A Study On Improving Extraction Efficiency Of Organic Light Emitting Diodes

Organic light-emitting diodes (OLED) have attracted intensiveintentions in both flat-panel displays and lighting due to the advantages ofshort response time, wide viewing angles, high contrast ratio, low powerconsumption, low cost and so on. By using phosphorescence emitters, theinternal quantum efficiency of OLED can reach almost100%. However,because of the existence of total internal reflection, waveguide effects andso on, OLED’s out-coupling efficiency still remains small and needs to befurther improved. In order to extract more light out and boost theapplication of OLED, it is effective to increase the out-coupling efficiencywith optical means.This article studied two ways to improve the extraction efficiency ofOLED in detail: The one was adding a photonic crystal layer between ITOlayer and glass substrate to couple out the waveguide mode in ITO layerand organic layers and thus, improve the light efficiency; The other was adding glass hemispheres on glass substrate to reduce the reflection at theglass/air interface and improve the light out-coupling efficiency.In the first part of research, finite difference time domain (FDTD)algorithm was used to model and simulate the energy distribution ofOLED and analyze the way photonic crystal increase the OLED extractionefficiency with mode coupling theory. Meanwhile, by studying theextraction efficiency of photonic crystal OLED with the source placed indifferent places in a period, it was found photonic crystal OLED’sextraction efficiency is closely related to sources’ position. There was onlya source placed under the center of high refractive index material ofphotonic crystal in traditional FDTD simulation model, while OLED’slighting is caused by millions of dipoles. According to the research in thisarticle, the mismatch of model and actual structure will cause significanterror. To solve this problem, a modified simulation model was proposedand it was used in the optimization of photonic crystal’s parameters.In the second part of research, the effects of adding a glasshemisphere on single OLED device and adding glass hemispherescorrespondingly on the surface of OLED arrays were studied. Theinfluence of different characteristics of refractive index match fluid, size ofemitting element, and size of hemispheres on the extraction efficiency of OLED was verified by experiments. In order to study the emitting arrays, itwas necessary to make OLED arrays and ITO patterns were obtained bylithography. Based on these, the most suitable structure of OLED arrayswith glass hemispheres under certain conditions was found by LightToolsmodeling and simulation and experiments. It was also found the properadhesive, the NOA65UV curing adhesive, which can permanently fixglass hemispheres.This study proved photonic crystal and glass hemispheres couldimprove the extraction efficiency of OLED by theoretical analysis andexperimental verification. On one hand, the principle of photonic crystalimproving OLED’s extraction efficiency was analyzed in mode coupling,which is a new perspective and the modified FDTD simulation wasproposed to make up for deficiencies of traditional model, which ismeaningful to the further study of photonic crystal OLED. On the otherhand, the OLED array with glass hemispheres was well designed and made.The problems of the process were well resolved, and these would be usefulin the making of OLED lighting panel.