Analysis and optimization of light outcoupling in OLEDs with different emitting dipole orientations

Abstract

External light-outcoupling structures in organic light-emitting diodes (OLEDs) present an efficient and cost-effective solution to improve optical performance of devices. External structures can be realized, for example, by applying a microtextured foil on the emitting side of the device. We employ advanced three-dimensional optical simulations to investigate the effects of OLED structures with different external textures in relation to different emitting dipole orientations and layer thicknesses of the OLED stack. We investigate light outcoupling of a red, ITO free, large-area, bottom-emitting OLED, by applying various sinusoidal and pyramid-like external microtextures in periodic (rectangular and hexagonal) and random arrangements. It is shown that there are minimal differences in outcoupling efficiency for the optimized textures for different texture feature shapes, under condition that sufficiently high aspect ratio (height / period) of the texture is applied. With optimized external texture with three-sided micropyramids, horizontally aligned dipole sources and optimized layer thicknesses of OLED stack more than 62 % outcoupling efficiency is predicted. Interestingly, the results show that the highest efficiencies for individual orientation of dipoles, can be achieved with the same shape and size of the texture, regardless of the preferential orientation of the emitting dipoles, which simplifies further research and development of external light extraction.