We applied thiophene/phenylene co-oligomers (TPCOs) to a major component of organic solar cells. We made sandwich-type devices combining a p-type TPCO with fullerene (C-60) or an n-type TPCO. As an option we made a device that combined a mixed layer of p-type and n-type TPCOs with the C-60 layer. All devices indicated rectification properties in the dark and generated electric powers under the white light illumination. The device comprising methoxy-substituted biphenyl-capped bithiophene and C-60 achieved the best power conversion efficiency of 1.20% with a short-circuit current of 4.28mA/cm(2) and fill factor of 0.46. The results demonstrate that the TPCO materials are useful as the major component of the organic solar cells.