Progress of Light Extraction Technology for Organic Light-Emitting Diodes

Organic light-emitting diodes (OLEDs) have been developing rapidly in terms of materials and design over the past 30 years. They have been applied on large scales in displays, especially in high-end smartphones. However, in the field of lighting, the industry is not sufficiently mature because of the low efficiency, short life, and high cost of OLEDs. The main reason for this is that the optical waveguide effect and surface plasmon losses lead to the consumption of most of the optical energy coupled to the non-radiative thermal energy, resulting in a huge gap between the internal and external quantum efficiency. It is also one of the main causes for the poor efficiency and longevity. Therefore, it is necessary to recover the part of the light trapped inside OLED lighting devices via light extraction technology. In this paper, we introduce optical analytic methods for OLEDs by analyzing the four modes of light coupling channels and the mechanisms of light energy loss based on the ray-optical model. Then, based on the research achievements of some scientific institutions and recent representative patents in the industry, we summarize the relevant light extraction technology considering two aspects: external light extraction and internal light extraction. Finally, we discuss the development route and prospects of the light extraction technology.