Asymmetric Host Molecule Bearing Pyridine Core for Highly Efficient Blue Thermally Activated Delayed Fluorescence OLEDs

In this study, two host materials, pCzBzbCz and pCzPybCz , are synthesized to achieve a high efficiency and long lifetime of blue thermally activated delayed fluorescence organic light-emitting diodes (TADF-OLEDs). The molecular design strategy involves the introduction of a pyridine group into the core structure of pCzPybCz as an electron-withdrawing unit, and an electron-donating phenyl group into the structure of pCzBzbCz . These host materials demonstrate good thermal stability and high triplet energy (T₁=3.07 eV for pCzBzbCz and 3.06 eV for pCzPybCz ) for the fabrication of blue TADF-OLEDs. In particular, pCzPybCz -based OLED devices demonstrate an external quantum efficiency (EQE) of 22.7 % and an operational lifetime of 24 h (LT₉₀, time to attain 90 % of initial luminance) at an initial luminance of 1000 cd m⁻². This superior lifetime could be explained by the C−N bond dissociation energy (BDE) in the host molecular structure. Furthermore, a mixed-host system using the electron-deficient 2,4-bis(dibenzo[b,d]furan-2-yl)-6-phenyl-1,3,5-triazine (DDBFT) is proposed to inhibit the formation of the anion state of our host materials. In short, the device operational lifetime is further improved by applying DDBFT. The carbazole-based asymmetric host molecule containing a pyridine core realizes a high-efficiency blue TADF-OLED showing a positive effect on the operating lifetime, and can provide useful strategies for designing new host materials.     

蓝色延迟荧光材料及器件的研究进展

有机发光二极管(OLEDs)作为新一代的显示技术,因其启亮电压低、厚度薄、能效高、响应速度快等优点,而被世界各国研究人员所关注。 有机电致发光材料及器件的发展一直备受关注,根据发光机理的不同,有机发光材料可以分为荧光材料和磷光材料。 蓝色荧光材料作为OLEDs发展中的重要部分,拥有磷光材料无法比拟的优势,近年来成为研究热点。 本文介绍了近年来国内外蓝色延迟荧光材料的研究进展,并展望了其发展前景和趋势。     

喹喔啉类红光热活化延迟荧光材料研究进展

红光材料是显示三基色材料之一, 具有发射能量小、穿透能力强和背景荧光干扰小等优点, 被广泛应用于全彩显示、生物传感和光动力治疗等领域. 红光材料目前存在的问题主要有: (1)跃迁能级小, 其非辐射跃迁速率快, 导致量子效率普遍较低; (2)分子共轭较大, 存在较强的π-π堆积作用, 容易导致荧光淬灭; (3)分子设计需要更大的共轭, 在分子合成上较为困难. 热活化延迟荧光(thermally activated delayed fluorescence, TADF)材料作为具有全新的发光机制的材料能够通过反向系间窜越过程利用三重态激子发射荧光, 极大地提高了量子效率, 因此, 基于TADF机制的红光材料成为了近年来人们研究的热点. 由于结构的特点, 喹喔啉及其衍生物非常适合用来构建红光TADF分子. 从喹喔啉类TADF红光材料的分子结构视角出发, 对红光TADF材料的近年来的研究进行概述, 讨论分子结构对材料性能影响, 并且对其发展进行展望.     

有机电致发光器件中的双极性蓝光荧光材料

双极性蓝光荧光材料因其双极传输特性和发光特性,为有机电致发光器件性能提升及结构简化提供了新途径。大多数双极性蓝光荧光材料在结构上符合电子给体-π桥-电子受体(D-π-A),根据电子受体单元,本文将其分为二苯磷/磺酰类、二米基硼类、五元杂环类、六元氮杂环类等,讨论了各类材料结构特点及在器件中的应用性能,对非D-π-A型材料也进行了总结。同时,介绍了热活化延迟荧光特性的双极性蓝光荧光材料的进展情况。最后,对双极性蓝光荧光材料中存在的问题进行了提炼,并展望了其发展前景。     

热激活延迟荧光材料的光物理行为及性能预测

热激活延迟荧光(Thermally activated delayed fluorescence, TADF)材料由于三线态激子可通过反系间窜越(Reverse intersystem crossing, RISC)转换为单线态激子,在有机发光二极管(Organic light-emitting diodes, OLEDs)中理论上可达到100%的激子利用率而被广泛关注。但实验上开发设计高性能TADF材料较为复杂且研究周期较长,理论研究可以从本质上建立材料结构-性能的关系,预测材料的性质并提供一定的分子设计策略。本文围绕高性能TADF材料的开发,从发光原理出发,系统阐述了分子的设计策略及光物理参数如材料单-三线态能级差(Single-triplet energy gap,ΔE_ST)、系间/反系间窜越速率、吸收/发射光谱、辐射/非辐射速率等的计算原理、计算方法和研究进展。最后我们探讨了TADF材料理论研究面临的机遇和挑战,通过对TADF材料的理论研究综述和研究前景的展望,期待吸引更多的研究工作者,推动该领域的发展和突破。     

The Design of Dual Emitting Cores for Green Thermally Activated Delayed Fluorescent Materials

Dual emitting cores for thermally activated delayed fluorescent (TADF) emitters were developed. Relative to the corresponding TADF emitter with a single emitting core the TADF emitter with a dual emitting core, 3,3′,5,5′‐tetra(carbazol‐9‐yl)‐[1,1′‐biphenyl]‐2,2′,6,6′‐tetracarbonitrile, showed enhanced light absorption accompanied by a high photoluminescence quantum yield. The quantum and power efficiencies of the TADF devices were enhanced by the dual emitting cores.     

基于给-受体结构的热活化延迟荧光材料

热活化延迟荧光（TADF）材料由于第一单线态（S₁）与三线态激发态（T₁）之间的能级差较小,使得三线态激子能够有效地系间窜越至单线态发光,实现100%的激子利用率,在有机发光二极管（OLED）等领域得到广泛应用,是目前有机电子学研究的热点之一。基于给-受体（D-A）结构构建TADF材料具有分子设计简便、易于制备、性能优异等特点,引起了人们的普遍关注。本文综述了基于D-A结构设计TADF材料的基本原则,依据给受体构筑单元的不同,概括了各类TADF材料的结构和性能特点以及在器件应用等方面的最新研究进展,最后总结了D-A结构型TADF材料尚存在的问题,并对其未来的关键研究方向进行了分析和展望。     

Tetradentate Gold(III) Complexes as Thermally Activated Delayed Fluorescence (TADF) Emitters: Microwave‐Assisted Synthesis and High‐Performance OLEDs with Long Operational Lifetime

Structurally robust tetradentate gold(III)‐emitters have potent material applications but are rare and unprecedented for those displaying thermally activated delayed fluorescence (TADF). Herein, a novel synthetic route leading to the preparation of highly emissive, charge‐neutral tetradentate [C^C^N^C] gold(III) complexes with 5‐5‐6‐membered chelate rings has been developed through microwave‐assisted C?H bond activation. These complexes show high thermal stability and with emission origin (³IL, ³ILCT, and TADF) tuned by varying the substituents of the C^C^N^C ligand. With phenoxazine/diphenylamine substituent, we prepared the first tetradentate gold(III) complexes that are TADF emitters with emission quantum yields of up to 94 % and emission lifetimes of down to 0.62 μs in deoxygenated toluene. These tetradentate Au^III TADF emitters showed good performance in vacuum‐deposited OLEDs with maximum EQEs of up to 25 % and LT₉₅ of up to 5280 h at 100 cd m^?2.     

基于铜金属有机配合物的热活化延迟荧光材料

具有热活化延迟荧光(thermally activated delayed fluorescence, TADF)特性的配合物可以同时利用单重态和三重态激子,因此发光量子效率较高,近年来受到广大科研工作者的关注。特别是铜金属有机配合物,最低单重态和最低三重态的能量差较小,又可以通过不同配体或取代基进行调节,所以具有较好TADF性能。本文根据配位原子的类型,汇总和分析了近5年具有TADF性质的铜配合物的结构特点和发光性能,并简要讨论了其在有机发光二极管(organic light-emitting diodes,OLEDs)中的潜在应用。     

聚合物热激活延迟荧光材料的分子设计与器件性能

聚合物热激活延迟荧光(TADF)材料应用于有机发光二极管(OLEDs)中以来,取得了飞速发展,迄今为止已经报道了多种不同分子结构及性能优异的聚合物TADF发光材料.它们具有不含重金属的化学结构、100%的理论内量子效率和易于通过溶液加工进行大面积制造的优势.本文从分子结构和发光颜色2个角度总结了不同结构TADF聚合物的研究进展,重点介绍了我们课题组在长链型TADF聚合物设计与OLEDs器件性能方面的研究工作,探究TADF聚合物颜色调控与效率提升的途径,论述了TADF聚合物存在的问题与未来发展.     

CN-Modified Imidazopyridine as a New Electron Accepting Unit of Thermally Activated Delayed Fluorescent Emitters

Two efficient thermally activated delayed fluorescent (TADF) emitters were developed by utilizing CN-modified imidazopyridine as an acceptor unit. The CN-modified imidazopyridine acceptor was combined with either an acridine donor or a phenoxazine donor through a phenyl linker to produce two TADF emitters, Ac-CNImPy and PXZ-CNImPy. The acridine-based Ac-CNImPy emitter exhibited sky-blue emission with a CIE coordinate of (0.18, 0.38), whereas the phenoxazine-donor-based PXZ-CNImPy showed greenish-yellow emission with a CIE coordinate of (0.32, 0.58). A high photoluminescence quantum yield of 80 % was observed for the PXZ-CNImPy emitter compared with 40 % for the Ac-CNImPy emitter. Organic light-emitting diodes based on the PXZ-CNImPy emitter demonstrated high external quantum efficiency of 17.0 %. Hence, the CN-modified imidazopyridine unit can be considered as a useful electron acceptor for the future design of highly efficient TADF emitters.     

High Performance Thermally Activated Delayed Fluorescence Sensitized Organic Light‐Emitting Diodes

Recently, organic light‐emitting diodes (OLEDs) employing thermally activated delayed fluorescence (TADF) materials have aroused huge attention in both academia and industry. Compared with fluorescent and phosphorescent materials, TADF materials can theoretically capture 100 % excitons without incorporating noble metals, making them effective emitters and hosts for OLEDs simultaneously. Here, in this review, our recent works on mechanisms and materials of high performance TADF‐sensitized phosphorescent (TSP) OLEDs, TADF‐sensitized fluorescent (TSF) OLEDs and TADF‐sensitized TADF (TST) OLEDs are summarized. Finally, we propose the outlook for the further development and application of TADF‐sensitized OLEDs.     

Studies on Annihilation and Coreactant Electrochemiluminescence of Thermally Activated Delayed Fluorescent Molecules in Organic Medium

Very recently, there is a great research interest in electrochemiluminescence (ECL) featuring thermally activated delayed fluorescence (TADF) properties, i.e., TADF-ECL. It is appealing since the earlier reports in this topic well-confirmed that this strategy has a great potential in achieving all-exciton-harvesting ECL efficiency under electrochemical excitation, which is a breakthrough in the topic of organic ECL. However, organic phase electrochemistry and ECL studies surrounding TADF-ECL are still extremely rare. Especially, the ECL spectra of previous reported TADF emitters are still very different from their PL spectra. In this work, we systematically measure and discuss the liquid electrochemistry and ECL behavior of two typical TADF molecules in organic medium. Most importantly, we verify for the first time that the ECL spectra of them (coreactant ECL mode) are identical to their PL spectra counterparts, which confirms the effectiveness of TADF photophysical properties in the coreactant ECL mode in practice.     

Peripheral Amplification of Multi‐Resonance Induced Thermally Activated Delayed Fluorescence for Highly Efficient OLEDs

Multi‐resonance induced by boron and nitrogen atoms in opposite resonance positions endows a thermally activated delayed fluorescence (MR‐TADF) emitter with a strikingly small full width at half maximum of only 26 nm and excellent photoluminescence quantum yield of up to 97.48 %. The introduction of a carbazole unit in the para position of the B‐substituted phenyl‐ring can significantly boost up the resonance effect without compromising the color fidelity, subsequently enhancing the performances of the corresponding pure blue TADF‐OLED, with an outstanding external quantum efficiency (EQE) up to 32.1 % and low efficiency roll‐off, making it one of the best TADF‐OLEDs in the blue region to date. Furthermore, utilizing this material as host for a yellow phosphorescent emitter, the device also shows a significantly reduced turn‐on voltage of 3.2 V and an EQE_max of 22.2 %.     

Pyridinyl-Carbazole Fragments Containing Host Materials for Efficient Green and Blue Phosphorescent OLEDs

Pyridinyl-carbazole fragments containing low molar mass compounds as host derivatives H1 and H2 were synthesized, investigated, and used for the preparation of electro-phosphorescent organic light-emitting devices (PhOLEDs). The materials demonstrated high stability against thermal decomposition with the decomposition temperatures of 361–386 °C and were suitable for the preparation of thin amorphous and homogeneous layers with very high values of glass transition temperatures of 127–139 °C. It was determined that triplet energy values of the derivatives are, correspondingly, 2.82 eV for the derivative H1 and 2.81 eV for the host H2. The new derivatives were tested as hosts of emitting layers in blue, as well as in green phosphorescent OLEDs. The blue device with 15 wt.% of the iridium(III)[bis(4,6-difluorophenyl)-pyridinato-N,C2′]picolinate (FIrpic) emitter doping ratio in host material H2 exhibited the best overall characteristics with a power efficiency of 24.9 lm/W, a current efficiency of 23.9 cd/A, and high value of 10.3% of external quantum efficiency at 100 cd/m². The most efficient green PhOLED with 10 wt% of Ir(ppy)₃ {tris(2-phenylpyridine)iridium(III)} in the H2 host showed a power efficiency of 34.1 lm/W, current efficiency of 33.9 cd/A, and a high value of 9.4% for external quantum efficiency at a high brightness of 1000 cd/m², which is required for lighting applications. These characteristics were obtained in non-optimized PhOLEDs under an ordinary laboratory atmosphere and could be improved in the optimization process. The results demonstrate that some of the new host materials are very promising components for the development of efficient phosphorescent devices.     

A Silane-Based Bipolar Host with High Triplet Energy for High Efficiency Deep-Blue Phosphorescent OLEDs with Improved Device Lifetime

A high triplet energy host is developed using a silane moiety, 9-(4-(triphenylsilyl)dibenzo[b,d]furan-2-yl)-9H-carbazole (SiDBFCz), is designed through extensive density functional theory (DFT) calculations to obtain appropriate hole and electron injection barriers. The chemical hardness and the charge transport characteristics are comprehensively investigated to realize a bipolar host with high triplet energy over 2.9 eV for deep blue phosphorescent organic light-emitting diodes (PHOLEDs). The synthesized SiDBFCz clearly exhibits the bipolar characteristics especially with emitter molecules doped. An external quantum efficiency over 19 % without any microcavity optimization is achieved thanks to the good charge balance in the SiDBFCz PHOLED. The device lifetime of the SiDBFCz PHOLED is improved more than 1000 %, compared to the unipolar control devices at an initial luminance of 500 cd m⁻². The dramatic enhancement of the operational stability of the deep blue PHOLED is also thoroughly investigated in terms of electrochemical stability of host molecules in charged or excited states. The results clearly indicate that the device lifetime is strongly correlated with the bond dissociation energy and the activation energy for the bond dissociation reaction in triplet excited state.     

Asymmetric Thermally Activated Delayed Fluorescence Materials Rendering High-performance OLEDs Through both Thermal Evaporation and Solution-processing

Exploring high-efficiency thermally activated delayed fluorescence(TADF) materials is of great importance regarding to organic light-emitting diode(OLED). Herein, we present a design strategy for developing asymmetric TADF materials based on a diphenyl sulfone-phenoxazine structure, resulting in efficient TADF emitters(CzPXZ and t-CzPXZ) with aggregation-induced emission properties, while t-CzPXZ is modified with tert-butyl groups. The two compounds exhibit high solid-state luminescence, efficient TADF, and significantly impressive device performances by both thermal evaporation and solution processing. For an instance, CzPXZ and t-CzPXZ enable the thermally-evaporated OLEDs with high external quantum efficiencies(EQEs) of over 20%. Meanwhile, t-CzPXZ allows the solution-processed device with a high EQE of 16.3% with low-efficiency roll-off, attributing to the enhanced molecular solubility and suppressed excitons quenching through tert-butyl modification on t-CzPXZ. The results reveal that the proposed asymmetric structure is a promising approach for developing high-efficiency TADF materials and OLEDs.     

高分子热活化延迟荧光材料研究进展

高分子热活化延迟荧光材料能够利用热活化的反向系间窜越过程将三线态激子转变为单线态激子而发出荧光,理论上可以实现100%的内量子效率,突破了传统高分子荧光材料内量子效率不超过25%的极限,因而代表了未来低成本高效率高分子发光材料的发展方向。 近年来,高分子热活化延迟荧光材料在分子设计方面取得了重要进展,形成了主链型、侧链型和树枝状高分子热活化延迟荧光材料等材料体系,同时其器件性能得到了大幅提升,部分材料的器件效率达到了高分子磷光材料的水平。 本文从材料和器件两个方面,围绕高分子热活化延迟荧光材料的分子结构、光物理特性和器件性能,总结和评述了国内外研究者在该领域方向的研究进展,并分析了未来发展面临的机遇和挑战。     

圆偏振发光性质的热活化延迟荧光材料及电致发光器件

具有圆偏振发光性质的热活化延迟荧光(circularly polarized thermally activated delayed fluorescence,CP-TADF)材料,因其在数据存储、生物成像以及3D显示等领域的应用前景,受到学者们的广泛关注。基于此类材料所制备的圆偏振热活化延迟荧光器件展现出优异的器件性能。本文从圆偏振热活化延迟荧光材料的发光机理及分子设计策略出发,依据CP-TADF材料构筑方法的不同,概括了其结构设计策略,系统地综述了各种类型CP-TADF材料的分子结构和光电性能的关系及其在电致发光器件领域的应用,最后探讨了目前CP-TADF材料存在的问题,并展望了其未来发展前景及挑战。