Study On The Doped Organic Light Emitting Diodes Based On Thermally Activated Delayed Fluorescence Materials And Related Electroluminescence Mechanisms

In the filed of both display and illumination,increasing the luminescence efficiency of the device has always gained continuous attention.Recently a new mechanism named thermally activated delayed fluorescence(TADF)has been systematically studied and reported by Adachi's group.According the theory of TADF,it provides a promising approach for high internal quantum efficiency closed to 100%.Futhermore,most TADF materials don't contain rare metal atoms,leading to a fact that TADF materials have been regarded as lower-cost and greener substitutes for phosphorescent materials.Up to now,numerous TADF OLEDs which emit light ranging from blue to red have already been discovered.However,most of these state-of-art reports are mainly attributed to the enormous progress in the TADF molecule design.Fundamental investigations in the detailed emission mechanism and device construction are relatively rare.The related points of the work were summarized as followed:In the first chapter,we introduce the basic knowledge and the research progress of organic electroluminescent technology in recent years.And then the design idea and teoretical basis were expounded.In the second chapter,by fine-tuning the host-guest compatibility in OLEDs based on the green TADF guest 4CzPN,we introduce two different electroluminescence mechanisms and investigate the influence of them on device perormances.At last,by facilitating the trap-assisted recombination mechanism and reducing the accumulation of interfacial charges in OLEDs based on the emissive layer of 3-CzTHZ doped with 4CzPN,we realized an efficient green TADF OLEDs with a maximum external quantum efficiency(EQE),power efficiency(PE)and current efficiency(CE)of 22.2%,65.9 lm/W and 76.0 cd/A,respectively.In the third chapter,by tuning the doping concentration in OLEDs based on a new designed and synthesized TADF guest SPFPTr,we investigate the relationship between the doping concentration,electroluminescence mechanism and device performance.At last,by facilitating the trap-assisted recombination mechanism in OLEDs based on the emissive layer of DPEPO doped with SPFPTr,we realized an efficient blue TADF OLEDs with a maximum EQE,PE and CE of 21.4%,32.70 lm/W and 37.52 cd/A,respectively.In the fourth chapter,we choose two host materials,CBP and SPFPTr for a new designed and synthesized TADF guest DTPATXO and construct a series of OLEDs.According to our work,we demonstrate that the host?guest energy transfer mechanism is more suitable for TADF guests which should be doped at a low concentration.At last,we realized an efficient yellow TADF OLEDs based on the emissive layer of SPFPTr:DTPATXO/DPEPO:SPFPTr/SPFPTr:DTPATXO,with a maximum EQE,PE and CE of 21.69%,67.28 lm/W and 64.85cd/A,respectively.In the fifth chapter,we combined external heavy atom effect with TADF and investigated the influences of synergistic effect on device performances based on the TADF guest 4CzPN and an ultra thin FIrpic layer.At last,we realized an efficient green TADF OLEDs with a maximum EQE,PE and CE of 17.9%,50.5 lm/W and 61.1 cd/A,respectively.In comparison with the control device without heavy atoms,the enhancement of maximum external quantum efficiency is 38%.