Improved Hole-Injection and Power Efficiency of Organic Light-Emitting Diodes Using an Ultrathin Li-Doped ZnO Buffer Layer

Abstract

We report on the advantages of an anode buffer layer of Li-doped ZnO (LZO) on the electro-optical properties of organic light-emitting diodes (OLEDs). LZO layers with different thicknesses were prepared by thermally evaporating the LZO powders and then treating them with ultraviolet (UV) ozone exposure. The turn-on voltage of OLEDs decreased from , the maximum luminance value increased from and the power efficiency increased from when a thick LZO layer was inserted between indium-tin oxide (ITO) anodes and β-naphthylphenylbiphenyl diamine hole-transporting layers. X-ray and ultraviolet photoelectron spectroscopy were performed to show that the formation of the LZO layer and the work function increased by when the LZO/ITO layer was treated by UV-ozone for . The surface of the LZO/ITO film became smoother after the UV-ozone treatment. Thus, the hole-injection energy barrier was lowered by inserting an LZO buffer layer, resulting in the decrease of the turn-on voltage and the increase of the power efficiency in OLEDs.