New dithieno[3,2-b:2',3'-d]thiophene (DTT)-based copolymers were designed and synthesized for use as donor materials in polymer solar cells (PSCs). The optical, electrochemical, and photovoltaic properties of the copolymers were investigated. The results indicate that the acceptor units in the copolymers influenced the band gap, electronic energy levels, and photovoltaic properties of the copolymers significantly. The band gaps of the copolymers were in the range 1.85-2.02 eV. Under optimized conditions, the DTT-based polymers showed power conversion efficiencies (PCEs) for the PSCs in the range 0.97-1.19% under AM 1.5 illumination (100 mW/cm2). Among the copolymers, P2, which contained a pyrrolo[3,4-f]isoindole-tetraone acceptor unit, showed a power conversion efficiency of 1.19% with a short circuit current of 4.18 mA/cm2, open circuit voltage of 0.77 V, and a fill factor of 0.37, under AM 1.5 illumination (100 mW/cm2).