Application of two-dimensional conjugated benzo[1,2-b:4,5-b']dithiophene in quinoxaline-based photovoltaic polymers

Two new donor-acceptor (D-A) alternative copolymers, PBDTDTQx-T and PBDTDTQx-O, were designed and synthesized to investigate the influence of two-dimensional conjugated structure on photovoltaic properties of conjugated polymers. In these two polymers, PBDTDTQx-O was used as control material, which is an alternative copolymer based on a quinoxaline derivative (DTQx) and alkoxy-substituted benzo[1,2-b:4,5-b']dithiophene (BDT-O) unit; PBDTDTQx-T has an identical conjugated backbone as PBDTDTQx-O, but a simple two-dimensional conjugated BDT unit (BDT-T) was used to replace the alkoxy-BDT. The polymers were characterized by TGA, UV-vis absorption, electrochemical cyclic voltammetry, hole mobility of space-charge-limited current (SCLC) model, and photovoltaic measurements. It was found that PBDTDTQx-T exhibits similar molecular energy levels and higher hole mobility than PBDTDTQx-O. The power conversion efficiency (PCE) of the polymer solar cells (PSCs) based on PBDTDTQx-T: [6,6]-phenyl-C-71-butyric acid methyl ester (PC ₇₁BM) = 1/2 (w/w) reached ∼5%, which is 60% higher than that of PBDTDTQx-O-based PSC. On the basis of these results, it can be concluded that the application of two-dimensional conjugated structure would be a feasible approach to improve photovoltaic properties of conjugated polymers.