In response to the structural and electronic limitations of the popular benzo[1,2-b:4,5-b']dithiophene-thieno[3,4-b]thiophene (PBnDT-TT) polymer series, this study explores the design and synthesis of a thienothiazole (TTz) moiety. The synthesis of TTz was streamlined down to four high-yielding steps, resulting in the new polymer PBnDT-TTz for organic solar cells. By incorporating TTz, a nitrogen is directly introduced into the polymer backbone which tunes the HOMO level and eliminates the reliance on external substituents. Compared to its TT analogue, PBnDT-TTz exhibits the same HOMO level of -5.06 eV and the same V(oc) of 0.69 eV, yet a higher power conversion efficiency of 2.5%. These promising results demonstrate the benefits of backbone modification and the great potential of TTz in the design of new polymers for organic photovoltaics.