Synthesis, characterization and photovoltaic properties of conjugated copolymers based on 2-alkyl-thieno[3,4-b]imidazole

A new acceptor unit, 2-alkyl-thieno[3,4-b]imidazole, was synthesized through an annulation reaction between a 3,4-diaminothiophene derivative and a carboxyl acid or an acyl chloride, and was used for developing donor–acceptor conjugated copolymers. Copolymerization of 2-alkyl-thieno[3,4-b]imidazole and benzo[1,2-b:3,4-b′]dithiophene (BDT) or dithieno[3,2-b:2′,3′-d]silole (DTS) afforded 6 new low bandgap polymers P1–P6. These polymers possess intensive absorption at 400–650 nm and the optical bandgaps are in the range of 1.69–1.88 eV. They were firstly applied in polymer solar cells and the best efficiency of 0.72% was obtained from P2/PC71BM cells fabricated using a dip-coating method.