Synthesis and photovoltaic properties of alkoxy-benzimidazole containing low band gap polymers

• A series of benzimidazole-based copolymers was synthesized.
• The copolymers exhibited good thermal stabilities.
• The copolymers had good solubility.
• The best device provided a power conversion efficiency of 0.76%.

Dioctyloxy substituted dimethyl-2H-benzimidazole (4,7-dibromo-5,6-dioctyloxy-2,2-dimethyl-2H-indene, DOMBI) was prepared and copolymerized with thiophene (or bithiophene) and carbazole using both Stille and Suzuki polymerization. In DOMBI, the sulfur at 2-position of 2,1,3-benzothiadiazole (BT) unit was replaced with dialkyl substituted carbon and dioctyloxy groups were introduced at 5,6-position of dimethyl-2H-benzimidazole (MBI) unit. DOMBI unit is still keeping 1,2-quinoid form with two alkyl groups on the carbon which substitute the sulfur of BT. In addition to this, dioctyloxy groups on 5,6-position will improve the absorption at the longer wavelength region. The poly(2,2-dimethyl-5,6-bis(octyloxy)-4-(2-thienyl)-2H-bezimidazole) (PTDOMBI) thin films show absorption band with peaks at 357 and 656 nm and an absorption onset at 775 nm, corresponding to a band gap of 1.60 eV. The devices with PTDOMBI with [6,6]phenyl-C71-butyric acid methyl ester showed power conversion efficiency of 0.76%.