-molecule acceptors involving two benzo[1,2-b:4,5-b′]dithiophene moieties for efficient polymer solar cells

Abstract Benzo[1,2-b:4,5-b′]dithiophene (BDT) is demonstrated as a very effective building block to construct acceptor-donor-acceptor (A-D-A) small-molecule acceptors (SMAs) involving one BDT unit for high-performance polymer solar cells (PSCs). Herein, we successfully developed two new A-D-A SMAs, namely IDBDT-O and IDBDT-T, with the same BDT-fused nonacyclic indacenobis(benzo[1,2-b:4,5-b′]dithiophene) (IDBDT) central D unit and 1,1-dicyanomethylene-3-indanone (IC) terminal A group, but different side chains (alkoxyl vs alkylthiophene) attached on the BDT units. The IDBDT central core is formed by the fusion of one indacene segment and two BDT moieties, which obviously differs from the previously studied BDT-fused SMAs. Compared to IDBDT-T with alkylthiophene side chains, IDBDT-O with alkoxyl side chains exhibits a broader absorption, smaller band gap, and higher-lying HOMO energy level. Moreover, these IDBDT-based SMAs have upshifted LUMO energy levels relative to the reported BDT-fused SMAs, which is beneficial to increase open-circuit voltage (Voc). Bulk-heterojunction PSCs fabricated from polymer donor PBDB-T and IDBDT-T achieve the best power conversion efficiency (PCE) of 7.14% with a high Voc of 0.96 V, a short-circuit current density (Jsc) of 15.20 mA cm−2, much larger than a PCE of 1.83%, a Voc of 0.89 V and a Jsc of 5.21 mA cm−2 for the counterpart of PBDB-T:IDBDT-O. The enhanced device performance could be ascribed to higher and more balanced carrier mobility, more efficient exciton dissociation, and weaker bimolecular recombination as well as better active layer morphology in the IDBDT-T-based PSCs relative to IDBDT-O. Our results provide a new molecular design strategy to exploit BDT-fused A-D-A SMAs for high-performance PSCs.