Copolymer derived from dihexyl-2H-benzimidazole and carbazole for organic photovoltaic cells

Abstract A new accepter unit, dihexyl-2H-benzimidazole, was prepared and utilized for the synthesis of the conjugated polymers containing electron donor–acceptor pair for OPVs. Dihexyl-2H-benzimidazole unit was designed to substitute the BT unit of PCDTBT. A new series of copolymers with carbazole as the electron-rich unit and dihexyl-2H-benzimidazole as the electron-deficient unit are synthesized. To obtain absorption in the longer wavelength region, bithiophene units without any alkyl group are incorporated as one of the monomers, which may result in low solubility of the polymers. In dihexyl-2H-benzimidazole, sulfur at 2-position of BT unit was replaced with dialkyl substituted carbon, while keeping the 1,2-quinoid form, to improve the solubility of the polymers. The spectra of the solid films show absorption bands with maximum peaks at 446–457 nm and the absorption onsets at 527–539 nm, corresponding to bandgaps of 2.30–2.35 eV. Under white light illumination (AM 1.5 G, 100 mW/cm2), the devices with PCBBTHBIs:PCBM layers showed open-circuit voltages (VOC) of 0.13–0.23 V, short-circuit current densities (JSC) of 3.52–5.69 mA/cm2, and fill factors (FF) of 0.36–0.40, giving power-conversion efficiencies of 0.21–0.47%.