Thiadiazole quinoxaline-based copolymers with ∼1.0 eV bandgap for ternary polymer solar cells

A new strong electron-deficient acceptor unit, 4,9-bis(5-bromothiophen-2-yl)-6,7-bis(5-dodecylthiophen-2-yl)-[1,2,5]thiadiazolo[3,4-g]quinoxaline (TQxT), is designed. Three TQxT based new conjugated polymers have been further developed by alternating with benzo[1,2-b:4,5-b′]dithiophene or naphtho[1,2-b:5,6-b′]difuran. Featuring ultra low bandgaps of 1.03-1.10 eV, these polymers exhibit absorption spectra beyond near-infrared (NIR) region. The polymer:PC₇₁BM (1:2) solar cells deliver a best power conversion efficiency (PCE) of 0.43%. The doping of 9 wt% PBDTT-TQxP into P3HT:PC₇₁BM (1:2) blend, however, leads to a highest PCE of 3.58% for the resultant ternary solar cells, corresponding to ∼22% improvement in comparison to 2.93% PCE for P3HT:PC₇₁BM binary cells.