Thin-film encapsulated white organic light top-emitting diodes using a WO3/Ag/WO3 cathode to enhance light out-coupling

An alternative design of a semitransparent cathode for top-emission white-fluorescent organic light-emitting diodes (OLEDs) has been investigated. The scope of this study was to improve the luminance of OLEDs used for displays while keeping the current density versus voltage characteristic unchanged for addressing purposes. The use of an optical simulation tool allowed the optimization of the tri-layer cathode WO3/Ag/WO3 to increase the light out-coupling coefficient of the device leading to an increased white emission compared with a reference device with a Ca/Ag cathode. An increase of similar to 40% in luminance has been calculated by simulation and experimentally confirmed. The p-i-n OLED structure underneath the tri-layer cathode allowed an efficient injection of electrons independently from the work function of WO3. The WO3/Ag/WO3 cathode has been also confirmed to be compatible with the atomic layer deposition technique for thin film encapsulation. Finally, lifetime measurements up to 600h have been carried out to quantify the enhancements induced by the new cathode compared with the control device. It has been found that lifetimes of both cathode architectures are similar on this time scale, while the WO3/Ag/WO3 cathode shows a lower voltage drift versus aging.