Study On White Thin-film Electroluminescent Devices With Doping-free Phosphorescent EMLs

White organic light-emitting diodes(WOLEDs)have attracted much attention because of their advantages of low-cost,light weight,high-contrast,flexible and fullcolor.With thirty years of rapid development,WOLEDs have seen tremendous progress due to their great potential application in flat-panel displays and solid-state lighting.In this paper,three-colors WOLEDs with doping-free emitting layers are took as the research object.The effects of the emitting layer thickness on the Electroluminescent characteristics and spectral stability of WOLEDs are systematically studied.Firstly,three-colors WOLEDs with anode//B-G-O//cathode sequence are studied.The thickness of the emitting layers is less than 1 nm.3 nm bipolar co-evaporating interlayers are prepared between the emitting layers.The thickness of blue,green and orange emitting layers are changed respectively,and the influence of the emitting layer thickness on the Electroluminescent characteristics and spectral stability of WOLEDs are studied.With the increase of blue and orange emitting layer thickness,the device efficiency decreases gradually.With the increase of blue emitting layer(on the anode side)thickness or the decrease of orange emitting layer thickness(on the cathode side),the spectral stability increases gradually.The green emitting layer plays an important role in the formation of efficient,stable and continuous energy transfer,and its thickness has little effect on the spectral stability.We preliminarily believe that spectral stability is related to energy transfer and carriers trapping on emitting materials.In addition,we use TCTA and TmPyPB as unipolar spacers respectively,the advantages of bipolar coevaporation interlayer in device performance and spectral stability are proved.The current efficiency and power efficiency of the best device are 62.8 cd/A and 65.9 lm/W,respectively.When the brightness ranges from 1000 cd/A to 10000 cd/A,the change of Commission Internationale de L'Eclairage(CIE)coordinates is(0.001,0.002).Then,we change the order of emitting layers and study three-colors WOLED devices with anode//O-G-B//cathode sequence.With the increase of orange and blue emitting layer thickness,the efficiency of the devices decrease gradually.The stability of the spectrum increases with the increase of orange emitting layer(on the anode side)thickness or the decrease of blue emitting layer(on the cathode side)thickness.The thickness of green emitting layer has little effect on the spectral stability.In the sight of the ratio of emitting layer thickness,the spectral stability of the devices decreases with the decrease of the ratio of the anode side emitting layer thickness to the cathode side emitting layer thickness.However,with the same ratio of the emitting layer thickness,the spectral stability of the device does not change significantly with the change of the emitting layer thickness.The current efficiency and power efficiency of the best device are 59.9 cd/A and 60.4 lm/W,respectively.When the brightness ranges from 1000 cd/A to 10000 cd/A,the change of CIE coordinates is(0.004,0.002).Finally,we study the mechanism between the emitting layer thickness and spectral stability.The energy transfer process between different emitting material is verified by studying the transient photoluminescence lifetime.By comparing the performances and spectra of thin film blue probe devices with different locations of blue emitting layer and reverse order three-colors WOLEDs,it determines that the exciton concentration decreases gradually from the anode side to the cathode side.The results show that the spectral stability is affected by energy transfer,exciton concentration and trapping effect of emitting material.Therefore,the same ratio of emitting layer thickness has little effect on device performance,while the thicker the anode side emitting layer or the thinner the cathode side emitting layer is,the better the spectral stability becomes.