Synthesis and characterization of 2H-benzimidazole- and terthiophene-based polymer for organic photovoltaics

New semiconducting polymer with alkoxy substituted terthiophene, as the electron rich unit, and 2,2-(1,5-pentamethylene)-2H-benzimidazole (PMBI), as the electron deficient unit, was synthesized using Stille polymerization. The advantage of PMBI is higher solubility of the polymer while keeping the coplanarity of the backbone as compared to the benzothiadiazole moiety of PCDTBT. In PMBI, the sulfur at 2-position of BT unit was replaced with the carbon of cyclohexane to make a highly soluble electron deficient moiety while keeping the 1,2-quinoid form of BT unit. The polymer, PTBHOTPMBI, exhibits more red shifted absorption peak as compared to the polymers with benzothiadiazole unit. The PTBHOTPMBI thin film shows two broad absorption bands with peaks at 475 and 755 nm and an absorption onset at 943 nm, corresponding to a band gap of 1.32 eV. The device of PTBHOTPMBI with PC71BM showed an open-circuit voltage (VOC) of 0.25 V, a short-circuit current density (JSC) of 2.30 mA/cm2, and a fill factor (FF) of 0.30, giving a power conversion efficiency of 0.17%.