Saturated Red‐Light‐Emitting Gold(III) Triphenylamine Dendrimers for Solution‐Processable Organic Light‐Emitting Devices

A novel isoquinoline‐containing C^N^C ligand and its phosphorescent triphenylamine‐based alkynylgold(III) dendrimers have been synthesized. These alkynylgold(III) dendrimers serve as phosphorescent dopants in the fabrication of efficient solution‐processable organic light‐emitting devices (OLEDs). The photophysical, electrochemical, and electroluminescence properties were studied. A saturated red emission with CIE coordinates of (0.64, 0.36) and a high EQE value of 3.62 % were achieved. Unlike other red‐light‐emitting iridium(III) dendrimers, a low turn‐on voltage of less than 3 V and a reduced efficiency roll‐off at high current densities were observed; this can be accounted for by the enhanced carrier transporting ability and the relatively short lifetimes in the high‐generation dendrimers. This class of alkynylgold(III) dendrimers are promising candidates as phosphorescent dopants in the fabrication of solution‐processable OLEDs.     

Doping‐Free White Organic Light‐Emitting Diodes

Doping‐free white organic light‐emitting diodes (WOLEDs) have great potential to the next‐generation solid‐state lighting and displays due to the excellent properties, such as high efficiency, bright luminance, low power consumption, simplified structure and low cost. In this account, our recent developments on doping‐free WOLEDs have been summarized. Firstly, fundamental concepts of doping‐free WOLEDs have been described. Then, the effective strategies to develop doping‐free WOLEDs have been presented. Particularly, the manipulation of charges and excitons distribution in different kinds of doping‐free WOLEDs have been highlighted, including doping‐free fluorescent/phosphorescent hybrid WOLEDs, doping‐free thermally activated delayed fluorescent WOLEDs and doping‐free phosphorescent WOLEDs. In the end, an outlook for the future development of doping‐free WOLEDs have been clarified.     

Electrophosphorescent Heterobimetallic Oligometallaynes and Their Applications in Solution‐Processed Organic Light‐Emitting Devices

By combining the iridium(III) ppy‐type complex (Hppy=2‐phenylpyridine) with a square‐planar platinum(II) unit, some novel phosphorescent oligometallaynes bearing dual metal centers (viz. Ir^III and Pt^II) were developed by combining trans‐[Pt(PBu₃)₂Cl₂] with metalloligands of iridium possessing bifunctional pendant acetylene groups. Photophysical and computational studies indicated that the phosphorescent excited states arising from these oligometallaynes can be ascribed to the triplet emissive Ir^III ppy‐type chromophore, owing to the obvious trait (such as the longer phosphorescent lifetime at 77 K) also conferred by the Pt^II center. So, the two different metal centers show a synergistic effect in governing the photophysical behavior of these heterometallic oligometallaynes. The inherent nature of these amorphous materials renders the fabrication of simple solution‐processed doped phosphorescent organic light‐emitting diodes (PHOLEDs) feasible by effectively blocking the close‐packing of the host molecules. Saliently, such a synergistic effect is also important in affording decent device performance for the solution‐processed PHOLEDs. A maximum brightness of 3 356 cd m^?2 (or 2 708 cd m^?2), external quantum efficiency of 0.50 % (or 0.67 %), luminance efficiency of 1.59 cd A^?1 (or 1.55 cd A^?1), and power efficiency of 0.60 Lm W^?1 (or 0.55 Lm W^?1) for the yellow (or orange) phosphorescent PHOLEDs can be obtained. These results show the great potential of these bimetallic emitters for organic light‐emitting diodes.     

Self‐Host Blue‐Emitting Iridium Dendrimer with Carbazole Dendrons: Nondoped Phosphorescent Organic Light‐Emitting Diodes

A blue‐emitting iridium dendrimer, namely B‐G2 , has been successfully designed and synthesized with a second‐generation oligocarbazole as the dendron, which is covalently attached to the emissive tris[2‐(2,4‐difluorophenyl)‐pyridyl]iridium(III) core through a nonconjugated link to form an efficient self‐host system in one dendrimer. Unlike small molecular phosphors and other phosphorescent dendrimers, B‐G2 shows a continuous enhancement in the device efficiency with increasing doping concentration. When using neat B‐G2 as the emitting layer, the nondoped device is achieved without loss in efficiency, thus giving a state‐of‐art EQE as high as 15.3 % (31.3 cd A^?1, 28.9 lm W^?1) along with CIE coordinates of (0.16, 0.29).     

High‐Performance Organic Electroluminescence: Design from Organic Light‐Emitting Materials to Devices

Organic electroluminescence is considered as the most competitive alternative for the future solid‐state displays and lighting techniques owing to many advantages such as self‐luminescence, high efficiency, high contrast, high color rendering index, ultra‐thin thickness, transparency, flat and flexibility, etc. The development of high‐performance organic electroluminescence has become the continuing focus of research. In this personal account, a brief overview of representative achievements in our study on the design of highly efficient novel organic light‐emitting materials (including fluorescent materials, phosphorescent iridium(III) complexes and conjugated polymers bearing phosphorescent iridium(III) complex) and high‐performance device structures together with working principles are given. At last, we will give some perspectives on this fascinating field, and also try to provide some potential directions of research on the basis of the current stage of organic electroluminescence.     

An Easy Route to Red Emitting Homoleptic IrIII Complex for Highly Efficient Solution‐Processed Phosphorescent Organic Light‐Emitting Diodes

A thiophene‐phenylquinoline‐based homoleptic Ir^III complex, [Ir(Th‐PQ)₃], has been synthesised by a simple route and utilised as a dopant in solution‐processed phosphorescent organic light‐emitting diodes (PhOLEDs). It shows the current efficiency of approximately 26 cd A^?1 and the external quantum efficiency of about 21 %, which are the highest values reported to date for PhOLEDs prepared by solution‐process.     

Star‐Shaped Trinuclear Cyclometalated Platinum(II) Complexes as Single‐Component Emitters in White‐Emitting PLEDs

Two star‐shaped phosphorescent small molecules, Ph‐3FPt(pic) and 4Ph‐3FPt(pic), are single‐component emitters in polymer white‐light‐emitting diodes (WPLEDs) that are comprised of three blue–light‐emitting phosphorescent chromophores of FPt(pic) and are attached to benzene‐1,3,5‐trioxy‐ and 1,3,5‐tri(4‐oxyphenyl)benzene cores through a hexyloxy chain, respectively. Compared to their corresponding mono‐ or dinuclear platinum complexes, this class of star‐shaped homotrinuclear cyclometalated platinum(II) complexes exhibited controllable excimer emission. Stable white/near‐white emission was obtained in single‐emissive‐layer PLEDs by using the Ph‐3FPt(pic) or 4Ph‐3FPt(pic) as a single dopant and a blend of poly(vinylcarbazole) and 2‐(4‐biphenyl)‐5‐(4‐tert‐butyl‐phenyl)‐1,3,4‐oxadiazole as a host matrix at dopant concentrations of 1–4 wt. %. Our results provide an efficient way to control excimer formation and to obtain a single‐component emitter for use in WPLEDs.     

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.     

Benzimidazobenzothiazole‐Based Bipolar Hosts to Harvest Nearly All of the Excitons from Blue Delayed Fluorescence and Phosphorescent Organic Light‐Emitting Diodes

Much effort has been devoted to developing highly efficient organic light‐emitting diodes (OLEDs) that function through phosphorescence or thermally activated delayed fluorescence (TADF). However, efficient host materials for blue TADF and phosphorescent guest emitters are limited because of their requirement of high triplet energy levels. Herein, we report the rigid acceptor unit benzimidazobenzothiazole (BID‐BT), which is suitable for use in bipolar hosts in blue OLEDs. The designed host materials, based on BID‐BT, possess high triplet energy and bipolar carrier transport ability. Both blue TADF and phosphorescent OLEDs containing BID‐BT‐based derivatives exhibit external quantum efficiencies as high as 20 %, indicating that these hosts allow efficient triplet exciton confinement appropriate for blue TADF and phosphorescent guest emitters.     

8‐Hydroxy Quinoline Derivatives as Auxiliary Ligands for Red‐Emitting Cyclic‐Platinum Phosphorescent Complexes: Synthesis and Properties

Heavy metal complexes exhibit high phosphorescent efficiency and have been used extensively for electrophosphorescent emitters in the past 16 years. In 2006, we initially reported the use of the popular ligand, 8‐hydroxyquinoline (Q) to coordinate with the heavy metal ions and obtained the red‐infrared phosphorescent emission. In this paper, 8‐hydroxyquinoline has been modified at the 5‐position by electron‐donating and attracting groups and platinum complexes based on 2‐phenylpyridine and 8‐hydroxyquinoline derivatives were synthesized. The electron‐withdrawing group CF ₃ and NO ₂ lowers the HOMO level of the Q ligand and results in a N^O centered enhanced red‐infrared phosphorescence emission. The complex with CF ₃ modification exhibits the highest phosphorescence quantum yield in solid state with a life time of 1.17 μs.     

Molecular and Device Design Strategies for Ideal Performance White Organic Light‐Emitting Diodes

The progress of white organic light‐emitting diodes (WOLEDs) via adopting fluorescent and phosphorescent organic materials have attracted commercial interest for their broad range of visible spectrum and potential of 100 % internal quantum efficiency. In this account, smart molecular designs for developing efficient phosphorescent host and good color purity blue fluorescent emitters are prepared to be discussed, especially donor‐acceptor modification to regulate their triplet states and bipolar transport properties. Rational device configuration design strategies were also introduced by cooperating with efficient conventional fluorescent and thermally activated delayed fluorescent emitting molecules to achieve full exciton utilization and simplified device structures, further suggesting perspectives of potentially low‐cost, ideal performance and promoted operational lifetime in WOLED devices.     

One‐Step Borylation of 1,3‐Diaryloxybenzenes Towards Efficient Materials for Organic Light‐Emitting Diodes

The development of a one‐step borylation of 1,3‐diaryloxybenzenes, yielding novel boron‐containing polycyclic aromatic compounds, is reported. The resulting boron‐containing compounds possess high singlet‐triplet excitation energies as a result of localized frontier molecular orbitals induced by boron and oxygen. Using these compounds as a host material, we successfully prepared phosphorescent organic light‐emitting diodes exhibiting high efficiency and adequate lifetimes. Moreover, using the present one‐step borylation, we succeeded in the synthesis of an efficient, thermally activated delayed fluorescence emitter and boron‐fused benzo[6]helicene.     

Bipolar Host Materials for Organic Light‐Emitting Diodes

It is important to balance holes and electrons in the emitting layer of organic light‐emitting diodes to maximize recombination efficiency and the accompanying external quantum efficiency. Therefore, the host materials of the emitting layer should transport both holes and electrons for the charge balance. From this perspective, bipolar hosts have been popular as the host materials of thermally activated delayed fluorescent devices and phosphorescent organic light‐emitting diodes. In this review, we have summarized recent developments of bipolar hosts and suggested perspectives of host materials for organic light‐emitting diodes.     

Novel Red Phosphorescent Polymers Bearing Both Ambipolar and Functionalized IrIII Phosphorescent Moieties for Highly Efficient Organic Light‐Emitting Diodes

A series of novel red phosphorescent polymers is successfully developed through Suzuki cross‐coupling among ambipolar units, functionalized Ir^III phosphorescent blocks, and fluorene‐based silane moieties. The photophysical and electrochemical investigations indicate not only highly efficient energy‐transfer from the organic segments to the phosphorescent units in the polymer backbone but also the ambipolar character of the copolymers. Benefiting from all these merits, the phosphorescent polymers can furnish organic light‐emitting diodes (OLEDs) with exceptional high electroluminescent (EL) efficiencies with a current efficiency (η _L) of 8.31 cd A⁻¹, external quantum efficiency (η _ext) of 16.07%, and power efficiency (η _P) of 2.95 lm W⁻¹, representing the state‐of‐the‐art electroluminescent performances ever achieved by red phosphorescent polymers. This work here might represent a new pathway to design and synthesize highly efficient phosphorescent polymers.

     

A Prototype of a Volumetric Three‐Dimensional Display Based on Programmable Photo‐Activated Phosphorescence

A proof‐of‐principle prototype of a volumetric 3D‐displaying system is demonstrated by utilizing the photo‐activated phosphorescence of two long‐lived phosphorescent metal‐porphyrins in dimethyl sulfoxide (DMSO), a photochemically deoxygenating solvent. The first phosphorescent sensitizer, Pt(TPBP), absorbs a light beam with a wavelength of 635 nm, and the sensitized singlet oxygen is scavenged by DMSO. The second phosphorescent emitter, Pt(OEP), absorbs a light beam with a wavelength of 532 nm and visibly phosphoresces only in the deoxygenated zone generated by the first sensitizer. The phosphorescent voxels, 3D images, and animations are well‐defined by the intersections of the 635‐nm and 532‐nm light beams that are programmable by tuning of the excitation‐power densities, the beam shapes, and the kinetics. As a pivotal selection rule for the phosphorescent molecular couple used in this 3D‐displaying system, their absorptions and emissions must be orthogonal to each other, so that they can be excited and addressed independently.     

Highly Efficient Near‐Infrared Delayed Fluorescence Organic Light Emitting Diodes Using a Phenanthrene‐Based Charge‐Transfer Compound

Significant efforts have been made to develop high‐efficiency organic light‐emitting diodes (OLEDs) employing thermally activated delayed fluorescence (TADF) emitters with blue, green, yellow, and orange–red colors. However, efficient TADF materials with colors ranging from red, to deep‐red, to near‐infrared (NIR) have been rarely reported owing to the difficulty in molecular design. Herein, we report the first NIR TADF molecule TPA‐DCPP (TPA=triphenylamine; DCPP=2,3‐dicyanopyrazino phenanthrene) which has a small singlet–triplet splitting (ΔE_ST) of 0.13 eV. Its nondoped OLED device exhibits a maximum external quantum efficiency (EQE) of 2.1 % with a Commission International de L′Éclairage (CIE) coordinate of (0.70, 0.29). Moreover, an extremely high EQE of nearly 10 % with an emission band at λ=668 nm has been achieved in the doped device, which is comparable to the most‐efficient deep‐red/NIR phosphorescent OLEDs with similar electroluminescent spectra.     

A Bipolar Transporter as an Efficient Green Fluorescent Emitter and Host for Red Phosphors in Multi‐ and Single‐Layer Organic Light‐Emitting Diodes

Multifunctional donor–acceptor compound 4,4′‐bis(dibenzothiophene‐S,S‐dioxide‐2‐yl)triphenylamine ( DSTPA ) was obtained by linking a strongly electron‐withdrawing core and a strongly electron‐donating core with a biphenyl bridge in linear spatial alignment. DSTPA not only has suitable HOMO and LUMO levels for easily accepting both holes and electrons, it was also demonstrated to have a high fluorescence quantum yield of 0.98 and a high triplet energy level of 2.39 eV. Versatile applications of DSTPA for bipolar transport, green fluorescent emission, and sensitizing a red phosphor were systematically investigated in a series of multi‐ and single‐layer organic light‐emitting devices. In traditional multilayer devices, it shows excellent performance both in an undoped fluorescent device (used as a green emitter and achieving maximum current and power efficiencies (CE and PE) of 12.6 cd A^?1 and 9.4 Lm W^?1, respectively) and in a red phosphorescent device (used as a host and achieving maximum CE and PE of 26.4 cd A^?1 and 26.3 Lm W^?1, respectively). Furthermore, DSTPA was also simultaneously used as an emitter, a hole transporter, and an electron transporter in a single‐layer device showing CE and PE of 5.1 cd A^?1 and 4.7 Lm W^?1, respectively. A single‐layer red phosphorescent device with efficiencies of 11.7 cd A^?1 and 12.6 Lm W^?1 was obtained by doping DSTPA with a red phosphor. The performances of all of the devices in this work are comparable to the best of their corresponding classes in the literature.     

Synthesis of Unsymmetric Bipyridine–PtII–Alkynyl Complexes through Post‐Click Reaction with Emission Enhancement Characteristics and Their Applications as Phosphorescent Organic Light‐Emitting Diodes

Two unsymmetric bipyridine–platinum(II)–alkynyl complexes have been synthesised by a post‐click reaction. These metal complexes are found to exhibit emission enhancement properties. The photoluminescence quantum yield can be significantly increased from 0.03 in solution to 0.72 in solid‐state thin films. Efficient solution‐processable organic light‐emitting diodes have been fabricated by utilizing these complexes as phosphorescent dopants. A high external quantum efficiency of up to 5.8 % has been achieved.     

Effective Host Materials for Blue/White Organic Light‐Emitting Diodes by Utilizing the Twisted Conjugation Structure in 10‐Phenyl‐9,10‐Dihydroacridine Block

Two new 10‐phenyl‐9,10‐dihydroacridine derivatives attached by dibenzothiophene (DBT) and dibenzofuran (DBF) were synthesized. The influence of the substituents of these materials was studied by theoretical calculations (DFT calculation) and experimental measurements. Owing to the twisted N‐phenyl ring, both molecules possess sufficiently high triplet energies and are suitable as hosts for phosphorescent organic light‐emitting diodes. To evaluate the electroluminescent (EL) performance of these materials, FIrpic‐based blue PHOLEDs and two‐color white PHOLEDs (FIrpic and PO‐01 as the dopants) were fabricated using the common device structures. High external quantum efficiencies (EQE) of 21.1 % and 20.9 % for FIrpic‐based blue PHOLEDs were achieved by FPhAc and TPhAc, respectively. The white device based on the host FPhAc achieved a higher performance, with a maximum EQE of 24.7 % than the device with TPhAc as host material.     

Supramolecular Sky‐Blue Phosphorescent Polymer Iridium Complexes for Single‐Emissive‐Layer Organic Light‐Emitting Diodes

Novel supramolecular phosphorescent polymers (SPPs) are synthesized as a new class of solution‐processable electroluminescent emitters. The formation of these SPPs takes advantage of the efficient non‐bonding assembly between bis(dibenzo‐24‐crown‐8)‐functionalized iridium complex monomer and bis(dibenzylammonium)‐tethered co‐monomer, which is monitored by ¹H NMR spectroscopy and viscosity measurements. These SPPs show good film morphology and an intrinsic glass transition with a T_g of 94–116 °C. Noticeably, they are highly photoluminescent in solid state with quantum efficiency up to ca. 78%. The photophysical and electroluminescent properties are strongly dependent on the molecular structures of the iridium complex monomers.