Fused ring thiophene-based poly(heteroarylene ethynylene)s for organic solar cells

Alternating small band gap π-conjugated polymers consisting of alkylated cyclopentadithiophene or dithienopyrrole units linked via ethynylene bonds to 2,3-diphenylthieno[3,4-b]pyrazine or 2,1,3-benzothiadiazole have been prepared in high yields by Sonogashira condensation polymerization. The copolymers were characterized by NMR, GPC, UV–vis, and cyclic voltammetry. In thin films these polymers exhibit an optical band gap of ∼1.46−1.72 eV. In bulk heterojunction solar cells with PCBM as an acceptor these donor polymers provide estimated power conversion efficiencies of 1.34−1.74%. Because of the ethynylene bonds, the polymers lead to the solar cells with a relatively high open circuit voltage (0.72−0.82 V) in relation to the reduced optical band gap, which helps in reducing energy losses.