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 (Ir Ox) and ruthenium oxide (Ru Ox), 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 100 mA cm2 decreased from 17 to 11 V for OLEDs with 3-nm -thick Ir Ox interlayers and from 17 to 14 V for OLEDs with 2-nm -thick Ru Ox ones. The maximum luminance value increased about 50% in OLED using Ir Ox and 108% in OLED using Ru Ox. 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 0.4 nm for Ir Ox and 0.6 nm for Ru Ox using x-ray reflectivity measurements. Secondary electron emission spectra showed that the work function increased by 0.6 eV for Ir Ox and by 0.4 eV for Ru Ox. 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.