Research On Efficient And Stable Thermally Activated Delayed Fluorescent Emitters And Their Relationship With Hosts

High performance thermally activated delayed fluorescence(TADF)materials have attracted much attention in both academic and industry fields.Albeit that extremely high efficiencies have been achieved in TADF organic light-emitting diodes(OLEDs),they still suffer from serious efficiency roll-off and relatively short device lifetimes,which limit their further practical application.Thus,how to suppress their efficiency roll-off and improve their device lifetimes without sacrificing their efficiencies has become an urgent problem to be solved.To answer it,this paper designs and synthesizes a series of TADF emitters and host materials,and fabricates their corresponding TADF devices.The effect of TADF emitters,host materials,and host-guest interaction on the performance of TADF devices are thoroughly discussed.The main results are as follows:1.By introducing heteroatoms O,S and N,six deep-blue/blue TADF materials are designed and synthesized.It is revealed that,for TADF material,reducing the distance from the heteroatom in the donor moiety to the acceptor moiety can effectively increase the contribution of the heteroatom to the natural transition orbits(NTOs)of singlet and triplet,promotes their n-?*transition proportion,and enhances their spin-orbital coupling(SOC)thereby.This will further increase the reverse intersystem crossing rate of TADF material,reduce its triplet exciton concentration,and eventually suppress the efficiency roll-off of its corresponding TADF device without reducing the device's maximum efficiency.As a result,the fabricated deep-blue and blue TADF devices based on BFCZPZ1 and BTCZPZ1 achieve maximum EQEs of 6.5%and 21.1%,respectively.Even at 1000 cd m^-2,they could still maintain 3.8%and 10.6%,respectively.2.By introducing triphenylene moieties,eight N-type host materials are designed and synthesized.It is revealed that,strong?-?interaction between triphenylene moieties in adjacent molecules can make these hosts form an ordered arrangement,which effectively suppresses the fluctuation of intermolecular distance and the fluctuation of hopping-site energy.By further controlling the lowest unoccupied molecular orbitals(LUMOs)to be located on the triphenylene moieties,electron transport pathways along the stacking direction of triphenylene can be formed,which greatly increase the electron mobilities of host materials and the highest value could reach 9.66×10^-4 cm² V^-1 s^-1,As a result,the maximum EQE of fabricated green TADF device is as high as 16.3%,and it can still maintain 15.3%and 13.8%at 5000 cd m^-2 and 10000 cd m^-2.Such low efficiency roll-off can be attributed to suppressed annihilation processes.3.Five host materials with different triplet energy levels,frontier molecular orbital energy levels,and charge mobilities are designed and synthesized.By adopting them,the charge recombination pathways of TADF devices can be controlled.Research on efficiency roll-off reveals that,Langevin recombination(LR)dominated TADF devices exhibit lower efficiency roll-off than trap-assisted recombination(TAR)dominated devices as a result of suppressed triplet-triplet annihilation and singlet-triplet annihilation.Research on device lifetime reveals that,triplet excitons of hosts play a dominant role in device aging.By selecting host materials which exhibit slightly higher triplet energy levels than TADF emitters and device optimization,LR dominated TADF devices can exhibit longer lifetimes than TAR dominated TADF devices.Based on this strategy,an efficient and stable LR dominated blue TADF device is fabricated.At 1000 cd m^-2,its EQE and half-life reaches 17.9%and 269 h.