Synthesis And Properties Of Thermally Activated Delayed Fluorescence Materials Based On Quinoxaline Derivatives

Organic light emitting diodes?OLEDs?based on thermally activated delayed fluorescence?TADF?materials without any noble-metals are attracting growing interest due to their potential in achieving 100%internal quantum efficiency?IQE?in theory via efficient up-conversion from non-radiative triplet?T1?to radiative singlet?S1?,Over the past few years,the record-high external quantum efficiencies?EQEs?over 30%have been realized for blue and green TADF-OLEDs,and the performance of some TADF-OLEDs even exceeds the level of that of POLEDs.Red TADF-OLEDs have a wide range of applications in bio-imaging,sensors,communications,night vision and other fields.But,the EQEs of orangered to red TADF-OLEDs with emission maxima beyond 580 nm rarely exceed 25%.Due to the increase of non-radiative transition rate caused by the energy gap law and the serious triplet-triplet exciton annihilation under high current density,the development of red light is relatively slow.Therefore,it is necessary to design new types of acceptors,by introducing different types of donors to adjust the molecular structure and photophysical properties to obtain excellent OLED device performance.This thesis mainly studies a new type of thermally activated delayed fluorescent material system based on quinoxaline derivatives as electron acceptors.It mainly includes the following research work:In chapter ?,by using triphenylamine as the electron donor and cyanoline as the electron acceptor,we designed and synthesized two"cross"type D-A-D orange-red to red TADF materials,named TPA-APQDCN and TPA-DBPDCN.We prepared devices with different doping concentrations based on these two materials,and their optimal doping concentrations were both 10 wt%.TPA-DBPDCN-based device exhibited a standard red emission with peak at 648 nm,corresponding to the Commission International de L'Echairage?CIE?coordinates of?0.66,0.32?,and with EQE_max of 13.0%,CE_max of 6.3 cd A^-1,PE_max of 5.5 lm W^-1.However,it shows a serious efficiency roll-off at high current densities.The EQE values only remained 1.1%at 1000 cd m^-2.TPA-APQDCN-based device exhibited an extremely high EQE_max of 31.3%with peak emission of 610 nm,corresponding to CIE coordinate of?0.59,0.41?,corresponding to CE_max of 49.8 cd A^-1,PE_max of 59.0 lm W^-1.Similarly,the EQE values only remained 2.7%at 1000 cd m^-2.In chapter ?,we designed and synthesized three orange-red-to-red TADF materials based on benzopyrazine acceptors and phenoxazine donors by adjusting the conjugation degree of the acceptor and the number of electron donors,named PXZ-PQM,DPXZ-PQM and DPXZ-DPPM.Theoretical calculations and tests show that these three molecules have high fluorescence quantum efficiency while the HOMO and LUMO are highly separated.The singlet-triplet energy level differences(?E_ST)are 0.03,0.02,and 0.05 eV,respectively.Among them,the molecular DPXZ-PQM exhibits the best device performance.The doped film showed high fluorescence quantum efficiency of 88%.The DPXZ-PQM-based deviec display orange-red emission with peak at 610nm,corresponding to CIE coordinates of?0.51,0.48?.And it shows a maximum external quantum efficiency(EQE_max)of 26.0%.Obviously,a state-of-the-art EQE of 26.0%is among the highest values for TADF-OLEDs emitting in orange-red to red range without any light out-coupling enhancement More importantly,at luminance of 100 and 1000 cd m^-2,the EQE values are still remains at a high level of 20.1%and 13.7%.Therefore,this research report provides some design ideas for the development of orange-red to red TADF-OLEDs which can achieve both high EQE and low efficiency roll-off.