Two donor–acceptor (D–A) alternative copolymers of benzodithiophene (BDT) donor unit and an acceptor unit of naphtho[2,3-c]thiophene-4,9-dione (NTDO) with different alkyl side chains, PBDTNTDO-1 and PBDTNTDO-2, were synthesized for application as donor materials in polymer solar cells (PSCs). The copolymers show good solubility in common organic solvents, broad visible absorption from 350 nm to 670 nm, and relatively lower HOMO energy levels at −5.14 eV for PBDTNTDO-1 and −5.19 eV for PBDTNTDO-2. The PSCs based on PBDTNTDO-2 as donor and PC₇₀BM as acceptor demonstrated power conversion efficiency of 1.52% with an open circuit voltage of 0.88 V and a short circuit current of 5.67 mA cm⁻², under the illumination of AM1.5, 100 mW cm⁻².