Enhancement of Light Extraction Efficiency in Organic Light Emitting Diode with Nano-Scaled Random Patterns

Abstract

Fast response, true color representation, and applicability to flexible display, organic light emiting diodes (OLED) become prominent next generation display devices. However, light extraction efficiency of OLED is still limited to below 30% due to surface plasmon polaritons (SPP) and total internal reflection phenomena at each layer. In order to improve the lower value of light extraction efficiency, various works are investigated. The main reasons of trapped light inside OLED device are total internal reflection and waveguide and SPP mode. Total internal reflection effect can be decreased by application of micro-lenz array and surface scattering structures. However, the clarity of image can also be damaged as an unwanted side effect. Captured light inside the device, due to waveguide and SPP mode can be decreased by photonic crystal pattern or moth-eye pattern. In both cases, device fabrication process is the key issue.In order to fabricate highly efficient OLED display without blurring of image, nano-scaled random pattern was formed on glass substrate using direct ptinting method[1]. Nano-scaled random pattern has low haziness and smoothed curvatures on the surface. It was formed by direct printing of spin on glass (SOG)materials such as hydrogen silsesquioxane (HSQ). After proper heat treatment, HSQ becomes SiO2. Current efficiency and power efficiency of OLED, formed on glass substrate with nano-scaled random pattern, exhibit 25% and 35% increase, compared to OLED device on flat glass substrate, respectively. External quantum efficiency of OLED device on nano-patterned substrate also shows 28% of increase[1]. Keywords: optical scattering pattern, nanoimprint, OLED, light extraction efficiency