Three novel conjugated copolymers containing alkoxylated 4,7-diphenyl-2,1,3-benzothiadiazole and dialkylfluorene or dialkyloxyphenylene or dialkylthiophene units were prepared by Horner polycondensation reactions. They are all soluble in common organic solvents such as chloroform, tetrahydrofuran, and chlorobenzene. The novel copolymers were characterized by NMR, GPC, and elemental analysis. Thermogravimetric analysis of the copolymers showed they have good thermal stability with the decomposition temperature higher than 350 degrees C. Cyclic voltammetric study shows that the HOMO energy levels of the three copolymers are deep-lying which implies that these copolymers have good stability in the air and the relatively low HOMO energy level assures a higher open circuit potential when they are used in photovoltaic cells. Bulk-heterojunction photovoltaic cells were fabricated with the copolymers as the donors and PCBM as the acceptor. The cells based on the three copolymers exhibited power conversion efficiencies of 0.65, 1.25, 1.62% with high open circuit potential of 0.76, 0.96, and 1.04 V under one sun of AM 1.5 solar simulator illumination (100 mW/cm(2)).