Theoretical Simulation And Experimental Study Of Light Extraction Efficiency Of Organic Light-Emitting Device

In 1963,since Pope and his mates observed the electroluminescence of anthracene for the first time, Organic Light-Emitting Diodes ,as a new kind of technology, have entered into the scientist's view.After almost three decades of development,organic light emitting devices have obtain application in a certain extent.Comparing with the traditional lighting and display devices, OLEDs possess a lot of advantages such as ultrathin, light weight,wide viewing angle,fast response and so on.In addition,other than traditional devices,OLED is cold light source, that is, in light of the process will not take the initiative to give out heat.All these advantages lead the OLEDs a very broad market prospects.But so far,OLEDs still can't achieve universal access to applications ,the most important reason is its low efficiency.Therefore,in this paper,we will mainly summarize the methods of improving the light extraction efficiency of the devices which are commonly used around the world.Then we explore the role of interference and micro-lens played on the light extraction efficiency of the devices we used in the experiments in order to find an available way to improve the light extraction efficiency of OLEDs.Until now,there has been a lot of ways to improve the light extraction efficiency such as widening the distance between organic layers and the cathode in order to diminish the effect of surface plasma mode.Add some micro-structures,for instance, photonic crystal,periodic tubular aluminum oxide,gold nanowires,grid structure of low refraction,and so on,to decrease the effect of waveguide mode.Making some special structures on the back of the substrate,such as microlens with different shapes and sizes,anti-reflection film,light guide plate and so on,all the methods above can lead a increase of critical angle so that to out-coupling the light bounded in the substrate.In the experiment, we first study the varying light extraction efficiency caused by the effect of interference.The structure we used in the experiment is :ITO/NPB/Alq3/LiF/Al.First,we fix the thickness of NPB as 50nm,making the thickness of Alq3 to vary from 10nm to 100nm,increasing 10nm at a time.And then,fix the thickness of Alq3 as 50nm,making the thickness of NPB to vary from 10nm to 100nm,increasing 10nm at a time as well.In addition,we still make the simulation of the devices using microcavity theory which only take the effect of interference in account (without considering the electrical properties of the situation).By comparing this two kind of results,we found that when the varying layers are thin,the brightness is low,that is due to the effect of the surface plasmon mode,the lower the distance between the light-emitting layer and the cathode is,the greater the effect of the surface plasmon mode is.As it gets thicker,the brightness increasing gradually,for the effect of the surface plasmon mode becomes smaller.However,as it is too thicker,the brightness of the device appears on a downward trend again,for at this time,the effect caused by absorption and total reflection become stronger.And at last,we found that the optimal thickness of NPB and Alq3 for the best light out-coupled efficiency is 70nm.Since then, we have studied the impact of micro-lens on the device.We first evaluate the impact of the dimensions and packing factor of hemispheric microlens,using 3D-FDTD to simulate its photometric characteristics,and then make components of three different colors: red, yellow, and blue, and attached the microlens to the back of the substrates.The result showed that the efficiency of all the devices after attaching the microlens increased in a certain extent.For the blue components,it increased by 15%, for the yellow ones,it increased by 53% ,and 80% for the red ones.