A Universal Method of Producing Transparent Electrodes Using Wide‐Bandgap Materials

A UV light‐emitting diode (LED) is an eco‐friendly optical source with diverse applications. However, currently, the external quantum efficiency (EQE) of AlGaN‐based UV LEDs, particularly in the UV‐C band (<280 nm), is very low (<11%) mainly due to a large optical absorption via p‐GaN contact layers. A direct Ohmic contact to p‐AlGaN layers should be obtained using UV‐transparent conductive electrodes (TCEs) to solve this problem. A universal method is presented here to make such contact using electrical breakdown, with wide‐bandgap materials, to form conductive filaments (CFs), providing a current path between the TCEs and the p‐(Al)GaN layers. The contact resistance between the TCEs and the p‐GaN layers (or p‐AlGaN) is found to be on the order of 10^?5 Ω cm² (or 10^?3 Ω cm²), while optical transmittance is maintained up to 95% for AlN‐based TCEs at 250 nm. These findings could be a critical turning point delivering a breakthrough in UV LED technologies.     

Stable White Light‐Emitting Biocomposite Films

The demonstration of reliable and stable white light‐emitting diodes (LEDs) is one of the main technological challenges of the LED industry. This is usually accomplished by incorporation of light‐emitting rare‐earth elements (REEs) compounds within an external polymeric coating of a blue LED allowing the generation of white light. However, due to both environmental and cost issues, the development of low‐cost REE‐free coatings, which exhibit competitive performance compared to conventional white LED is of great importance. In this work, the formation of an REE‐free white LED coating is demonstrated. This biocomposite material, composed of biological (crystalline nanocellulose and porcine gastric mucin) and organic (light‐emitting dyes) compounds, exhibits excellent optical and mechanical properties as well as resistance to heat, humidity, and UV radiation. The coating is further used to demonstrate a working white LED by incorporating it within a commercial blue LED.     

Optical and Structural Properties of InGaN–AlGaN Ultraviolet Light-Emitting Diodes

We fabricated and investigated the performances of InGaN–AlGaN ultraviolet (UV) light-emitting diodes (LEDs) emitting at 380 nm. The output power of a conventional LED, a patterned sapphire substrate LED (PSS LED), and a PSS flip-chip LED (PSS FCLED) were about 0.94, 1.86, and 5.18 mW, respectively, at a forward injection current of 20 mA. These results indicate that the light output–powers of the PSS LED and PSS FCLED were enhanced as much as 97% and 451% compared to the conventional LED. Subsequent optical simulations confirm the remarkable enhancements in optical power of the PSS FCLED at UV wavelengths.      

Graphene/SiO2/p‐GaN Diodes: An Advanced Economical Alternative for Electrically Tunable Light Emitters

Advanced materials that combine novel functionality and ease of applicability are central to the development of light‐emitting diodes (LEDs), which is of ever increasing commercial importance. Here a new metal‐insulator‐semiconductor (MIS) LED structure that combines economical fabrication with novel device properties is reported. The presented MIS‐LED consists of a graphene electrode on p‐GaN substrate separated by an insulating SiO₂ layer. It is found that the MIS‐LED possesses a unique tunability of the electroluminescence spectra depending on the bias conditions. Tunnel injection from graphene into the p‐GaN can explain the difference in luminescence spectra under forward and reverse bias. The demonstrated MIS‐LED expands the use of graphene and also possibly allows the direct integration of light emitters with other circuit elements.     

Wavelength‐Emission Tuning of ZnO Nanowire‐Based Light‐Emitting Diodes by Cu Doping: Experimental and Computational Insights

The band‐gap engineering of doped ZnO nanowires is of the utmost importance for tunable light‐emitting‐diode (LED) applications. A combined experimental and density‐functional theory (DFT) study of ZnO doping by copper (Zn²⁺ substitution by Cu²⁺) is presented. ZnO:Cu nanowires are epitaxially grown on magnesium‐doped p‐GaN by electrochemical deposition. The heterojunction is integrated into a LED structure. Efficient charge injection and radiative recombination in the Cu‐doped ZnO nanowires are demonstrated. In the devices, the nanowires act as the light emitters. At room temperature, Cu‐doped ZnO LEDs exhibit low‐threshold emission voltage and electroluminescence emission shifted from the ultraviolet to violet–blue spectral region compared to pure ZnO LEDs. The emission wavelength can be tuned by changing the copper content in the ZnO nanoemitters. The shift is explained by DFT calculations with the appearance of copper d states in the ZnO band‐gap and subsequent gap reduction upon doping. The presented data demonstrate the possibility to tune the band‐gap of ZnO nanowire emitters by copper doping for nano‐LEDs.     

Highly Efficient Green ZnAgInS/ZnInS/ZnS QDs by a Strong Exothermic Reaction for Down‐Converted Green and Tripackage White LEDs

Highly efficient bright green‐emitting Zn?Ag?In?S (ZAIS)/Zn?In?S (ZIS)/ZnS alloy core/inner‐shell/shell quantum dots (QDs) are synthesized using a multistep hot injection method with a highly concentrated zinc acetate dihydrate precursor. ZAIS/ZIS/ZnS QD growth is realized via five sequential steps: a core growth process, a two‐step alloying–shelling process, and a two‐step shelling process. To enhance the photoluminescence quantum yield (PLQY), a ZIS inner‐shell is synthesized and added with a band gap located between the ZAIS alloy‐core and ZnS shell using a strong exothermic reaction. The synthesized ZAIS/ZIS/ZnS QDs shows a high PLQY of 87% with peak wavelength of 501 nm. Tripackage white down‐converted light‐emitting diodes (DC‐LEDs) are realized using an InGaN blue (B) LED, a green (G) ZAIS/ZIS/ZS QD‐based DC‐LED, and a red (R) Zn?Cu?In?S/ZnS QD‐based DC‐LED with correlated color temperature from 2700 to 10 000 K. The red, green, and blue tripackage white DC‐LEDs exhibit high luminous efficacy of 72 lm W^?1 and excellent color qualities (color rendering index (CRI, R_a) = 95 and the special CRI for red (R₉) = 93) at 2700 K.     

Highly Efficient Blue‐Emitting Bi‐Doped Cs2SnCl6 Perovskite Variant: Photoluminescence Induced by Impurity Doping

Lead halide perovskites show excellent optoelectronic properties but are unsatisfactory in terms of stability and toxicity. Herein, bismuth (Bi)‐doped lead‐free inorganic perovskites Cs₂SnCl₆:Bi are reported as blue emissive phosphors. Upon Bi doping, the originally nonluminous Cs₂SnCl₆ exhibits a highly efficient deep‐blue emission at 455 nm, with a Stokes shift of 106 nm and a high photoluminescence quantum yield (PLQY) close to 80%. Hybrid density functional theory calculations suggest the preferred formation of [Bi_Sn+V_Cl] defect complex, which is believed to be responsible for the optical absorption and the associated blue emission. The Cs₂SnCl₆:Bi also shows impressive thermal and water stability due to its inorganic nature and the formation of protective BiOCl layer. White light‐emitting diodes (LEDs) are constructed using Cs₂SnCl₆:Bi and commercial yellow phosphors combined with commercial UV LED chips, giving the Commission Internationale de I'Eclairage (CIE) color coordinates of (0.36, 0.37). This work represents a significant step toward the realization of highly efficient, stable, and environmentally benign next‐generation solid‐state lighting.     

Quasi‐2D Colloidal Semiconductor Nanoplatelets for Narrow Electroluminescence

The first functional light‐emitting diodes (LEDs) based on quasi 2D colloidal core/shell CdSe/CdZnS nanoplatelets (NPLs). The solution‐processed hybrid devices are optimized with respect to their electroluminescent characteristics, first, by improving charge injection through exchanging the as‐synthesized NPL long‐chain ligands to shorter ones such as 3‐mercaptopropionic acid, and second, by comparing different hole‐transporting layers. NPL‐LEDs exhibit a maximum luminance of 4499 cd m^‐2 and external quantum efficiencies of 0.63%. In particular, over different applied voltages, systematically narrow electroluminescence of full width at half maximum (FWHM) in the range of 25–30 nm is observed to be independent from the choice of device configuration and NPL ligands. As spectrally narrow electroluminescence is highly attractive in terms of color purity in the context of LED applications, these results emphasize the unique potential of this new class of colloidal core/shell nanoplatelet in achieving bright and functional LEDs of superior color purity.     

Transparent InP Quantum Dot Light‐Emitting Diodes with ZrO2 Electron Transport Layer and Indium Zinc Oxide Top Electrode

Because of outstanding optical properties and non‐vacuum solution processability of colloidal quantum dot (QD) semiconductors, many researchers have developed various light emitting diodes (LEDs) using QD materials. Until now, the Cd‐based QD‐LEDs have shown excellent properties, but the eco‐friendly QD semiconductors have attracted many attentions due to the environmental regulation. And, since there are many issues about the reliability of conventional QD‐LEDs with organic charge transport layers, a stable charge transport layer in various conditions must be developed for this reason. This study proposes the organic/inorganic hybrid QD‐LEDs with Cd‐free InP QDs as light emitting layer and inorganic ZrO₂ nanoparticles as electron transport layer. The QD‐LED with bottom emission structure shows the luminescence of 530 cd m^?2 and the current efficiency of 1 cd/A. To realize the transparent QD‐LED display, the two‐step sputtering process of indium zinc oxide (IZO) top electrode is applied to the devices and this study could fabricate the transparent QD‐LED device with the transmittance of more than 74% for whole device array. And when the IZO top electrode with high work‐function is applied to top transparent anode, the device could maintain the current efficiency within the driving voltage range without well‐known roll‐off phenomenon in QD‐LED devices.     

Enhancement of Light Extraction Through the Wave‐Guiding Effect of ZnO Sub‐microrods in InGaN Blue Light‐Emitting Diodes

The improvement of the light extraction efficiency (LEE) of a conventional InGaN blue light‐emitting diode (LED) by the incorporation of one‐dimensional ZnO sub‐microrods is reported. The LEE is improved by 31% through the wave‐guiding effect of ZnO sub‐microrods compared to LEDs without the sub‐microrods. Different types of ZnO microrods/sub‐microrods are produced using a simple non‐catalytic wet chemical growth method at a low temperature (90 °C) on an indium‐tin‐oxide (ITO) top contact layer with no seed layer. The crystal morphologies of needle‐like or flat‐top hexagonal structures, and the ZnO microrods/sub‐microrod density and size are easily modified by controlling the pH value and growth time. The wave‐guiding phenomenon within the ZnO rods is observed using confocal scanning electroluminescence microscopy and micro‐electroluminescence spectra.     

Sunlight‐Induced Cross‐Linked Luminescent Films Based on Polysiloxanes and d‐Limonene via Thiol–ene “Click” Chemistry

The increasing pursuit of biocontained elastic materials led the investigation of the potential use of the monoterpene limonene in film synthesis via thiol–ene reaction. Poly[(mercaptopropyl)methylsiloxane] (PMMS) is first synthesized. By controlling the molar ratio of PMMS and functional monomers, such as polyethylene glycol allyl methyl ether or rhodamine‐B, PMMS is partially functionalized while leaving spare mercapto groups that could be further used as cross‐linking sites. On the basis of the functionalized PMMS, novel transparent silicone luminescent films with hydrophilic tunable properties are prepared by natural‐sunlight‐triggered thiol–ene “click” chemistry by using d ‐limonene as a cross‐linker. Their structures and properties are thoroughly characterized. Transparent luminescent films are coated on commercially available UV‐light emitting diode (LED) cell from solution medium followed by an in situ cross‐linking step; a colorful LED cell is obtained through this facile and efficient method. The UV‐LED coated by films show very intense photoluminescence under normal visible light or the light is on, and has very high coloric purity.     

Plasmonic Nanocavity Organic Light‐Emitting Diode with Significantly Enhanced Light Extraction,Contrast, Viewing Angle,Brightness, and Low‐Glare

One central challenge in LEDs is to increase light extraction; but for display applications, other factors may have equal significance, such as ambient‐light absorption, contrast, viewing angle, image sharpness, brightness, and low‐glare. However, current LED structures enhance only some of the factors, while degrading the others. Here, a new organic LED (OLED) structure is proposed and demonstrated, with a novel plasmonic nanocavity, termed “plasmonic cavity with subwavelength hole‐array” (PlaCSH), and exhibits experimentally significant enhancements of all above factors with unprecedented performances. Compared to the conventional OLEDs (the same but without PlaCSH), PlaCSH‐OLEDs achieve experimentally: i) 1.57‐fold higher external‐quantum‐efficiency and light‐extraction‐efficiency (29%/32% without lens, 55%/60% with lens)—among the highest reported; ii) ambient‐light absorption not only 2.5‐fold higher but also broad‐band (400 nm) and nearly angle and polarization independent, leading to lower‐glare; iii) fivefold higher contrast (12 000 for 140 lux ambient‐light); iv) viewing angle tunable by the cavity length; v) 1.86‐fold higher normal‐view‐brightness; and vi) uniform color over all emission angles. The PlaCSH is an excellent optical antenna—excellent in both radiation and absorption of light. Furthermore, PlaCSH‐OLEDs, a simple structure to produce, are fabricated using nanoimprint over large‐area (≈1000 cm²), hence scalable to wallpaper size.     

White Light‐Emitting Diode From Sb‐Doped p‐ZnO Nanowire Arrays/n‐GaN Film

A whole interfacial transition of electrons from conduction bands of n‐type material to the acceptor levels of p‐type material makes the energy band engineering successful. It tunes intrinsic ZnO UV emission to UV‐free and warm white light‐emitting diode (W‐LED) emission with color coordinates around (0.418, 0.429) at the bias of 8–15.5 V. The W‐LED is fabricated based on antimony (Sb) doped p‐ZnO nanowire arrays/Si doped n‐GaN film heterojunction structure through one‐step chemical vapor deposition with quenching process. Element analysis shows that the doping concentration of Sb is ≈1.0%. The I–V test exhibits the formation of p‐type ZnO nanowires, and the temperature‐dependent photoluminescence measurement down to 4.65 K confirms the formation of deep levels and shallow acceptor levels after Sb‐doping. The intrinsic UV emission of ZnO at room temperature is cut off in electroluminescence emission at a bias of 4–15.5 V. The UV‐free and warm W‐LED have great potential application in green lights program, especially in eye‐protected lamp and display since television, computer, smart phone, and mobile digital equipment are widely and heavily used in modern human life, as more than 3000 h per year.     

2D Ruddlesden–Popper Perovskite Nanoplate Based Deep‐Blue Light‐Emitting Diodes for Light Communication

Ruddlesden–Popper perovskite, (PEA)₂PbBr₄ (PEA = C₈H₉NH₃), is a steady and inexpensive material with a broad bandgap and a narrow‐band emission. These features make it a potential candidate for deep‐blue light‐emitting diodes (LEDs). However, due to the weak exciton binding energy, LEDs based on the perovskite thin films usually possess a very low external quantum efficiency (EQE) of <0.03%. Here, for the first time, the construction of high‐performance deep‐blue LEDs based on 2D (PEA)₂PbBr₄ nanoplates (NPs) is demonstrated. The as‐fabricated (PEA)₂PbBr₄ NPs film shows a deep‐blue emission at 410 nm with excellent stability under ambient conditions. Impressively, LEDs based on the (PEA)₂PbBr₄ NPs film deliver a bright deep‐blue emission with a maximum luminance of 147.6 cd m^?2 and a high EQE up to 0.31%, which represents the most efficient and brightest perovskite LEDs operating at deep‐blue wavelengths. Furthermore, the LEDs retain over 80% of their efficiencies for over 1350 min under ≈60% relative humidity. The steady and bright deep‐blue LEDs can be used as an excitation light source to realize white light emission, which shows the potential for light communication. The work provides scope for developing perovskite into efficient and deep‐blue LEDs for low‐cost light source and light communication.     

Half‐duplex visible light communication using an LED as both a transmitter and a receiver

In typical visible light communication (VLC) systems, light‐emitting diodes (LEDs) are used as optical transmitters and photodiodes are used as optical receivers. Currently, many communication devices such as smart phones have a built‐in LED lamp whereas they usually do not have a built‐in photodiode. If we find a way to receive VLC signals without the need to add an additional photodiode on the communication devices, it will contribute to the spread of VLC. Therefore, we propose and demonstrate a VLC scheme without the need for a photodiode. As the first step, we investigate the characteristics of an LED as a VLC receiver and find out that an LED can also be used as a VLC receiver in certain conditions. Then, we demonstrate a half‐duplex VLC system using an LED as both an optical transmitter and an optical receiver, without the need for a photodiode. This technique could be used in various applications such as the VLC between smart phones with a built‐in LED lamp and the VLC between LED traffic lights. Copyright © 2015 John Wiley & Sons, Ltd.     

Three‐Dimensional Branched Nanowire Heterostructures as Efficient Light‐Extraction Layer in Light‐Emitting Diodes

A facile method to fabricate three‐dimensional branched ZnO/MgO nanowire heterostructures and their application as the efficient light‐extraction layer in light‐emitting diodes are reported. The branched MgO nanowires are produced on the hydrothermally‐grown ZnO nanowires with a small tapering angle towards the tip (≈6°), by the oblique angle flux incidence of MgO. The structural evolution during the growth verifies the formation of the MgO nanoscale islands with strong (111) preferred orientation on very thin (5–7 nm) MgO (110) layer. The MgO nanobranches, then grown on the islands, are polycrystalline consisting of many grains oriented in specific directions of <200> and <220>, supported by the nucleation theory. The LEDs with the branched ZnO/MgO nanowire arrays show a remarkable enhancement in the light output power by 21% compared with that of LEDs with pristine ZnO nanowires. Theoretical calculations using a finite‐difference time‐domain method reveal that the nanostructure is very effective in breaking the wave‐guiding mode inside the ZnO nanowires, extracting more light especially in radial direction through the MgO nanobranches.     

Micro Light‐Emitting Diodes for Display and Flexible Biomedical Applications

Inorganic‐based micro light‐emitting diodes (µLEDs) have witnessed significant improvements in terms of display and biomedical applications, which can shift the paradigm of future optoelectronic systems. In particular, µLED displays are on the verge of becoming the next big interface platform for visual communications, expanding to various internet of things and wearable/bioapplications. Novel µLED concepts need to be upgraded to be able to satisfy their potential optoelectric applications, such as virtual reality, smart watches, and medical sensors for individual computing in this hyperconnected society. Here, representative progresses in the field of flexible µLEDs are reviewed with regard to device structures, massive µLED transfers, methods for performance enhancement, and applications.     

Towards realization of a large‐area light‐emitting diode‐based solar simulator

Solar simulators based on light‐emitting diodes (LEDs) have shown great promise as alternative light sources for indoor testing of photovoltaic cells with certain characteristics that make them superior to the traditional solar simulators. However, large‐area uniform illumination more suitable for larger cells and module measurements still remain a challenge today. In this paper, we discuss the development and fabrication of a scalable large‐area LED‐based solar simulator that consists of multiple tapered light guides. We demonstrate fine intermixing of many LED light rays and power delivery in the form of a synthesized AM 1.5 spectrum over an area of 25 cm × 50 cm with better than 10% spatial nonuniformity. We present the spectral output, the spatial uniformity, and the temporal stability of the simulator in both the constant current mode and the pulsed‐mode LED operation, and compare our data with the International Electrotechnical Commission standards on solar simulators for class rating. Although the light intensity with our current design and settings falls short of the standard solar AM 1.5 intensity, this design and further improvements open up the possibility of achieving large‐area, high‐power indoor solar simulation with various desired spectra. Copyright © 2011 John Wiley & Sons, Ltd.     

Extracting Light from Polymer Light‐Emitting Diodes Using Stamped Bragg Gratings

In this paper, we describe a method for increasing the external efficiency of polymer light‐emitting diodes (LEDs) by coupling out waveguided light with Bragg gratings. We numerically model the waveguide modes in a typical LED structure and demonstrate how optimizing layer thicknesses and reducing waveguide absorption can enhance the grating outcoupling. The gratings were created by a soft‐lithography technique that minimizes changes to the conventional LED structure. Using one‐dimensional and two‐dimensional gratings, we were able to increase the forward‐directed emission by 47 % and 70 %, respectively, and the external quantum efficiency by 15 % and 25 %.     

Light‐Emitting Electrochemical Cells Based on Color‐Tunable Inorganic Colloidal Quantum Dots

Large‐area, ultrathin light‐emitting devices currently inspire architects and interior and automotive designers all over the world. Light‐emitting electrochemical cells (LECs) and quantum dot light‐emitting diodes (QD‐LEDs) belong to the most promising next‐generation device concepts for future flexible and large‐area lighting technologies. Both concepts incorporate solution‐based fabrication techniques, which makes them attractive for low cost applications based on, for example, roll‐to‐roll fabrication or inkjet printing. However, both concepts have unique benefits that justify their appeal. LECs comprise ionic species in the active layer, which leads to the omission of additional organic charge injection and transport layers and reactive cathode materials, thus LECs impress with their simple device architecture. QD‐LEDs impress with purity and opulence of available colors: colloidal quantum dots (QDs) are semiconducting nanocrystals that show high yield light emission, which can be easily tuned over the whole visible spectrum by material composition and size. Emerging technologies that unite the potential of both concepts (LEC and QD‐LED) are covered, either by extending a typical LEC architecture with additional QDs, or by replacing the entire organic LEC emitter with QDs or perovskite nanocrystals, still keeping the easy LEC setup featured by the incorporation of mobile ions.