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.     

Glass-Ceramics and Solid-State Lighting

Yttrium aluminum garnet (YAG) doped with Ce³⁺ ion is known as an excellent phosphor for light-emitting diode (LED), usually used as a powder form dispersed in organic resins. We have developed translucent glass-ceramics (GC) of YAG: Ce³⁺ microcrystals in 2004. The GC sheet with half millimeter thick can work efficiently to make identical emission spectra with conventional white LED when combined with a blue LED. This report reviews the development history of the GC materials and impact for all inorganic solution for solid-state lighting.     

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.     

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.     

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.     

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.     

Crystal structure,energy transfer mechanism and tunable luminescence in Ce3+/Dy3+ co-activated Ca20Mg3Al26Si3O68 nanophosphors

Ce³⁺, Dy³⁺ and Ce³⁺/Dy³⁺ co-doped Ca₂₀Mg₃Al₂₆Si₃O₆₈ (CMAS) nanophosphors were synthesized via modified solution-combustion method. Sharp X-ray diffraction patterns confirmed the formation of pure crystalline phase of Ca₂₀Al₂₆Mg₃Si₃O₆₈ as an orthorhombic crystal system having space group Pmmn. The phase purity of as synthesized material has allowed reliable structural parameters to be obtained from the Rietveld analysis of its powder diffraction pattern. The Ce³⁺, Dy³⁺ and Ce³⁺/Dy³⁺ emission at different lattice sites in CMAS host has been identified and discussed. Under ultra-violet (UV) excitation, optical properties and the energy transfer mechanism from Ce³⁺ to Dy³⁺ in CMAS: Ce³⁺/Dy³⁺ nanophosphors have been elaborated by photoluminescence spectroscopy. Also, the effects of doping and sintering temperature on the structure of prepared CMAS host samples have been investigated in detail. The Ce³⁺/Dy³⁺ concentration quenching mechanism due to multipole–multipole interaction has been studied and the critical energy-transfer distance was calculated to be 7.8 Å. The band gap of the synthesized phosphors was calculated from diffuse reflectance spectra using the Kubelka–Munk function. A uniform layered structure network has been revealed in scanning electron microscopy images of the CMAS phosphor. Transmission electron microscopy results indicate nanocrystalline nature of synthesized phosphors. CMAS: 1 m% Ce³⁺ and CMAS: 0.5 m% Dy³⁺ nano-luminescent powders are promising candidate as a blue and blue–yellow emitting UV convertible phosphor for application in white light emitting diodes. By utilizing the energy transfer mechanism in present CMAS: Ce³⁺/Dy³⁺ nanophosphors, with an appropriate tuning of the activator content, these phosphors can exhibit great potential for white light emission, as single-emitting component phosphors in solid state lighting technology.     

Synthesis and luminescent characteristics of yellow emitting GdSr2AlO5:Ce phosphor for blue light based white LED

A series of Ce³⁺ ions doped GdSr₂AlO₅ (GSA) phosphors were synthesized by a citric acid based sol–gel method. The X-ray diffraction patterns confirmed their tetragonal structure after the samples were annealed at 1300 °C, and the scanning electron microscope image showed the closely packed particles. The excitation spectra revealed that the GSA phosphor effectively excited with blue light of 442 nm due to the 4f¹→5d¹ transition and exhibited yellow emission corresponding to the 5d¹→4f¹ transition of Ce³⁺ ions. The optimum doping concentration of Ce³⁺ ions was 5 mol% and the critical distance was calculated to be ~17 Å. White LEDs were fabricated by combining blue LED (465 nm) chip with Ce³⁺:GSA phosphor. The CIE chromaticity coordinates (0.34, 0.31) provide their emission potentiality in the white light region.     

Long-lasting luminescence in garnet-based phosphors prepared by combustion synthesis

Recently long-lasting luminescence (LL) has been reported in YAG doped with Pr³⁺ or Ce³⁺ processed in reducing atmosphere. However, YAG:Ce emission is yellow which is not attractive for long lasting applications. On the other hand, Ga substituted garnets give green emission at shorter wavelengths. We explored Ga substituted garnets prepared by combustion synthesis for LL. These phosphors do exhibit green LL, although the intensities are lower compared to that of YAG:Ce.     

Spectral dependency of Eu2+-activated silicate phosphors on the composition for LED application

An Sr₂SiO₄-Ba₂SiO₄ material system doped by Eu²⁺ was studied for light emitting diodes (LEDs) application. The main concern was the precise control of excitation and emission spectra for maximum light yield and color coordinate, which was carried out by changing the composition of the alkaline earth ions in host lattice. The Sr₂SiO₄ : Eu-Ba₂SiO₄ : Eu system was found to be excellent for white LED applications with excitation in the 380–465 nm region. Especially, the yellow light intensity from (Sr,Ba)SiO₄ : Eu phosphors was comparable to YAG : Ce phosphors in case of blue LED excitation.     

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.     

Broadband excited Na3Tb(PO4)2:Ce3+/Eu2+ green/yellow-emitting phosphors with high color purity for LED-based application

To enhance the display quality of light-emitting diodes (LEDs), it is of great significance to exploit green/yellow-emitting phosphors with narrow emission band, high quantum yield, and excellent color purity to satisfy the application. Herein, orthophosphate-based green/yellow-emitting Na₃Tb(PO₄)₂:Ce³⁺/Eu²⁺ (NTPO:Ce³⁺/Eu²⁺) phosphors have been successfully synthesized by a facile solid-state reaction method. The absorption band of NTPO samples was extended to the near-ultraviolet region and the absorption efficiency was significantly improved owing to a highly efficient energy transfer from Ce³⁺/Eu²⁺ ion to Tb³⁺ ion in NTPO host certified by time-resolved PL spectra. Upon 300 nm excitation, the NTPO:Ce³⁺ is characterized by ultra-narrow-band green emission of Tb³⁺ with an absolute quantum yield of 94.5%. Unexpectedly, NTPO:Eu²⁺ emits bright yellow light with a color purity of 73% as a result of the blending of green light emission from Tb³⁺ and red light emission from Eu³⁺. The thermal stability has been improved by controlling the stoichiometric ratio of Na⁺. The prototype white LED used yellow-emitting NTPO:Eu²⁺ phosphor has higher color-rendering index (Ra = 83.5), lower correlated color temperature (CCT = 5206 K), and closer CIE color coordinates (0.338, 0.3187) to the standard white point at (0.333, 0.333) than that used green-emitting NTPO:Ce³⁺ phosphor, indicating the addition of the yellow light component improved the Ra of the trichromatic (RGB) materials.     

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.     

Enhanced red emission and improved thermal quenching of CaAlSiN3:Eu2+ phosphors via boron doping

The development of high-performance phosphors is required for phosphor-converted white light-emitting diodes. However, most approaches are unable to achieve optimum emission intensity and thermal quenching simultaneously. Here, a series of CaAlSiN₃:Eu²⁺ (CASN:Eu²⁺) red-emitting phosphors doped with B were synthesized using field-assisted sintering technology. Compared with CASN:Eu²⁺, the B-doped phosphor exhibited high external quantum efficiency (EQE) and good thermal quenching performance. With boron doping, the EQE of CaAlSiN₃:Eu²⁺ shows an obvious growth, increasing from 48.83% to 70.68%. Meanwhile, thermal quenching performance has also been greatly improved, which is strongly associated with the band structure of Eu²⁺ and the crystal structure of CASN. The location of B in the crystal lattice was studied and the mechanism of improving thermal quenching via B doping was discussed in detail. Finally, a white LED fabricated by the combination of a GaN blue chip (450 nm) with the as-synthesized red phosphors and Y₃(Al, Ga)₅O₁₂:Ce³⁺ green phosphors (531 nm), shows a high color rendering index (Ra =91.6). This study offers a novel method to improve luminescence properties of CASN:Eu²⁺ red-emitting phosphors, which may broaden their application in solid-state lighting devices.     

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.     

A broadband yellow-emitting nitridoalumosilicate Ca4SiAl3N7:Ce3+ phosphor for solid-state lighting

Developing an efficient broadband yellow phosphor with more red-light components and small thermal quenching is of great significance for solid-state lighting. In this study, a broadband yellow-emitting nitridoalumosilicate Ca₄SiAl₃N₇:Ce³⁺ phosphor was successfully synthesized by a solid-phase method at comparatively low temperature (1350 °C) and normal pressure. The crystal structure and electronic structure of Ca₄SiAl₃N₇ were studied using Rietveld refinement and density functional theory. The photoluminescence properties of the Ca₄SiAl₃N₇:Ce³⁺ phosphor were studied, including excitation and emission spectra, time-resolved photoluminescence spectra and temperature-dependent emission spectra. The results show that the Ca₄SiAl₃N₇:Ce³⁺ phosphor can be effectively excited by the blue chip and emit a strong broadband yellow light with maximum at 568 nm and the half width of 142 nm. Moreover, the Ca₄SiAl₃N₇:Ce³⁺ phosphor exhibits good thermal stability, which can still maintain 75% and 68% of the strength at room temperature when at 150 °C and 200 °C, respectively, and without spectral shift. A warm WLED can be realized by combining Ca₄SiAl₃N₇:Ce³⁺ yellow phosphor and blue LED chip. This study provides insights into developing novel broadband yellow nitridoalumosilicate phosphor with more red-light components, small thermal quenching and simple synthesis conditions.     

Crystal Structure and Optical Performance of Al3+ and Ce3+ Codoped Ca3Sc2Si3O12 Green Phosphors for White LEDs

Ca₃Sc₂Si₃O₁₂:Ce³⁺ (CSS:Ce) green phosphors used for white light‐emitting diodes (LEDs) are synthesized and codoped with Al³⁺ via a solid‐state reaction method. The crystal structure and vibrational modes are analyzed by X‐ray diffraction, Fourier transform infrared spectroscopy, and Raman scattering spectroscopy. The energy transfer behavior and optical performance are characterized by photoluminescence and excitation spectra, quantum efficiency, and time‐resolved photoluminescence. The incorporation of Al³⁺ into CSS:Ce can inhibit the formation of the impurity phases Sc₂O₃ and CeO₂, improve crystallinity, and enhance the photoluminescence intensity as well as quantum efficiency. The substitution of Sc³⁺ with Al³⁺ increased the crystal field splitting of Ce³⁺ and resulted in the red shift of photoluminescence. The results show that Ca₃Sc_2?xAl_xSi₃O₁₂:Ce³⁺ has high quantum efficiency, making it a promising green phosphor that can be collocated with a commercial 450 nm blue LED and a red phosphor for solid‐state lighting applications.     

Stable and Color‐Tailorable White Light from Blue LEDs Using Color‐Converting Phosphor–Glass Composites

Commercial Ce³⁺:YAG phosphors were embedded in glass frits. Thermal condition for the viscous sintering of the composite materials was optimized. The phosphor–glass composites had maximum external efficiency of 30% and maximum light extraction efficiency of 39%. Color temperatures of the composites composed of fluorescent glass frits containing Eu³⁺ and Mn²⁺ combined with blue LEDs shifted from ~7000 to ~4000 K.     

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.