Recent advances in functional structures for light extraction of organic light-emitting diodes

Organic light-emitting diodes (OLEDs) have been extensively studied in recent years with the advent of display and lighting applications, resulting in great achievements. Through the material and structure optimization of OLEDs, the internal quantum efficiency has approached 100%, and the device stability has been greatly increased. The efficiency of external light extraction has been increased through material and structure optimization, but has very low efficiency because of total internal reflection and metal surface absorption. Because the outcoupling efficiency must be increased for the industrialization of OLEDs, considerable research has been performed in this area. For example, studies have been conducted to improve the light-extraction efficiency by applying an optical-functional structure to the inside and outside of OLEDs. Herein, we review the results of studies on the application of various methods for optically functional structures to OLEDs and discuss the improvement of the light-extraction efficiency.