Enhancement of photovoltaic performance by increasing conjugation of the acceptor unit in benzodithiophene and quinoxaline copolymers

Copolymers based on benzodithiophene and quinoxaline, represented by 4,8-bis(5-(3,4,5-tris(octyloxy)phenyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene (TOBDT) and 2,3-diphenyl-5,8-di(thiophen-2-yl)quinoxaline (TQ1) or 10,13-bis(4-(2-ethylhexyl)thiophen-2-yl)dibenzo[a,c]phenazine (TQ2), were synthesized via a Stille coupling reaction. By increasing the conjugation in the TQ2 unit, the polymer based on TQ2 showed a narrower band gap (Eg), a lower highest occupied molecular orbital energy level and enhanced interchain π–π interactions. Polymer solar cells based on TQ2 showed a simultaneous enhancement of the open-circuit voltage (Voc), short-circuit current density (Jsc) and fill factor (FF) compared with polymer solar cells based on the TQ1 polymer. A good power conversion efficiency of 4.24% was achieved by solar cells based on the TQ2 polymer and [6,6]-phenyl-C71-butyric acid methyl ester composite. These preliminary results indicate that increasing the acceptor unit (quinoxaline) conjugation is an effective way of improving the performance of polymer solar cells.