Synthesis and photovoltaic properties of copolymers based on 2,2-(1,5-pentamethylene)-2H-benzimidazole

We report synthesis and photovoltaic properties of two new conjugated copolymers with benzimidazole prepared by Stille coupling reaction. The advantage of PMBI is high solubility of the polymer while keeping the coplanarity of the backbone. The electrochemical bandgaps of PCDTPMBI5 and PCBBTPMBI5, calculated from cyclic voltammetry data, are about 2.11 and 2.00 eV, respectively. The optical band gap of the synthesized PCDTPMBI5 (1.43 eV) was smaller than that of PCBBTPMBI5 (1.56 eV), caused by lower ICT effect contributed by low molecular weight of PCBBTPMBI5. The devices comprising PCDTPMBI5:PC61BM (1:2) and PCBBTPMBI5:PC61BM (1:3) with thermal treatment showed an open-circuit voltage (VOC) of 0.27 and 0.30 V, a short-circuit current density (JSC) of 1.09 and 1.75 mA/cm2, and a fill factor (FF) of 0.29 and 0.38, giving an power conversion efficiency of 0.09 and 0.20%, respectively.