A series of organic photovoltaic devices consisting of concentration gradients of poly (3-octylthiophene) (P3OT) and [6,6]-phenyl-C(61) butyric acid methyl ester (PCBM) were fabricated by thermally-induced interdiffusion of consecutively spin-cast layers of P3OT and PCBM from solvents of chloroform and pyridine, respectively. The device performance was evaluated as a function of the layer thicknesses, interdiffusion temperature, and interdiffusion time. A maximum power conversion efficiency of 1.0% under AM1.5G simulated solar spectrum was obtained for 70nm P3OT thickness, 45nm PCBM thickness, and interdiffusion at 150 degrees C for 20 min. Auger spectroscopy depth profiling measurements indicated that the optimal devices consist of concentration gradients of P3OT and PCBM extending across the entire film in opposite directions. (C) 2009 Elsevier B.V. All rights reserved.