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Three novel copolymers (PT-ID, PT-DTBT, and PT-DTBTID) of benzo[1,2-b:4,5-b′]dithiophene and thiophene with different conjugatedside groups (1,3-indanedione (ID), 4,7-dithien-5-yl-2,1,3-benzodiathiazole(DTBT), and DTBT-ID) were synthesized anddeveloped for polymer solar cell applications. The effects of thedifferent conjugated side groups on the thermal, photophysical, electrochemicaland photovoltaic properties of these copolymers were investigated.As the length of the conjugated side groups increased, the absorptionof the UV–vis region in solution was red-shifted. By changingthe different side groups, the energy levels and band gaps of theresulted copolymers were effectively tuned. The three copolymers exhibitdeep HOMO energy level and relatively high open-circuit voltage (Voc). Bulk heterojunction solarcells with these copolymers as electron donors and (6,6)-phenyl-C61-butyric acid methyl ester (PC61BM) as an electronacceptor exhibit power conversion efficiencies of 2.48%, 4.18% and1.16% for PT-ID, PT-DTBT, and PT-DTBTID, respectively. [ABSTRACT FROM AUTHOR]