A new series of polysilafluorene-type low-bandgap polymers containing 3,6-dialkyloxy-9,9-dialkylsilafluorene and 4,7-di-2-thieny1-2,1,3-benzothiadiazole units has been synthesized. UV/vis absorption spectroscopy and grazing incident X-ray diffraction results showed that the alkoxy moiety on the silafluorene unit broadens the absorption band of the polymers because of its electron-donating property, enabling more efficient harvesting of photons from the solar spectrum. Furthermore, the silicon atoms of the polymers lead to a highly ordered structure, which is essential for high charge-carrier mobility. In addition, high molecular weight polymers can be prepared by using long octyloxy/hexyl solubilizing groups. The blend of new poly[2,7-(3,6-dioctyloxy-9,9-dihexylsilafluorene)-alt-5,5-(4',7'-di-2-th ienyl-2',1',3'-benzothiadiazole)] (P4H) and [6,6]-phenyl-C-71-butyric acid methyl ester (PC71BM) exhibited a power-conversion efficiency of 4.05% with an open-circuit voltage of 0.67 V, a short-circuit current density of 11.1 mA cm(-2), and a fill factor of 54.3% under simulated 100 mW cm(-2) air mass 1.5 global (AM1.5G) illumination.