A Medium Bandgap D–A Copolymer Based on 4‑Alkyl-3,5-difluorophenyl Substituted Quinoxaline Unit for High Performance Solar Cells

Development of high-performance donor–acceptor (D–A) copolymers has been indicated as a promising strategy to improve the power conversion efficiencies (PCEs) of organic solar cells (OSCs). In this work, a new medium bandgap conjugated D–A copolymer, HFAQx-T, based on 4,8-bis­(5-(2-ethylhexyl)­thiophen-2-yl)­benzo­[1,2-<i>b</i>:4,5-<i>b</i>′]­dithiophene (BDT-T) as donor unit, 4-alkyl-3,5-difluorophenyl substituted quinoxaline (HFAQx) as the acceptor unit, and thiophene as the spacer, was designed and synthesized. HFAQx-T is a well-compatible donor polymer; OSCs based on HFAQx-T exhibit excellent performance in both fullerene and fullerene-free based devices. The optimized conventional single junction bulk heterojunction (BHJ) OSCs of HFAQx-T:PC₇₁BM showed a PCE of 9.2%, with an open circut voltage (<i>V</i>_oc) of 0.9 V, a short circuit current (<i>J</i>_sc) of 14.0 mA cm^–2, and a fill factor (FF) of 0.74. Also, when blended with 3,9-bis­(2-methylene-(3-(1,1-dicyano­methylene)­indanone)-5,5,11,11-tetrakis­(4-hexylphenyl)­dithieno­[2,3-<i>d</i>:2′,3′-<i>d</i>′]-<i>s</i>-indaceno­[1,2-<i>b</i>:5,6-<i>b</i>′]-dithiophene (ITIC), the HFAQx-T-based device exhibited a PCE of 9.6%. HFAQx-T is among a few D–A copolymers that can deliver >9% efficiency in both fullerene and fullerene-free solar cells. This work demonstrates that the 4-alkyl-3,5-difluorophenyl substituted quinoxaline (Qx) is a promising electron-accepting building block in constructing ideal D–A copolymers for OSCs