Dithienosilole-benzothiadiazole-based ternary copolymers with a D1-A-D2-A structure for polymer solar cells

A series of narrow band gap conjugated copolymers comprising two electron-rich (donor, D) and one electron-deficient (acceptor, A) moieties regularly alternating along the polymer backbone were designed and synthesized. The ternary copolymers with the repeating unit in a D-1-A-D-2-A manner were constructed by copolymerizing a bisstannyled-D-1 (D-1 = cyclopentadithiophene or benzodithiophene-derivatives) and a dibromo-monomer (Br-A-D-2-A-Br, D-2 = dithienosilole, A = benzothiadiazole) through a palladium-catalyzed Stille polymerization. It was recognized that the optical properties, frontier molecular orbital energy levels and the photovoltaic performance of the resulting copolymers can be influenced by modifying the copolymerized D-1 moiety. By carefully optimizing the electron-donating behaviours and substitutions of the D-1 unit, bulk-heterojunction solar cells with a power conversion efficiency of 4.11% were achieved based on an inverted device configuration of ITO/PFN-OX/PBDT-O-ADA: PC71BM/MoO3/Al. These results demonstrated that the construction of regularly alternating narrow band gap conjugated ternary copolymers can be an effective strategy for the development of electron-donating materials for polymer solar cells.