Four benzodithiophene (BDT)–thiophene (T) copolymers with conjugated side chains containing electron-withdrawing ethylrhodanine acceptor units, PHDBDT-T-R, PEHBDT-T-R, PHDBDT-T-TR, and PEHBDT-T-TR, were designed and synthesized for investigating the effect of side chains on the physicochemical properties and photovoltaic performance of the conjugated polymers. All the four copolymers possess an identical conjugated backbone of alternative benzodithiophene–thiophene, but different side chains on BDT and thiophene units, respectively. Polymer solar cells (PSCs) with these polymers as donors and PC₇₀BM as acceptors exhibit an initial power conversion efficiency (PCE) of 0.61% for PHDBDT-T-R, 2.32% for PEHBDT-T-R, 1.46% for PHDBDT-T-TR, and 2.36% for PEHBDT-T-TR. After the treatment with 3 vol% DIO additive and with methanol, the highest PCE was increased up to 1.01%, 4.04%, 3.47%, and 4.25% for PHDBDT-T-R, PEHBDT-T-R, PHDBDT-T-TR, and PEHBDT-T-TR, respectively, with significantly increased J_sc and FF. The effects of methanol treatment on the photovoltaic performance of the PSCs can be ascribed to the increased carrier transport, improved exciton dissociation and optimized phase separation of the active layer. This work indicates that side-chain engineering plays a key role in molecular structures and optoelectronic properties.