Efficient polymer solar cells based on a new quinoxaline derivative with fluorinated phenyl side chain

A novel donor-π-acceptor (D-π-A) type polymer of PBDTT-DTFPQx composed of a medium electron-donating 5,8-dialkylthienyl substituted benzo[1,2-b:4,5-b′]dithiophene (BDTT) moiety and a strong electron-accepting 4-fluorophenyl substituted 6,7-dioctyloxyquinoxaline (FPQx) with thiophene π-bridge units was synthesized and characterized, as a donor material for polymer solar cells (PSCs). This polymer exhibits a low optical bandgap of 1.66 eV with an absorption onset of 745 nm, a low-lying HOMO energy level of -5.52 eV, and a hole mobility of 5.05 × 10^-4 cm² V^-1 s^-1. Compared to the reported analogues, PSCs based on PBDTT-DTFPQx/PC₇₁BM demonstrated an outstanding fill factor (FF) value. With an optimized blend ratio of PBDT-TFQ:PC₇₁BM (1:4, w/w), a high power conversion efficiency (PCE) of 7.2% was obtained, with an open-circuit voltage (V_oc) of 0.87 V, a short-circuit current (J_sc) of 11.4 mA cm^-2, and a FF of up to 73% under AM 1.5G irradiation. The results demonstrate that the introduction of two fluoride atoms onto 4-positions of the phenyl group at the quinoxaline unit by side-chain engineering into the BDTT-alt-DTQx type polymers would be a feasible approach to improve photovoltaic properties in PSCs.