A wide bandgap conjugated polymer donor based on alkoxyl-fluorophenyl substituted benzodithiophene for high performance non-fullerene polymer solar cells

Abstract

A new wide bandgap (WBG) conjugated polymer PFOPB based on a fluorinated-alkoxyphenyl benzodithiophene donor unit and a benzodithiophene-4,8-dione acceptor unit was designed and synthesized for application as a donor in non-fullerene (NF) polymer solar cells (PSCs). In order to investigate the effect of side chain fluorination on the photovoltaic performance, a corresponding non-fluorinated polymer POPB was also synthesized as a comparison material. Compared to POPB with a bandgap of 1.81 eV, a highest occupied molecular orbital (HOMO) energy level of −5.38 eV, an extinction coefficient of 5.31 × 10⁴ cm⁻¹ at 577 nm and a hole mobility of 3.96 × 10⁻⁴ cm² V⁻¹ s⁻¹, the fluorinated polymer PFOPB possesses a wider bandgap of 1.86 eV, a deeper HOMO level of −5.50 eV, a higher extinction coefficient of 8.45 × 10⁴ cm⁻¹ at 612 nm and a higher hole mobility of 1.51 × 10⁻³ cm² V⁻¹ s⁻¹. The PFOPB:IT-4F-based PSCs exhibit a much higher power conversion efficiency (PCE) of 11.7% than POPB:IT-4F-based devices (6.2%). These results suggest that side-chain fluorination is an effective strategy to improve the optoelectronic properties of WBG polymers in NF-PSCs.