Warm white light-emitting diodes using organic–inorganic halide perovskite materials coated YAG:Ce3+ phosphors

This work reports warm white light-emitting diodes (WLEDs) using organic–inorganic halide perovskite materials coated YAG:Ce³⁺ phosphors by a liquid phase synthesis method. The perovskite MAPbBr_3?xI_x-coated YAG:Ce³⁺ phosphors have more red light contribution than YAG:Ce³⁺ phosphors without the perovskite coating. The chromaticity coordinate of white LED with YAG:Ce³⁺ remote phosphor is (x = 0.3134, y = 0.3497) 6359. However, those of MAPbBr_2.5I_0.5 and MAPbBr_2.0I_1.0-coated YAG:Ce³⁺ remote phosphor shift to (x = 0.4220, y = 0.3725) 2908 and (x = 0.4067, y = 0.4028) 3525, respectively. The colors of perovskite-coated samples are more red and warm. Therefore, the perovskite-coated YAG:Ce³⁺ method is useful for warm WLED.     

Combustion synthesis of multicomponent ceramic phosphors for solid state lighting

About 50% of the world’s need for lighting is provided by artificial lighting. For over the last decade, the world has witnessed rapid shift from conventional Hg-based lighting to LED-based solid state lighting (SSL). SSL technology extensively uses YAG:Ce³⁺ phosphor for production of white light emitting devices (LEDs). Part of the blue light from the (In,Ga)N LED chip is absorbed by a thin layer of Ce³⁺-doped YAG and is converted into yellow light. The combination of blue and yellow gives a bright white light source with an overall energy efficiency exceeding that of a compact fluorescent lamp. Several soft chemical routes have been explored for synthesis of YAG but were discarded due to their complex nature, high cost for the industrialization, phase impure materials, etc. In this paper we describe rapid one-step modified combustion synthesis of YAG:Ce³⁺ and related phosphors carried out at 500°C using a mixed fuel. Photoluminescence spectra of YAG:Ce³⁺ and LED prepared thereof are comparable with those of commercial phosphors.     

Transparent YAG:Ce ceramics for WLEDs with high CRI: Ce3+ concentration and sample thickness effects

Transparent YAG ceramics with different Ce³⁺ doping concentrations and various sample thickness have been fabricated via solid-state sintering under vacuum, for the purpose of high power white light emitting diodes (WLEDs). Their phase compositions were checked by X-ray diffraction (XRD). Optical and luminescence characteristics were investigated by transmittance, absorption spectra and photoluminescence examinations. It is found that by altering the Ce³⁺ concentration and sample thickness, the CIE color coordinates of the assembled LED devices can be tailored to white light region. More importantly, the color rendering index (CRI) of the LED devices got higher with decreased Ce³⁺ doping concentration and sample thickness. Meanwhile, the effect of Ce³⁺ concentration on the CRI was found more significant compared to that of the sample thickness. This study provides an efficient approach to tailor the luminescence properties, especially to improve the CRI of the WLEDs.     

Spectrum regulation of YAG:Ce transparent ceramics with Pr,Cr doping for white light emitting diodes application

YAG:Ce transparent ceramics with high luminous efficiency and color render index were prepared via a solid state reaction-vacuum sintering method. Cr³⁺and Pr³⁺ were applied to expand the spectrum of YAG:Ce transparent ceramics. As prepared ceramics exhibit luminescence spectrum ranging from 500 nm to 750 nm, which almost covers full range of visible light. After the concentration optimization of Ce³⁺, Pr³⁺ and Cr³⁺, high quality white light was obtained by coupling the YAG:Ce,Pr,Cr ceramics with commercial blue LED chips. Color coordinates of the YAG:Ce,Pr,Cr ceramics under 450 nm LED excitation vary from cold white light to warm white light region. The highest luminous efficiency of WLEDs encapsulated by transparent YAG:Ce,Pr,Cr ceramic was 89.3 lm/W, while its color render index can reach nearly 80. Energy transfers between Ce³⁺ → Pr³⁺ and Ce³⁺ → Cr³⁺ were proved in co-doped ceramic system. Transparent luminescence ceramics accomplished in this work can be quite prospective for high power WLEDs application.     

Eu3+‐doped glass as a color rendering index enhancer in phosphor‐in‐glass

A new method for improving color rendering index (CRI) and low correlated color temperature (CCT) in high‐power white‐light‐emitting diodes (WLEDs) is proposed. We used a configuration of phosphor‐in‐glass (PIG) and studied light output changes with the increment in concentration of yellow‐emitting Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce³⁺) phosphor. The PIG was coupled on the top of blue‐light‐emitting diodes (LED) chip (465 nm). To compensate the lack of red emission in the phosphor, Eu³⁺‐doped tellurium glass with different europium content was employed as a red emitter. The suitable contents of YAG:Ce³⁺ and Eu³⁺ were 7.5 weight percent (wt%) and 3 mol percent (mol%), respectively. The CRI value went from 72 to 82, whereas the CCT was reduced from 24 933 to 6434 K. The proposed structure can improve CCT as well as CRI of WLEDs just by placing a glass on top.     

The Cr‐doping effect on white light emitting properties of Ce:YAG phosphor ceramics

The Cr/Ce‐doped YAG transparent ceramic was fabricated by the solid‐state reaction in vacuum. The Cr/Ce‐doped YAG ceramic phosphor effectively complement the red spectral component and improve the color rendering performance when excited by blue light that is due to the effective energy transfer between Cr³⁺ ion and Ce³⁺ ion. However, the energy transfer from Ce³⁺ to Cr³⁺ ion leads to energy loss and therefore the luminous efficacy of the WLED which is composed of blue LED chip and the Cr/Ce‐doped YAG ceramic phosphor decreases. The composite phase structure of ceramic phosphor is designed for improving the extraction efficacy and increasing the luminous efficacy by breaking the total internal reflection (TIR) at the interface between air and ceramic.     

Decay Behavior in Ce3+‐doped La3Si6N11 and Lu3Al5O12 Phosphors

Ce³⁺‐activated light emitting diode (LED) phosphors have been extensively examined for photoluminescence, and have been the focus of many detailed structural studies. However, reports of the decay curves of Ce³⁺‐activated LED phosphors are rare. Although we have reported the decay behaviors of several Eu²⁺‐activated LED phosphors such as Sr₂SiO₄, Sr₂Si₅N₈, and CaAlSiN₃, we have never conducted an in‐depth study into the decay behavior for Ce³⁺‐activated LED phosphors. For this study, we investigated the decay curves of well‐known Ce³⁺‐activated LED phosphors such as La₃Si₆N₁₁ and Lu₃Al₅O₁₂. Similar to Eu²⁺‐activated LED phosphors, the decay behavior of Ce³⁺‐activated LED phosphors was sensitive to the Ce³⁺ concentration and to the detection wavelength. There was active nonradiative energy transfer between the Ce³⁺ activators located at different sites.     

One-step combustion synthesis of Tb3Al5O12:Ce phosphor

A simple, one-step, and fast method based on exothermic reactions is described for synthesis of Tb₃Al₅O₁₂:Ce phosphor. Light-emitting diodes (LEDs) were fabricated by depositing this phosphor on a blue chip. Photoluminescence and LED emission are compared with respective results for well-known YAG:Ce phosphor. A significant improvement in color rendering index (CRI) attributed to the red shift of Ce³⁺ emission was observed. Persistent emission is also reported for the first time in the Tb₃Al₅O₁₂:Ce annealed in reducing atmosphere. It well correlates with Ce³⁺ emission and a peak around 80°C in the thermoluminescence glow. The long-lasting emission was associated with host-related electron traps.     

Stable and efficient all-inorganic color converter based on phosphor in tellurite glass for next-generation laser-excited white lighting

Laser lighting is considered as a next-generation high-power lighting due to its high-brightness, directional emission, and quasi-point source. However, thermally stable color converter is an essential requirement for white laser diodes (LDs). Herein, we proposed a stable and efficient phosphor-in-glass (PiG) in which YAG:Ce³⁺ and MFG:Mn⁴⁺ phosphors were embedded into tellurite glass matrixes. The glass matrixes with low-melting temperature and high refractive index were prepared by designing their composition. The luminescence of YAG:Ce³⁺ PiGs was adjusted by controlling phosphor thickness. Aiming to compensate for red emission, multi-color PiGs were realized by stacking MFG:Mn⁴⁺ layers on YAG:Ce³⁺ layer. The phosphor crystals are chemically stable and maintain intact in the glass matrix. Furthermore, white LDs were fabricated by combining the PiGs with blue LDs. As the phosphor thickness increases, the chromaticity of white LDs shifts from cool to warm white, and the white LDs exhibit excellent thermal stability under different excitation powers.     

Discovery of new broadband yellow-emitting nitridoalumosilicate phosphor and its pc-WLED application

Developing a yellow phosphor with broadband emission covering more red-light areas is an effective approach to achieve high-quality solid-state lighting. In this study, a novel yellow-emitting nitride phosphor, Ca₅Si₂Al₂N₈:Ce³⁺, was successfully prepared at atmospheric pressure and lower temperatures (1300°C), and its structure-property relation was revealed using crystal refinement, photoluminescence (PL) spectra, time-resolved PL spectra, and density-functional theory calculations. The results demonstrate that Ca atoms occupy three different crystallographic sites in the lattice, which are substituted by Ce³⁺ to form multiple luminescence centers. Thus, Ca₅Si₂Al₂N₈:Ce³⁺ emits strong yellow light with a maximum peak at 585 nm and a wide emission band. Compared with YAG:Ce³⁺, Ca₅Si₂Al₂N₈:Ce³⁺ has a wider emission band with a FWHM of 150 nm, which can effectively cover the green and red areas. Moreover, the sample can be fully excited by a blue LED chip due to its broad excitation band. Notably, the Ca₅Si₂Al₂N₈'s tight crystal structure composed of edge-sharing AlN₄ and SiN₄ tetrahedra pairs guarantee its thermochemical stability and quantum efficiency. Furthermore, Ca₅Si₂Al₂N₈:Ce³⁺ exhibits better thermal stability than YAG:Ce³⁺. The results indicate that Ca₅Si₂Al₂N₈:Ce³⁺ is a promising yellow phosphor for WLEDs.     

Synthesis of K2SiF6:Mn4+ Phosphor from SiO2 Powders via Redox Reaction in HF/KMnO4 Solution and Their Application in Warm‐White LED

In this article, we propose a facile method for synthesis of K₂SiF₆:Mn⁴⁺ phosphor and discuss its promising application in warm‐white light emitting diodes (LED). The K₂SiF₆:Mn⁴⁺ was synthesized from SiO₂ powders through redox reaction in HF/KMnO₄ solution. The optical properties of LEDs containing different ratios of K₂SiF₆:Mn⁴⁺ phosphor and commercial Ce³⁺‐doped garnets (YAG‐40) yellow–green phosphor were studied. A warm‐white LED, with color temperature of 3510 K and color rendering index of 90.9 and efficacy of 81.56 lm/W was demonstrated.     

Sandwich structured phosphor-in-glass films enabling laser lighting with superior optical properties

YAG:Ce³⁺ PiG film cannot produce high color-rendering laser lighting due to the deficiency in red spectral component, but adding nitride-based red phosphors usually leads to dramatic decrease of brightness and efficiency as a result of their obvious luminance saturation. In this work, a sandwich structured PiG (SS-PiG) film was created using green-emitting YAGG:Ce³⁺ and orange-emitting GdYAG:Ce³⁺ phosphors, where two types of PiG films were separately co-fired on each surface of a sapphire substrate. The color-rendering index of SS-PiG film can reach 81.4 upon blue laser excitation, improved by 24.3% when compared with that of YAG:Ce³⁺ PiG film (65.5). Driven by blue laser diodes with a flux density of 7.69 W mm^-2, the SS-PiG film shows a luminous emittance of 1362 lm mm^-2, which is 179% higher than traditional multilayer structured PiG film (489 lm mm^-2). The SS-PiG film enables to enhance both of color rendition and luminance of laser-phosphor-converted lighting.     

Effect of Boron Nitride (BN) on Luminescent Properties of Y3Al5O12:Ce Phosphors and their White Light‐Emitting Diode Characteristics

This article reports a low‐cost yellow‐emitting Y₃Al_5‐xB_xO_12‐xN_x:Ce³⁺ phosphor with an enhanced luminescent intensity and excellent thermal stability for white light‐emitting diodes (LEDs). It was synthesized by a simple gas‐pressure sintering (GPS) process. The effect of B³⁺–N^3? incorporation on the optical properties of Y₃Al₅O₁₂:Ce³⁺ phosphor was investigated. The addition of appropriate amounts of boron nitride (BN) leads to a marked increase in photoluminescent intensity and a slight shift of its emission spectra toward the blue region, which is assigned to the improved crystallinity and increased particle size. Especially, the prepared oxynitride phosphor does not exhibit any thermal quenching under high temperature, and the emission intensity at 250°C even increases up to 175% of that measured at 20°C. Finally, the white LED flat lamp with luminous efficiency as high as 101 lm/W, color rendering index of 72, and correlated color temperature of about 6600 K is successfully realized by using YAG:Ce³⁺ phosphor doped with 0.5 molar ratio BN, which is acceptable and promising for general indoor illuminations to replace fluorescent or incandescent lamps.     

Fabrication of Ce:YAG,Ce,Cr:YAG and Ce:YAG/Ce,Cr:YAG dual-layered composite phosphor ceramics for the application of white LEDs

(Ce_0.001Y_0.999)₃Al₅O₁₂ and (Ce_0.001Y_0.999)₃(Cr_xAl_1−x)₅O₁₂ (x=0.001−0.005) transparent ceramics were synthesized by the solid state reaction and vacuum sintering and their optical properties were measured. High quality white light was obtained when the Ce:YAG/Ce,Cr:YAG dual-layered composite ceramic was directly combined with commercial blue LED chip. A maximum luminous efficacy exceeding 76 lm/W at a low correlated color temperature of 4905 K was obtained. The color temperature can be controlled by variations of Cr³⁺ concentration and the ceramic thickness. Hence, the Ce:YAG/Ce,Cr:YAG dual-layered composite phosphor ceramic may be a promising candidate for white LEDs.     

NIR Enhancement Based on Energy Transfer Process of Ce3+–Yb3+ in Inverse Opal Photonic Crystals

In this article, we fabricated Ce³⁺, Yb³⁺‐coped yttrium aluminum garnet (YAG) inverse opal photonic crystals by a self‐assembly technique in combination with a sol–gel method. The photonic band gap was located at 545 nm. With the influence of the photonic band gap, cooperative down‐conversion energy transfer process of Ce³⁺‐Yb³⁺ was investigated. It is significant that the energy transfer efficiency between Ce³⁺ and Yb³⁺ is enhanced from 29.6% to 47.6% by suppression of the yellow spontaneous emission of Ce³⁺. The mechanisms for the influence of the photonic band gap on cooperative down‐conversion process of Ce³⁺ and Yb³⁺ are discussed.     

The effect of Lu3+ doping upon YAG:Ce phosphor ceramics for high-power white LEDs

Intense green emission is extremely significant to the color rendering index (CRI) of white LEDs. Various green-emitting YLuAG:Ce phosphor ceramics were successfully prepared by vacuum sintering. The effects of Lu³⁺ doping on structure and luminescence property were investigated in detail. In comparison with YAG:Ce, YLuAG:Ce ceramics own smaller grain size, better luminescence performance and higher thermal stability. The photoluminescence (PL) intensity of YLuAG:Ce ceramics increases by 23.6 % due to the “light scattering enhanced effect”. Furthermore, the Ce³⁺ emission is obviously blue-shifting from 533 nm to 519 nm, and the intensity of YLuAG:Ce ceramics reduces only about 8.9 % at 250 °C, showing better thermal stability (vs 11.1 % of YAG:Ce). The LE of LED packaged by YLuAG:Ce ceramic is up to 148.88 lm/W when the doping Lu³⁺ y is 2.1. The above results show that tailored YLuAG:Ce phosphor ceramic is a potential green-emitting color converter for high-power LEDs (hp-LEDs).     

Dy3+/Ce3+ Codoped YAG Transparent Ceramics for Single‐Composition Tunable White‐Light Phosphor

Superior optical, thermal, and mechanical properties of transparent ceramics are very important in the applications of solid lasers, solid‐state lighting, and transparent armors. Herein, a series of (Dy_0.03Ce_xY_0.97?x)₃Al₅O₁₂ transparent ceramics were fabricated using vacuum reactive sintering method. Importantly, these Dy³⁺/Ce³⁺ codoped yttrium aluminum garnet (YAG) transparent ceramics served as single‐composition tunable white‐light phosphors for UV‐LEDs is developed for the first time. By combining with commercially available UV‐LEDs directly, the optimal chromaticity coordinates and correlated color temperature (CCT) are (x = 0.33, y = 0.35) and 5609 K, respectively. Notably, the codoping of Ce³⁺ enhances the luminescent intensity of Dy³⁺ ions while excited at 327 nm. The emission color of YAG transparent ceramics can be tuned from white to yellow through energy transfer between Dy³⁺ and Ce³⁺. These new phosphors, possessing of pure CIE chromaticity and environmentally friendly nature, are promising for applications in white UV‐LEDs.     

Effect of the Ce3+ concentration on laser-sintered YAG ceramics for white LEDs applications

This paper describes the laser sintering and luminescence properties of Y₃Al₅O₁₂ (YAG) phosphors doped with different concentrations of Ce³⁺ and synthesized using the polymeric precursor method. The ceramics were sintered by a new laser sintering technique in which a CO₂ laser is employed as the heating source. The resultant ceramics exhibited a homogeneous microstructure with narrow grain size distribution and high relative density. The introduction of Ce³⁺ ions led to luminescence quenching at a concentration above 0.5 mol% and a redshift of the emission spectrum band with increasing cerium concentration. The excitation spectrum showed two characteristic bands centered at 340 nm and 460 nm and a relative change in their intensity by change the Ce³⁺ concentration. The presence of a single valence (Ce³⁺) of cerium was determined by X-ray absorption near edge structure measurements.     

Growth of YAG:Ce3+-Al2O3 eutectic ceramic by HDS method and its application for white LEDs

The resin-free YAG:Ce³⁺-Al₂O₃ eutectic ceramic phosphor for white light emitting diodes (WLEDs) was successfully grown in vacuum by Horizontal Directional Solidification method (HDS). X-ray diffraction and scanning electron microscopy indicate that this material has a typical eutectic structure of interpenetrating sapphire and garnet phases. The excitation spectra, emission spectra and temperature characteristics of the eutectic show that it is characterized by a wide excitation band and it has good stability in high temperature. In X-ray photoelectron spectroscopy, annealing in an air atmosphere could eliminate the oxygen vacancies and didn’t change the Ce³⁺ valence in the eutectic. The YAG:Ce³⁺-Al₂O₃ eutectic ceramic with different thickness was fixed in COB (chip on board) element for researching the performance of the WLEDs with the phosphor. The electroluminescence characterization of the WLEDs show that the WLEDs with the eutectic ceramic are more excellent than the common commercial WLEDs.     

Optical performances of mono‐dispersed spherical YAG:Ce3+ nano‐phosphor achieved by one‐pot synthesis

Mono‐dispersed spherical YAG:Ce³⁺ nano‐phosphors were successfully synthesized by a one‐pot glycol‐thermal process using aluminum isopropoxide, yttrium, and cerium acetate hydrates as the precursor, (1,4)‐butanediol as the solvent, ethidenediamine as the additive agent that can both control the morphology and improve the optical performances of the as‐achieved products. The as‐prepared YAG:Ce³⁺ nano‐phosphors displayed mono‐dispersed spheres of about 150 nm and an improved optical performance with a quantum yield (QY) of 41% and good photostability, indicating that they have a considerable potential to be applied in solid‐state lighting or used as coatings in other optical electronic devices.