A new fused-ring building block dithieno[3,2-f:2',3'-h]-phthalimide was synthesized and used in synthesis of conjugated polymers. On the basis of this building block, we designed and synthesized a series of conjugated copolymers containing electron-rich unit benzo[1,2-b:4,5-b']dithiophene (P1 and P2) or with electron-deficient unit 2,2'-bithiazole (P3 and P4) or benzo[c][1,2,5]thiadiazole (P5) and homopolymer (P6). P1 and P4 P6 have good solubility in common organic solvents, while P2 and P3 have poor solubility. P1-P6 exhibit good thermal stability with 5% weight loss temperatures of 193-417 degrees C. PI P6 in films exhibit absorption maxima of 482-532 nm with optical bandgaps of 1.88-2.25 eV and weak emission peaked at 597-721 nm. P1-P6 have estimated HOMOs of -5.3 to -6.2 eV and LUMOs of -2.8 to -3.4 eV. The optical properties and HOMO/LUMO levels are influenced by the comonomers. The polymer solar cells based on PI:P1:PC6IBM (1:1, w/w) exhibit preliminary power conversion efficiency of 0.30% under an AM 1.5 simulated solar light at 100 mW/cm(2).