We demonstrated that driving voltages, external quantum efficiencies, and power conversion efficiencies of organic light-emitting diodes (OLEDs) are improved by inserting a wide-energy-gap interlayer of (4,4'-N,N'-dicarbazole)biphenyl (CBP) between a hole-transport layer of N,N-di(naphthalen-1-yl)-N,N'-diphenylbenzidine (alpha-NPD) and a light-emitting layer of tris(8-hydroxyquinoline)aluminum. By optimization of CBP thicknesses, the device with a 3-nm-thick CBP layer had the lowest driving voltage and the highest power conversion efficiency among the OLEDs. We attributed these improvements to enhancement of a carrier recombination efficiency and suppression of exciton-polaron annihilation. Moreover, we found that the degradation of the OLEDs is caused by decomposition of CBP molecules and excited-state alpha-NPD molecules. (C) 2009 Elsevier B.V. All rights reserved.