Donor–acceptor-type conjugated copolymers containing a thienoisoindigo unit are synthesized and characterized. In combination with benzodithiophene donor units, this strong acceptor moiety can be used to extend the light absorption of their conjugated copolymers to longer wavelengths. The absorption spectra of the copolymers show features that extended up to 1200 nm in the near infrared (NIR). The resulting optical energy gaps are very narrow, ranging from 1.20 to 1.29 eV. The copolymers are used as the donor material with PC61BM acceptor in the active layer of bulk-heterojunction organic photovoltaic cells, and the devices exhibit photocurrent at NIR wavelengths with a power-conversion efficiency of 0.52% under air mass (AM) 1.5 illumination.