Wide bandgap dithienobenzodithiophene-based π-conjugated polymers consisting of fluorinated benzotriazole and benzothiadiazole for polymer solar cells

Abstract

Two wide bandgap donor–acceptor type π-conjugated polymers based on dithienobenzodithiophene as the donor unit and difluorobenzotriazole or difluorobenzothiadiazole as the acceptor unit were designed and synthesized. The copolymer based on difluorobenzothiadiazole exhibited more pronounced aggregations in chlorobenzene solutions than that of the copolymer based on difluorobenzotriazole. Both copolymers exhibited relatively wide bandgaps with deep highest occupied molecular orbitals, leading to high open circuit voltages of over 0.95 V for the fabricated polymer solar cells. These copolymers exhibited quite analogous hole mobility of about 0.1 cm² V⁻¹ s⁻¹ as measured by organic field effect transistors. Bulk heterojunction polymer solar cells based on these copolymers as the electron-donating materials and PC₇₁BM as the electron-accepting material exhibited relatively high performance, with the best power conversion efficiency of 7.45% attained for the copolymer based on the difluorobenzothiadiazole unit. These results demonstrated that the constructed wide bandgap π-conjugated polymers can be promising candidates for the fabrication of high performance solar cells with multi-junction architectures.