Phosphorescent top-emitting organic light-emitting devices with improved light outcoupling

Abstract

A dielectric capping layer has been used to increase the light output and to tune the spectral characteristics of top-emitting, phosphorescent organic light-emitting devices (OLEDs). By controlling the thickness of the dielectric layer deposited on top of a thin metal cathode, the transmittance of the top electrode can be adjusted. Maximum light output is not achieved at highest cathode transmittance, indicating that the interplay between different interference effects can be controlled by means of the capping-layer thickness. Furthermore, we demonstrate that the electrical device characteristic is not influenced by the capping layer. The strength of our concept in particular lies in the fact that the optical and the electrical device performance can be optimized separately. Using the capping-layer concept, we have achieved an OLED efficiency of 64 cd/A with pure green emission.