Optimization of external coupling and light emission in organic light-emitting devices: modeling and experiment

Abstract

The emission of light and external coupling after the appropriate excitons have been formed in the organic light-emitting devices (OLEDs) has been investigated. The internally emitted light can be classified into three modes: externally emitted, substrate waveguided, and indium–tin–oxide (ITO)/organic waveguided. A combined classical and quantum mechanical microcavity model is used to calculate the distribution of light emission into these three modes in an OLED on planar substrates. The ITO/organic modes maybe suppressed due to the thinness of the ITO/organic layers. Consequently, as much as over 50% of the internally generated light is emitted externally in some structures, much greater than the ∼20% figure given by classical ray optics. This model is used to examine how this distribution varies with exciton to cathode distance, the thickness of the ITO layer, and the index of refraction of the substrate. It can also be applied to OLEDs on shaped substrates where an increase in the total external emission up to a factor of 2.3 has been demonstrated. The numerical results agree well with experimentally measured far-field intensity profiles, edge emissions, and increase in external emission due to shaped substrates. Finally, based on these results, we discuss different approaches to device optimization, depending on the fluorescence efficiency of the emitter and whether a shaped substrate is used.