We demonstrated that the stability of organic solar cells (OSCs) under light irradiation is markedly enhanced by inserting a molybdenum trioxide (MoO(3)) buffer layer between an anode layer of indium tin oxide (ITO) and a p-type layer of 5,10,15,20-tetraphenylporphyrin (H(2)TPP) or N,N'-di(1-naphthyl)-N,N'-diphenylbenzidine (alpha-NPD). The use of the MoO(3) layer also enhanced open-circuit voltages and power conversion efficiencies of the OSCs due to an increase in built-in potential. From results of stability test of hole-only alpha-NPD devices, we concluded that the OSC degradation occurs near the ITO/p-type layer interface and that the use of the MoO(3) layer can prevent the degradation at this interface. (C) 2009 Elsevier FIN. All rights reserved.