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-2yObenzo[1,2-b:4,5-bldithiophene (BDT-T) as donor unit, 4-alkyl3,5-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 FIFA.Qx-T:PC71BM showed a PCE of 9.2%, with an open circut voltage (1/,) of 0.9 V, a short circuit current (Jj 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-hexylpheny1)dithieno[2,3-d:2',3'-dl-s-indaceno[1,2-b:5,6-1/1-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.