The synthesis of conjugated polymers 1-5 functionalized with 4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiophene-4,9-dione in the backbone is reported and their use in the construction of organic solar cells is demonstrated. Increasing the molar ratio of 2,7-dibromo-3,8-dihexyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiophene4 ,9-dione, relative to 4,4'-dihexyl-5,5'-dibromo-2,2'-bithiophene, in the copolymer synthesis significantly lowers the solubility of these polymers. The incorporation of highly conjugated 3,8-dihexyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiophene-4,9-dione unit into the polymer backbone has been confirmed by UV-vis absorption. The observation of decreasing quantum yield for the emission in the order of 1, 2, 3 is consistent with copolymers with different comonomer content. The power conversion efficiencies of solar cells using blends of these polymers with PCBM ([6,6]-phenyl C-61-butyric acid methyl ester) were determined to be 0.11% for polymer 1, 0.33% for 2, and 0.26% for 3, respectively. Under identical white light illumination, the power conversion efficiency of the device based on polymer 2/PCBM as the active layer was three times higher compared to that of device based on polymer 1/PCBM. Owing to the limited solubility and poor film-forming ability of polymer 3, the power conversion efficiency of solar cell based on 3/PCBM blend is lower than that of 2/PCBM blend, but is still larger than that of 1/PCBM blend. (c) 2008 Wiley Periodicals, Inc.