Novel benzo[1,2-b:4,5-b']difuran-based copolymer enables efficient polymer solar cells with small energy loss and high VOC

Abstract

Abstract Benzo[1,2-b:4,5-b']difuran (BDF)-based copolymers have displayed some success in fabricating efficient polymer solar cells (PSCs). However, the PSCs suffer from significant energy loss (Eloss) that results in low open-circuit voltage (Voc) and greatly hinders the further improvement of photovoltaic performance of PSCs. Herein, we designed and synthesized a novel copolymer PBDFP-Bz based on BDF and benzotriazole, where the BDF unit was modified by (2-ethylhexyl)(2-fluorophenyl)sulfane side chain. PBDFP-Bz displays large coplanar structure and deep-lying highest occupied molecular orbital energy level of −5.50 eV. PSCs based on PBDFP-Bz as donor and small molecule ITIC as acceptor achieved a power conversion efficiency (PCE) of 11.10% with Voc of 0.97 V and Eloss of 0.60 eV. Moreover, when adopting small molecule IT-M to replace ITIC, PBDFP-Bz:IT-M based PSCs delivered a larger Voc of 1.02 V and a smaller Eloss of 0.57 eV due to the shallower lowest unoccupied molecular orbital energy level of IT-M versus that of ITIC. Importantly, a remarkable PCE of 12.93% was also obtained for PBDFP-Bz:IT-M based PSCs, which is among attractive photovoltaic performance of PSCs containing BDF-based copolymers. Our work not only suggests that PBDFP-Bz is a promising copolymer for efficient PSCs fabrication, but also sheds light on the design rule that managing the molecular structure of photovoltaic material and its energy level is an effective strategy to fabricate highly efficient PSCs.