新型TADF材料可以使外量子效率提高3倍

正一国际联合研究团队成功研制出一种新型热激发延迟荧光(TADF)材料,这是世界上首次通过系间窜越从单重态激子到三重态激子发射出绿色至深红色光波。这项研究结果将推动白色TADF发光设备在室内外的应用研究。在过去的几十年中,对有机电致发光器件(OLED)的研究取得了很大进展。实际上,很多类型的OLED获得了广泛的市场应用,从照明光源到智能手机等都有OLED的身影。目前,几     

有机小分子电致磷光及发光机理研究进展

在过去10余年对小分子和聚合物电致发光器件的研究中,由于器件三线态激子能量没有得到充分的利用,使器件的内量子效率存在25%的理论极限,大大限制了其发光效率。为突破这一理论极限,在小分子主体材料中掺杂磷光染料制成电致磷光器件是近几年研究的热点,磷光染料的掺杂可以充分利用单线态和三线态激子,理论上器件的内量子效率可以达到100%。本文针对有机小分子电致磷光器件的发展、发光机理以及主客体分子间的能量传递等方面作了简明的讨论,指出了在器件设计时应该注意的一些问题。     

有机小分子电致磷光材料研究进展

在过去20年对小分子电致发光器件的研究中,由于没有充分利用三线态激子能量,器件的内量子效率存在25%的理论极限.由于有机磷光染料可以同时利用其单线态和三线态激子,理论上可以使器件的内量子效率达到100%,突破了25%的理论极限,因而近几年在小分子主体材料中掺杂磷光染料制成器件的研究备受关注.综述了近几年金属有机电致磷光材料的研究进展,重点评述了金属铱配合物在分子设计上的研究进展,同时论述了其发光机理和掺杂剂材料以及器件制作的研究进展,展望了金属有机配合物电致磷光材料的发展前景,并提出了今后磷光材料的发展方向.     

热激活延迟荧光器件性能影响因素研究进展

热激活延迟荧光(TADF)材料能够充分利用单重态和三重态激子,其理论内量子效率可达100%,突破了传统荧光材料的极限。TADF材料具有生产成本低、易合成、效率高等优点,近年来受到了广泛关注。介绍了TADF材料分子设计和器件结构设计的基本原则,综述了近年来TADF器件的最新研究进展。     

基于咔唑类给体分子的给-受体型热活化延迟荧光材料研究进展

热活化延迟荧光(Thermally activated delayed fluorescence, TADF)材料是新一代发光材料，可以通过吸收环境中的热量使分子的三重态转换为单重态，理论上激子利用率达到100%,量子效率大大提高，在有机发光二极管(Organic light-emitting diode, OLED)中有广阔的应用前景。给-受体(Donor-acceptor, D-A)型的纯有机分子是关注度较高的一类TADF分子。其中咔唑作为一种给体单元，易经其他取代基修饰形成新给体，使D-A分子具有较小的最低三重态和单重态的能级差ΔE_ST,是经常选用的给体基团。另外，理论计算在研究咔唑衍生物分子的TADF性质，预测其在OLED中的性能方面发挥了重要作用。本文综述了基于不同咔唑类给体构筑的D-A结构的TADF分子，依据咔唑分子上取代基的不同，具体介绍了近五年各类TADF分子的结构特点和发光效率，重点讨论了这些分子在器件应用方面的性能，并且结合理论计算分析的结果总结了可能改变D-A型TADF性质的因素，期望能够为未来设计和合成性能更加优异的含咔唑给体的D-A型TA...     

"光子-买一赠三”--简评有机三线态电致发光材料与器件进展*

有机电致发光材料与器件的研究已取得了重要进展,但要实现高信息含量的应用,器件的稳定性和效率仍须进一步提高。基于量子统计理论的研究结果表明,只有25%的电子空穴复合能量生成单线态激子,对于一个纯荧光的发光材料,在理论上,其器件效率的上限是光致发光效率的25%。三线态发光材料的应用,理论上可有效利用所有的复合能量,从而大幅度提高器件效率,目前已成为有机电致发光领域的研究热点。综述了有机三线态电致发光材料与器件的进展。     

热激发延迟荧光分子的受体基团研究进展

近年来,热激发延迟荧光材料(Thermally activated delayed fluorescence,TADF)及其电致发光器件取得了快速发展。TADF材料应具有小的单线态-三线态能级差,从而使其三线态激子可以通过反向系间窜越过程到达单线态,进而辐射发光。因此,与传统的荧光及磷光材料相比,TADF材料除了理论上可以实现100%的内量子效率(Internal quantum efficiency,IQE)和电生激子利用率外,它还具有更高的发光效率,且大部分TADF分子为纯有机给-受体体系,分子结构简单,基团选择范围广。通过增强TADF受体单元的吸电子能力,可以有效促进前线轨道的分离,从而降低单重态-三重态能级差,提高反向系间窜越速率。同时,调节受体的分子构型还可以抑制分子间的相互作用,改善载流子注入传输。然而,相对于给体单元,受体单元类型多样且功能差异较大,这对选择合适的受体基团以实现有效的光电性质调控造成了一定影响。近年来,人们构建热激发延迟荧光材料更侧重于受体基团的选择,且取得了显著成果。常见的受体基团有膦氧、氰基、三嗪和羰基等。深入研究受体基团对调控分子内电子效应和分子间相互作用至关重要。本文对近年来报道的TADF体系中主要的受体基团进行了梳理,对其结构和光电性质之间的关系进行了总结,以期为高效TADF分子的设计开发提供借鉴。     

红色热激活延迟荧光材料的研究进展

通过反向系间窜越,热激活延迟荧光(TADF)材料可以同时利用三线态激子和单线态激子,使器件的理论内量子效率突破传统荧光材料的25%,达到理论上的100%,可与磷光材料相媲美,且材料价格便宜无需贵金属,因而受到人们广泛关注。近年来,对绿光及蓝光材料的研究进展较快,而红光材料由于分子结构的合理设计比较困难,研究进展相对较慢。从受体的类型出发,综述了近年来有关红色TADF材料的研究进展,并结合现有工作,对红色TADF材料的发展前景进行了展望。     

有机半导体照明(有机照明)研究进展

采用有机发光二极管(OLEDs)的有机半导体照明(有机照明)是绿色环保、健康安全的新型面光源,有望在固态照明领域得到广泛的应用。有机照明的发展是随着有机发光材料的不断进步而进步的。有机发光材料从最初的荧光材料发展到磷光材料以及最近提出的热活化延迟荧光材料,其性能在不断地提升。基于这些材料的白光OLEDs的性能也在不断提升。最早的白光器件基于荧光小分子材料,但是由于只能利用单线态激子发光,效率很低。随后磷光材料的引入使得白光器件的效率大幅度提升,但是由于蓝色磷光材料本身的稳定性问题,全磷光白光器件的寿命较短。为了结合荧光和磷光的优点,人们提出了荧光/磷光杂化的白光器件,这是目前最有前景的一类白光器件结构。目前针对有机照明的研究,已从早期只关注效率突破阶段,进入到综合提高效率和寿命阶段。从荧光白光、磷光白光以及荧光/磷光混合白光3个方面对有机照明的研究状况、发展趋势进行了介绍。     

含卤素的纯有机小分子室温磷光材料研究概述

在最近几年中,纯有机小分子构建的室温磷光材料因其能够同时利用单线态激子和三线态激子,且相较于重金属掺杂体系和高分子体系,具有低毒性、制备简单等优点引起了人们极大的兴趣。通过介绍近几年含有卤素的有机小分子材料的室温磷光发光类型,讨论了室温磷光小分子在分子设计时需要综合考虑的各种因素。     

Highly Efficient Blue Fluorescent OLEDs Based on Upper Level Triplet–Singlet Intersystem Crossing

Purely organic electroluminescent materials, such as thermally activated delayed fluorescent (TADF) and triplet–triplet annihilation (TTA) materials, basically harness triplet excitons from the lowest triplet excited state (T₁) to realize high efficiency. Here, a fluorescent material that can convert triplet excitons into singlet excitons from the high‐lying excited state (T₂), referred to here as a “hot exciton” path, is reported. The energy levels of this compound are determined from the sensitization and nanosecond transient absorption spectroscopy measurements, i.e., small splitting energy between S₁ and T₂ and rather large T₂–T₁ energy gap, which are expected to impede the internal conversion (IC) from T₂ to T₁ and facilitate the reverse intersystem crossing from the high‐lying triplet state (hRISC). Through sensitizing the T₂ state with ketones, the existence of the hRISC process with an ns‐scale delayed lifetime is confirmed. Benefiting from this fast triplet–singlet conversion, the nondoped device based on this “hot exciton” material reaches a maximum external quantum efficiency exceeding 10%, with a small efficiency roll‐off and CIE coordinates of (0.15, 0.13). These results reveal that the “hot exciton” path is a promising way to exploit high efficient, stable fluorescent emitters, especially for the pure‐blue and deep‐blue fluorescent organic light‐emitting devices.     

Recent Progress of Singlet‐Exciton‐Harvesting Fluorescent Organic Light‐Emitting Diodes by Energy Transfer Processes

The external quantum efficiency (EQE) of organic light‐emitting diodes (OLEDs) has been dramatically improved by developing highly efficient organic emitters such as phosphorescent emitters and thermally activated delayed fluorescent (TADF) emitters. However, high‐EQE OLED technologies suffer from relatively poor device lifetimes in spite of their high EQEs. In particular, the short lifetimes of blue phosphorescent and TADF OLEDs remain a big hurdle to overcome. Therefore, the high‐EQE approach harvesting singlet excitons of fluorescent emitters by energy transfer processes from the host or sensitizer has been explored as an alternative for high‐EQE OLED strategies. Recently, there has been a big jump in the EQE and device lifetime of singlet‐exciton‐harvesting fluorescent OLEDs. Recent progress on the materials and device structure is discussed herein.     

Combined Inter‐ and Intramolecular Charge‐Transfer Processes for Highly Efficient Fluorescent Organic Light‐Emitting Diodes with Reduced Triplet Exciton Quenching

Inter‐ and intramolecular charge‐transfer processes are combined using an exciplex‐forming host and a thermally activated delayed fluorescent dopant, for fabricating efficient fluorescent organic light‐emitting diodes along with the reduced efficiency roll‐off at high current densities. Extra conversion on the host from triplet exciplexes to singlet exciplexes followed by energy transfer to the dopant reduces population of triplet excitons on dopant molecules, thereby reducing the triplet exciton annihilations at high current densities.     

Extrafluorescent electroluminescence in organic light-emitting devices

Organic light-emitting devices (OLEDs) are a promising technology for flat-panel displays and solid-state lighting. While OLED efficiencies have increased dramatically in recent years, further progress is complicated by the fact that the vast majority of organic materials are fluorescent and therefore emit only from molecular excited states ('excitons') with spin 0, or 'singlet' spin symmetry. Here, we demonstrate the ability to manipulate the fraction of excitons which form as singlets in fluorescent materials by altering the OLED structure. We insert a mixing layer that affects only charge-transfer (CT) states, which are the precursors to excitons. As a result, we triple the singlet fraction and the efficiency of the red fluorophore DCM2. We term fluorescence enhanced by CT spin mixing 'extrafluorescence', and show that its origin is in part an inversion of the usual energetic ordering of the singlet and triplet CT states.     

Singlet exciton fission in nanostructured organic solar cells

Singlet exciton fission is an efficient multiexciton generation process in organic molecules. But two concerns must be satisfied before it can be exploited in low-cost solution-processed organic solar cells. Fission must be combined with longer wavelength absorption in a structure that can potentially surpass the single junction limit, and its efficiency must be demonstrated in nanoscale domains within blended devices. Here, we report organic solar cells comprised of tetracene, copper phthalocyanine, and the buckyball C(60). Short wavelength light generates singlet excitons in tetracene. These are subsequently split into two triplet excitons and transported through the phthalocyanine. In addition, the phthalocyanine absorbs photons below the singlet exciton energy of tetracene. To test tetracene in nanostructured blends, we fabricate coevaporated bulk heterojunctions and multilayer heterojunctions of tetracene and C(60). We measure a singlet fission efficiency of (71 ± 18)%, demonstrating that exciton fission can efficiently compete with exciton dissociation on the nanoscale.     

Bipolar host materials: a chemical approach for highly efficient electrophosphorescent devices

The future of organic light-emitting devices (OLEDs) is drifting from electrofluorescence toward electrophosphorescence due to the feasibility of realizing 100% internal quantum efficiency. There is limited availability of transition metals (TMs) such as Ir, Os, and Pt, which are used for color-tunable phosphorescent emitters, and the use of the host-guest strategy is necessary for suppressing the detrimental triplet-triplet annihilation inherently imparted by the TM-centered emitters. The inevitable demands of organic host materials provide organic chemists with tremendous opportunities to contribute their expertise to this technology. With suitable molecular design and judicious selection of chemical structures featured with different electronic nature, the incorporation of hole-transporting (HT) and electron-transporting (ET) moieties combines the advantages of both functional units into bipolar host materials, which perform balanced injection/transportation/recombination of charge carriers and consequentially lead the OLEDs to have higher performances and low roll-off efficiencies. This review highlights recently developed bipolar host materials with the focus on molecular design strategies and the structure-property-performance relationships of various classes of bipolar host materials, which are classified into several categories according to the structural features of their constituents (HT/ET blocks and spacers).     

Donor–acceptor-structured 1,4-diazatriphenylene derivatives exhibiting thermally activated delayed fluorescence: design and synthesis,photophysical properties and OLED characteristics

A new series of luminescent 1,4-diazatriphenylene (ATP) derivatives with various peripheral donor units, including phenoxazine, 9,9-dimethylacridane and 3-(diphenylamino)carbazole, is synthesized and characterized as thermally activated delayed fluorescence (TADF) emitters. The influence of the donor substituents on the electronic and photophysical properties of the materials is investigated by theoretical calculations and experimental spectroscopic measurements. These ATP-based molecules with donor–acceptor–donor (D–A–D) structures can reduce the singlet–triplet energy gap (0.04–0.26 eV) upon chemical modification of the ATP core, and thus exhibit obvious TADF characteristics in solution and doped thin films. As a demonstration of the potential of these materials, organic light-emitting diodes containing the D–A–D-structured ATP derivatives as emitters are fabricated and tested. External electroluminescence quantum efficiencies above 12% and 8% for green- and sky-blue-emitting devices, respectively, are achieved.     

Recent progresses on materials for electrophosphorescent organic light-emitting devices

Although organic light-emitting devices have been commercialized as flat panel displays since 1997, only singlet excitons were emitted. Full use of singlet and triplet excitons, electrophosphorescence, has attracted increasing attentions after the premier work made by Forrest, Thompson, and co-workers. In fact, red electrophosphorescent dye has already been used in sub-display of commercial mobile phones since 2003. Highly efficient green phosphorescent dye is now undergoing of commercialization. Very recently, blue phosphorescence approaching the theoretical efficiency has also been achieved, which may overcome the final obstacle against the commercialization of full color display and white light sources from phosphorescent materials. Combining light out-coupling structures with highly efficient phosphors (shown in the table-of-contents image), white emission with an efficiency matching that of fluorescent tubes (90 lm/W) has now been realized. It is possible to tune the color to the true white region by changing to a deep blue emitter and corresponding wide gap host and transporting material for the blue phosphor. In this article, recent progresses in red, green, blue, and white electrophosphorescent materials for OLEDs are reviewed, with special emphasis on blue electrophosphorescent materials.