The Investigation On The Light Out-coupling Efficiency Of Organic Light-emitting Devices

Organic light-emitting diodes(OLEDs)have gradually come into our own life,for example,the recent OLEDs display screen used in iphone X.Nowadays,OLEDs has an internal quantum efficiency of nearly 100%,but in OLEDs,only 20% of the light can emit.Therefore,the light out-coupling has become a research hotspot.The traditional OLEDs devices use ITO as the transparent anode,because of the high refractive index of ITO.The light with a large incident angle is totally reflected when being incident on the glass substrate from ITO,resulting in nearly 50% light is confined between the metal cathode and the ITO.And another nearly 30% of the light is confined in the glass substrate due to total reflection.The key factors that result in lower out-coupling efficiency are the low refractive index glass substrate and the high refractive index ITO anode.In order to extract light from the OLEDs device,the simulation and experiment are combined to find the new device structure to enhance the light out-coupling efficiency and optimize the parameters through simulation in this work.Preparing the OLEDs devices by experiment to verify the results which obtained by the simulation.Using silver thin film and silver nanowire thin film instead of ITO to prepare OLEDs devices.The starting point and the content of this work are as follows:(1)Traditionally,it is time-consuming and labor-intensive to optimize the parameters of high-efficiency OLEDs by experimentally preparing OLEDs,resulting in inefficient research and development of new device structures.In this work,FDTD Solutions software was used to design the structure of a new type of OLEDs and to perform optical simulation.The calculation of the substrate refractive index,the thickness of the silver thin film and the position of the light emission that how to affect the proportion of each mode light in the transparent silver anode OLEDs device,which provide the theoretical guidance for the preparation of high efficient silver film OLEDs devices.The simulation results are validated by experimentally preparing the silver thin-film anode OLEDs.The simulation results show that the actual device performance can be accurately reflected by the structural parameters of the devices.When the thickness of the silver film is 20 nm and the voltage is about 8 V,the highest current efficiency is 5.3 cd/A,and the maximum luminance reaches 57596 cd/m2.In addition,a novel device structure of top-emission OLEDs was designed and simulated,and the effects of different light-emitting surface covering structures on the coupling output efficiency of the device were investigated.(2)For the poor stability of pure silver nanowires thin film,the prepared OLEDs can not work.In this work,we proposed a method to process pure silver nanowires thin film using metal oxide sol-gel to obtain silver nanowire composite transparent conductive film with high thermal stability and high substrate adhesion.We study deeply in the underlying mechanism of strong substrate adhesion and use the XPS spectrum to reveal the intrinsic mechanism.(3)In view of the problem that the high refractive index of ITO confines a large amount of light in OLEDs devices,this work proposes to use silver nanowire thin film as an anode instead of ITO and to prepare corresponding OLEDs devices which can work stably on glass and flexible PET substrate.The current efficiency of the PET substrate relative to the glass substrate is increased by 32%,and the current efficiency of the glass substrate OLEDs device is up to 4.3 cd/A.