Faculty of Engineering, Kyoto Sangyo University
Kamigamo, Kyoto, Japan
题目: Working mechanism of charge generation layers in tandem organic light-emitting diodes
White organic light-emitting diodes (OLEDs) have attracted much attention because of applications to displays and lighting. A lot of studies have been made for OLEDs with a single electroluminescence (EL) unit. The EL unit consists of electron injection layer, electron transporting layer, emitting layer, hole transporting layer, and hole injection layer. Recently OLEDs with tandem structure have been studied to improve the current efficiency, external quantum efficiency, and the operational lifetime. The tandem OLED consists of two or multiple EL units connected in series (i.e., vertical stacking) with intermediate connectors between adjacent two EL units. The connector acts as charge generation layer (CGL) which generates both electrons and holes under external voltage applied across the electrodes of OLEDs. The CGL consists of two components, a p-type layer and an n-type layer. The CGL is formed by an organic/metal-oxide bilayer or organic/organic bilayer. The device structure of tandem OLED with two EL units is anode/EL1/n-type layer/p-type layer/EL2/cathode.
The working mechanism of CGL, including an answer to question “which of n- and p-type layers actually generates carriers?”, is explained using the energy level diagram of CGL and its adjacent EL layers. The energy levels are determined from the ultraviolet photoelectron spectroscopy (UPS). Discussion is made on the electronic processes in CGL based on recent UPS studies.