Study Of Recombination Dynamics And Stability In High Efficient Thermally Activated Delayed Fluorescence Organic Light Emitting Diodes

Servicing as third generation of Organic light emitting materials for organic lightemitting devices(OLEDs),thermally activated delayed fluorescent(TADF)materials exhibit Excellent performance,strong system compatibility and enormous market potential,and have been widely regarded in both academia and industry as a promising technology for solid-state lighting,flat-panel displays and flexible displays.Although the tremendous progress has been made,OLEDs are still facing challenges related to its efficiency and stability for practical application and commercialization.Leaving out the external elements such as temperature and the erosion of water or oxygen in the air,all these challenges are related to fundamental dynamics of excited-state in materials and devices,such as the formation and decay of excitons and their dynamic process.Following electrons and holes recombination,singlet excitons,triplet excitons and polarons are generated,after which they relax to ground state or interact with each other,which has significant influence on device performance and stability.Therefore,it is necessary to fully investigate and understand the fundamental mechanisms of the dynamics of these excitons and polarons.In this dissertation,we base on OLEDs using TADF materials,aim at investigating both photo-induced and electric-induced dynamic process of singlet and triplet excitons in TADF based devices,the excitation characteristics of organic semiconductors and the basic physical mechanisms controlling the performance and stability of OLEDs.There are 3 key issues discussed we focus on:At first we studied the luminance mechanism of TADF OLEDs based on solution process.By employing TADF material 4Cz IPN,we fabricate high performance TADF OLEDs without using heavy metal atoms.Through transient EL measurement,we systematically studied the carrier injection and exciton movement,the luminance mechanism is subsequently clarified.The underlying cause and general physical laws inside the OLEDs are investigated to offer some guidance for device optimization.Secondly we foucs on the excited-state dynamics upon both photo-and electroexcitation of TADF molecules.We characterize the TADF properties of 4Cz IPN and its host dependence of photophysical properties to determine a suitable host for solutionprocesseed OLEDs.We also report the dynamics of singlet and triplet exctions in TADF OLEDs via transient electroluminescence(EL)measurement,clarify the underlying reason for the efficiency roll-off of the TADF OLEDs.The further understanding of the excited state dynamics upon both photo-excitation and electro-excitation offer systematic analysis to the operating mechanism in the TADF based OLEDs.Thirdly,by using TADF materials as host matrix,we fabricate OLEDs with outstanding efficiency as well as stability.We compare the stability performance of OLEDs in which one of two TADF materials are used as hosts in the emitting layer against those using the well-known host material CBP.Phosphorescent or fluorescent emitters are used as guest materials in the EMLs of these devices(referred to herein respectively as Ph OLEDs and FOLEDS).We realize lifetime of more than 230 hours at an initial luminance of 10,000 cd/m2 and enhanced external quantum efficiency(EQE)in TADFhost OLEDs.We found that the use of TADF materials doesn’t significantly reduce T1 concentration in the emitting layer(EML),the enhancement in operational lifetime is most related to the morphological stability of TADF materials.Moreover,by doping TADF material into hole transport layer to suppress the molecule aggregation,the device stability is further increased to 350 hours.