Phosphor-converted White Light-emitting Diodes Research Articles

A systematic study on optoelectronic properties of Mn4+-activated Zr-based hexafluoride red phosphors X2ZrF6 (X = K, Na, Cs): first-principles investigation and prospects for warm-white LEDs applications

The quest for developing energy-efficient and environment-friendly phosphors for lighting devices such as light-emitting diodes (LEDs) is on rise to meet the future energy challenges. In this connection, phosphor-converted white LEDs are promising candidates for the next generation state-of-the-art solid-state lighting technology to substitute the traditional lighting devices such as fluorescent lamps, incandescent lamps, halogen lamps, and backlights for liquid crystal displays. Hereby, we report a systematic analysis on the optoelectronics properties of Zr-based Mn4+-activated phosphors X2ZrF6 (X = K, Na, Cs) for potential optoelectronics and photoluminescence device applications such as LEDs. For the comparative analysis of the first-principles calculations based on full-potential linearized augmented plane wave DFT procedure, we employed spin polarized GGA and GGA + U scheme of exchange and correlation energy potentials. In contrast to parent X2ZrF6 compounds, the band gaps of the Mn-doped X2MnF6 compounds have been lowered considerably. The improvement on band-gap values with GGA + U method reaffirms the famous drawback of GGA-based DFT methods regarding the underestimation of band gaps in highly correlated systems. For the prospects of materials regarding optoelectronics and photoluminescence applications, we first time report a detailed systematic analysis of optical properties such as dielectric functions, energy loss function, reflectivity, absorption coefficient, refractive index and optical conductivity. The materials are weakly photons reflector in IR and visible regions while they are strong photons absorbers in the UV region. In the absence of experimental evidences, indirect evidence of the wide band-gaps of K2ZrF6 and Na2ZrF6 compounds is ascertained via doping of Mn4+ ions. Besides, as all the energy levels of Mn4+ ions are around 4 eV these can also be observed in the experimental absorption spectra.

Hetero-valent substitution strategy toward orange-red luminescence in Bi3+ doped layered perovskite oxide phosphors for high color rendering index white light-emitting diodes

Controllable efficient emission tuning from 553 nm (yellow) to 609 nm (orange-red) in novel Sr 1- x La 1+ x ZnO 3.5+ x /2 :Bi 3+ (0 ≤ x ≤ 0.4) phosphors with layered perovskite structure is first designed for high color rendering index pc-WLEDs via a hetero-valent cation substitution and controlling over site selective excitation strategies. • Hetero-valent substitution strategy is designed in novel yellow SrLaZnO 3.5 :Bi 3+ . • Luminescence tuning from 558 nm (yellow) to 585 nm (orange) is achieved. • Orange-red luminescence (609 nm) is achieved by site-selective excitation strategy. • White LED shows a superior color rendering index (Ra = 97.1, R9 = 98). The equivalent or hetero-valent substitution strategy is an efficient method for stimulating photoluminescence tuning or optimizing the photoluminescence performances of phosphor materials. A new K 2 NiF 4 -type layered perovskite SrLaZnO 3.5 :Bi 3+ phosphor with broad-band yellow emission (558 nm) is first explored. Based on an efficient hetero-valent substitution strategy, a series of Sr 1- x La 1+ x ZnO 3.5+ x /2 :Bi 3+ (0 ≤ x ≤ 0.4) solid solution phases are successfully constructed. The different Bi 3+ local environments at two crystallographic sites and vacancy oxygen make the various distributions of emission and excitation spectra. Consecutive emission tuning from yellow (558 nm) to orange (585 nm), and improvement of emission intensity and internal quantum efficiency (IQE = 45%-64%) are achieved through La 3+ hetero-valent substitution for Sr 2+ in Sr 1- x La 1+ x ZnO 3.5+ x /2 :Bi 3+ (0 ≤ x ≤ 0.4). The structural stability and Bi 3+ occupation distribution in solid solution have been revealed by structural elaboration and crystal field regulation. Based on the both hetero-valent substitution and site-selective excitation strategies, a novel orange-red luminescence can be observed peaking at ~609 nm under near-ultraviolet light (~390 nm) excitation without visible absorption. The fabricated full-spectrum phosphor-converted white light-emitting diodes (pc-WLEDs) show outstanding warm white light, and a high color rendering index (Ra = 97.1, R9 = 98) is achieved, implying that the as-prepared Sr 1- x La 1+ x ZnO 3.5+ x /2 :Bi 3+ (0 ≤ x ≤ 0.4) phosphors can used as yellow/orange-red components in pc-WLED lighting. These results indicate that hetero-valent substitution can predict and control site preference of luminescent centers, providing a new way for optimizing phosphor materials for solid-state lighting.

Improving thermal stability and quantum efficiency through solid solution for Ce3+-activated (Ba1-xSrx)3Y2(BO3)4 phosphors

Thermal stability and quantum efficiency are crucial parameters for phosphors when they act as components in phosphor-converted white light-emitting diodes (pc-wLEDs). Therefore, it is important to explore phosphors with highly thermal stability and quantum efficiency. Blue-emitting Ce3+-activated (Ba1-xSrx)3Y2(BO3)4 phosphors were synthesized through high-temperature solid-state reaction. X-ray powder diffraction suggests the success of complete solid solution in the phosphor series. Scanning electron microscope (SEM) image and elemental mapping suggest uniform distribution of Ba and Sr atoms in the phosphor particles. Rietveld refinement indicates that cell volume decreases with increasing the content of Sr2+, while the related emission peak exhibits a slight blue shift, which implies the existence of remote control effect. Both thermal stability and quantum efficiency increase with the evolution of solid solution from Ba3Y2(BO3)4:Ce3+ to Sr3Y2(BO3)4:Ce3+, which is due to the increase of structural rigidity. The emission intensity of Sr3Y1.94(BO3)4:0.06Ce3+ at 423 K remains ∼56% of the room-temperature value. The internal and external quantum efficiencies of Sr3Y1.94(BO3)4:0.06Ce3+ are ∼93% and ∼67%, respectively. A pc-wLED was fabricated with a 365 nm UV chip, Sr3Y1.94(BO3)4:0.06Ce3+, commercial green phosphor, and commercial red phosphor, which exhibits a color rendering index of 94.5. This work provides an example on the adjustment of luminescent properties through solid solution.

Efficient Energy Transfer from Trap Levels to Eu3+ Leads to Antithermal Quenching Effect in High-Power White Light-Emitting Diodes.

The most critical aspect in the assembly of phosphor-converted white light-emitting diodes (pc-WLEDs) is how to stabilize the device in a practical environment. The high applied currents can generate enormous heat up to more than 100 °C, and such a continuous illumination process will lead to serious effects concerning the stability of the device. Therefore, the new search for examples to fully suppress thermal quenching effect is a real challenge. In this study, a novel Eu3+-activated CaMgGeO4 (CMGO) phosphor of olivine type is developed via a conventional solid-state reaction. The results reveal that Eu3+ occupies the low symmetric Ca2+ site of this host. Upon visible-light sensitization at 464 nm, a dominant red emission band with maximum at 612 nm is witnessed. Its full width at half-maximum (fwhm) is merely ∼4.37 nm, and a high color purity of around 94% is achieved. Their corresponding Commission Internationale de L'Eclairage (CIE) coordinates are very close to standard red color coordinates (0.666, 0.333). The influence of concentration and temperature on the optical property has been explored. It has been discovered that the optimized sample (CMGO:0.01Eu3+) is not influenced by the thermal quenching effect and its fluorescent intensity is improved even up to 473 K, which is mainly attributed to the incorporation of abundant trap sites generated by the nonequivalent substitution Eu3+ for Ca2+. After it is integrated into commercially available YAG:Ce3+ phosphor-based pc-WLEDs, the excellent optical parameters of the fabricated WLEDs are evaluated. The correlated color temperature (CCT) varies from cool white (6458 K) to warm (4370 K), and the color rendering index (CRI) increases from 78 to 86 under a high flux operating current of 200 mA. Furthermore, the chromaticity coordinates remain almost stable with the increasing drive current from 200 mA to 1000 mA. It is highly expected that CaMgGeO4:0.01Eu3+ will become a suitable red phosphor for the preparation of white LEDs with high efficiency.

Digital Luminaire Design Using LED Digital Twins—Accuracy and Reduced Computation Time: A Delphi4LED Methodology

Light-emitting diode (LED) digital twins enable the implementation of fast digital design flows for LED-based products as the lighting industry moves towards Industry 4.0. The LED digital twin developed in the European project Delphi4LED mimics the thermal-electrical-optical behavior of a physical LED. It consists of two parts: a package-level LED compact thermal model (CTM), coupled to a chip-level multi-domain model. In this paper, the accuracy and computation time reductions achieved by using LED CTMs, compared to LED detailed thermal models, in 3D system-level models with a large number of LEDs are investigated. This is done up to luminaire-level, where all heat transfer mechanisms are accounted for, and up to 60 LEDs. First, we characterize a physical phosphor-converted white high-power LED and apply LED-level modelling to produce an LED detailed model and an LED CTM following the Delphi4LED methodology. It is shown that the steady-state junction temperature errors of the LED CTM, compared to the detailed model, are smaller than 2% on LED-level. To assess the accuracy and the reduction of computation time that can be realized in a 3D system-level model with a large number of LEDs, two use cases are considered: (1) an LED module-level model, and (2) an LED luminaire-level model. In the LED module-level model, the LED CTMs predict junction temperatures within about 6% of the LED detailed models, and reduce the calculation time by up to nearly a factor 13. In the LED luminaire-level model, the LED CTMs predict junctions temperatures within about 1% of LED detailed models and reduce the calculation time by about a factor of 4. This shows that the achievable computation time reduction depends on the complexity of the 3D model environment. Nevertheless, the results demonstrate that using LED CTMs has the potential to significantly decrease computation times in 3D system-level models with large numbers of LEDs, while maintaining junction temperature accuracy.

Ultra-high color rendering warm-white light-emitting diodes based on an efficient green-emitting garnet phosphor for solid-state lighting

High light-quality and low color temperature are two crucial factors for a comfortable healthy illumination. Phosphor-converted white light-emitting diodes (LEDs) could be readily realized by combining near-UV (380–420 nm) LED chips with tri-color phosphors, and thus near-UV-excitable inorganic phosphors with efficient visible luminescence are highly demanded for fabricating high-performance near-UV-pumped white LEDs. Herein, we demonstrated a novel highly efficient green-emitting phosphor based on Ce3+/Tb3+ ions co-activated Ca2GdHf2Al3O12 (abbreviated as: CGHAO) garnet compound, which enabled the realization of ultra-high color rendering warm-white LEDs. Notably, the CGHAO:Ce3+,Tb3+ phosphors could be efficiently excited by near-UV light around 408 nm and exhibited intense green emissions around 543 nm with high internal quantum efficiency of 82.7% and external quantum efficiency of 60.6%. The energy transfer from Ce3+ to Tb3+ ions was studied in detail, and the composition of CGHAO:Ce3+,Tb3+ phosphors was optimized. Utilizing the optimal CGHAO:0.04Ce3+,0.4Tb3+ green phosphor as color converter, a prototype near-UV-pumped white LED device was fabricated, and upon 120 mA driving current it demonstrated bright warm-white light with excellent CIE chromaticity coordinates of (0.391, 0.360), low correlated color temperature of 3575 K, ultra-high color rendering index (Ra = 94.4, R12 = 89.0, and R9 = 80.6) and good luminous efficacy of 27.40 lm·W−1. This work offers a new strategy for designing efficient LED phosphors for high-color-quality solid-state lighting.

Highly thermal stable phosphor LiSrPO4:Eu2+ with a new crystal structure

ABPO4-type structures are attractive host candidates for phosphors owing to their versatile crystal structures, which makes the great possibility for tuning of luminescent properties. Up till now, there are still some related compounds lack detailed crystal structure informations. And there are also upcoming ABPO4-type structures and related phosphors are under discovering. In this work, Eu2+-activated LiSrPO4 phosphor with a new crystal structure is discovered. Rietveld refinement suggests that it belongs to a monoclinic (m-) system, which exhibits a higher symmetry than the already reported monoclinic one (PDF#53–1238). The Eu2+-activated phosphor exhibits a narrow emission band at 420 nm with a full width at half maximum of 28 nm, which is different from Eu2+ in the old monoclinic or hexagonal (h-) LiSrPO4 with an emission band at around 450 nm. The difference is discussed by comparing the crystal structures especially the coordination environments between the newly found m- and h-LiSrPO4. Eu3+ ion was also used to probe the difference in crystal structure. An additional thermal treatment at 900 °C for the as-prepared samples greatly enhances their emission and quantum efficiency, which is due to the increase of Eu2+/Eu3+ ratio. Integrated emission intensity at 150 °C remains ~94% of the room-temperature value for either one-step or two-step sintering process. Results suggest that the as-prepared phosphor can be a good candidate for phosphor-converted white light-emitting diodes.

Mn2+- doped Zn2SnO4 green phosphor for WLED applications

Eco-friendly phosphors based on non-rare-earth elements have received increasing interest in the field of fabricating white light-emitting diodes (WLEDs). In this work, Mn2+- activated Zn2SnO4 green phosphors were synthesized by high energy planetary ball milling followed by annealing at 1000 °C in a reducing gas environment. The effects of the annealing temperature and the Mn2+ doping concentration on the structural and optical properties were studied. Photoluminescence spectrum displays a green emission band peaking at 523 nm with full width at half maximum of about 32 nm, which is assigned to the spin forbidden d-d transition (4T1(4G) → 6A1(6S)) of the Mn2+ ions. The photoluminescence excitation spectrum of Zn2SnO4:5%Mn2+, monitored at 523 nm, presents five distinct peaks at 359, 380, 424, 435, and 444 nm, which are in good agreement with Mn2+ absorption transitions. The absorption peak at 444 nm well coincides with the peak wavelength of a blue LED emission. By coating the prepared phosphor on a blue LED chip, a green-emitting LED was successfully fabricated. Moreover, a series of white LEDs were made based on 450 nm chip coated with a blend of the Zn2SnO4:5%Mn2+ phosphor with a red phosphor (Zn2SnO4:3%Cr3+,0.6%Al). Warm white light with the correlated color temperature of 3858 K and a high color rendering index of 91 was achieved. These results indicate that the Zn2SnO4:5%Mn2+ phosphor is a promising material for applications in phosphor-converted WLEDs.

Multiple Energy Transfer in Luminescence-Tunable Single-Phased Phosphor NaGdTiO4: Tm3+, Dy3+, Sm3.

Advances in solid-state white-light-emitting diodes (WLEDs) necessitate the urgent development of highly efficient single-phase phosphors with tunable photoluminescence properties. Herein, the Tm3+, Dy3+, and Sm3+ ions are incorporated into the orthorhombic NaGdTiO4 (NGT) phosphors, resulting in phosphors that fulfill the aforementioned requirement. The emission spectrum of Tm3+ ions overlaps well with the adsorption spectra of both Dy3+ and Sm3+ ions. Under the excitation at 358 nm, the single-phase NaGdTiO4: Tm3+, Dy3+, Sm3+ phosphor exhibits tunable emission peaks in the blue, yellow, and red regions simultaneously, resulting in an intense white-light emission. The coexisting energy transfer behaviors from Tm3+ to Dy3+ and Sm3+ ions and the energy transfer from Dy3+ to Sm3+ ions are demonstrated to be responsible for this phenomenon. The phosphors with multiple energy transfers enable the development of single-phase white-light-emitting phosphors for phosphor-converted WLEDs.

High-efficiency YAG:Ce3+ glass-ceramic phosphor by an organic-free screen-printing technique for high-power WLEDs

Abstract Y3Al5O12:Ce3+ glass-ceramic phosphor (YAG-GC), as a high thermal stability material, provides an efficient way to resolve the aging of phosphor-converted white light-emitting diodes in the high-power field. In the present work, transparent YAG-GCs are successfully prepared via a novel organic-free screen-printing route. The results reveal that the degradations of YAG phosphors are negligible in the as-prepared glass ceramic material. Favorable transmittance is achieved and the value is near 50%. The PL spectra intensities of YAG-GCs increase with increasing YAG doping concentration and film thickness. Superior thermal stability (87.9% of the initial value at 150 °C) and high quantum efficiency of 88.91% (decreased only 2.33% compared with the YAG raw materials) are obtained. The LE of YAG-GCs increases, while the CCT and CRI decrease with increases in film thicknesses and YAG doping concentrations. A high LE of 150.1 lm/W and an acceptable CCT of 5836 K, a CRI of 67.6 are achieved in YAG-GCs. All the results indicate that the as-prepared YAG-GCs are promising candidates in the field of high-power WLEDs.

Double relaxation emission of Sn2+ activator in tin fluorophosphate glass for optoelectronic device applications

In this work, in addition to the presence of cool white luminescence under ultraviolet illumination, after removing illumination, transitory phosphorescence (the afterglow times were about ∼1 s) was observed in pure tin fluorophosphate glass prepared by traditional melting methods. By dynamic spectroscopy analysis, its novel coexistence of fluorescence and phosphorescence phenomenon was attributed to the double relaxation behavior of T1 → S0, and the glass composition was considered to be an important factor affecting the S1/S1′→T1 conversion rate of Sn2+ activator. The electron paramagnanetic resonance test suggested that the oxygen vacancy surrounded in Sn2+ activator captured electronics to form the F+-like center. As this defect center as energy trap, the brand-new triplet (T1)-singlet (S1) relaxation band model of Sn2+ activator was proposed to describe this double relaxation emission. It was demonstrated that tin fluorophosphate glass possessed excellent potential in the application of light-responsive optoelectronic devices and phosphor-converted white light-emitting diodes (pc-WLED).

Preparation of red-emitting BaSiF6:Mn4+ phosphors for three-band white LEDs

BaSiF6:Mn4+ phosphors were synthesized to improve the color purity of phosphor-converted white light-emitting diodes (LEDs). The optimal synthesis conditions for attaining the brightest red emission spectrum from BaSiF6:Mn4+ were investigated. The BaSiF6:Mn4+ phosphor exhibited a strong absorption band at approximately 465 nm because of the 4A2g → 4T2g transition of Mn4+ ions. In addition, the phosphor exhibited three strong emission peaks, at 615, 632, and 650 nm, because of a combination of vibrational shifts and the 2Eg → 4A2g transition in Mn4+ ions. Thus, BaSiF6:Mn4+ is suitable for use as the red-emitting phosphor in three-band white LEDs pumped by a blue LED. The optical properties of BaSiF6:Mn4+ were examined under excitation from a commercial blue LED by coating the phosphor onto the blue LED. Similarly, a three-band white LED was also fabricated by coating the red-emitting phosphor BaSiF6:Mn4+ and green-emitting phosphor CsPbBr3 onto a commercial blue LED.

Discovery of blue-emitting Eu2+-activated sodium aluminate phosphor with high thermal stability via phase segregation

Thermally stable phosphors are crucial for the application in phosphor-converted white light-emitting diodes. This paper reports the evolution of phase compositions in nominal Na2−xAl2B2O7:xEu (x = 0.02–2.0) upon replacing Na atoms by Eu atoms. Eu2+-activated blue-emitting phosphor was discovered through phase segregation combined with related analyses. The structure, phase composition and related luminescent properties were investigated by X-ray powder diffraction, backscattered electron image combined with elemental mapping, cathodoluminescence-combined scanning electron microscope, and temperature-dependent emission spectra, etc. Analyses results suggest that EuBO3 phase segregates upon the gradual replacement of Na by Eu atoms, which is accompanied by phase transformation from Na2Al2B2O7:Eu2+ to NaAl11O17:Eu2+ and dramatic enhancement of blue emission. The broad emission band is peaking at around 470 nm under UV irradiation. The internal quantum efficiencies of nominal Na0.4Al2B2O7:1.6Eu and Na0.2Al2B2O7:1.8Eu are 58.6% and 60.2%, respectively, which contain both NaAl11O17 and EuBO3 phase. The emission intensity for these two samples at 150 °C remains 96% (x = 1.6) and 83% (x = 1.8) of the room-temperature values, respectively.

Near-ultraviolet chip-based phosphor-converted solar-spectrum white light-emitting diode

As efficient artificial light sources, nitride-based light-emitting diodes (LEDs) have been widely used. However, III nitride white light-emitting diodes (WLEDs) are mostly fabricated by combining a blue LED chip with yellow phosphors, which results in inevitable problems, such as low color-rendering index (CRI) and detrimental effect to human eyes. As a solution for healthy lighting, we report a strategy to produce a WLED with an emission spectrum perfectly matched with natural daylight. By utilizing near-ultraviolet LED chips and a mixture of blue/cyan/amber/red phosphors, a CRI (Ra) of 97.9 is achieved for the WLEDs at a correlated color temperature of around 5000 K. The resemblance ratio of these solar-spectrum WLEDs with the standard normalized daylight spectrum (5000 K) is found to as high as 93.4%. Residual UV light in the normalized spectrum is <5 % . The method will benefit the development of high-efficiency healthy artificial light sources.

Improving LED Efficiency with the New Polymorph β-Ca1-x Srx AlSiN3 :Eu2.

Innovative materials for phosphor-converted white light-emitting diodes (pc-LEDs) are much sought after due to the huge potential of the LED technology to reduce energy consumption worldwide. One of the main levers for further improvements are the conversion phosphors. The system Ca1-x Srx AlSiN3 :Eu2+ currently provides one of the most important red-emitting phosphors for pc-LEDs. We report the discovery of the new polymorph β-Ca1-x Srx AlSiN3 :Eu2+ which allows for significant improvements to LED efficacies. It crystallizes in the orthorhombic space group Pbcn with lattice parameters a=982.43(10) pm, b=575.2(1) pm and c=516.12(5) pm. Compared to α-Ca1-x Srx AlSiN3 :Eu2+ , its emission shows a significantly reduced spectral full-width at half maximum (FWHM). With that, we demonstrated 3 % efficacy increase for white light-emitting pc-LEDs. The new polymorph can easily be industrialised, because the synthesis works on the same equipment as α-Ca1-x Srx AlSiN3 :Eu2+ .

A novel tunable extra-broad yellow-emitting nitride phosphor with zero-thermal-quenching property

Phosphor-converted white light-emitting diodes (pc-WLEDs) are bringing revolutionary changes to the lighting industry. To promote technological development, phosphors with tunable photoluminescence and excellent thermal stability are required. In our work, a novel tunable yellow-green emitting phosphor Li2CaSi2N4:Ce3+ is successfully synthesized through solid-state reaction. With the increase of Ce3+ concentration, the emission light could transfer from green to yellow region and the full width at half-maximum (FWHM) increases from 123 to 165 nm. Meanwhile, the phosphor does not exhibit thermal quenching up to 200 °C. The phosphor possesses a wider emission band and contains more cyan and red components than YAG:Ce3+. Combing Li2CaSi2N4:Ce3+ phosphor with a blue chip, pc-WLED with excellent technical indicators was obtained. Under the low voltage and current conditions, the phosphors also exhibit perfect stability with high current saturation.

A single-phase full-visible-spectrum phosphor for white light-emitting diodes with ultra-high color rendering.

Excellent luminous performance and high color rendering are the keys to white light-emitting diode (WLED) illumination. This work reports a single-phase full-visible-spectrum Y2Mg2Al2Si2O12(YMAS):Eu2+,Mn2+ phosphor for WLEDs with ultra-high color rendering. The luminescence of a single Mn2+ doped YMAS phosphor is very weak due to the spin-forbidden transition of Mn2+, while it can be dramatically enhanced in the YMAS:Eu2+,Mn2+ system through efficient energy transfer from the sensitizer Eu2+. Meanwhile, the luminescent color of this phosphor can be tuned from cyan to cold white, to warm white, and finally close to the yellow region by controlling the activator concentration and energy transfer process. Its good thermal and chromaticity stability meet the requirements of application in WLEDs. Its stable photochromic performance at different excitation wavelengths (365-395 nm) indicates that it can be used in different ultraviolet chips. The YMAS:0.03Eu2+,0.30Mn2+ phosphor-converted WLED achieves an ultra-high color rendering index (Ra = 93.3), near-standard chromaticity coordinates (CIE = (0.3343, 0.3388)) and a suitable correlated color temperature (CCT = 5417 K).

Ultrabroadband red luminescence of Mn4+ in MgAl2O4 peaking at 651 nm.

Blue light pumped red luminescence with broadband and high photon-energy emission is highly desired for phosphor-converted white light-emitting diodes (pc-wLEDs), to achieve a high color rendering index and high luminous efficacy. Mn4+-doped red-emitting phosphors generally exhibit sharp vibronic emissions associated with the parity- and spin-forbidden 2Eg→4A2g transitions. In this paper, two abnormal luminescence behaviors were observed for Mn4+ in the MgAl2O4:Mn4+ spinel phosphor with a short wavelength emission band peaking at 651 nm. Firstly, the Mn4+ 2Eg→4A2g transition exhibits ultrabroadband luminescence in MgAl2O4 and the large full-width at half-maximum (FWHM) is dependent both on the calcination temperature and on the partial substitution of Al3+ with Ga3+. Secondly, the thermal quenching behavior of the Mn4+ 2Eg→4A2g luminescence in MgAl2O4 shows a dependence on its thermal treatment and preparation method. The Rietveld refinement and Raman results demonstrate that the variation in the FWHM of the luminescence spectra is a sum effect of structural ordering (i.e., isotropic displacement decrease of constituent atoms) and the Mg ↔ Al anti-site disorder. A model for the observed varying thermal quenching of luminescence was tentatively proposed. The intrinsic thermal quenching temperature of Mn4+ luminescence in MgAl2O4 was found to be 390-400 K using the samples prepared by the co-precipitation and molten salt methods. The present work gives a novel perspective to understand the luminescence spectra of Mn4+ 2Eg→4A2g transition.

Closing the Cyan Gap Toward Full-Spectrum LED Lighting with NaMgBO3:Ce3+

The photoluminescence spectrum generated by an ordinary phosphor-converted white light-emitting diode (pc-wLED) that combines a blue LED chip with a yellow phosphor or a near-UV LED with red, green...

White Light-Emitting Diodes With Ultrahigh Color Rendering Index by Red/Green Phosphor Layer Configuration Structure

Currently, white light-emitting diodes (WLEDs) have been widely applied in lighting, display, and medical fields. However, there still exists the problem of low color rendering index (CRI, ${R}_{a}$ ). In this work, phosphor-converted WLEDs (pc-WLEDs) with ultrahigh ${R}_{a}$ were fabricated using Y3Al3Ga2O12:Ce3+ green phosphor and CaAlSiN3:Eu2+ red phosphor configuration structure. The broad emission spectrum of the green phosphor compensates for the gap of the cyan-emitting region that realizes full-spectrum white-light emission. The weight ratio of green/red phosphors was controlled to optimize the optical performances of pc-WLEDs. At the ratio of 0.15/0.012, the fabricated pc-WLED exhibits a natural white light with an ultrahigh ${R}_{a}$ of 95.2 and an incredibly small International Commission on Illumination (CIE) chromaticity coordinates deviation ( ${D}_{{uv}} = -0.0034$ ) at 350 mA. The corresponding correlated color temperature (CCT) and luminous efficiency (LE) are 4526 K and 62.34 lm/W, respectively. Furthermore, two separated phosphor configuration structures were constructed for pc-WLEDs. The R up/G down structure achieves the highest LE of 64.93 lm/W, and the corresponding CCT and ${R}_{a}$ are 5714 K and 95.8, respectively, chiefly due to the compensation for reabsorption effect caused by the large difference in the luminous efficacy of radiation (LER) between dichromatic phosphors. The results demonstrate that by adjusting the phosphor configuration structure, the pc-WLEDs with high color rendering can be obtained, which have great applications in high-quality lighting.