Improved photovoltaic performance of a 2D-conjugated benzodithiophene-based polymer by the side chain engineering of quinoxaline

To investigate the influence of side chains of quinoxaline on the photovoltaic performances, a novel D–A-type polymer of PBDTDT(Qx-3)-T was synthesized and characterized, in which 5,8-dioctylthienyl substituted benzo[1,2-b:4,5-b′]dithiophene (BDT-T), thiophene (T) and 6,7-dioctyloxy-2,3-diphenylquinoxaline (Qx-3) were used as the donor (D) unit, π-bridge and acceptor (A) unit, respectively. The resulting polymer exhibited good thermal stability with a high decomposition temperature of 357 °C, a low optical bandgap of 1.78 eV with an absorption onset of 696 nm, a low-lying highest occupied molecular orbital (HOMO) energy level of −5.51 eV, and a high carrier mobility of 2.19 × 10−4 cm2 V−1 s−1. Compared to the reported analogues, polymer solar cells (PSCs) based on PBDTDT(Qx-3)-T/PC71BM demonstrated the highest open-circuit voltages (Voc) up to 0.96 V. The maximum power conversion efficiency (PCE) of 6.9% with a Voc of 0.94 V, short-circuit current (Jsc) of 11.28 mA cm−2 and a fill factor (FF) of 64.7% was obtained with a delicate balance among the above factors for the polymer in PSCs. On the basis of these results, it can be concluded that the appending two octyloxy side chains at 6,7-positions of quinoxaline in the BDT-T-alt-DTQx type polymers would be a feasible approach to improve photovoltaic properties.