Design And Synthesis Of Novel Exciplex Emitters For High Performance Organic Light-emitting Diodes

Thermally activated delayed fluorescent（TADF）emitters have been extensively studied in organic light emitting diodes（OLED）because they can enable efficient upconvert triplet excitons to singlet excitons and radiative decay from singlet state to ground state,realizing the 100%internal quantum efficiency.As an important category of TADF emitters,exciplex emitters are extensively applied in organic optoelectronics owing to their outstanding optical and electronic properties.However,the development of exciplex emitters and their corresponding OLEDs is still confronted with some serious challenges:i)poor device efficiency;ii)much-needed boost for the device stability;iii)limited luminous region.To address these problems,based on the working mechanisms of exciplex emitters and the conrresponding OLEDs,this dissertation mainly focuses on the core issue of exciton conversion and utilization during the luminescence process,and a series of new exciplex emitters and highly efficient exciplex-based OLEDs were successfully developed.Some achievements are listed as below:1.High efficiency and stability of exciplex-based OLEDs were prepared by innovatively introducing intermolecular hydrogen bonds（HBs）between D and A molecules to suppress non-radiative transitions.For the exciplex emitters in this chapter,the 13PXZB:B4Py MPM with more intermolecular HBs exhibits a higher photoluminescence quantum yield(Φ_PL)of 69.6%and lower triplet non-radiative decay rate constant(k^T_nr)of 3.4×10⁵ s^-1.In OLEDs,13PXZB:B4Py MPM realizes a maximum external quantum efficiency（EQE）of 14.6%.Meanwhile,the half lifetime(LT₅₀)of13PXZB:B4Py MPM-based OLED is 4.8 and 8.2 times higher than those of13PXZB:B3Py MPM-and 13PXZB:B2Py MPM-based OLEDs.By comparing the performances of 13AB-and 13PXZB-based OLEDs,the huge difference demonstrates that intermolecular HBs between D and A molecules can directly enhance the exciplex-based device performance,and provide a new routine for the development of efficient and stable exciplex emitters.2.Considering that the exciplex emitters processed by solution process would encounter ineffective contact between D and A molecule and harmful intermolecular aggregation,a strategy of modulating the intermolecular interactions of the solution-processed exciplex by intermolecular HBs formed between D and A was proposed.Highly-efficient exciplexes were constructed benefiting from the suppression of non-radiative energy loss.Accordingly,three new materials namely t MCz-DPy,t MCz-DPm and t MCz-TRZ were firstly designed and synthesized to act as acceptors and constructed three exciplex emitters BP-PXZ:t MCz-DPy,BP-PXZ:4MCz-DPm and BP-PXZ:4MCz-TRZ.By comparing their photophysical of their solution-processed and vacuum-deposited films,with intermolecular HBs assisted,BP-PXZ:t MCz-DPy and BP-PXZ:t MCz-DPm prapred by solution-processed showed a higherΦ_PLs of 68.7%and 57.4%as well as lower k^T_nrs of 1.33×10⁵ s^-1 and 2.11×10⁵ s^-1.In the vacuum-deposited and solution-porcesed OLEDs using the three exciplex emitters,the BP-PXZ:t MCz-DPy-based solution-processed device exhibited a maximum EQE of 15.3%and long-term LT₅₀of 18.2 hours at an initial of rough 100 cdm^-2,further demonstrating the significance of intermolecular HBs between D and A molecules for the development of solution-processed exciplex emitters.3.Proposing a novel strategy of constructing exciplex emitters by introducing a suitable third component to realize multiple reverse intersystem crossing（RISC）channels in the exciplex systems,both the conrresponding excitons utilization and the reverse intersystem crossing efficiency of triplet excitons were further enhaced.Accordingly,a new TADF emitter DBT-SADF with a high T₁ level and suitable HOMO level was designed as a component to build a series of exciplex emitters.Among them,tricomponent exciplex DBT-SADF:PO-T2T:CDBP with three RISC channels showed a much higherΦ_PL of 61.0%and a rate constants of RISC process(k _RISC)of 14.2×10⁵ s^-1than the corresponding bicomponent exciplexes.In OLEDs,DBT-SADF:PO-T2T:CDBP exhibited a maximum EQE of 20.5%.The outstanding performance is among the best result of the reported exciplex-based OLEDs at that time,further proving the significance of multiply channels between D and A molecules for the development of high efficient exciplex emitters.4.Innovatively introduced dendritic large molecules as the third component into exciplex emitters to relax the obligatory requirement on the solubility of constituent materials in solution-processed exciplexes and improve the exciton utilization of exciplex emitters,ultimately high efficient and stable exciplex-based OLEDs were successfully realized.With the the introduction of dendritic large molecules,DBT-DMAC,SSP,DMAC-DPS with good solubity as well as SOSP with a poor solubility were designed to be as donors and successfully construct a series of tricomponent exciplex emitters with acceptor PO-T2T.Compared with binary exciplex emitters without the dendritic large molecules,theΦ_PLs,k^T_nrs and film quality of the tricomponent exciplex emitters were remarkably improved.In solution-processed OLEDs,DBT-DMAC:PO-T2T:Sim CP3-Ph,SSP:PO-T2T:Sim CP3-Ph and SOSP:PO-T2T:Sim CP3-Ph by using the dendritic large molecule Sim CP3-Ph as third component respectively exhibited a maximum EQEs of26.1%,22.8%,28.6%and 24.1%,and LT₅₀ of 3.4,4.1,10.2 and 0.8 hours at an initial of rough 200 cdm^-2.The outstanding performance is among the best result of the reported exciplex-based OLEDs at that time.5.Considering the D:A mixed exciplexes with deep-red/near-infrared（NIR）emissions were still rarely studied and reported,an efficient method was accordingly presented by introducing a phosphor as one component.The non-radiative transitions of triplet excitons were inhibited owing to spin-orbit coupling of heavy metal,further improving the light efficiency of deep-red and NIR exciplex emitters.Accordingly,a suitable phorphos Ir-817 was used as donor to farbricate a series of exciplex emitters with different electron-accepting materials.Among them,B4Py MPM:Ir-817-based OLED showed a deep-red Electroluminescence（EL）emission peaking at 672 nm and a maximum EQE of 1.03%.By combing Ir-817 with stronger electron-accepting materials,the TRZ-3SO₂:Ir-817-based device realized an EL peak at 746 nm and a maximum EQE of 0.20%.These results demonstrate the phosphor-involved exciplexes provide a reliable approach to develop efficient near-infrared exciplex emitters.