Exploiting Novel Unfused-Ring Acceptor for Efficient Organic Solar Cells with Record Open-Circuit Voltage and Fill Factor

Unfused-ring acceptors (UFAs) show bright application prospects in organic solar cells (OSCs) thanks to their easy synthesis, low cost, and good device performance. The selection of central-core building block and suitable side chain are the key factors to achieve high-performance UFAs. Current tremendous endeavors for the development of UFAs mainly concentrate on obtaining higher short-circuit current density (J(sc)), albeit accompanied by low open-circuit voltage (V-oc) and modest fill factor (FF). Herein, two novel A-D-A '-D-A type UFAs (BTCD-IC and BTCD-2FIC), which have the same new electron-withdrawing central-core dithieno[3 ',2 ':3,4;2 '',3 '':5,6]-benzo[1,2-c][1,2,5]thiadia-zole (DTBT) and cyclopentadithiophene unit (CPDT, substituted by 2-butyl-1-octyl alkyl chain) coupling with different terminals, were designed and synthesized. Two UFAs showed strong and broad light absorption in the wavelength range of 300-850 nm owing to the strong intramolecular charge transfer effect favorable by DTBT core. Compared with BTCD-IC, BTCD-2FIC with F-containing terminal group exhibited higher molar extinction coefficient, lower energy level, higher charge mobility, stronger crystallinity, more ordered molecular stacking, and better film morphology. As a result, when blended with donor polymer PBDB-T (poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b ']dithiophene)-co-(1,3-di(5-thiophene-2-yl)-5,7-bis(2-ethylhexyl)benzo[1,2-c:4,5-c ']-dithiophene-4,8-dione)]), the BTCD-2FIC-based OSC achieved a superior power conversion efficiency (PCE) of 11.32 %, with a high V-oc of 0.85 V, a J(sc) of 18.24 mA cm(-2), and a FF of 73 %, than BTCD-IC-based OSC (PCE=8.96 %). Impressively, the simultaneously enhanced V-oc and FF values of the PBDB-T:BTCD-2FIC device were the highest values of the A-D-A '-D-A-type UFAs. The results demonstrate the application of electron-withdrawing DTBT central-core unit in efficient UFAs provides meaningful molecular design guidance for high-performance OSCs.