Enhancement of hole-injection and power efficiency of organic light emitting devices using an ultra-thin ZnO buffer layer

The advantages of using an anode buffer layer of ZnO on the electro-optical properties of organic light emitting devices (OLEDs) are reported. ZnO powders were thermal-evaporated and then treated with ultra-violet (UV) ozone exposure to make the ZnO layers. The turn-on voltage of OLEDs decreased from 4 V (4.2 cd/m(2)) to 3 V (3.4 cd/m(2)) and the power efficiency increased from 2.7 Im/W to 4.7 Im/W when a 1-nm-thick ZnO layer was inserted between indium tin oxide (ITO) anodes and alpha-naphthylphenylbiphenyl diamine (NPB) hole-transporting layers. X-ray and ultra-violet photoelectron spectroscopy (XPS and UPS) results revealed the formation of the ZnO layer and showed that the work function increased by 0.59 eV when the ZnO/ITO layer was treated by UV-ozone for 20 min. The surface of the ZnO/ITO film became smoother than that of bare ITO film after the UV-ozone treatment. Thus, the hole-injection energy barrier was lowered by inserting an ZnO buffer layer, resulting in a decrease of the turn-on voltage and an increase of the power efficiency of OLEDs. (C) 2009 Elsevier B.V. All rights reserved.