Three novel copolymers (PT-ID, PT-DTBT, and PT-DTBTID) of benzo[1,2-b:4,5-b']dithiophene and thiophene with different conjugated side groups (1,3-indanedione (ID), 4,7-dithien-5-yl-2,1,3-benzodiathiazole (DTBT), and DTBT-ID) were synthesized and developed for polymer solar cell applications. The effects of the different conjugated side groups on the thermal, photophysical, electrochemical and photovoltaic properties of these copolymers were investigated. As the length of the conjugated side groups increased, the absorption of the UV-vis region in solution was red-shifted. By changing the different side groups, the energy levels and band gaps of the resulted copolymers were effectively tuned. The three copolymers exhibit deep HOMO energy level and relatively high open-circuit voltage (V-oc). Bulk heterojunction solar cells with these copolymers as electron donors and (6,6)-phenyl-C-61-butyric acid methyl ester (PC61BM) as an electron acceptor exhibit power conversion efficiencies of 2.48%, 4.18% and 1.16% for PT-ID, PT-DTBT, and PT-DTBTID, respectively.