We report a series of fused pyrrolo[3,2-d:4,5-d′]bisthiazole (PBTz)-containing polymers as OPV donor materials, which exhibit a combination of small optical band gap, good solubility and relatively high molecular weight. The performance of the organic photovoltaic cells (OPV) made using the PBTz-containing polymers is studied. The highest performance was obtained with the polymer exhibiting the strongest crystallinity in the blend film, a PBTz-fused bithiophene polymer. Blend devices made from this polymer with PC70BM delivered a short-circuit current density of up to 9.4 mA cm−2, an open-circuit voltage of 0.61 V, a fill factor of 53% and a power conversion efficiency of 3%.