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.     

Characterization and Field‐Emission Properties of Needle‐like Zinc Oxide Nanowires Grown Vertically on Conductive Zinc Oxide Films

Needle‐like ZnO nanowires with high density are grown uniformly and vertically over an entire Ga‐doped conductive ZnO film at 550 °C. The nanowires are grown preferentially in the c‐axis direction. The X‐ray diffraction (XRD) θ‐scan curve shows a full width at half maximum (FWHM) value of 2°. This indicates that the c‐axes of the nanorods are along the normal direction of the substrate surface. The investigation using high‐resolution transmission electron microscopy (HRTEM) confirmed that each nanowire is a single crystal. A room‐temperature photoluminescence (PL) spectrum of the wires consists of a strong and sharp UV emission band at 380 nm and a weak and broad green–yellow band. It reveals a low concentration of oxygen vacancies in the ZnO nanowires and their high optical quality. Field electron emission from the wires was also investigated. The turn‐on field for the ZnO nanowires was found to be about 18 V μm^–1 at a current density of 0.01 μA cm^–2. The emission current density from the ZnO nanowires reached 0.1 mA cm^–2 at a bias field of 24 V μm^–1.     

High‐Performance Dibenzoheteraborin‐Based Thermally Activated Delayed Fluorescence Emitters: Molecular Architectonics for Concurrently Achieving Narrowband Emission and Efficient Triplet–Singlet Spin Conversion

Thermally activated delayed fluorescence (TADF) materials, which enable the full harvesting of singlet and triplet excited states for light emission, are expected as the third‐generation emitters for organic light‐emitting diodes (OLEDs), superseding the conventional fluorescence and phosphorescence materials. High photoluminescence quantum yield (Φ_PL), narrow‐band emission (or high color purity), and short delayed fluorescence lifetime are all strongly desired for practical applications. However, to date, no rational design strategy of TADF emitters is established to fulfill these requirements. Here, an epoch‐making design strategy is proposed for producing high‐performance TADF emitters that concurrently exhibiting high Φ_PL values close to 100%, narrow emission bandwidths, and short emission lifetimes of ≈1 µs, with a fast reverse intersystem crossing rate of over 10⁶ s^?1. A new family of TADF emitters based on dibenzoheteraborins is introduced, which enable both doped and non‐doped TADF‐OLEDs to achieve markedly high external electroluminescence quantum efficiencies, exceeding 20%, and negligible efficiency roll‐offs at a practical high luminance. Systematic photophysical and theoretical investigations and device evaluations for these dibenzoheteraborin‐based TADF emitters are reported here.     

Origin of the Exclusive Ternary Electroluminescent Behavior of BN‐Doped Nanographenes in Efficient Single‐Component White Light‐Emitting Electrochemical Cells

White‐light‐emitting electrochemical cells (WLECs) still represent a significant milestone, since only a few examples with moderate performances have been reported. Particularly, multiemissive white emitters are highly desired, as a paradigm to circumvent phase separation and voltage‐dependent emission color issues that are encountered following host:guest and multilayered approaches. Herein, the origin of the exclusive white ternary electroluminescent behavior of BN‐doped nanographenes with a B₃N₃ doping pattern (hexa‐perihexabenzoborazinocoronene) is rationalized, leading to one of the most efficient (≈3 cd A^?1) and stable‐over‐days single‐component and single‐layered WLECs. To date, BN‐doped nanographenes have featured blue thermally activated delayed fluorescence (TADF). This doping pattern provides, however, white electroluminescence spanning the whole visible range (x/y CIE coordinates of 0.29–31/0.31–38 and average color rendering index (CRI) of 87) through a ternary emission involving fluorescence and thermally activated dual phosphorescence. This temperature‐dependent multiemissive mechanism is operative for both photo‐ and electroluminescence processes and holds over the device lifespan, regardless of the device architecture, active layer composition, and operating conditions. As such, this work represents a new stepping‐stone toward designing a new family of multiemissive white emitters based on BN‐doped nanographenes that realizes one of the best‐performing single‐component white‐emitting devices compared to the prior‐art.     

Solution‐Processed Extremely Efficient Multicolor Perovskite Light‐Emitting Diodes Utilizing Doped Electron Transport Layer

A specially designed n‐type semiconductor consisting of Ca‐doped ZnO (CZO) nanoparticles is used as the electron transport layer (ETL) in high‐performance multicolor perovskite light‐emitting diodes (PeLEDs) fabricated using an all‐solution process. The band structure of the ZnO is tailored via Ca doping to create a cascade of conduction energy levels from the cathode to the perovskite. This energy band alignment significantly enhances conductivity and carrier mobility in the CZO ETL and enables controlled electron injection, giving rise to sub‐bandgap turn‐on voltages of 1.65 V for red emission, 1.8 V for yellow, and 2.2 V for green. The devices exhibit significantly improved luminance yields and external quantum efficiencies of, respectively, 19 cd A^?1 and 5.8% for red emission, 16 cd A^?1 and 4.2% for yellow, and 21 cd A^?1 and 6.2% for green. The power efficiencies of these multicolor devices demonstrated in this study, 30 lm W^?1 for green light‐emitting PeLED, 28 lm W^?1 for yellow, and 36 lm W^?1 for red are the highest to date reported. In addition, the perovskite layers are fabricated using a two‐step hot‐casting technique that affords highly continuous (>95% coverage) and pinhole‐free thin films. By virtue of the efficiency of the ETL and the uniformity of the perovskite film, high brightnesses of 10 100, 4200, and 16,060 cd m^?2 are demonstrated for red, yellow, and green PeLEDs, respectively. The strategy of using a tunable ETL in combination with a solution process pushes perovskite‐based materials a step closer to practical application in multicolor light‐emitting devices.     

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.     

Enhancing Light Emission of Nanostructured Vertical Light‐Emitting Diodes by Minimizing Total Internal Reflection

Nanostructured vertical light‐emitting diodes (V‐LEDs) with a very dense forest of vertically aligned ZnO nanowires on the surface of N‐face n‐type GaN are reported with a dramatic improvement in light extraction efficiency (～3.0×). The structural transformation (i.e., dissociation of the surface nitrogen atoms) at the nanolevel by the UV radiation and Ozone treatments contributes significantly to the initial nucleation for the nanowires growth due to the interdiffusion of Zn into GaN, evident by the scanning photoemission microscopy (SPEM), high‐resolution transmission electron microscopy (HR‐TEM), and ultraviolet photoelectron spectroscopy (UPS) measurements. This enables the growth of densely aligned ZnO nanowires on N‐face n‐type GaN. This approach shows an extreme enhancement in light extraction efficiency (>2.8×) compared to flat V‐LEDs, in good agreement with the simulation expectations (～3.01×) obtained from 3D finite‐difference time‐domain (FDTD) tools, explained by the wave‐guiding effect. The further increase (～30%) in light extraction efficiency is also observed by optimized design of nanogeometry (i.e., MgO layer on ZnO nanorods).     

Halide‐Transport Chemical Vapor Deposition of Luminescent ZnS:Mn2+ One‐Dimensional Nanostructures

ZnS:M²⁺ (M = Mn, Co, or Cu) single‐crystal one‐dimensional nanostructures have been prepared via a simple halide‐transport chemical vapor deposition (HTCVD) process at a relatively low temperature. The obvious phase transition suggests that doping with Mn favors the formation of the hexagonal phase at a relative low temperature. The strong photoluminescence from blue to green and the yellow–orange emission, which was caused by the doping of various elements in ZnS nanowires and nanobelts, suggests possible applications of the one‐dimensional nanostructures in nanoscale optoelectronic devices.     

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.     

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.     

Localized Surface Plasmon Enhanced All‐Inorganic Perovskite Quantum Dot Light‐Emitting Diodes Based on Coaxial Core/Shell Heterojunction Architecture

This work presents a strategy of combining the concepts of localized surface plasmons (LSPs) and core/shell nanostructure configuration in a single perovskite light‐emitting diode (PeLED) to addresses simultaneously the emission efficiency and stability issues facing current PeLEDs' challenges. Wide bandgap n‐ZnO nanowires and p‐NiO are employed as the carrier injectors, and also the bottom/upper protection layers to construct coaxial core/shell heterostructured CsPbBr₃ quantum dots LEDs. Through embedding plasmonic Au nanoparticles into the device and thickness optimization of the MgZnO spacer layer, an emission enhancement ratio of 1.55 is achieved. The best‐performing plasmonic PeLED reaches up a luminance of 10 206 cd m^?2, an external quantum efficiency of ≈4.626%, and a current efficiency of 8.736 cd A^?1. The underlying mechanisms for electroluminescence enhancement are associated with the increased spontaneous emission rate and improved internal quantum efficiency induced by exciton–LSP coupling. More importantly, the proposed PeLEDs, even without encapsulation, present a substantially improved operation stability against water and oxygen degradation (30‐day storage in air ambient, 85% humidity) compared with any previous reports. It is believed that the experimental results obtained will provide an effective strategy to enhance the performance of PeLEDs, which may push forward the application of such kind of LEDs.     

Tunable Band‐Selective UV‐Photodetectors by 3D Self‐Assembly of Heterogeneous Nanoparticle Networks

Accurate detection of ultraviolet radiation is critical to many technologies including wearable devices for skin cancer prevention, optical communication systems, and missile launch detection. Here, a nanoscale architecture is presented for band‐selective UV‐photodetectors, which features unique tunability and miniaturization potential. The device layout relies on the 3D integration of ultraporous layers of tailored nanoparticles. By tailoring the transmittance window between the indirect band gap of TiO₂ nanoparticles and the sharp edge of the direct band gap of ZnO, a band‐selective photoresponse is achieved with tunable bandwidth to less than 30 nm and photo‐ to dark‐current ratios of several millions at a light intensity of 86 μW cm^?2 and operation bias of 1 V. The potential of this integrated morphology is shown by fabrication of the first inherent UVA photodetector with selectivity against the edge of the UVB and visible light of nearly 60 times. This tunable architecture and nanofabrication approach are compatible with state‐of‐the micromachining technologies and provide a flexible solution for the engineering of wearable band‐selective photodetectors.     

High‐efficiency Sb2S3‐based hybrid solar cell at low light intensity: cell made of synthesized Cu and Se‐doped Sb2S3

Cu‐doped (as p‐doped) and Se‐doped (as n‐doped) Sb₂S₃ were synthesized from undoped Sb₂S₃ using a newly developed technique, simple colloidal synthesis method. X‐ray diffraction measurements detected no peaks related to any of the Cu and Se compounds in Cu and Se‐doped samples. Energy dispersive X‐ray analysis, however, confirmed the presence of Cu and Se ions in the doped samples. Diffuse reflectance spectroscopy revealed the optical band gap energy changes because of doping effect, as reported for both the p‐type and the n‐type material. The valence‐band X‐ray photoelectron spectroscopy data showed a significant shift in the valence band to higher (Se‐doped; +0.53 eV) and a shift to lower (Cu‐doped; −0.41 eV) binding energy, respectively, when compared with the undoped sample. We report here on an inexpensive solar cell designed and made entirely of a synthesized material (indium tin oxide/p‐doped Sb₂S₃ + polyaniline (PANI)/amorphous/undoped Sb₂S₃ + PANI/n‐doped Sb₂S₃ + PANI/PANI/electrolyte (0.5 M KI + 0.05 M I₂)/Al). The cell has a high efficiency of 8% to 9% at a very low light intensity of only 5% sun, which makes it particularly suitable for indoor applications. As found, the cell performance at the intensity of 5% sun is governed by high shunt resistance (R_SH) only, which satisfies standard testing conditions. At higher light intensities (25% sun), however, the cell exhibits lower but not insignificant efficiency (around 2%) governed by both the series (R_S) and the R_SH. Minimal permeability in the UV region (up to 375 nm) and its almost constant value in the visible and the NIR region at low light intensity of 5% sun could be the reasons for higher cell efficiency. Copyright © 2015 John Wiley & Sons, Ltd.     

Synthesis and Optical Properties of Europium‐Doped ZnS: Long‐Lasting Phosphorescence from Aligned Nanowires

Quasi‐aligned Eu²⁺‐doped wurtzite ZnS nanowires on Au‐coated Si wafers have been successfully synthesized by a vapor deposition method under a weakly reducing atmosphere. Compared with the undoped counterpart, incorporation of the dopant gives a modulated composition and crystal structure, which leads to a preferred growth of the nanowires along the [01 0] direction and a high density of defects in the nanowire hosts. The ion doping causes intense fluorescence and persistent phosphorescence in ZnS nanowires. The dopant Eu²⁺ ions form an isoelectronic acceptor level and yield a high density of bound excitons, which contribute to the appearance of the radiative recombination emission of the bound excitons and resonant Raman scattering at higher pumping intensity. Co‐dopant Cl^– ions can serve not only as donors, producing a donor–acceptor pair transition with the Eu²⁺ acceptor level, but can also form trap levels together with other defects, capture the photoionization electrons of Eu²⁺, and yield long‐lasting (about 4 min), green phosphorescence. With decreasing synthesis time, the existence of more surface states in the nanowires forms a higher density of trap centers and changes the crystal‐field strength around Eu²⁺. As a result, not only have an enhanced Eu²⁺ 4f⁶5d¹–4f⁷ intra‐ion transition and a prolonged afterglow time been more effectively observed (by decreasing the nanowires' diameters), but also the Eu²⁺ related emissions are shifted to shorter wavelengths.     

Beryllium‐Assisted p‐Type Doping for ZnO Homojunction Light‐Emitting Devices

A key step in realization of a ZnO homojunction light‐emitting diode is the effective p‐type doping in ZnO:N. In this article, a feasible route is demonstrated to enhance hole doping in ZnO:N films by the assistance of Beryllium. The newly synthesized p‐type ZnO is applied in light‐emitting devices. The corresponding p–i–n junction exhibits excellent diode characteristics, and strong near band edge ultraviolet emissions is also observed even at temperatures as high as 400 K under the injection of continuous current. The results represent a critical advance toward the development of high‐efficiency and stabilized p‐type ZnO, which is also a desirable key step for future ZnO‐based optoelectronic applications.     

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.     

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.     

The UV and blue light emission properties of Mn doped ZnO nanocrystals

We present a study of the light emission properties over wavelengths from UV to blue of Mn doped ZnO nanocrystals fabricated by means of a thermal evaporation vapor phase deposition process. The samples were grown with a Mn mole ratio in the Zn/Mn mixed source of 0% (pure ZnO sample, used as a reference), 5%, 10%, or 15% in a constant O₂/Ar gas mixture flowing at 500 °C. The pure ZnO nanocrystals exhibited a strong and predominantly UV emission peaking at 377 nm. In the photoluminescence spectra of mixed ZnO:Mn nanocrystals the major UV emission shifts from 377 to 408 nm, and a strong blue emission appears at 435 nm. The former is mainly induced by the impurity levels of Mn introduced in the band gap of the ZnO nanocrystals, while the latter is closely related to defect and Mn²⁺ ions. With increasing Mn concentration the blue emission is enhanced due to the strong exchange interaction in the short range spin system and the excess impurities on the surface. The results show that the optical properties of ZnO can be tuned by the doping concentration of Mn. Mn doped ZnO nanocrystals with strong blue emission can be used in the fabrication of blue light devices.     

Dual Emission of Water‐Stable 2D Organic–Inorganic Halide Perovskites with Mn(II) Dopant

Moisture‐delicate and water‐unstable organic–inorganic halide perovskites (OI‐HPs) create huge challenges for the synthesis of highly efficient water‐stable light‐emitting materials for optoelectronic devices. Herein, a simple acid solution–assisted method to synthesize quantum confined 2D lead perovskites through Mn doping is reported. The efficient energy transfer between host and dopant ions in orange light‐emitting Mn²⁺‐doped OI‐HPs leads to the most efficient integrated luminescence with a photoluminescence quantum yield over 45%. The Mn²⁺ substitution of Pb²⁺ and passivation with low dielectric constant molecules such as phenethylamine, benzylamine, and butylamine enhance water resistivity, leading to water stability. The dual emission process of this water‐stable 2D Mn‐doped perovskite will help in developing highly efficient 2D water‐stable perovskites for practical applications.     

溶胶-凝胶旋涂法制备In掺杂ZnO薄膜结构和光学性能的研究

采用溶胶-凝胶(sol-gel)旋涂法在常规玻璃衬底 上生长了In掺杂浓度分别为1at%、2at%、3at%、4at%、5at%的ZnO薄膜。借助X射线衍射仪(X RD)、扫描电子显微镜(SEM)、紫外- 可见分光光度计(UV-Vis)对样品的晶粒生长、结构以及光学性能进行表征。结果如下:所 制 备的薄膜均沿(002)方向择优生长,且随着In³⁺掺杂浓度增加 ,衍射峰的峰型及半高宽均呈 先降低后升高的趋势;In³⁺掺入后,ZnO薄膜晶粒由原来的六边形状发展成类似蠕虫 状,同 时粒径变小且大小不一;与本征样品相比,掺杂后的ZnO光透过率提高了10%,且吸收边向短 波长方向偏移,同时随着In³⁺的掺入,薄膜的光学带隙值从3.49 eV增加到3.80 eV。当In³⁺掺 杂浓度为4at%时,薄膜(002)峰的峰形最为尖锐、峰值最大,晶粒较为均匀、 晶格间距更小,光透过率最高,光学带隙值相对较大为3.77 eV。