In a device structure of ITO/hole-injection layer/N,N'-biphenyl-N,N'-bis-(1-naphenyl)-[1,1'-biphthyl]4,4'-diamine(NP B)/tris(8-hydroxyquinoline)aluminum(Alq(3))/Al, we investigated the effect of the hole-injection layer on the electrical characteristics and external quantum efficiency of organic light-emitting diodes. Thermal evaporation was performed to make a thickness of NPB layer with a rate of 0.5-1.0 angstrom/s at a base pressure of 5 x 10(-6) Torr. We measured current voltage characteristics and external quantum efficiency with a thickness variation of the hole-injection layer. CuPc and PVK buffer layers improve the performance of the device in several aspects, such as good mechanical junction, reducing the operating voltage, and energy band adjustment. Compared with devices without a hole-injection layer, we found that the optimal thickness of NPB was 20 nm in the device structure of ITO/NPB/Alq(3)/Al. By using a CuPc or PVK buffer layer, the external quantum efficiencies of the devices were improved by 28.9% and 51.3%, respectively. (c) 2006 Elsevier B.V. All rights reserved.