【作者】 张明；

【导师】 郑才俊；

【作者基本信息】 电子科技大学 ， 光学工程， 2022， 博士

【摘要】 热活化延迟荧光（Thermally activated delayed fluorescence,TADF）材料可以通过逆系间窜越通道（Reverse intersystem crossing process,RISC）将三线态激子上转化至单线态,再通过单线态激子的辐射跃迁实现100%内量子效率,成为近年来有机电致发光器件（Organic light-emitting diodes,OLED）领域的研究热点。激基复合物作为一类重要的TADF发光材料,凭借其独特的光电特性,被广泛应用在有机光电子学领域。然而,激基复合物及其OLED器件的发展仍面临着严重的挑战:i)较低的器件效率;ii)亟需提升的稳定性;iii)局限的发光区域。针对这些问题,本论文从激基复合物及其OLED器件的工作机制出发,通过重点研究发光过程中激子的转化和利用这一核心问题,构建了一系列全新的激基复合物发光体系,并在此基础上成功发展了高性能的OLED器件。具体研究成果如下:1.创新性地提出在激基复合物的给体材料（Donor,D）和受体材料（Acceptor,A）之间引入分子间氢键的策略来提升激基复合物体系的刚性,进而抑制其非辐射跃迁过程,获得高效且稳定的激基复合物发光体系。对比本工作中发展的系列激基复合物,拥有更多D和A分子间氢键的13PXZB:B4Py MPM呈现了最高69.6%的荧光量子产率(Photoluminescence quantum yield,Φ_PL)和最低3.4×10⁵ s^-1的三线态非辐射跃迁速率常数(Non-radiative process of triplet excitons,k^T_nr)。在OLED中,13PXZB:B4Py MPM实现了14.6%的最大外量子效率（External quantum efficiency,EQE）。同时,基于13PXZB:B4Py MPM制备的OLED的半衰期寿命(Half lifetime,LT₅₀)分别是13PXZB:B3Py MPM和13PXZB:B2Py MPM的4.8和8.2倍。通过进一步对比以13PXZB和13AB作为D材料的激基复合物发光体系,其OLED器件性能的巨大差异揭示了在D和A之间引入分子间氢键对激基复合物性能的重要影响,为发展高效且稳定的激基复合物提供了新的路径。2.针对溶液法激基复合物在制备过程中产生的D与A分子间的无效接触及过度分子聚集而导致的激子猝灭和非辐射跃迁,提出了利用D和A之间形成分子间氢键调控溶液法激基复合物的分子间相互作用的策略,通过降低非辐射能量损失来构建高效溶液法激基复合物发光体系。据此,本工作设计合成了三个新材料t MCz-DPy、t MCz-DPm和t MCz-TRZ作为A材料并构建了系列激基复合物BP-PXZ:t MCz-DPy、BP-PXZ:t MCz-DPm和BP-PXZ:t MCz-TRZ。对比三组激基复合物的溶液法和真空蒸镀混合薄膜,拥有分子间氢键的BP-PXZ:t MCz-DPy和BP-PXZ:t MCz-DPm溶液法薄膜呈现了68.7%和57.4%的高Φ_PLs及1.33×10⁵和2.11×10⁵s^-1的低k^T_nrs。利用三组激基复合物制备的真空蒸镀法和溶液法OLED中,基于BP-PXZ:t MCz-DPy的溶液法OLED实现了最高15.3%的EQE和18.2小时的LT₅₀,进一步证明了分子间氢键对发展溶液法激基复合物的重要作用。3.创新性地提出通过引入合适的第三元组分构建具有多个RISC通道的新型三元激基复合物发光体系,来提升体系的逆系间窜越效率进而提高其发光效率。据此,本工作设计合成了一个具有高三线态能级和合适HOMO能级的TADF材料DBT-SADF作为组分构建了系列激基复合物。其中,具有三个RISC通道的三元激基复合物DBT-SADF:PO-T2T:CDBP实现了最高的Φ_PL（61.0%）和逆系间窜越速率常数(14.2×10⁵ s^-1)。在OLED中,DBT-SADF:PO-T2T:CDBP实现了高达20.5%的EQE,其性能指标居于当时文献报道的同类型激基复合物OLED器件中的领先水平,证明了多个RISC通道对于发展高效激基复合物发光体系的重要意义。4.创新性地提出利用树枝状大分子作为第三元组分构建溶液法激基复合物的策略,在降低溶液法对组成材料高溶解性要求的同时提高激基复合物激子利用率,进而获得高效且稳定的溶液法激基复合物。随着系列树枝状大分子的引入,具有高溶解性的DBT-DMAC、SSP、DMAC-DPS和低溶解性的SOSP均作为D材料配合A材料PO-T2T成功构建了系列三元溶液法激基复合物。相较于未添加树枝状大分子的二元激基复合物,三元溶液法激基复合物的Φ_PLs、k^T_nrs及薄膜质量均得到了显著的提升。在溶液法OLED中,基于树枝状大分子Sim CP3-Ph的三元激基复合物DBT-DMAC:PO-T2T:Sim CP3-Ph、SSP:PO-T2T:Sim CP3-Ph、DMAC-DPS:PO-T2T:Sim CP3-Ph和SOSP:PO-T2T:Sim CP3-Ph分别实现了高达26.1%、22.8%、28.6%和24.1%的EQEs和3.4、4.1、10.2和0.8小时的LT₅₀s,这些性能指标居于文献报道的激基复合物OLED器件中的领先水平,为溶液法激基复合物的发展指明了方向。5.针对具有深红光及近红外光发射的高效激基复合物难以构建这一问题,提出了引入磷光材料作为组分单元的构建策略,通过磷光材料中重金属原子的自旋耦合效应抑制三线态激子的非辐射跃迁,从而提升深红光及近红外激基复合物的发光效率。据此,本工作以磷光材料Ir-817作为D材料与不同的A材料相结合发展了系列激基复合物。其中,以B4Py MPM:Ir-817为发光层制备的OLED器件呈现了672 nm的深红光发射和1.03%的EQE。而与具有更强缺电子特性的A材料结合,基于TRZ-3SO₂:Ir-817制备的OLED实现了746 nm的近红外光发射和0.2%的EQE。这些结果证明引入磷光材料是发展高效近红外激基复合物的可靠途径。更多还原

【Abstract】 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.更多还原