Six novel conjugated copolymers (P1−P6) containing coplanar cyclopentadithiophene (CPDT) units (incorporated with bithiazole/thienyl-based monomers) were synthesized and developed for the applications of polymer solar cells (PSCs). Copolymers P1−P6 covered broad absorption ranges from UV to near-infrared (400−800 nm) with narrow optical band gaps of 1.70−1.94 eV, which are compatible with the maximum solar photon reflux. Partially reversible p- and n-doping processes of P1−P6 in electrochemical experiments were observed. Compared with those previously reported CPDT-based narrow band gap polymers, the proper molecular design for HOMO/LUMO levels of P1−P6 induced relatively high photovoltaic open-circuit voltages in the PSC devices. Powder X-ray diffraction (XRD) analyses suggested that these copolymers formed highly self-assembled π−π stackings. Under 100 mW/cm2 of AM 1.5 white-light illumination, bulk heterojunction PSC devices containing an active layer of electron donor copolymers P1−P6 blended with ele...