New Trends in the Use of Transition Metal–Ligand Complexes for Applications in Electroluminescent Devices

The advantage of using phosphorescent transition metal–ligand complexes in optoelectronic applications such as organic light‐emitting diodes (OLEDs) and light‐emitting electrochemical cells (LECs) are described and evaluated. Additionally, different device constructions utilizing phosphorescent transition‐metal complexes like iridium(III ) mixed‐ligand complexes and ruthenium(II ) systems are reviewed and specified. Diverse host materials in which the phosphorescent emitters can be placed are discussed, such as small organic molecules and a few polymeric systems, and alternative processing technologies are briefly compared. Recent developments in the synthesis of iridium(III ) triplet emitters are discussed. Different device architectures require different kinds of metal–ligand complexes. The different synthetic routes leading to charged and non‐charged complexes are briefly discussed.     

Recent Progress in Ionic Iridium(III) Complexes for Organic Electronic Devices

Ionic iridium(III) complexes are emerging with great promise for organic electronic devices, owing to their unique features such as ease of molecular design and synthesis, excellent photophysical properties, superior redox stability, and highly efficient emissions of virtually all colors. Here, recent progress on new material design, regarding photo‐ and electroluminescence is highlighted, including several interesting topics such as: i) color‐tuning strategies of cationic iridium(III) complexes, ii) widespread utilization in phosphorescent light‐emitting devices fabricated by not only solution processes but also vacuum evaporation deposition, and iii) potential applications in data record, storage, and sercurity. Results on anionic iridium(III) complexes and “soft salts” are also discussed, indicating a new related subject. Finally, a brief outlook is suggested, pointing out that ionic iridium(III) complexes should play a more significant role in future organic electronic materials technology.     

Microwave synthesis of novel Ir(III) complexes and their application to an electroluminescence device

A microwave synthetic method has been developed for novel electroluminescent Ir(III)-polypyridine complexes. The novel Ir(III)-polypyridine complexes give intensively multi-colored luminescence whose emission peaks are occurred ranging from 450 to 630 nm. The chemical properties and electroluminescence character of these Ir(III) complexes are studied. These Ir complexes exhibit red electrophosphorescence and they might be promising red emitting materials for EL devices.     

Deep Blue Phosphorescent Organic Light‐Emitting Diodes with CIEy Value of 0.11 and External Quantum Efficiency up to 22.5%

Organic light‐emitting diodes (OLEDs) based on red and green phosphorescent iridium complexes are successfully commercialized in displays and solid‐state lighting. However, blue ones still remain a challenge on account of their relatively dissatisfactory Commission International de L'Eclairage (CIE) coordinates and low efficiency. After analyzing the reported blue iridium complexes in the literature, a new deep‐blue‐emitting iridium complex with improved photoluminescence quantum yield is designed and synthesized. By rational screening host materials showing high triplet energy level in neat film as well as the OLED architecture to balance electron and hole recombination, highly efficient deep‐blue‐emission OLEDs with a CIE at (0.15, 0.11) and maximum external quantum efficiency (EQE) up to 22.5% are demonstrated. Based on the transition dipole moment vector measurement with a variable‐angle spectroscopic ellipsometry method, the ultrahigh EQE is assigned to a preferred horizontal dipole orientation of the iridium complex in doped film, which is beneficial for light extraction from the OLEDs.     

Polar‐Electrode‐Bridged Electroluminescent Displays: 2D Sensors Remotely Communicating Optically

A novel geometry for electroluminescent devices, which does not require transparent electrodes for electrical input, is demonstrated, theoretically analyzed, and experimentally characterized. Instead of emitting light through a conventional electrode, light emission occurs through a polar liquid or solid and input electrical electrodes are coplanar, rather than stacked in a sandwich configuration. This new device concept is scalable and easily deployed for a range of modular alternating‐current‐powered electroluminescent light sources and light‐emitting sensing devices. The polar‐electrode‐bridged electroluminescent displays can be used as remotely readable, spatially responsive sensors that emit light in response to the accumulation and distribution of materials on the device surface. Using this device structure, various types of alternating current devices are demonstrated. These include an umbrella that automatically lights up when it rains, a display that emits light from regions touched by human fingers (or painted upon using a mixture of oil and water), and a sensor that lights up differently in different areas to indicate the presence of water and its freezing. This study extends the dual‐stack, coplanar‐electrode device geometry to provide displays that emit light from a figure drawn on an electroluminescent panel using a graphite pencil.     

Purely Organic Thermally Activated Delayed Fluorescence Materials for Organic Light‐Emitting Diodes

The design of thermally activated delayed fluorescence (TADF) materials both as emitters and as hosts is an exploding area of research. The replacement of phosphorescent metal complexes with inexpensive organic compounds in electroluminescent (EL) devices that demonstrate comparable performance metrics is paradigm shifting, as these new materials offer the possibility of developing low‐cost lighting and displays. Here, a comprehensive review of TADF materials is presented, with a focus on linking their optoelectronic behavior with the performance of the organic light‐emitting diode (OLED) and related EL devices. TADF emitters are cross‐compared within specific color ranges, with a focus on blue, green–yellow, orange–red, and white OLEDs. Organic small‐molecule, dendrimer, polymer, and exciplex emitters are all discussed within this review, as is their use as host materials. Correlations are provided between the structure of the TADF materials and their optoelectronic properties. The success of TADF materials has ushered in the next generation of OLEDs.     

Correlation of Controllable Aggregation with Light‐Emitting Property in Polymer Blend Optoelectronic Devices

The control of solution‐processed emitting layers in organic‐based optoelectronic devices enables cost‐effective processing and highly efficient properties. However, a solution‐based protocol for emitter fabrication is highly complex, and the link between the device performance and internal nanoscale features as well as three associated fabricating parameters (e.g., the employed solvents, annealing temperatures, and molecular concentration) needs to be understood. Here, this study investigates the influence of the solution‐processing parameters on the nanostructure–property relationship in light emitters that consist of iridium complexes doped in polymer. The boiling points and evaporation rates of the selected solvents govern the nanomorphology of molecular aggregation in the as‐processed state, and the aggregation is either needle‐like, spherical, or even a mixture of needles and spheres. Furthermore, a direct observation via in situ heating microscopy indicates that annealing of emitters containing a needle‐type aggregation promotes the associated molecular transport, leading to a substantial reduction in the surface roughness. Consequently, a nearly threefold increase in the current efficiency of the device is induced. These findings have important implications for the tuning of the aggregation of iridium complexes for emitters used in the new evolution of high‐performance organic‐based optoelectronic devices.     

Electrical Scanning Probe Microscopy on Active Organic Electronic Devices

Polymer‐ and small‐molecule‐based organic electronic devices are being developed for applications including electroluminescent displays, transistors, and solar cells due to the promise of low‐cost manufacturing. It has become clear that these materials exhibit nanoscale heterogeneities in their optical and electrical properties that affect device performance, and that this nanoscale structure varies as a function of film processing and device‐fabrication conditions. Thus, there is a need for high‐resolution measurements that directly correlate both electronic and optical properties with local film structure in organic semiconductor films. In this article, we highlight the use of electrical scanning probe microscopy techniques, such as conductive atomic force microscopy (c‐AFM), electrostatic force microscopy (EFM), scanning Kelvin probe microscopy (SKPM), and similar variants to elucidate charge injection/extraction, transport, trapping, and generation/recombination in organic devices. We discuss the use of these tools to probe device structures ranging from light‐emitting diodes (LEDs) and thin‐film transistors (TFT), to light‐emitting electrochemical cells (LECs) and organic photovoltaics.     

Stretchable Organometal‐Halide‐Perovskite Quantum‐Dot Light‐Emitting Diodes

Stretchable light‐emitting diodes (LEDs) and electroluminescent capacitors have been reported to potentially bring new opportunities to wearable electronics; however, these devices lack in efficiency and/or stretchability. Here, a stretchable organometal‐halide‐perovskite quantum‐dot LED with both high efficiency and mechanical compliancy is demonstrated. The hybrid device employs an ultrathin (<3 µm) LED structure conformed on a surface‐wrinkled elastomer substrate. Its luminescent efficiency is up to 9.2 cd A^?1, which is 70% higher than a control diode fabricated on the rigid indium tin oxide/glass substrate. Mechanical deformations up to 50% tensile strain do not induce significant loss of the electroluminescent property. The device can survive 1000 stretch–release cycles of 20% tensile strain with small fluctuations in electroluminescent performance.     

Monolayer Transition Metal Dichalcogenides as Light Sources

Reducing the dimensions of materials is one of the key approaches to discovering novel optical phenomena. The recent emergence of 2D transition metal dichalcogenides (TMDCs) has provided a promising platform for exploring new optoelectronic device applications, with their tunable electronic properties, structural controllability, and unique spin valley–coupled systems. This progress report provides an overview of recent advances in TMDC‐based light‐emitting devices discussed from several aspects in terms of device concepts, material designs, device fabrication, and their diverse functionalities. First, the advantages of TMDCs used in light‐emitting devices and their possible functionalities are presented. Second, conventional approaches for fabricating TMDC light‐emitting devices are emphasized, followed by introducing a newly established, versatile method for generating light emission in TMDCs. Third, current growing technologies for heterostructure fabrication, in which distinct TMDCs are vertically stacked or laterally stitched, are explained as a possible means for designing high‐performance light‐emitting devices. Finally, utilizing the topological features of TMDCs, the challenges for controlling circularly polarized light emission and its device applications are discussed from both theoretical and experimental points of view.     

Highly sensitive color fine-tuning of diblock copolymeric nano-aggregates with tri-metallic cations,Eu(Ⅲ),Tb(Ⅲ),and Zn(Ⅱ),for flexible photoluminescence films(FPFs)

Luminescence materials have shown promise as display apparatus and lighting devices.The particularly interesting systems are photoluminescence materials that are capable of reversible colors emitting repeatedly on exposure to light.Here we report a series of color tunable flexible and transparent photoluminescence films consisting of multi-metals(Eu³⁺,Tb³⁺and Zn³⁺)induced polymer aggregates(MIPAs)which are distributed uniformly in the polyacrylonitrile(PAN)films without agglomeration.MIPAs have a unique spherical structure due to the self-assembly of polystyrene-block-polyacrylic acid(PS-b-PAA)induced by metal ions.Notably,when applied in photoluminescence devices,these photoluminescence films exhibit not only red,green,blue colors(RGB)light,but also other tuned various color light covering the whole visible range upon excitation of 345 nm through adjusting the relative ratios of metal complexes.As the most important key point,non-conductive polymers can be used in photoluminescence devices as host medium,which is not realized in electroluminescent devices.Thus,the flexible photoluminescence films(FPFs)innovated herein exhibit the great potential to apply for flexible light-color and light-energy transformation devices.     

Efficient Solid‐State Photoluminescence of Graphene Quantum Dots Embedded in Boron Oxynitride for AC‐Electroluminescent Device

Emerging graphene quantum dots (GQDs) have received much attention for use as next‐generation light‐emitting diodes. However, in the solid‐state, π‐interaction‐induced aggregation‐caused photoluminescence (PL) quenching (ACQ) in GQDs makes it challenging to realize high‐performance devices. Herein, GQDs incorporated with boron oxynitride (GQD@BNO) are prepared from a mixture of GQDs, boric acid, and urea in water via one‐step microwave heating. Due to the effective dispersion in the BNO matrix, ACQ is significantly suppressed, resulting in high PL quantum yields (PL‐QYs) of up to 36.4%, eightfold higher than that of pristine GQD in water. The PL‐QY enhancement results from an increase in the spontaneous emission rate of GQDs due to the surrounding BNO matrix, which provides a high‐refractive‐index material and fluorescence energy transfer from the larger‐gap BNO donor to the smaller‐gap GQD acceptor. A high solid‐state PL‐QY makes the GQD@BNO an ideal active material for use in AC powder electroluminescent (ACPEL) devices, with the luminance of the first working GQD‐based ACPEL device exceeding 283 cd m^?2. This successful demonstration shows promise for the use of GQDs in the field of low‐cost, ecofriendly electroluminescent devices.     

Highly efficient phosphorescent organic light emitting diodes based on iridium(III) complex with bulky substituent spacers

We developed highly efficient phosphorescent organic light emitting diodes (PHOLEDs) using iridium(III) complex, fac-tris[4-methyl-2-2(4'-trimethylsilylphenyl)pyridine] [Ir(msippy)3]. PHOLEDs based on Ir(msippy)3 complex exhibit the yellowish-green emission with CIE color coordinates of (0.31,0.64). These device performances were compared with those of the green emitting Ir(ppy)3-based devices. The higher external quantum efficiency (EQE) of 25.6% and the current efficiency of 84.4 cd/A were achieved for Ir(msippy)3-based device. The results show that the complete energy and/or charge transfer from the host to Ir(msippy)3 dopant in the emitting layer (EML) of the device resulted in the higher device efficiencies compared with those of Ir(ppy)3-based devices.     

Optical characteristics of organic light emitting diode with IrQ(ppy)2-5Cl and its emitter

The synthesis and photophysical study of a cyclometalated mixed-ligand iridium(III) complex are reported. The iridium complex (called IrQ(ppy)₂-5Cl) has two cyclometalated 2-phenylpyridine (ppy) ligands and one 8-hydroxyquinoline (Q) ligand, where one of the H atom is substituted by Cl atom. Absorption and photoluminescence spectra are studied for the neat film and films of IrQ(ppy)₂-5Cl doped in 4,4′-N,N′-dicarbazole-biphenyl and polystyrene, together with the electroluminescence spectra using multi-layer light emitting devices. The electronic states are studied using density functional theory calculations. Emission bands are observed at 502 and 666 nm, which arise from ppy and Q ligands, respectively.     

Phosphorescent Pt(II) and Pd(II) Complexes for Efficient,High‐Color‐Quality,and Stable OLEDs

Phosphorescent organic light‐emitting diodes (OLEDs) are leading candidates for next‐generation displays and solid‐state lighting technologies. Much of the academic and commercial pursuits in phosphorescent OLEDs have been dominated by Ir(III) complexes. Over the past decade recent developments have enabled square planar Pt(II) and Pd(II) complexes to meet or exceed the performance of Ir complexes in many aspects. In particular, the development of N‐heterocyclic carbene‐based emitters and tetradentate cyclometalated Pt and Pd complexes have significantly improved the emission efficiency and reduced their radiative lifetimes making them competitive with the best reported Ir complexes. Furthermore, their unique and diverse molecular design possibilities have enabled exciting photophysical attributes including narrower emission spectra, excimer ‐based white emission, and thermally activated delayed fluorescence. These developments have enabled the fabrication of efficient and “pure” blue OLEDs, single‐doped white devices with EQEs of over 25% and high CRI, and device operational lifetimes which show early promise that square planar metal complexes can be stable enough for commercialization. These accomplishments have brought Pt complexes to the forefront of academic research. The molecular design strategies, photophysical characteristics, and device performance resulting from the major advancements in emissive Pt and Pd square planar complexes are discussed.     

Iridium(III) Complex–Derived Polymeric Micelles with Low Dark Toxicity and Strong NIR Excitation for Phototherapy and Chemotherapy

Iridium(III) complexes are potent candidates for photodynamic therapy. However, their clinical usage is impeded by their poor water solubility, high dark toxicity, and negligible absorption in near‐infrared region (NIR region). Here, it is proposed to solve these challenges by developing an iridium(III) complexe‐based polymeric micelle system. This system is self‐assembled using an iridium(III) complex‐containing amphiphilic block polymer. The upconversion nanoparticles are included in the polymeric micelles to permit NIR excitation. Compared with the nonformulated iridium(III) complexes, under NIR stimulation, this polymeric micelle system exhibits higher ¹O₂ generation efficiency, negligible dark toxicity, excellent tumor‐targeting ability, and synergistic phototherapy–chemotherapy effect both in vitro and in vivo.     

Highly Efficient,Conventional, Fluorescent Organic Light‐Emitting Diodes with Extended Lifetime

Highly efficient, yellow‐fluorescent organic light‐emitting diodes with a maximum external quantum efficiency exceeding 25.0% and extended lifetime are reported using iridium‐complex sensitizers doped in an exciplex host. Energy transfer processes reduce the lifetime of the exciplex and excitons on the Ir complexes and enable an excited state to exist in a conventional fluorescent emitter, thereby increasing device lifetime. The device stability depends on the location of the excited state.     

Stretchable Light‐Emitting Diodes with Organometal‐Halide‐Perovskite–Polymer Composite Emitters

Intrinsically stretchable light‐emitting diodes (LEDs) are demonstrated using organometal‐halide‐perovskite/polymer composite emitters. The polymer matrix serves as a microscale elastic connector for the rigid and brittle perovskite and induces stretchability to the composite emissive layers. The stretchable LEDs consist of poly(ethylene oxide)‐modified poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate as a transparent and stretchable anode, a perovskite/polymer composite emissive layer, and eutectic indium–gallium as the cathode. The devices exhibit a turn‐on voltage of 2.4 V, and a maximum luminance intensity of 15 960 cd m^?2 at 8.5 V. Such performance far exceeds all reported intrinsically stretchable LEDs based on electroluminescent polymers. The stretchable perovskite LEDs are mechanically robust and can be reversibly stretched up to 40% strain for 100 cycles without failure.     

Perovskite Materials for Light‐Emitting Diodes and Lasers

Organic–inorganic hybrid perovskites have cemented their position as an exceptional class of optoelectronic materials thanks to record photovoltaic efficiencies of 22.1%, as well as promising demonstrations of light‐emitting diodes, lasers, and light‐emitting transistors. Perovskite materials with photoluminescence quantum yields close to 100% and perovskite light‐emitting diodes with external quantum efficiencies of 8% and current efficiencies of 43 cd A^?1 have been achieved. Although perovskite light‐emitting devices are yet to become industrially relevant, in merely two years these devices have achieved the brightness and efficiencies that organic light‐emitting diodes accomplished in two decades. Further advances will rely decisively on the multitude of compositional, structural variants that enable the formation of lower‐dimensionality layered and three‐dimensional perovskites, nanostructures, charge‐transport materials, and device processing with architectural innovations. Here, the rapid advancements in perovskite light‐emitting devices and lasers are reviewed. The key challenges in materials development, device fabrication, operational stability are addressed, and an outlook is presented that will address market viability of perovskite light‐emitting devices.     

聚合物发光材料喷墨打印成膜技术研究进展

喷墨打印具有成本低、定位精度高、可实现全彩色化等优点,被业界认为是实现聚合物电致发光器件(PLED)量产的重要技术之一。本文介绍了喷墨打印技术在PLED制备中的应用现状和研究进展,探讨了聚合物发光材料溶液喷墨打印流变性、薄膜均匀性等基础性问题。