The electroluminescence performance of devices as enhanced by using ruthenium oxide (RuOx ) as a hole injection layer (HIL ) between indium-tin-oxide ( ITO ) anodes and 4,4'-bis [N-(1-naphthyl)-N-phenyl-amino]biphenyl (alpha-NPD ) and using Mg-Al alloy as a cathode. Both the HIL and the Mg-Al alloy were applied to organic light emitting diodes (OLEDs ) on both glass and flexible substrates. The operation voltage at a current density of 100 mA/cm(2) decreased from 14.7 to 9.9 V when the RuOx interlayer and Mg-Al cathode were applied to OLEDs on glass substrate. Also, the operation voltage reduced from 17.1 to 10.8 V for OLED on polyethylene terephthalate ( PET ) substrate. The maximum luminance value increased from 1510 to 5400 cd/m(2) in OLEDs on glass substrate using the RuOx interlayer and Mg-Al cathode. Synchrotron radiation photoelectron spectroscopy results showed that core level of the C 1s spectra shifted 0.25 eV toward higher binding energy when alpha-NPD was deposited on RuOx-coated ITO/PET substrate. Secondary cutoff spectra revealed that the work function increased by 0.45 eV when the RuOx interlayer was deposited on ITO/PET substrate. Thus, the RuOx interlayer lowered the potential barrier for hole injection, reducing the turn-on voltage of OLEDs and increasing the quantum efficiency. (c) 2007 The Electrochemical Society.