Highly efficient organic light-emitting diodes with hole injection layer of transition metal oxides

We report on the advantage of interlayers using transition-metal oxides, such as iridium oxide (IrOx) and ruthenium oxide (RuOx), between indium tin oxide (ITO) anodes and 4′-bis[N-(1-naphtyl)-N-phenyl-amino]biphenyl (α-NPD) hole transport layers on the electrical and optical properties of organic light-emitting diodes (OLEDs). The operation voltage at a current density of 100mA∕cm2 decreased from 17to11V for OLEDs with 3-nm-thick IrOx interlayers and from 17to14V for OLEDs with 2-nm-thick RuOx ones. The maximum luminance value increased about 50% in OLED using IrOx and 108% in OLED using RuOx. Synchrotron radiation photoelectron spectroscopy results revealed that core levels of Ru 3d and Ir 4f shifted to high binding energies and that the valence band was splitting from metallic Fermi level as the surface of the transition metal was treated with O2 plasma. This provides evidence that the transition-metal surface transformed to a transition-metal oxide. The surface of the transition metal became smoother with the O2 plasma treatment. The thickness was calculated to be 0.4nm for IrOx and 0.6nm for RuOx using x-ray reflectivity measurements. Secondary electron emission spectra showed that the work function increased by 0.6eV for IrOx and by 0.4eV for RuOx. Thus, the transition-metal oxides lowered the potential barrier for hole injection from ITO to α-NPD, reducing the turn-on voltage of OLEDs and increasing the quantum efficiency.