【作者】 王鹏；

【导师】 赵谡玲；

【作者基本信息】 北京交通大学 ， 光学工程， 2020， 博士

【摘要】 热致活化延迟荧光(TADF)材料作为第三代OLEDs发光材料,其优秀的性能、强大的系统兼容性以及巨大的市场潜力,给OLEDs照明和显示带来了新的活力。虽然目前TADF材料的研究和开发发展迅速,但其器件的效率和稳定性依然面临挑战,和实际应用相差甚远。器件性能与材料和器件中激发态的形成、衰变、激子的动态行为等最基本物理过程紧密相关,当电子和空穴分别从有机发光器件的两极注入并相遇,形成单线态激子、三线态激子和极化子,其中激子以辐射形式复合是有机电致发光的基础,而激子和极化子的衰减过程以及在发光过程中它们之间的相互作用也是导致器件效率衰退和器件稳定性最为关键的因素。本论文以高效热致活化延迟荧光TADF OLEDs为切入点,从材料的电学特性、激发特性以及器件的动态特性来调控器件的性能,明确TADF OLEDs在工作状态下的基本物理过程,进而提高器件性能,是本论文的主要目标和工作内容。论文首先以小分子TADF材料4Cz IPN为发光客体,基于溶液法制备了TADF OLEDs,系统分析和研究了TADF OLEDs的发光过程及基本物理过程。通过改变4Cz IPN的掺杂浓度,发现4Cz IPN的双极性传输特性,这能够改善发光层内部的载流子平衡,器件效率随着客体掺杂浓度提高而上升,但高电流下效率roll-off也更加严重。为了改善效率和roll-off现象,我们对器件结构进行了优化,使用了深HOMO能级的电子传输材料B4PYMPM阻挡载流子,改善了器件性能,将器件的启亮电压降低至2.6V,优化后的器件电流效率最大值由8.9 cd/A上升至12.8 cd/A。并通过延迟发光和瞬态发光技术,阐明了TADF器件的发光机制,寻求深层的物理机制及有一般性的物理规律,为器件的优化提供参数。针对TADF OLEDs中的效率roll-off现象,从PL瞬态和EL瞬态两个角度对激子的运动过程和发光动态特性进行了讨论,分析了器件效率roll-off的原因。通过对比4Cz IPN在四种不同主体材料的薄膜中的光物理特性,及其瞬态PL的衰减曲线,我们提出了对同一客体TADF材料如何选择不同主体材料的一般规则。并分别制备了以m CP和CBP为主体的TADF OLED器件,其中m CP主体器件的亮度(23898 cd/m~2)、效率(25.2 cd/A)均高于CBP主体器件。利用EL瞬态测试手段研究了TADF OLEDs中的动态发光过程,系统分析了TADF器件效率roll-off的原因。最后我们利用TADF材料作为主体材料,制备了高效率、高稳定性的磷光和荧光器件,对影响TADF OLEDs稳定性的因素进行了研究,并在此基础上提出了提高器件稳定性的思路。制备的磷光器件电流效率达到了80cd/A,EQE为20.8%,稳定性从66小时提升到230小时。利用瞬态电致发光测试、薄膜高温稳定性测试等手段,我们确定TADF材料作主体时并不能显著降低三线态激子的浓度,TADF材料本身极强的形貌稳定性抑制了主客体之间随时间产生的相分离,提升了器件的稳定性。我们更进一步在空穴传输层中掺杂了更为稳定的TADF材料,成功抑制了HTL中的分子聚集现象,将器件的寿命进一步提升至350小时。图71幅,表6个,参考文献135篇。更多还原

【Abstract】 Servicing as third generation of Organic light emitting materials for organic lightemitting devices(OLEDs),thermally activated delayed fluorescent(TADF)materials exhibit Excellent performance,strong system compatibility and enormous market potential,and have been widely regarded in both academia and industry as a promising technology for solid-state lighting,flat-panel displays and flexible displays.Although the tremendous progress has been made,OLEDs are still facing challenges related to its efficiency and stability for practical application and commercialization.Leaving out the external elements such as temperature and the erosion of water or oxygen in the air,all these challenges are related to fundamental dynamics of excited-state in materials and devices,such as the formation and decay of excitons and their dynamic process.Following electrons and holes recombination,singlet excitons,triplet excitons and polarons are generated,after which they relax to ground state or interact with each other,which has significant influence on device performance and stability.Therefore,it is necessary to fully investigate and understand the fundamental mechanisms of the dynamics of these excitons and polarons.In this dissertation,we base on OLEDs using TADF materials,aim at investigating both photo-induced and electric-induced dynamic process of singlet and triplet excitons in TADF based devices,the excitation characteristics of organic semiconductors and the basic physical mechanisms controlling the performance and stability of OLEDs.There are 3 key issues discussed we focus on:At first we studied the luminance mechanism of TADF OLEDs based on solution process.By employing TADF material 4Cz IPN,we fabricate high performance TADF OLEDs without using heavy metal atoms.Through transient EL measurement,we systematically studied the carrier injection and exciton movement,the luminance mechanism is subsequently clarified.The underlying cause and general physical laws inside the OLEDs are investigated to offer some guidance for device optimization.Secondly we foucs on the excited-state dynamics upon both photo-and electroexcitation of TADF molecules.We characterize the TADF properties of 4Cz IPN and its host dependence of photophysical properties to determine a suitable host for solutionprocesseed OLEDs.We also report the dynamics of singlet and triplet exctions in TADF OLEDs via transient electroluminescence(EL)measurement,clarify the underlying reason for the efficiency roll-off of the TADF OLEDs.The further understanding of the excited state dynamics upon both photo-excitation and electro-excitation offer systematic analysis to the operating mechanism in the TADF based OLEDs.Thirdly,by using TADF materials as host matrix,we fabricate OLEDs with outstanding efficiency as well as stability.We compare the stability performance of OLEDs in which one of two TADF materials are used as hosts in the emitting layer against those using the well-known host material CBP.Phosphorescent or fluorescent emitters are used as guest materials in the EMLs of these devices(referred to herein respectively as Ph OLEDs and FOLEDS).We realize lifetime of more than 230 hours at an initial luminance of 10,000 cd/m2 and enhanced external quantum efficiency(EQE)in TADFhost OLEDs.We found that the use of TADF materials doesn’t significantly reduce T1 concentration in the emitting layer(EML),the enhancement in operational lifetime is most related to the morphological stability of TADF materials.Moreover,by doping TADF material into hole transport layer to suppress the molecule aggregation,the device stability is further increased to 350 hours.更多还原