Preparation And Properties Of Thermal Activated Delayed Fluorescence Based Solution-Processed Hybrid White Electroluminescent Devices

Since the first report of white organic light-emitting diodes(WOLEDs)in 1995,the application of WOLEDs in the fields of next-generation displays and comfort lighting sources has attracted extensive attention.Generally,white light devices mainly use vacuum vapor deposition process to build a series structure,and materials with different light colors are stacked together to form a wide spectrum emission.Although this kind of devices can obtain high efficiency electroluminescence,the vacuum vapor deposition preparation process has some disadvantages,such as high cost,complex process,serious waste of materials and difficulties in fabricating large-area devices.Compared with the vacuum vapor deposition process,the solution processing technology has the advantages of low cost,relatively simple process,precise control of doping concentration,etc.,and has broader application prospects in the preparation of large-area and flexible devices.In recent years,WOLEDs based on solution processing technology have achieved rapid development.However,solution-processed WOLEDs still face some urgent problems,such as the erosion and penetration of the solvent into the film,which will lead to defects in the internal film morphology of the device,poor solubility of luminescent materials,and poor device efficiency and stability.Therefore,it has great significance to explore an efficient and stable solution processing WOLEDs.In this dissertation,the red,green and blue primary color thermally activated delayed fluorescence(TADF)materials are used as the research subject.First,solution-processed TADF-OLEDs are prepared,and the devices performance is optimized.Further,the corresponding complementary color luminescent materials are selected to construct efficient and stable solutionprocessed hybrid WOLEDs.This paper focuses on the construction of white light devices,performance optimization and device luminescence mechanism,and provides ideas for obtaining high-efficiency solution-processed hybrid WOLEDs.The research content of the dissertation is divided into the following three parts:(1)An efficient solution-processed TADF-OLED was prepared by using the green TADF material DPS-24Ac as the dopant.The research results show that when mCP is used as the host material,the device achieves the best performance,with a maximum external quantum efficiency(EQE)of 21.73%,an emission wavelength of 504 nm and a Commission International de l’Eclairage(CIE)coordinate of(0.22,0.44).Further,a hybrid white light device was prepared by using red fluorescent,phosphorescent or TADF materials as complementary color light-emitting dopants.When DPS-24Ac and TS-1 were used as green and red dopants,respectively,the device obtained a maximum EQE of 13.36%,a maximum power efficiency(PE)of 18.42 lm/W,and a CIE coordinate of(0.31,0.46),the color rendering index(CRI)of 76 warm white light emission.The experimental results show that by optimizing the doping concentration of the material,the doping of green and red guests can effectively construct warm white OLEDs.(2)An efficient solution-processed TADF-OLED was prepared by using the orange-red TADF material TS-1 as the dopant.The research results show that when CBP is used as the host material,the device achieves the best performance,with a maximum EQE of 12.58%,an emission wavelength of 608 nm and a CIE coordinate of(0.58,0.41).The carrier transport test proves the exciton recombination region.The change affects the efficiency of the device.Furthermore,a hybrid white light device was prepared by using sky blue phosphorescence or TADF material as a complementary color luminescent dopant.When TS-1 and CzAcSF were used as orange-red and sky-blue dopants,respectively,the device obtained a maximum EQE of 13.52%,a maximum PE of 18.08 lm/W,a CIE coordinate of(0.31,0.37),and a color rendering index of 77 standard white light emission.The experimental results show that the addition of the hole transport layer by the orthogonal solvent method can effectively improve the performance of solution-processed OLEDs;and by optimizing the doping concentration of the materials,the doping of sky blue and orange-red light dopants can effectively construct standard white light OLEDs.(3)An efficient solution-processed TADF-OLED was prepared by using the sky-blue TADF material TTT-Ph-Ac as the dopant.The research results show that when mCPCN is used as the host material,the device achieves the best performance,with a maximum EQE of 23.23%,an emission wavelength of 492nm,and a CIE coordinate of(0.19,0.36).We demonstrate that carrier transport balance can improve device efficiency through detailed mobility tests.Further,a hybrid white light device was prepared by using red fluorescent,phosphorescent or TADF material as a complementary color light-emitting dopant.When TTT-Ph-Ac and Ir(piq)2acac were used as sky-blue and red dopants,respectively,the device obtained a maximum EQE of 22.57%,a maximum PE of 20.88 lm/W,a color coordinate of(0.34,0.35)and the standard white light emission with a color rendering index of 73.The experimental results show that by changing the thickness of the hole transport layer,the effective recombination of excitons can be promoted and the quenching of exciton concentration can be reduced;the doping of sky-blue light and red light dopants can effectively construct standard white light emission;the short luminescence lifetime of the red dopant helps to improve the spectral stability of the device at different voltages.