In Situ Analysis of Hole Injection Barrier of Molybdenum-Oxide-Coated Anode with Organic Materials Using Synchrotron Radiation Photoemission Spectroscopy

Abstract

We report the enhancement of the electroluminescent property of organic light-emitting diodes (OLEDs) using MoO(x) as a hole injection layer between indium tin oxide (ITO) anodes and 4,4(')-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (alpha-NPD). The operation voltage for 100 mA/cm(2) of OLEDs with a 10 A degrees thick MoO(x) layer decreased from 24.5 to 14.7 V, and the luminescence value at 190 mA/cm(2) increased from 1280 to 9750 cd/m(2). alpha-NPD was deposited in situ on O(2)-plasma-treated ITO (O(2)-ITO) anodes and O(2)-ITO anodes coated with an O(2)-plasma-treated MoO(x) layer (O(2)-MoO(x)), and their interface dipole energies were quantitatively determined using synchrotron radiation photoemission spectroscopy. Secondary electron emission spectra after the deposition of alpha-NPD on anodes revealed that the work function of O(2)-MoO(x) is 0.55 eV higher than that of O(2)-ITO. The corresponding interface dipole energies were -0.20 eV for O(2)-ITO and -0.50 eV for O(2)-MoO(x). The hole injection barrier at the interface was reduced from 1.15 to 0.85 eV by the deposition of MoO(x), resulting in an enhancement of the electroluminescence property of OLEDs.