Synthesis And Electroluminescent Properties Of Novel Nitrogen Heterocyclic-Based Thermally Activated Delayed Fluorescence Materials And Host Materials

Organic light-emitting diodes?OLEDs?have attracted extensive academic and industrial research since they have been widely used as screens in flagship mobile phones of major manufacturers and their great potential in solid-state lighting.The vast market demands accelerated the search for newly efficient and stable materials.Thermally activated delayed fluorescence?TADF?materials,due to the intrinsic small singlet-triplet energy gap(?E_ST),can efficiently utilize the high concentration triplet excitons through the reverse intersystem crossing?RISC?process in electroluminescence?EL?that traditional fluorescence materials cannot utilize,thus achieving nearly 100%high internal quantum efficiency?IQE?.At the same time,compared with phosphorescence materials using heavy metal atoms,purely organic TADF OLEDs have achieved similar high efficiency as phosphorescent OLEDs,which shows considerable application prospects.However,vacuum evaporation is still the main processing technology,and the efficiency roll-off control of solution processed devices is still not ideal in the cheaper methods.Considering the molecular design strategies,especially for non-doped systems,were mainly confined to aromatic amine units with large torsion,which limits the expansion of TADF materials and narrows the scope of screening commercially efficient and stable materials.In this paper,we mainly developed the host materials which can be used to improve the device efficiency roll-off of the solution processed ones and the highly efficient non-doped single-molecule TADF fluorescence systems based on aromatic amine free units.In the second chapter,we have developed a series of bipolar transporting host materials based on nitrogen heterocyclic acceptors and multi-generation dendronized carbazole units.By introducing dendritic units,on the one hand,the molecular weight of the materials is increased,thereby improving their thermal stability and solvent resistance,making them more suitable for solution processing.On the other hand,the solubility of the materials is increased,and the film forming ability of solution processed system is improved to avoid the damages of morphological defects to the efficiency and stability of devices.The introduction of tert-butyl terminal group improved the electrochemical stability of the material,which is complementary to the photophysical stability of the material and effectively avoided the luminescent quenching caused by the host material.Among them,solution-processed OLED devices with bipolar transporting material mCDtCBPy as the host still remain 80.40%of the EQE_max at 1000 cd m^-2,and can achieve the maximum brightness of 15571 cd m^-2.It is one of the reported best results of efficiency roll-off control and device brightness of solution processed sky blue TADF devices.In Chapter 3,we designed and synthesized a series of D-A structured luminescent molecules based on aromatic amines free dibenzothiophene,thianthrene donor and diphenyltriazine acceptor.Charge is transferred through the aryl linker and through space simultaneously in the ortho connected molecule:oTE-DRZ.The combined charge-transfer path and n-?*transition characteristics of the triazine unit allow very small?E_ST and enough large transition dipole moment at the same time.Thanks to its unique molecular stacking,the quenching effect caused by F?rster energy transfer is completely restrained,to give high photoluminescence quantum yield in both doped and neat films.Through theoretical calculations,photophysical property characterization,single crystal structure analysis,etc.,we compared the performance of traditional fluorescent molecule oDBT-DRZ,mTE-DRZ with the TADF molecule oTE-DRZ,and explained the relationship between the single molecular TADF luminescence phenomena and the molecular design strategies.The green emission?CIE=0.31,0.57?with a maximum external quantum efficiency of 20.45%can be achieved by using oTE-DRZ neat film as the emission layer of a non-doped OLED device.This is the first high-efficiency single-molecular TADF based on aromatic amine free donors and used in highly efficient non-doped OLED devices.It broke the limitation of high-efficiency TADF luminescent molecules confined to aromatic amine units and provided a new idea for expanding the high-efficiency TADF luminescent systems.In Chapter 4,according to the design idea of TADF molecule based on non-aromatic amines unit in Chapter 3,we regulated the luminescent color by adjusting the electron donating/accepting ability of the donor/acceptor,or change the number of donor units,and realized the emissions from blue to yellow,so as to meet the requirements of flat panel display for RGB color emission and the needs of solid-state lighting for white emission.At the same time,the introduction of multi-donor unit increases the radiation transition channel,shortens the lifetime of triplet excitons,effectively avoids the bimolecular quenching effect and improves the device efficiency roll-off.By the means of theoretical calculation,the steady-state spectra and the characterization of transient PL decay curves under different conditions,we have verified our molecular design strategies.Non-doped OLED devices with oPXT-DRZ and 3oTE-DRZ as emission layers can achieve maximum external quantum efficiencies of 15.92%and 11.39%,respectively.For the first time,non-doped highly efficiency blue and yellow TADF emission based on unconventional aromatic amine donors are realized,and the molecular system based on non-aromatic amine TADF has been expanded,which proves the feasibility of our molecular design idea and provides a possibility of realizing simple,high-efficiency white emission OLEDs.