We have developed polymer solar cells featuring a buffer layer of polythiophene (PT) sandwiched between the active layer and the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) layer. We attribute the improvement in power conversion efficiency of these polymer solar cells, relative to that of those based on poly(3-hexylthiophene):[6,6]-phenyl-C(61)-butyric acid methyl ester (P3HT:PCBM), to a reduction in the degree of carrier recombination at the junction interface. Because the conductivity and the energy level of PT can be tuned simply by applying a bias to it in an electrolytic solution, we also investigated the effect of the energy level on the devices' performances. The power conversion efficiency of a solar cell containing a PT buffer layer reached 4.18% under AM 1.5 G irradiation (100 mW/cm(2)).