Organic P3HT:PCBM heterojunction solar cells with indium tin oxide (ITO) anodes modified by platinum nanoparticles (NPs) were fabricated. The open-circuit voltage (V-oc) of the cells was increased compared with the reference cell without Pt NPs. An enhanced power conversion efficiency of similar to 12% was obtained for the optimum sputtering deposition duration of 10 s. A double junction model of the Schottky junction and the p-n junction is proposed to describe the frequency dependence of the capacitance in the modified cell. The formation of the front Schottky junction allows efficient collection of holes from the active layer, and enhances the V-oc. Moreover, the air stability of organic solar cells was improved. The modification of ITO with Pt NPs shows promise as a technique for fabrication of high-efficiency stable photovoltaic devices. (C) 2012 Elsevier Ltd. All rights reserved.