Here we report, the synthesis, the spectroscopic and photovoltaic characterization of two alternating copolymers based on (octylsulfanyl)bithiophene and differently substituted benzodithiophene units (BDT). Bulk heterojunction (BHJ) polymer solar cells (PSCs) having as active layer a blend of these polymers as donor materials and [70]PCBM as acceptor material were prepared and investigated. The geometry of the device is: glass/ITO/PEDOT:PSS/blend/Ca/Al. The electrical performances of the cells were compared in order to study both the influence of the insertion of a benzodithiophene unit in the (alkylsulfanyl)bithiophene backbone and the effect of different substituents on benzodithiophene unit. The best realized device shows a power conversion efficiency of 2.3% under 100 mW/cm2 AM 1.5G illumination.