Synthesis And Optoelectronic Properties Of Phosphorescent Iridium Complexes Sensitized By Thermally Activated Delayed Fluorescence

Organic light-emitting diodes(OLEDs)have aroused wide interest in academia and industry over the last three decades due to their great potential in the new-generation flat-panel displays and solid-state lighting.The phosphorescent iridium complex is considered to be the most promising second-generation organic emitter.Attributed to the strong spin-orbital coupling(SOC)effect of the heavy metal atom,singlet and triplet excitons can be simultaneously utilized and nearly 100%internal quantum efficiency(IQE)can be achieved.Thermally activated delayed fluorescence(TADF)material is generally considered to be the third-generation organic emitter and is the most active filed of OLED at present.The TADF materials can also utilize singlet and triplet excitons through the fast reverse intersystem crossing(RISC)process,which results in the realization of 100%IQE.In general,the organic emitter tends to aggregate at high concentration due to the strong intermolecular interaction,which inevitably results in concentration quenching and triplet annihilation.In order to get better device performance,the organic emitters are generally doped in the host-matrix with a low concentration.However,the energy transfer from the conventional fluorescent host to the phosphorescent iridium complex is inefficient,which lead to the triplet decay through non-radiative transition and is not favorable for the improvement of device performance.Besides,the wide emission spectrum of TADF material results in the low color purity and the long triplet lifetime of TADF material results in the significant efficiency roll-off.In this thesis,we employ the TADF-sensitized phosphorescence strategy to deal with these challenges.A series of high-efficiency phosphorescent iridium complexes sensitized by various of TADF materials were designed and synthesized to achieve better device performance.These results provided new insights into the development of highly efficient OLEDs.In chapter 1,it starts with a brief introduction of the development and basic principles of OLEDs.Then,the three kinds of organic emitters are introduced,and the energy transfer mechanism of the TADF-sensitized phosphorescence is elaborated.Finally,the main design tactics and contents of this thesis are illustrated.In chapter 2,two novel yellow iridium complexes namely(EtCztPy)₂Ir(acac)and(tpCztPy)₂Ir(acac)were designed and synthesized by the combination of thieno[3,2-c]pyridine with a rigid carbazole moiety.By blending a TADF sensitizer in the emitting layer,device performance was dramatically improved due to the intermolecular sensitization.The maximum external quantum efficiencies(EQE_max)were 6.3%for the sensitized device based on(EtCztPy)₂Ir(acac)and 5.8%for that of(tpCztPy)₂Ir(acac),respectively,which were 4.8-7.8 times higher than those of the devices with traditional single host.In chapter 3,employing 6-phenylphenanthroidine derivative as the main ligand,two deep-red iridium complexes namely(TP-BQ)₂Ir(PXZ)and(TP-BQ)₂Ir(DMAC)were designed and synthesized by directly introducing two TADF-typed ancillary ligands with?-diketone units.The results of theoretical calculation demonstrated that the lowest excited states of the two iridium complexes are largely governed by the iridium cores and less affected by the ancillary ligands.Besides,the structures of the ancillary ligands are not affected by the coordination,and the RISC processes of the TADF-typed ancillary ligands can be preserved.Due to the effective self-sensitization of the TADF-typed ligands,the electroluminescent performances of(TP-BQ)₂Ir(PXZ)and(TP-BQ)₂Ir(DMAC)were almost twice higher than that of the prototyped(TP-BQ)₂Ir(acac).The optimized device based on(TP-BQ)₂Ir(DMAC)achieved an EQE_maxax of 10.5%with the emission peak at 650 nm.In chapter 4,employing the typical blue iridium complex FIrpic as the emissive core,an efficient blue iridium complex namely FIrpic-DMAC was designed and synthesized,in which the TADF unit was grafted into the phosphorescent iridium complex with a flexible alkyl chain.By grafting the TADF sensitizer into the iridium complex,FIrpic-DMAC displayed the improved film-forming ability,the suppressed interaction between emissive iridium cores and the self-sensitization due to the effective intramolecular energy transfer.As a result,the non-doped and doped device based on FIrpic-DMAC achieved the EQE_max of 11.5%and 18.9%,respectively.Due to the effective intramolecular self-sensitization and intermolecular sensitization in the emitting layer,the two-component warm-white device based on FIrpic-DMAC achieved an EQE_max of 21.1%.These efficiency values were significantly improved in comparison with the commercial phosphor of FIrpic.