Light extraction from organic light emitting diode

Abstract

Organic light emitting diodes (OLEDs) have attracted much attention for several applications, such as light source and display. It is of both commercial and scientific interests to improve external quantum efficiency of such light emitters. The external quantum efficiency of OLED is determined by the combination of charge balance, singlet-triplet ratio and light extraction efficiency. Application of phosphorescent emitting materials can produce internal quantum efficiency very close to theoretical limitation. However, due to the refractive index mismatch between air and organic emitting layer, most of the emitted light is lost through total internal reflection into substrate and indium-tin-oxide (ITO) waveguiding modes and to self-absorption. Therefore, there is a large space for improvement on the extraction efficiency of the devices. In this paper, A Monte Carlo simulation of external emitted light has been developed. The light extraction factor for planar OLED is 17.17%. This result demonstrates that the light extraction from planar OLEDs can be quantitatively modeled by a simple ray-tracing algorithm. Microlens arrays are introduced on glass substrates to suppress waveguiding loss in the substrate. In this work, we propose to use an etched glass master for fabricating microlens. The glass master is fabricated using a simple wet etching method. A photoresist/Cr/ITO multiplayer mask is made by lithography on the glass substrate and then the glass substrate is etched with HF/HCl solution for improving the quality of generated surface. The isotropic etching profile of the glass master is utilized for microlens replication. Lens arrays are replicated on polymer (PDMS) substrates. With the use of microlens arrays, the light extraction factor is increased experimentally, without detrimental effect to the electrical performance of the OLED.