| Organic light-emitting diodes?OLED?was widly applied in the field of solid-state light source and backlight of full-color display due to its unique advantages of high efficiency,wide viewing angle,high contrast,fast response,lightweight,large-area emission and flexible display,and so on.Usually,fluorescent materials,phosphorescent materials,and thermally activated delayed fluorescence materials?TADF?are the mainly materials for fabricating OLED.According to the theory of electron spin statistics,fluorescence materials can use only singlet?S1?exciton to emission,the internal quantum efficiency of its composed device does not exceed 25%and the power efficiency?PE?is quite low.While phosphorescent materials can achieve 100%internal quantum efficiency by harvested both singlet and triplet?T1?excitons for effectively emission.However,the stability of phosphorescent materials is very poorly,resulting in the lifetime of device cannot meet the requirements of practical applications.Thermally activated delayed fluorescence materials have attracted much attention in recent years are attribute to its no heavy metals,small singlet–triplet splitting(?EST)and can harveste triplet excitons through reverse intersystem crossing from the triplet state to the singlet statefor achieving 100%internal quantum efficiency.At present,the external quantum efficiency?EQE?of monochromatic OLED based on thermally excited delayed fluorescent materials is close to or beyond 30%.However,blue and red thermally excited delayed fluorescent materials composed device can realize high efficiency,but their brightness are very low and stability are very poor,which is one of the bottlenecks affecting on the industrialization of TADF OLED.In this thesis,blue?1,2-bis?carbazol-9-yl?-4,5-dicyanobenzene?2CzPN??and orange?2,3,5,6-tetrakis?3,6-diphenylcarbazol-9-yl?-1,4-dicyanobenzene?4CzTPN-Ph??TADF materials are doped in a bipolar host material as emitters to fabricate monochromatic OLED,and thermally excited delayed fluorescent/phosphorescent hybrid white organic light-emitting diodes?WOLED?is designed to study the effect of exciton behavior on the stability of TADF OLED.Firstly,we designed blue OLED based on the phosphorescent material iridium?III?bis?4,6-?difluorophenyl?-pyridinato-N,C20?picolinate?FIrpic?and TADF material 2CzPN.By optimizing the thickness of hole transport layer,the doping concentration of emitter,the thickness of electron transport layer and exciton blocking layer,we got the highest EQE,PE,and current efficiency?CE?of 26.4%,43.26 lm/W,and 41.33 cd/A for the device based on FIrpic emitter.and the maximum EQE,PE,and CE of 20.2%,41.7 lm/W,and 39.8 cd/A for the device based on 2CzPN emitter,respectively.We also found that removing the exciton blocking layer can slow down the efficiency roll-off,but the efficiency also decreased.Based on this study,we then use 4CzTPN-Ph as the emitter to fabricated orange OLED.Through modulating the doping concentration of the emitter,replacing material and changing the thickness of the exciton barrier layer and electron transport layer,EQEmax,PEmax,and CEmax of the optimized device are 15.56%,39.71 lm/W,and 41.08 cd/A,respectively.The EQEmax of the most stable device is 14.74%,it can still maintain the value of 12.59%at 1000 cd/m2.Finally,thermally excited delayed fluorescent/phosphorescent WOLED was design and optimize with fully considering the efficiency and stability of single-color devices,and the best performance of stable device are 17.40%,32.62 lm/W,and 38.32cd/A for EQEmax,PEmax,and CEmax,respectively.EQE is 17.29%at a brightness of 1000 cd/m2,the quantum efficiency roll-off rate is about0.65%,and Commission Internationale de L'Eclairage?CIE?coordinates is?0.30,0.38?.The reason for the difference in device performance was analyzed by using energy transfer and transmission theory.The research results of this paper will provide a theoretical basis for the preparation of TADF OLED with high efficiency and very stability. |
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