Focused on the quest for always increasing the out-coupling efficiency of organic LEDs (OLEDs) for lighting applications, this paper introduces original hierarchical structures embossed into plastic foils that are laminated on OLED substrates. These foils have structures with a simultaneously small and large periodicity, providing a large design freedom for optimizing the light output. Thorough experimental characterization is carried out to quantify the out-coupling enhancement, which turns out to be a challenge since finite-size pixel and substrate effects influence the measurement results. In that context, the employed multi-scale optical out-coupling model proves handy. It combines the wave-optical aspects of the thin film OLED stack and the ray-optical light propagation in the out-coupling structures, thereby being able to reproduce the efficiency reductions observed in the finite-size OLED samples.
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