Contents I. Introduction
Organic light-emitting diodes (oleds) have attracted considerable attention because of their unsurpassable advantages for the applications in solid-state lighting and large flat-panel display since the pioneer work of Tang and VanSlyke [1]. The commercial applications of OLEDs require high brightness, efficiency, and extended lifetime. To meet these requirements, light emission at a low driving voltage is very necessary because high driving voltage will reduce device lifetime due to thermal aging and impurity diffusion problems [2], [3]. The quality of the electron injection layer in an OLED is a crucial factor for the driving voltage as well as for efficiencies of the devices. In comparison, due to the fact that electron mobility in electron transport materials is much lower than hole mobility in typical hole transport materials such as N,N'-bis-(1-naphthyl)-N,N'-diphenyl-N,1'-biphenyl-4,4'-diamine (NPB). In order to balance the carriers in the recombination zone, an intensive and detailed study on the enhancement of carrier injection would be quite meaningful and practical.
Devices with various thicknesses of CuPc buffer layers. (a) J–V–L characteristics. The inset shows a microphotograph of the test devices and the device structure. (b) Dependence of current efficiency and power efficiency on current density. The inset shows the energy level diagram of the devices.
