High‐Brightness Blue Light‐Emitting Diodes Enabled by a Directly Grown Graphene Buffer Layer

Single‐crystalline GaN‐based light‐emitting diodes (LEDs) with high efficiency and long lifetime are the most promising solid‐state lighting source compared with conventional incandescent and fluorescent lamps. However, the lattice and thermal mismatch between GaN and sapphire substrate always induces high stress and high density of dislocations and thus degrades the performance of LEDs. Here, the growth of high‐quality GaN with low stress and a low density of dislocations on graphene (Gr) buffered sapphire substrate is reported for high‐brightness blue LEDs. Gr films are directly grown on sapphire substrate to avoid the tedious transfer process and GaN is grown by metal–organic chemical vapor deposition (MOCVD). The introduced Gr buffer layer greatly releases biaxial stress and reduces the density of dislocations in GaN film and In_xGa_1?xN/GaN multiple quantum well structures. The as‐fabricated LED devices therefore deliver much higher light output power compared to that on a bare sapphire substrate, which even outperforms the mature process derived counterpart. The GaN growth on Gr buffered sapphire only requires one‐step growth, which largely shortens the MOCVD growth time. This facile strategy may pave a new way for applications of Gr films and bring several disruptive technologies for epitaxial growth of GaN film and its applications in high‐brightness LEDs.     

Enhancement of Light Output Power from LEDs Based on Monolayer Colloidal Crystal

One of the major challenges for the application of GaN‐based light emitting diodes (LEDs) in solid‐state lighting lies in the low light output power (LOP). Embedding nanostructures in LEDs has attracted considerable interest because they may improve the LOP of GaN‐based LEDs efficiently. Recent advances in nanostructures derived from monolayer colloidal crystal (MCC) have made remarkable progress in enhancing the performance of GaN‐based LEDs. In this review, the current state of the art in this field is highlighted with an emphasis on the fabrication of ordered nanostructures using large‐area, high‐quality MCCs and their demonstrated applications in enhancement of LOP from GaN‐based LEDs. We describe the remarkable achievements that have improved the internal quantum efficiency, the light extraction efficiency, or both from LEDs by taking advantages of diverse functions that the nanostructures provided. Finally, a perspective on the future development of enhancement of LOP by using the nanostructures derived from MCC is presented.     

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.     

Nanostructuring Multilayer Hyperbolic Metamaterials for Ultrafast and Bright Green InGaN Quantum Wells

Semiconductor quantum well (QW) light‐emitting diodes (LEDs) have limited temporal modulation bandwidth of a few hundred MHz due to the long carrier recombination lifetime. Material doping and structure engineering typically leads to incremental change in the carrier recombination rate, whereas the plasmonic‐based Purcell effect enables dramatic improvement for modulation frequency beyond the GHz limit. By stacking Ag‐Si multilayers, the resulting hyperbolic metamaterials (HMMs) have shown tunability in the plasmonic density of states for enhancing light emission at various wavelengths. Here, nanopatterned Ag‐Si multilayer HMMs are utilized for enhancing spontaneous carrier recombination rates in InGaN/GaN QWs. An enhancement of close to 160‐fold is achieved in the spontaneous recombination rate across a broadband of working wavelengths accompanied by over tenfold enhancement in the QW peak emission intensity, thanks to the outcoupling of dominating HMM modes. The integration of nanopatterned HMMs with InGaN QWs will lead to ultrafast and bright QW LEDs with a 3 dB modulation bandwidth beyond 100 GHz for applications in high‐speed optoelectronic devices, optical wireless communications, and light‐fidelity networks.     

Highly Deformable and See‐Through Polymer Light‐Emitting Diodes with All‐Conducting‐Polymer Electrodes

Despite the high expectation of deformable and see‐through displays for future ubiquitous society, current light‐emitting diodes (LEDs) fail to meet the desired mechanical and optical properties, mainly because of the fragile transparent conducting oxides and opaque metal electrodes. Here, by introducing a highly conductive nanofibrillated conducting polymer (CP) as both deformable transparent anode and cathode, ultraflexible and see‐through polymer LEDs (PLEDs) are demonstrated. The CP‐based PLEDs exhibit outstanding dual‐side light‐outcoupling performance with a high optical transmittance of 75% at a wavelength of 550 nm and with an excellent mechanical durability of 9% bending strain. Moreover, the CP‐based PLEDs fabricated on 4 µm thick plastic foils with all‐solution processing have extremely deformable and foldable light‐emitting functionality. This approach is expected to open a new avenue for developing wearable and attachable transparent displays.     

Progress in Piezo‐Phototronic‐Effect‐Enhanced Light‐Emitting Diodes and Pressure Imaging

Wurtzite materials exhibit both semiconductor and piezoelectric properties under strains due to the non‐central symmetric crystal structures. The three‐way coupling of semiconductor properties, piezoelectric polarization and optical excitation in ZnO, GaN, CdS and other piezoelectric semiconductors leads to the emerging field of piezo‐phototronics. This effect can efficiently manipulate the emission intensity of light‐emitting diodes (LEDs) by utilizing the piezo‐polarization charges created at the junction upon straining to modulate the energy band diagrams and the optoelectronic processes, such as generation, separation, recombination and/or transport of charge carriers. Starting from fundamental physics principles, recent progress in piezo‐phototronic‐effect‐enhanced LEDs is reviewed; following their development from single‐nanowire pressure‐sensitive devices to high‐resolution array matrices for pressure‐distribution mapping applications. The piezo‐phototronic effect provides a promising method to enhance the light emission of LEDs based on piezoelectric semiconductors through applying static strains, and may find perspective applications in various optoelectronic devices and integrated systems.     

Next‐Generation Narrow‐Band Green‐Emitting RbLi(Li3SiO4)2:Eu2+ Phosphor for Backlight Display Application

The discovery of high efficiency narrow‐band green‐emitting phosphors is a major challenge in backlighting light‐emitting diodes (LEDs). Benefitting from highly condensed and rigid framework structure of UCr₄C₄‐type compounds, a next‐generation narrow green emitter, RbLi(Li₃SiO₄)₂:Eu²⁺ (RLSO:Eu²⁺), has emerged in the oxide‐based family with superior luminescence properties. RLSO:Eu²⁺ phosphor can be efficiently excited by GaN‐based blue LEDs, and shows green emission at 530 nm with a narrow full width at half maximum of 42 nm, and very low thermal quenching (103%@150 °C of the integrated emission intensity at 20 °C), however its chemical stability needs to be improved later. The white LED backlight using optimized RLSO:8%Eu²⁺ phosphor demonstrates a high luminous efficacy of 97.28 lm W^?1 and a wide color gamut (107% National Television System Committee standard (NTSC) in Commission Internationale de L'Eclairage (CIE) 1931 color space), suggesting its great potential for industrial applications as liquid crystal display (LCD) backlighting.     

Semiconductor Nanowire Light‐Emitting Diodes Grown on Metal: A Direction Toward Large‐Scale Fabrication of Nanowire Devices

Bottom‐up nanowires are attractive for realizing semiconductor devices with extreme heterostructures because strain relaxation through the nanowire sidewalls allows the combination of highly lattice mismatched materials without creating dislocations. The resulting nanowires are used to fabricate light‐emitting diodes (LEDs), lasers, solar cells, and sensors. However, expensive single crystalline substrates are commonly used as substrates for nanowire heterostructures as well as for epitaxial devices, which limits the manufacturability of nanowire devices. Here, nanowire LEDs directly grown and electrically integrated on metal are demonstrated. Optical and structural measurements reveal high‐quality, vertically aligned GaN nanowires on molybdenum and titanium films. Transmission electron microscopy confirms the composition variation in the polarization‐graded AlGaN nanowire LEDs. Blue to green electroluminescence is observed from InGaN quantum well active regions, while GaN active regions exhibit ultraviolet emission. These results demonstrate a pathway for large‐scale fabrication of solid state lighting and optoelectronics on metal foils or sheets.     

Electron Holographic Study of Semiconductor Light‐Emitting Diodes

Semiconductor light‐emitting diodes (LEDs), especially GaN‐based heterostructures, are widely used in light illumination. The lack of inversion symmetry of wurtzite crystal structures and the lattice mismatch at heterointerfaces cause large polarization fields with contributions from both spontaneous polarization and piezoelectric polarization, which in turn results in obvious quantum confined stark effect. It is possible to alleviate this effect if the local electrostatic fields and band alignment induced charge redistribution can be quantitatively determined across the heterostructures. In this Concept, the applications of electron holography to investigate semiconductor LEDs are summarized. Following the off‐axis electron holography scheme, the GaN‐based LED heterostructures including InGaN/GaN‐based quantum wells, other GaN‐based quantum wells, and other forms of GaN‐based LED materials are discussed, focusing on the local potential drops, polarization fields, and charge distributions. Moreover, GaAs‐based LED heterostructures are briefly discussed. The in‐line electron holography scheme emphasizes the capability of large area strain mapping across LED heterostructures with high spatial resolution and accuracy, which is combined with quantitative electrostatic measurements and other advanced transmission electron microscopy characterizations to provide an overall nanometer scale perspective of LED devices for further improvement in their electric and optical properties.     

大功率白光发光二极管光源的制作(英文)

采用GaN基蓝色发光芯片为激发源,结合黄色硅酸盐系列荧光粉封装成大功率白光发光二极管(W-LEDs).利用24颗大功率5W白光发光二极管制作了两种不同连接方式的W-LEDs路灯:2并12串,和4并6串.设计了相应的驱动电路,对这两种不同连接方式的大功率W-LEDs路灯的光电特性及其在照明光源中的应用条件作了深入地研究和对比,测试了它们的伏安特性,发光效率以及功效,结果表明2并12串连接方式的W-LEDs路灯具有更加稳定的伏安特性,更高的照度以及更高的功效.与高压钠灯和荧光灯的特性相比较,W-LEDs路灯作为绿色环保光源灯,具有更高的显色指数,更加环保,节能.     

Ultrathin Quantum Dot Display Integrated with Wearable Electronics

An ultrathin skin‐attachable display is a critical component for an information output port in next‐generation wearable electronics. In this regard, quantum dot (QD) light‐emitting diodes (QLEDs) offer unique and attractive characteristics for future displays, including high color purity with narrow bandwidths, high electroluminescence (EL) brightness at low operating voltages, and easy processability. Here, ultrathin QLED displays that utilize a passive matrix to address individual pixels are reported. The ultrathin thickness (≈5.5 µm) of the QLED display enables its conformal contact with the wearer's skin and prevents its failure under vigorous mechanical deformation. QDs with relatively thick shells are employed to improve EL characteristics (brightness up to 44 719 cd m^?2 at 9 V, which is the record highest among wearable LEDs reported to date) by suppressing the nonradiative recombination. Various patterns, including letters, numbers, and symbols can be successfully visualized on the skin‐mounted QLED display. Furthermore, the combination of the ultrathin QLED display with flexible driving circuits and wearable sensors results in a fully integrated QLED display that can directly show sensor data.     

Eu3+, Sm3+ Deep‐Red Phosphors as Novel Materials for White Light‐Emitting Diodes and Simultaneous Performance Enhancement of Organic–Inorganic Perovskite Solar Cells

The luminous efficiency of inorganic white light‐emitting diodes, to be used by the next generation as light initiators, is continuously progressing and is an emerging interest for researchers. However, low color‐rendering index (Ra), high correlated color temperature (CCT), and poor stability limit its wider application. Herein, it is reported that Sm³⁺‐ and Eu³⁺‐doped calcium scandate (CaSc₂O₄ (CSO)) are an emerging deep‐red‐emitting material with promising light absorption, enhanced emission properties, and excellent thermal stability that make it a promising candidate with potential applications in emission display, solid‐state white lighting, and the device performance of perovskite solar cells (PSCs). The average crystal structures of Sm³⁺‐doped CSO are studied by synchrotron X‐ray data that correspond to an extremely rigid host structure. Samarium ion is incorporated as a sensitizer that enhances the emission intensity up to 30%, with a high color purity of 88.9% with a 6% increment. The impacts of hosting the sensitizer are studied by quantifying the lifetime curves. The CaSc₂O₄:0.15Eu³⁺,0.03Sm³⁺ phosphor offers significant resistance to thermal quenching. The incorporation of lanthanide ion‐doped phosphors CSOE into PSCs is investigated along with their potential applications. The CSOE‐coated PSCs devices exhibit a high current density and a high power conversion efficiency (15.96%) when compared to the uncoated control devices.     

Room‐Temperature Triple‐Ligand Surface Engineering Synergistically Boosts Ink Stability,Recombination Dynamics,and Charge Injection toward EQE‐11.6% Perovskite QLEDs

Developing low‐cost and high‐quality quantum dots (QDs) or nanocrystals (NCs) and their corresponding efficient light‐emitting diodes (LEDs) is crucial for the next‐generation ultra‐high‐definition flexible displays. Here, there is a report on a room‐temperature triple‐ligand surface engineering strategy to play the synergistic role of short ligands of tetraoctylammonium bromide (TOAB), didodecyldimethylammonium bromide (DDAB), and octanoic acid (OTAc) toward “ideal” perovskite QDs with a high photoluminescence quantum yield (PLQY) of >90%, unity radiative decay in its intrinsic channel, stable ink characteristics, and effective charge injection and transportation in QD films, resulting in the highly efficient QD‐based LEDs (QLEDs). Furthermore, the QD films with less nonradiative recombination centers exhibit improved PL properties with a PLQY of 61% through dopant engineering in A‐site. The robustness of such properties is demonstrated by the fabrication of green electroluminescent LEDs based on CsPbBr₃ QDs with the peak external quantum efficiency (EQE) of 11.6%, and the corresponding peak internal quantum efficiency (IQE) and power efficiency are 52.2% and 44.65 lm W^?1, respectively, which are the most‐efficient perovskite QLEDs with colloidal CsPbBr₃ QDs as emitters up to now. These results demonstrate that the as‐obtained QD inks have a wide range application in future high‐definition QD displays and high‐quality lightings.     

Efficient Red Perovskite Light‐Emitting Diodes Based on Solution‐Processed Multiple Quantum Wells

This paper reports a facile and scalable process to achieve high performance red perovskite light‐emitting diodes (LEDs) by introducing inorganic Cs into multiple quantum well (MQW) perovskites. The MQW structure facilitates the formation of cubic CsPbI₃ perovskites at low temperature, enabling the Cs‐based QWs to provide pure and stable red electroluminescence. The versatile synthesis of MQW perovskites provides freedom to control the crystallinity and morphology of the emission layer. It is demonstrated that the inclusion of chloride can further improve the crystallization and consequently the optical properties of the Cs‐based MQW perovskites, inducing a low turn‐on voltage of 2.0 V, a maximum external quantum efficiency of 3.7%, a luminance of ≈440 cd m^?2 at 4.0 V. These results suggest that the Cs‐based MQW LED is among the best performing red perovskite LEDs. Moreover, the LED device demonstrates a record lifetime of over 5 h under a constant current density of 10 mA cm^?2. This work suggests that the MQW perovskites is a promising platform for achieving high performance visible‐range electroluminescence emission through high‐throughput processing methods, which is attractive for low‐cost lighting and display applications.     

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.     

Highly Efficient Spectrally Stable Red Perovskite Light‐Emitting Diodes

Perovskite light‐emitting diodes (LEDs) have recently attracted great research interest for their narrow emissions and solution processability. Remarkable progress has been achieved in green perovskite LEDs in recent years, but not blue or red ones. Here, highly efficient and spectrally stable red perovskite LEDs with quasi‐2D perovskite/poly(ethylene oxide) (PEO) composite thin films as the light‐emitting layer are reported. By controlling the molar ratios of organic salt (benzylammonium iodide) to inorganic salts (cesium iodide and lead iodide), luminescent quasi‐2D perovskite thin films are obtained with tunable emission colors from red to deep red. The perovskite/polymer composite approach enables quasi‐2D perovskite/PEO composite thin films to possess much higher photoluminescence quantum efficiencies and smoothness than their neat quasi‐2D perovskite counterparts. Electrically driven LEDs with emissions peaked at 638, 664, 680, and 690 nm have been fabricated to exhibit high brightness and external quantum efficiencies (EQEs). For instance, the perovskite LED with an emission peaked at 680 nm exhibits a brightness of 1392 cd m^?2 and an EQE of 6.23%. Moreover, exceptional electroluminescence spectral stability under continuous device operation has been achieved for these red perovskite LEDs.     

High‐Efficiency Perovskite Light‐Emitting Diodes with Synergetic Outcoupling Enhancement

Perovskite light‐emitting diodes (PeLEDs) show great application potential in high‐quality flat‐panel displays and solid‐state lighting due to their steadily improved efficiency, tunable colors, narrow emission peak, and easy solution‐processing capability. However, because of high optical confinement and nonradiative charge recombination during electron–photon conversion, the highest reported efficiency of PeLEDs remains far behind that of their conventional counterparts, such as inorganic LEDs, organic LEDs, and quantum‐dot LEDs. Here a facile route is demonstrated by adopting bioinspired moth‐eye nanostructures at the front electrode/perovskite interface to enhance the outcoupling efficiency of waveguided light in PeLEDs. As a result, the maximum external quantum efficiency and current efficiency of the modified cesium lead bromide (CsPbBr₃) green‐emitting PeLEDs are improved to 20.3% and 61.9 cd A^?1, while retaining spectral and angular independence. Further reducing light loss in the substrate mode using a half‐ball lens, efficiencies of 28.2% and 88.7 cd A^?1 are achieved, which represent the highest values reported to date for PeLEDs. These results represent a substantial step toward achieving practical applications of PeLEDs.     

Efficient and Tunable Electroluminescence from In Situ Synthesized Perovskite Quantum Dots

Semiconductor quantum dots (QDs) are among the most promising next‐generation optoelectronic materials. QDs are generally obtained through either epitaxial or colloidal growth and carry the promise for solution‐processed high‐performance optoelectronic devices such as light‐emitting diodes (LEDs), solar cells, etc. Herein, a straightforward approach to synthesize perovskite QDs and demonstrate their applications in efficient LEDs is reported. The perovskite QDs with controllable crystal sizes and properties are in situ synthesized through one‐step spin‐coating from perovskite precursor solutions followed by thermal annealing. These perovskite QDs feature size‐dependent quantum confinement effect (with readily tunable emissions) and radiative monomolecular recombination. Despite the substantial structural inhomogeneity, the in situ generated perovskite QDs films emit narrow‐bandwidth emission and high color stability due to efficient energy transfer between nanostructures that sweeps away the unfavorable disorder effects. Based on these materials, efficient LEDs with external quantum efficiencies up to 11.0% are realized. This makes the technologically appealing in situ approach promising for further development of state‐of‐the‐art LED systems and other optoelectronic devices.     

Roll‐to‐Roll Production of Graphene Hybrid Electrodes for High‐Efficiency,Flexible Organic Photoelectronics

Despite nearly two decades of research, the absence of ideal, flexible, and transparent electrodes has been the biggest bottleneck for realizing flexible and printable electronics via roll‐to‐roll (R2R) method. A fabrication of poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate):graphene:ethyl cellulose (PEDOT:PSS:G:EC) hybrid electrodes by R2R process, which allows for the elimination of strong acid treatment. The high‐performance flexible printable electrode includes a transmittance (T) of 78% at 550 nm and a sheet resistance of 13 Ω sq⁻¹ with excellent mechanical stability. These features arise from the PSS interacting strongly with the ethyoxyl groups from EC promoting a favorable phase separation between PEDOT and PSS chains, and the highly uniform and conductive G:EC enable rearrangement of the PEDOT chains with more expanded conformation surrounded by G:EC via the π–π interaction between G:EC and PEDOT. The hybrid electrodes are fully functional as universal electrodes for outstanding flexible electronic applications. Organic solar cells based on the hybrid electrode exhibit a high power conversion efficiency of 9.4% with good universality for active layer. Moreover, the organic light‐emitting diodes and photodetector devices hold the same level to or outperform those based on indium tin oxide flexible transparent electrodes.     

Recent Progress in GaN‐Based Light‐Emitting Diodes

In the last few years the GaN‐based white light‐emitting diode (LED) has been remarkable as a commercially available solid‐state light source. To increase the luminescence power, we studied GaN LED epitaxial materials. First, a special maskless V‐grooved c‐plane sapphire was fabricated, a GaN lateral epitaxial overgrowth method on this substrate was developed, and consequently GaN films are obtained with low dislocation densities and an increased light‐emitting efficiency (because of the enhanced reflection from the V‐grooved plane). Furthermore, anomalous tunneling‐assisted carrier transfer in an asymmetrically coupled InGaN/GaN quantum well structure was studied. A new quantum well structure using this effect is designed to enhance the luminescent efficiency of the LED to ～72%. Finally, a single‐chip phosphor‐free white LED is fabricated, a stable white light is emitted for currents from 20 to 60 mA, which makes the LED chip suitable for lighting applications.