Syntheses And Optoelectronic Properties Of Thermally Activated Delayed Fluorescent Emitters Based On Nitrogen-containing Heterocyclic Acceptors

After three decades of development,organic light-emitting diodes(OLEDs)have held great promise in the application of display and lighting.The traditional fluorescent materials can only utilize 25%singlet excitons.Owing to the strong spin-orbit coupling effect derived from the involved Ir(III)or Pt(II),the phosphorescent materials can achieve full utilization of singlet and triplet excitons generated during the electroluminescence process.However,the high cost of heavy-metal complexes and the instable defects of blue phosphors still extremely restrict their further application.Since2012,organic thermally activated delayed fluorescence(TADF)materials have attracted world-wide attention in academic and industrial communities.The OLEDs based on TADF materials can harvest both singlet and triplet excitons to approach 100%exciton utilization efficiency through an environmentally thermally activated reverse intersystem crossing(RISC)process.TADF materials are required to have small enough singlet-triplet gap to ensure rapid RISC rate and large radiation transition rate to guarantee the high photoluminescence quantum yield(PLQY).Such a critical design principle limits the species richness of TADF materials.With the intension of resolving these challenges,herein,we developed a series of efficient TADF materials based on nitrogen-containing heterocyclic acceptors and regulated the emission peaks from green to deep-red.The research enriches the variety of TADF materials,affording some new insights into the development of highly efficient TADF emitters and devices.In chapter 1,it begins with a brief introduction of the background and history of organic electroluminescence,the luminescent mechanism,composition and performance evaluation parameters of OLEDs.Then an emphasized review on the research progress of evaporation-or solution-process feasible TADF materials is elaborated.Finally,the main design strategy is exposited.In chapter 2,two green TADF materials,PyB-DMAC and PyB-DPAC,were constructed based on 4-benzoylpyridine acceptor and acridine donor.The large torsional angles between the donor and acceptor units not only endows them with small?E_ST for ensuring TADF character but also affords remarkable aggregation-induced emission(AIE)feature.The phenyl-substituted acridine as the donor endows the PyB-DPAC with higher molecular weight and larger rigidity.Therefore,the PyB-DPAC exhibits better thermal stability and higher PLQY.As a result,the non-doped vacuum-deposited device based on the emitter of PyB-DPAC achieves outstanding EL efficiencies with the maximum external quantum efficiencies(EQE_max)of 9.7%.Moreover,its EQE remains as high as 9.0%at the practical luminance of 1000 cd m^-2,accompanied with an impressive slow efficiency roll-off of 1.7%.What's more,PyB-DPAC-based solution-processed non-doped device accomplishes superior EL efficiencies with the EQE_max of 11.1%,which is outstanding among the non-doped solution-processed green devices.In chapter 3,naphthyridine or cyano-naphthyridine unit was chosen to be an electron acceptor,in virtue of its more rigid planar structure and stronger electron-withdrawing ability.Two new emitters,namely ND-AC and CND-AC,featuring an acridine donor were designed,synthesized and investigated.The nearly orthogonal molecular configuration of target emitters not only endows them with small energy differences between singlet and triplet states for ensuring TADF character but also afford remarkable AIE feature.Attributed to the high photoluminescence quantum yields,excellent TADF and AIE characteristics,the doped and non-doped OLEDs based on ND-AC exhibit outstanding performances with EQE_max of 16.8%and 12.0%,respectively.These efficiencies of the non-doped device are outstanding among previously reported non-doped fluorescent emitters with a similar color gamut.In chapter 4,two deep-red TADF emitters,HATNA-tCz and HATNA-tPCz,were constructed by combining carbazole-derivative donors with 5,6,11,12,17,18-hexaazatrinaphthylene(HATNA)acceptor core.HATNA was chosen as the acceptor unit for its extremely large and rigid?-conjugate structure with strong electron-deficiency property.3,6-bis(4-tert-butyl)carbazol(tCz)and 3,6-bis(4-tert-butylphenyl)carbazol(tPCz)were selected to be the donor units for their excellent hole-transporting capability and suitable electron-donating ability.In addition,the peripheral tertiary butyl can not only greatly improve the solubility but also partly diminish aggregation-induced quenching(ACQ)process.Both emitters exhibited distinct TADF characteristic and the solution-processed OLED based on HATNA-tPCz achieved a maximum EQE of 4.8%with emission peak of 692 nm,which is prominent among the reported solution-processed deep-red OLEDs.