Recent Advances in OLED Optical Design

Since the first demonstration in 1987,1 there has been rapid progress in the development of organic light-emitting diode (OLED) technology, resulting in commercialization of OLEDs for mobile and television displays. Over the last decade, there have been intense research works to achieve highly efficient devices, and as a result, nearly 100% internal quantum efficiency (IQE) has been realized using phosphorescent and thermally activated delayed fluorescent (TADF) emitters.2,3 The external quantum efficiency (EQE) of a typical OLED is estimated to be 20%–25% due to a large amount of light trapped inside the device. Therefore, it is important to understand the optics of an OLED stack and develop schemes to extract light out of the device in order to fully benefit the high IQE This review consists of four sections. In the first section, we review the basic concepts required to understand the fundamental of OLED optics. In the second section, we review how the refractive index of each layer in an OLED stack affect the outcoupling efficiency. The materials design for an OLED stack has been focused mainly on improving their luminescence and electrical properties,5–7 and there has been a little attention paid to the optical properties of materials. Here, we present the effect of refractive index of transport layers and the transparent electrode on the device efficiency and provide the perspective that could be helpful for establishing the optical design rules for OLEDs. In the third section, we review the various strategies to extract internal optical modes.