Synthesis and photovoltaic performance of donor-acceptor copolymers based on thieno[3,2-b]quinoxaline

Two new donor-acceptor copolymers containing carbazole and thieno[3,2-b]quinoxaline units with meta- and para-positioned alkoxy groups on the peripheral phenyl rings, PCTTQx-m and PCTTQx-p, were designed and synthesized. Their thermal stability, UV-Vis absorptions, energy levels and photovoltaic properties were investigated. Compared with their quinoxaline analogues using carbazole and quinoxaline derivatives as donor and acceptor units, by simply fusing a thiophene ring to the quinoxaline unit, PCTTQx-m and PCTTQx-p not only exhibit lower bandgaps (similar to 1.8 eV) but also maintain deep HOMO energy levels (similar to-5.3 eV). A polymer solar cell based on PCTTQx-m with the structure ITO/PEDOT:PSS/polymer:PC71BM/LiF/Al showed a promising PCE of 4.40% with a rather high open circuit voltage of 0.89 V, a short circuit current of 9.03 mA cm(-2) and a fill factor of 0.54, which was higher than that of PCTTQx-p (3.45%). After solvent vapor annealing treatment, the PCE of the polymer solar cell based on a PCTTQx-m/PC71BM blend film could be further improved to 5.31%.