Double Acceptor Block-Containing Copolymers with Deep HOMO Levels for Organic Solar Cells: Adjusting Carboxylate Substituent Position for Planarity.

A deep highest occupied molecular orbital (HOMO) level is a prerequisite for polymer donor material to boost the organic solar cells (OSCs) performance by achieving high open circuit voltage ( Voc). Abandoning the traditional concept of donor-acceptor (D-A) structure, two copolymers PBTZ-4TC and PBTZ-C4T based on acceptor1-π-acceptor2 (A1-π-A2) architecture, where thiophene as the bridge, the difluorinated benzotriazole (BTZ) as A1 unit alternating copolymerized with 4,4'-dicarboxylate-substituted difluorotetrathiophene (4TC) and 3,3'-dicarboxylate-substituted difluorotetrathiophene (C4T) as A2, respectively, are developed. Because of the double acceptor blocks with high electron affinity, both A1-π-A2 type copolymers possess the lower HOMO levels of 5.52-5.56 eV, which are lower than most D-A type donors. Polymer PBTZ-4TC and PBTZ-C4T have the same backbone but only differ with the position of carboxylate substituent on the A2 unit. Intriguingly, subtle optimizing the position of the carboxylate-substitute causes a significantly difference on the properties of the A1-π-A2 type copolymers. PBTZ-C4T with more planar geometry is demonstrated with better light absorption, higher crystallinity, more pronounced temperature-dependent aggregation effect, and favorable bulk heterojunction morphology but with slightly higher HOMO level and more emission energy loss relative to the PBTZ-4TC. The PBTZ-C4T device exhibits the higher power conversion efficiency (PCE) of 9.34% than the PBTZ-4TC-based one (8.75%). These results reveal that concept of A1-π-A2 type copolymers not only can afford more flexibility in tuning the energy levels to achieve the deep HOMO levels but also can provide a facial strategy to greatly enrich the types of polymer donors for high-performance OSCs.