Synthesis and Photovoltaic Properties of Donor−Acceptor Copolymers Based on 5,8-Dithien-2-yl-2,3-diphenylquinoxaline

Four donor−acceptor (D−A) type copolymers, namely, poly{9,9-dioctylfluorene-2,7-diyl-alt-5,8-dithien-2-yl-2,3-diphenylquinoxaline-5′,5′′-diyl} (PF-DTQx), poly{N-[1-(2′-ethylhexyl)-3-ethylheptanyl]carbazole-2,7-diyl-alt-5,8-dithien-2-yl-2,3-diphenylquinoxaline-5′,5′′-diyl} (PC-DTQx), poly{5,11-di(1-decylundecyl)indolo[3,2-b]carbazole-3,9-diyl-alt-5,8-dithien-2-yl-2,3-diphenylquinoxaline-5′,5′′-diyl} (PIC-DTQx), and poly{N-[1-(2′-ethylhexyl)-3-ethylheptanyl]-dithieno[3,2-b:2′,3′-d]pyrrole-2,6-diyl-alt-5,8-dithien-2-yl-2,3-diphenylquinoxaline-5′,5′′-diyl} (PDTP-DTQx), were synthesized by Suzuki or Stille coupling reactions. By changing the donor segment, the band gaps and energy levels of these 5,8-dithien-2-yl-2,3-diphenylquinoxaline (DTQx)-based polymers could be finely tuned. PDTP-DTQx exhibited the narrowest band gap of 1.56 eV and the absorption edge extended to 770 nm. We investigated bulk heterojunction type polymer solar cells (PSCs) based on these copolymers as the electron donor materials, and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) or [6,6]-phenyl C71 butyric acid methyl ester (PC70BM) as the acceptor. The power conversion efficiency (PCE) of the PSCs was in the range of 1.17−3.23% under AM 1.5 illumination (100 mW/cm2).