Nanocomposite layers consisting of poly(3,4,-ethylene dioxythiophene):polystyrene sulfonic acid (PEDOT:PSS) and inorganic nanoparticles (Ni, Cu, and NiO) were investigated as hole injection layers in organic light emitting diodes (OLEDs) consisting of N,N'-di(naphthalene-1-yl)-N,N'-diphenylbenzidine as a hole transporting and tris (8-hydroxyquinoline) aluminum as electron transporting and emitting layer. It was found that the addition of Cu and NiO nanoparticles resulted in inferior OLED performance for all nanoparticle concentrations, while for Ni nanoparticles, improved performance was obtained for optimized nanoparticle concentration compared to the devices with pure PEDOT:PSS. Reasons for the improved performance of devices with PEDOT:PSS:Ni layers are discussed. It appears that increase of the current driven into the diode is the reason for the improvement of OLED performances achieved by incorporating Ni-nanoparticles into the injection layer. (c) 2005 Elsevier B.V. All rights reserved.