Improvement of highly efficient organic light-emitting diodes using Mg-doped ZnO buffer layers

In this paper, we report on the improvement of the anode buffer layer of Mg-doped ZnO (MZO) on the electrical and optical properties of organic light-emitting diodes (OLEDs). The MZO layers with different thickness were prepared by thermal-evaporating the MZO powders and then treated by ultraviolet (UV) ozone exposure. The turn-on voltage of OLEDs decreased from 4 V (4.2 cd/m2) to 3 V (4.7 cd/m2), the maximum luminance value increased from 16780 cd/m2 to 35400 cd/m2 and the power efficiency increased from 2.74 lm/W to 5.77 lm/W as the 1-nm-thick MZO layer was inserted between indium tin oxide (ITO) anodes and α-naphthylphenylbiphenyl diamine (NPB) hole-transporting layers. The surface of the MZO/ITO became smoother with the UV-ozone treatment. X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) studies were performed to show that the formation of the MZO layer and the work function increased as the MZO/ITO layer was treated by UV-ozone for 20 min. Thus, the hole-injection energy barrier was lowered by inserting MZO buffer layer, consequently resulting in the decrease of the turn-on voltage and the increase of the power efficiency in OLEDs