Study On Efficiency Roll-off Regulation And Stability Of Solution-Processed Organic Electroluminescent Devices

Organic light-emitting diodes（OLEDs）have huge commercial potential in flat panel displays and solid-state lighting applications.Thermally activated delayed fluorescent（TADF）devices prepared by vacuum evaporation method can achieve an internal quantum efficiency（IQE）of nearly 100%with an external quantum efficiency（EQE）over 37%,avoiding the over-reliance on the phosphorescent materials containing noble metals.However,the solution-processed TADF-OLEDs still face many challenges.First of all,although many TADF molecules with high efficiency have been developed,only a few of them can be used in solution processes with sufficient retention properties.Secondly,almost all solution-processed TADF devices face severe efficiency roll-off.Finally,although the devices prepared by vacuum evaporation can achieve a long lifespan,especially phosphorescent devices,the lifetime of solution-processed devices is rarely reported.In order to realize the commercialization of solution-processed OLEDs,the lifespan of devices is an urgent problem to be solved.In view of this,this paper mainly explores the key problems in the above solution-processed devices from the two main directions of host materials and TADF guest materials.In chapter 2,we designed a series of blue-greenish TADF materials by introducing third-generation dendritic carbazole units with steric hindrance and hole-transporting ability to block the 3 and 6 active sites of acridine.These molecules simultaneously have excellent thermal stability,electrochemical stability and solvent resistance.In addition,by designing the sulfur-containing bispiral structure as the donor unit,the intramolecular charge transfer（ICT）interaction was weakened,and the blue-shift of emission peak was realized.More importantly,based on the above characteristics,these dendritic TADF molecules have a significantly shortened excited state lifetime which demonstrated a very fast reverse intersystem crossing（RISC）process,and finally achieved a significant improvement in efficiency roll-off of blue-greenish solution-processed devices.Among them,the t-Bu-DTAZ-based device exhibited an extremely low turn-on voltage of 2.7 V,achieving a maximum external quantum efficiency of10.77%with almost no efficiency decline at the luminance of 1000 cd m^-2,which is one of the best results of non-doped blue-greenish solution-processed devices.In chapter 3,we used three solution-processed host materials-DMBN-PDC,DMBN-PTC and MBN-PTC based on benzocyano acceptor units to achieve efficient sky-blue solution-processed TADF-OLED with low efficiency roll-off.These host materials showed gradually decreased triplet energy level to approaching the guest material-5TCz BN.In the process of pulling down the energy level of excited states,the extra-long-lifetime excitons of guest were gradually quenched.Despite the sacrifice of fluorescence quantum yield,the efficiencies of devices were preserved,and the efficiency roll-off at high brightness was also improved.Among them,the sky-blue solution-processed device based on DMBN-PTC with the moderate triplet energy level exhibited an EQE of 12.41%with a CIE coordinate of（0.18,0.36）.The efficiency can still maintain 68%of the maximum efficiency when operating at 1000 cd m^-2.Through the optimization of device structure,a more improved efficiency roll-off was achieved,that is,it can still maintain 91%of the maximum efficiency value at 1000 cd m^-2,which is superior to the most reported blue solution-processed TADF-OLEDs.In chapter 4,we evaluate the bond stability via quantitative calculations,the photo-stability via photophysical characterization and the electrochemical stability via cyclic voltammetry for comparing their chemical stability.From the investigation results,it is found that the chemical stability of m CPCN and Cz Si fall far behind the dendritic host materials.Therefore,we selected t-Bu TCz BP,t-Bu TCz BPy and t-Bu TCz BDPy for in-depth exploration of the relationship between host performance and device stability.Including the evaluation of the carrier transporting capacity of the hosts and photo/electrochemical oxidation experiments of the emissive layer,the further understanding of intrinsic factors in the device aging process was attained.Blue-greenish solution-processed devices based on t-Bu TCz BP:40%5TCz BN,t-Bu TCz BPy:40%5TCz BN and t-Bu TCz BDPy:40%5TCz BN can maintain 81%or more of the maximum external quantum efficiency at 1000 cd m^-2,which are better than most of the reported blue-greenish solution-processed TADF-OLEDs.Among them,t-Bu TCz BPy showed high anionic BDE of the C-N bond,excellent photo-stability,electrochemical stability and electron-transporting capability which promotes a balanced carrier transportation in doped devices,so its doped device showed the longest lifetime.After further optimizing the annealing temperature,the device lifetime was significantly increased to 28 h at the initial brightness of500 cd m^-2,which is the best performance of the currently reported blue-greenish solution-processed TADF-OLED.