We report the enhancement of the electroluminescent property of top emitting organic light emitting diodes (TEOLEDs) using thermally evaporated copper oxide (CuOx) as a hole injection layer (HIL) between UV ozone-treated silver (Ag) anodes and 4,4'-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (alpha-NPD). The operation voltage of TEOLEDs at 1 mA/cm(2) decreased from 6.2 to 5.0 V as a 2 nm thick CuOx layer was used as the HIL. alpha-NPD layers were separately in situ deposited on both Ag and thermally evaporated CuOx-coated Ag (Ag/CuOx) and their interface dipole energies were quantitatively determined using synchrotron radiation photoemission spectroscopy. Secondary electron emission spectra revealed that the work function of Ag/CuOx is higher by 0.53 eV than that of Ag. The corresponding interface dipole energies were -0.93 eV for Ag and -0.88 eV for Ag/CuOx. As a result, CuOx plays a role in reducing the hole injection barrier from 1.63 to 0.96 eV, resulting in a decrease in the turn-on voltage of TEOLEDs. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3476308] All rights reserved.