Synthesis and characterization of low-bandgap copolymers based on dihexyl-2H-benzimidazole and terthiophene

A series of new semiconducting polymers with 3-(hexyloxy)thiophene, 2,2-dihexyl-2H-benzimidazole (HBI) and thiophene units was synthesized using Stille polymerization. These random copolymers show good solubility at room temperature in organic solvents owing to the long alkyl chain in new acceptor, 2,2-dihexyl-2H-benzimidazole. In HBI, the sulfur at 2-position of 2,1,3-benzothiadiazole (BT) unit was replaced with the carbon to make a highly soluble electron deficient moiety while keeping the 1,2-quinoid form of BT unit. The spectra of the solid films show absorption bands with maximum peaks at about 408–526 nm and the absorption onsets at 550–692 nm, corresponding to band gaps of 1.79–2.25 eV. The onset wavelengths of the absorption spectra in thin films exhibit a gradual red-shift with decreased amount of dihexyl-2H-benzimidazol unit, that is, from 550 nm with PHOTDTHBI-9 to 692 nm with PHOTDTHBI-1. Under white light illumination (AM 1.5 G, 100 mW/cm²), the device with PHOTDTHBI-3:PCBM blend demonstrated a V_OC value of 0.36 V, a J_SC value of 1.20 mA/cm², and a FF of 0.37, leading to the efficiency of 0.16%.