Silole-containing polymers consisting of a dithienosilole homopolymer backbone (12) or an alternating dithienosilole and 4,7-bis(2-thienyl)-2,1,3-benzothiadiazole copolymer backbone (13) were synthesized. The presence of planar dithienosilole tricyclic units along these g.-conjugated polymer backbones lowered the band gap and led to strong absorption in the visible region of the solar spectrum. The introduction of electron-withdrawing benzothiadiazole moieties along the dithienosilole backbone further reduced the optical band gap and increased the interchain interaction. Bulk-heterojunction organic solar cells using 1:1 w/w polymer 12 or 13:PCBM (methanofullerene [6.6]-phenyl C61-butyric acid methyl ester) blends as the photoactive layers were prepared. Photovoltaic cells with copolymer 13 as the electron donor and PCBM as the electron acceptor exhibited an increased energy conversion efficiency by a factor of 3 up to 0.18% under an AM 1.5 simulated solar light at 100 mW/cm(2) after thermal annealing at 140 degrees C.