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-substitut...