Device efficiency of organic light-emitting diodes: Progress by improved light outcoupling

Organic light‐emitting diodes (OLEDs) are efficient large‐area light sources facing their market entry. Still, the development of stable and more efficient blue emitters and the enhancement of light outcoupling remain challenges for further device improvements. Here, we review the working principles of OLEDs and highlight ongoing efforts to improve their efficiency, in particular by coupling out more light. Organic light‐emitting diodes (OLEDs) are efficient large‐area light sources facing their market entry. Still, the enhancement of light outcoupling from the multi‐layer thin film structures remains a major challenge. This Review Article introduces the working principles of OLEDs and gives an overview of ongoing efforts to improve their efficiency, in particular by extracting more light. As an example, the photograph shows a white OLED where light emission from trapped modes is enabled by a high‐index prism.