Efficient organic photovoltaic tandem cells consisting of P3HT:bis[60]PCBM and P3HT:[70]PCBM active layers and Our original transparent conductive oxide interlayer are presented. The short circuit current and conversion efficiency are estimated to be more than 6 mA/cm(2) and 5% from simulations of devices with the thicknesses of both layers optimized. We clarify that actual devices fabricated utilizing nanoscale phase separation in the active layers and interlayer with indium tin oxide sandwiched by MoO3 and LiF buffer layers exhibit characteristics almost the same as those simulated for the ideal tandem cell. This is attributed to improve the efficiencies of carrier transport in the active layers and recombination in interlayer. (C) 2009 Elsevier B.V. All rights reserved.