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/PC71 BM cells fabricated using a dip-coating method. (c) 2012 Elsevier B.V. All rights reserved.