Synthesis and Characterization of Low-Bandgap Copolymers Based on Dihexyl-2H-benzimidazole and Cyclopentadithiophene

A series of new semiconducting polymers based on 4,4-dihexyl-4H-cyclopenta[2,1-b: 3,4-b'] dithiophene, 2,2-dihexyl-2H- benzimidazole, and thiophene units was synthesized. The polymers show good solubility at room temperature in organic solvents owing to long alkyl chain in new acceptor, 2,2dihexyl- 2H-benzimidazole. The advantage of dihexyl-2H-benzimidazole compared to the benzothiadiazole is to improve the solubility of the polymer. It was found that these polymers can finely be tuned for photovoltaic application by adjusting the contents ratio of the dihexyl-2H-benzimidazole unit. The spectra of the solid films show absorption bands with maximum peaks in the range of 421-577 nm and the absorption onsets at 588-683 nm, corresponding to band gaps of 2.11-1.82 eV. The devices with PCPDTDTHBI-1:PC71BM showed an open-circuit voltage (V-OC) of 0.46 V, a short-circuit current density (J(SC)) of 3.83 mA/cm(2), and a fill factor of 0.36, giving a power conversion efficiency of 0.64%. Decrease of the dihexyl-2H-benzimidazole contents in the polymers induced red-shift of the UV absorptions, and increased VOC and JSC values, to improve the efficiency of organic photovoltaics. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 4567-4573, 2010