Low Color Temperature Research Articles

High thermal stability of warm white emitting single phase GdPO4: Dy3+/ Sm3+ phosphor for UV excited wLEDs

Highly thermally stable warm white light emission has been obtained from a single component phosphor; GdPO4: Dy3+/Sm3+. A series of GdPO4: Sm3+ (1%)/Dy3+ (x%) (x = 0.1, 0.5, 1 and 2) phosphors have been synthesized by the co-precipitation method. The crystal structure and phase purity of the samples obtained were verified by X-ray diffraction. Photoluminescence spectra, luminescence lifetimes are used to study the effect of Dy3+ concentration and the energy transfer from Dy3+ to Sm3+ as a function of excitation wavelength. The nearest-neighbor contributed to the non-radiative energy transfer from Dy3+ to Sm3+ in GdPO4, under 273 and 363 nm excitation. A warm white light was obtained from all phosphors excited at 273 nm and for 1 and 2% of Dy3+ in GdPO4:Sm3+ under 363 nm excitation. Temperature-dependent luminescence properties are determined for GdPO4: Sm3+ (1%), Dy3+ (1%) under 273 nm and 363 nm excitation. The studied phosphor has a good luminescence thermal stability and the determined activation energy for thermal quenching is 1.26 and 0.75 eV for 273 nm and 363 nm excitation wavelengths, respectively. The effect of the Dy3+ concentration as an activator ion responsible for the white emission light, at a fixed concentration of Sm3+ as a red emission compensator in the Dy3+ emission spectrum, was discussed. The low color temperature and high thermal stability indicate that the studied material is a promising solid-state emitting phosphors for the UV chip excited warm wLED applications.

High-color-quality blue-light-pumped full-spectrum white-light-emitting diodes realized by efficient green-emitting CaY2HfScAl3O12:Ce3+ phosphors

Rare-earth doped inorganic phosphors are critical for solid-state white lighting. Particularly, efficient broadband, blue-light-excitable, green-emitting phosphors are highly needed for fabricating full-spectrum white-light-emitting diodes (WLEDs) with high-color-quality. Herein, we describe a new efficient blue-light-excited CaY2HfScAl3O12:Ce3+ green phosphor with high quantum efficiency (QE) and broad emission bandwidth. X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectrometer, excitation and emission spectra, CIE color coordinates, QE values, and temperature-dependent emission spectra have been used to characterize the samples. Rietveld refinement reveals that the CaY2HfScAl3O12 garnet compound has a cubic structure with lattice parameters of a = b = c = 12.38(13) Å, α = β = γ = 90°, and V = 1897.85(6) Å3. Impressively, the Ce3+ ions in the CaY2HfScAl3O12 host exhibit broadband excitation bands in the 300–500 nm spectral range with a maximum at 450 nm. Upon 450 nm excitation, the optimal phosphor doped with 2 mol% Ce3+ shows a broad green emission band in the wavelength range of 460–750 nm peaking around 532 nm, along with full width at half maximum of 121 nm as well as high internal QE of 78.9% and external QE of 46.6%. Ce3+ concentration-dependent luminescence properties and thermal stability at elevated temperature have been comprehensively studied. Finally, a WLED device is fabricated by utilizing CaY2HfScAl3O12:Ce3+ green phosphor, CaAlSiN3:Eu2+ red phosphor and a 450 nm blue LED chip, which generates bright full-spectrum warm-white light with ultrahigh color rendering index (Ra = 98.0, R9 = 97.5) and low correlated color temperature of 3602 K under 180 mA drive current. This work provides new insights into designing and developing efficient green phosphors for full-spectrum LED lighting.

Synthesis and luminescent properties of Eu(III)/Tb(III) rare earth complex-coated the core-shell structured silica microspheres: Unleashing the potential for white light emission

A rare earth organic complex, SiO2@ATPA-Si-RE (RE = Eu, Tb), exhibiting a core-shell structure, was synthesized using the ligand (HOOC)2C6H3NHCONH(CH2)3Si(OCH2CH3)3(ATPA-Si) to connect the silica microspheres core and coordinate with rare earth ions. The pH value of the reaction was adjusted to produce organic complex shells with thicknesses of 2.0 nm and 5.0 nm, while the SiO2 core measured a diameter of 125 nm. Luminescence spectra indicated that the emission intensity of the SiO2@ATPA-Si-RE (RE = Eu, Tb) core-shell structure was stronger than that of the ATPA-Si-RE organic complex due to a perfect match between the triplet energy level of SiO2@ATPA-Si and the excited energy level of rare earth ions (Eu3+, Tb3+). Remarkably, the core-shell structure with a 2.0 nm shell thickness possessed a greater emission intensity compared to the 5.0 nm shell thickness. Moreover, these findings unveiled the tunability of luminescence color, ranging from green or red to white, by adjusting the doping ratio of Eu3+ and Tb3+ ions and the coating thickness of complexes. Particularly, a warm white light emission, characterized by CIE coordinates (0.389, 0.347), a color rendering index value of 86.7, and a low correlated color temperature value of 3550 K, was achieved using the organic complex with a 2.0 nm thickness coated on SiO2 microspheres with a doping ratio of Eu3+ at 0.7 mmol and Tb3+ at 0.3 mmol. Conversely, the complexes with a 5.0 nm coating thickness and Tb3+ doping ratio of 0.3 mmol yielded a cold white light emission. These findings demonstrate the potential applications in next-generation lighting industry and display systems.

Novel Mn4+-activated fluoride red phosphors induced by alkaline metal ions replacement: Green synthesis, luminescence enhancement, high stability and application in warm WLED

Red-emitting Mn4+-doped fluoride phosphors are promising fluorescent material for use in high-performance warm white light emitting diodes (WLEDs). However, low stability and non-green synthesis method limit the development and commercialization of such phosphor. Herein, a series of novel (Li/Na/Cs)0.4Ba0.8SiF6:Mn4+ phosphors with optimized fluorescence properties induced by alkaline metal ions replacement are successfully synthesized via a green and eco-friendly approach. Under the irradiation of 460 nm blue light, Cs0.4Ba0.8SiF6:Mn4+ red phosphor with narrow band emission peak has the best optical performance. The optimal doping concentration of Mn4+ in Cs0.4Ba0.8SiF6 host and the reason for concentration quenching are systematically analyzed. Furthermore, the moisture resistance and thermal stability of this phosphor are discussed. After being soaked in deionized water for 90 min, the PL intensity of Cs0.4Ba0.8SiF6:Mn4+ phosphor can maintain at 40% of that in air, which is 4 times higher than commercial K2SiF6:Mn4+. At the operating temperature of LED (423 K), Cs0.4Ba0.8SiF6:Mn4+ red phosphor can exhibit stable light output intensity and color. A warm WLED with low correlated color temperature of 4336 K, high color rendering index of 88.4 and high lumen efficiency of 105.96 lm/W is fabricated via using Cs0.4Ba0.8SiF6:Mn4+ phosphor as a component of red-light, and the warm WLED can maintain stable output even at high driving currents. Overall, the alkali metal ion replacement and green synthesis strategies have opened up a new perspective for the synthesis of efficient Mn4+-doped fluoride phosphors, which has dual benefits for optimizing the performance of WLED and environmental protection.

Fabrication of Al2O3[sbnd]GAGG:Ce composite ceramic phosphors with excellent color quality for high-power laser-driven lighting

The application of high-power laser-driven lighting is limited by the insufficient heat dissipation of phosphor converters and the poor color quality of white light. In this study, the Al2O3(Gd,Ce)3(Al,Ga)5O12 (Al2O3GAGG:Ce) composite ceramics were designed to improve thermal performance of phosphors, while maintaining excellent color rendering under high-power laser excitation. Dense composite ceramics were fabricated by pre-sintering at 1600 °C in air followed by HIP at 1700 °C through the investigation of the phase evolution and sintering process. The emission spectrum of GAGG:Ce shows a red shift of 31 nm and a high full width at half maximum (FWHM) of 128 nm. Meanwhile, the thermal conductivity was increased to 13.2 W·m−1K−1 by addition of Al2O3, which is 72% higher than that of single-phase GAGG:Ce ceramic. Therefore, warm white light with high color rendering index (CRI) of 71.4 and low correlated color temperature (CCT) of 4352 K was attained under excitation of high-power LDs.

Preparation and photoluminescence properties of high thermal stability Sr3Ga2Sn1.5Si2.5O14:Eu3+ red phosphors for high-power light-emitting diodes application

A series of Sr3Ga2Sn1·5Si2·5O14:xEu3+ (SGSSO:xEu3+) phosphors with excellent thermal stability were prepared by the solid-state reaction method. The phase and luminescence properties of the sample were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), diffuse reflectance spectroscopy, and photoluminescence (PL) spectra. Under the excitation of 395 nm, the SGSSO:Eu3+ phosphor exhibits strong red emission at 618 nm, which is attributed to the 5D0 → 7F2 transition of the Eu3+ ions. When monitoring the 618 nm emission peak, the excitation spectrum contains several excitation peaks with a maximum value of 395 nm, which is well matched with the commercial violet LED chips. In addition, the emission spectra of SGSSO:Eu3+ at different temperatures have also been studied. Pleasantly, the luminous intensity of the SGSSO:Eu3+ sample at 150 °C still maintains 80.2% of that at room temperature, demonstrating its good thermal stability and application potential in high-power LEDs. The mechanism for the high thermal stability of the as-prepared phosphor was also discussed. Moreover, the photoluminescence quantum yield of the optimized SGSSO:0.06Eu3+ phosphor can reach 62.7%. Finally, a warm white LED device with high color rendering index and low color temperature was prepared using the prepared SGSSO:Eu3+ phosphor as a red component, verifying the application potential of SGSSO:0.06Eu3+ in LEDs.

Highly Efficient Hybrid Perovskite/Organic Tandem White Light Emitting‐Diodes with External Quantum Efficiency Exceeding 20%

AbstractPerovskite light‐emitting diodes (PeLEDs) have shown great potential for low‐cost display and lighting technologies, and all‐inorganic blue PeLEDs are recognized as a promising substitute for blue fluorescent/phosphorescent organic light‐emitting diodes (OLEDs) owing to their superior color purity and great stability potential. However, confined by the solution fabrication process, depositing multi‐layered white PeLEDs remains a challenge. Here, a newly designed hybrid perovskite/organic tandem white LED (POTWLED), integrated with a bottom blue PeLED unit and a top orange/(orange + red) OLED unit, to prevent damage to the underlying perovskite layer caused by the deposition of the top emitting layers, is reported. To optimize the performance of POTWLEDs, a conductive passivator of 2,8‐bis(diphenylphosphoryl)dibenzo[b,d] thiophene, with a strong surface binding group of P═O, is introduced to passivate perovskite defects, which promotes a maximum external quantum efficiency (EQE) of 17.3% for blue PeLEDs with an emission peak at 488 nm. As a result, based on the optimized blue PeLED units, a maximum EQE of 23.9% with a low color temperature of 2522 K is obtained for POTWLEDs, which is the highest efficiency for perovskite‐based WLEDs, illustrating the great potential of the hybrid tandem device in fabricating high‐performance perovskite‐based WLEDs.

Intense green light emission in Sr2 ZnGe2 O7 :Mn2+ phosphors by the design of high symmetry melilite structure.

A green phosphor Sr2 ZnGe2 O7 :Mn2+ with a melilite structure was prepared using a high-temperature solid-state reaction. When the 535 nm emission was monitored, the excitation spectrum of the Sr2 ZnGe2 O7 :Mn2+ was found to contain two excitation bands in the ultraviolet (UV) region. When excited by UV light, the sample shows bright green emission at 535 nm, which corresponds to the distinctive transition of Mn2+ (4 T1 →6 A1 ). Moreover, the quantum efficiency of Sr2 ZnGe2 O7 :Mn2+ could reach 67.6%. Finally, a high-performance white-light-emitting diode (WLED) with a low correlated colour temperature of 4632 K and a high colour rendering index (CRI) of 92.3 were packaged by coating commercial blue and red phosphors with an optimized Sr2 ZnGe2 O7 :Mn2+ sample on a 310 nm UV chip. This indicated that Sr2 ZnGe2 O7 :Mn2+ has the potential application as a green component in the WLED lighting field.

A Photoluminescence Study of Eu3+, Tb3+, Ce3+ Emission in Doped Crystals of Strontium-Barium Fluoride Borate Solid Solution Ba4-xSr3+x(BO3)4-yF2+3y (BSBF).

The present study is aimed at unveiling the luminescence potential of Ba4-xSr3+x(BO3)4-yF2+3y (BSBF) crystals doped with Eu3+, Tb3+, and Ce3+. Owing to the incongruent melting character of the phase, the NaF compound was used as a solvent for BSBF crystal growth. The structure of BSBF: Eu3+ with Eu2O3 concentration of about 0.7(3) wt% was solved in the non-centrosymmetric point group P63mc. The presence of Eu2O3 in BSBF: Eu3+ leads to a shift of the absorption edge from 225 nm to 320 nm. The photoluminescence properties of the BSBF: Ce3+, BSBF: Tb3+, BSBF: Eu3+, and BSBF: Eu3+, Tb3+, Ce3+ crystals have been studied. The unusual feature of europium emission in BSBF is the intensively manifested 5D0→7F0 transition at about 574 nm, which is the strongest for BSBF: Eu3+ at 370 nm excitation and for BSBF: Eu3+, Tb3+, Ce3+ at 300 nm and 370 nm excitations. No evidence of Tb3+→Eu3+ energy transfer was found for BSBF: Eu3+, Tb3+, Ce3+. The PL spectra of BSBF: Eu3+ at 77 and 300 K are similar with CIE chromaticity coordinates of (0.617; 0.378) at 300 nm excitation and (0.634; 0.359) at 395 nm excitation and low correlated color temperature which implies application prospects in the field of lightning. Due to the high intensity of 5D0→7F0 Eu3+ transition at 370 nm excitation, the BSBF: Eu3+ emission is yellow-shifted.

Energy transfer from VO43− to Sm3+ ions in ɑ-Sr2V2O7 phosphors for full-spectrum warm-WLED device and low-temperature luminescence lifetime thermometry

Novel ɑ-Sr2V2O7: Sm3+ phosphors with color-tunable performance were prepared in this work. The crystal structure, photoluminescence properties, and potential applications were introduced. The efficiency of the energy transfer from the [VO4]3- tetrahedral to Sm3+ was 22.67% at a low doping content of Sm3+ (9 mol%), driven by the dipole-dipole interaction. Interestingly, the ɑ-Sr2V2O7: Sm3+ phosphors emitted a full-spectrum (400–800 nm) emission, and the corresponding chromaticity coordinates were located in a near-warm light region. In the potential applications, the warm-white lighting-emitting diode (warm-WLED) based on ɑ-Sr2V2O7: 0.03Sm3+ phosphor was fabricated and further exhibited high-quality performances with the beneficial color rendering index (CRI = 78.67) and low correlated color temperature (CCT = 2257 K) for indoor illumination. Moreover, the low-temperature (38–298 K) optical thermometry performance was also analyzed with the help of Struck and Fonger's theory. Remarkably, the relative sensitivity has an outstanding value of 3.12% K−1 at 98 K. All the above results evidence that the full-spectrum ɑ-Sr2V2O7: Sm3+ phosphors are expected to apply in full-spectrum warm-WLED devices and low-temperature luminescence lifetime thermometry.

Synthesis of an efficient Mn4+-doped fluoride red phosphor with excellent fluorescence stability by green synthesis method for warm WLED

Currently, Mn4+-doped fluoride phosphors have become a research hotspot, which can help compensate for the missing red component in cold white light-emitting diodes (WLEDs). However, significant synthesis defects limit the development of such fluorescent powders due to the use of highly toxic solvent HF. To improve this situation, a simple green synthesis method should be explored to synthesize Mn4+-activated fluoride phosphor. In this study, a series of red-emitting Mn4+-activated BaTiF6 phosphors are successfully synthesized via the non-toxic reaction system (HNO3/HCl + NH4F). Its crystal structure, morphology, optical characteristics and thermal stability are systematically evaluated. Under blue light excitation, BaTiF6:Mn4+ exhibits a bright narrow-band red emission with exceptional correlated color temperature (3909 K) and color purity (90%). Additionally, the critical concentration of Mn4+ is determined to be 0.03 and the concentration quenching mechanism is also systematically discussed. By calculating the crystal field strength (Dq), Racah parameters (B and C) and nephelauxetic effect ratio parameter β1, it is confirmed that BaTiF6 host can provide a strong crystal field environment for Mn4+. Moreover, these results are compared with other matrix materials. Intriguingly, the optimal sample exists satisfactory thermal stability, whose fluorescence intensity at 423 K retains 49% of that at 298 K and the activation energy reaches up to 0.925 eV. In addition, the chromaticity shift and chromaticity coordinate variation are calculated as 17 × 10−3 and (−0.0063, 0.0063), respectively. Meanwhile, the feasibility of the as-fabricated WLEDs for interior illumination is examined. A high-performance warm WLED with low correlated color temperature (CCT = 4470 K) and high color rendering index (Ra = 88.3) is achieved by using BaTiF6:Mn4+ as a red-emitting material, moreover, WLED exhibits strong output stability under high driving current. The findings of this work provide a comprehensive understanding for rational design of high-performance Mn4+-activated fluoride red phosphors through a simple green synthesis method.

Ultra-narrow deep-red emitting AlN:Sm2+phosphor with nano-branched construct for wide color gamut backlight display and high-pressure sensing applications

Rare-earth (RE) doped AlN are excellent candidate materials for electroluminescent devices, full color displays and white lighting technology. In this paper, a deep red Sm2+ doped AlN (AlN:Sm2+) phosphor was synthesized for the first time by a one-step direct nitridation method. Detailed XRD and EDS studies show the presence of samarium (Sm) ions the AlN, and XPS measurements indicate Sm ions are divalent. SEM and TEM studies show that the AlN:Sm2+ have a branched nanostructure, consisting of a primary stem and secondary short nano-branches. AlN:Sm2+ phosphor has a broad and strong excitation bands in the range of 300–600 nm, ultra-narrow deep red emission at 686 nm, near unity color purity, and good thermal stability (78.2% at 413 K). A blue-pumped warm white light emitting diode with high color rendering index (Ra∼87.5) and low correlated color temperature (CCT∼4875 K) was fabricated. Moreover, a super-wide color gamut (117.6% of the NTSC) can be achieved by using AlN:Sm2+ as the red component. Furthermore, photoluminescence (PL) and Raman spectra of AlN:Sm2+ were studied under hydrostatic pressure up to 25 GPa. The shift of the 5D0→7F0 emission band (dλ/dP≈0.13 nm/GPa) and the decrease of PL intensity ratio (5D0→7F0/5D0→7F1, dIR(0/1)/dP≈−5.6%/GPa) with applied pressure can be used for optical pressure sensor. Raman spectroscopy revealed a phase transition of AlN:Sm2+ from wurtzite to rocksalt phase at 19.9 GPa. The large doping of Sm2+ ions and unique intrinsic geometry in branched nanostructure co-affect its compressibility and structural stability under high pressure. The results indicate that AlN:Sm2+ phosphor has promising applications in backlight displays and optical pressure sensors due to their excellent luminescent properties.

Efficient blue-light-excited cyan-emitting CaLu2HfScAl3O12:Ce3+ phosphors for full-visible-spectrum warm-white LEDs

Bright cyan-emitting phosphor is indispensable to fill the cyan gap for the fabrication of full-visible-spectrum warm-white light-emitting diodes (LEDs) with high color rendering index (CRI). In this study, we demonstrate a novel efficient cyan-emitting Ce3+-activated CaLu2HfScAl3O12 (CLHSA) garnet phosphor for application in blue-light-excited full-spectrum warm-white LEDs. CLHSA:Ce3+ phosphors show broadband excitation spectra in the 300–470 nm spectral range with a maximum peak at 408 nm, indicating they can be efficiently pumped by the blue LED chips. The optimal doping concentration of Ce3+ ions is found to be 1 mol%. Upon 408 nm excitation, the optimal CLHSA:1%Ce3+ phosphor gives rise to a bright broadband cyan emission (peak position: 498 nm; bandwidth: 97.5 nm) with CIE chromaticity coordinates of (0.2196, 0.4265) and high photoluminescence quantum yield of 64.5%. Finally, we have fabricated a blue-light-excited high-CRI warm-white LED device by employing the CLHSA:1%Ce3+ cyan phosphor, commercial YAG:Ce3+ yellow phosphor and commercial CaAlSiN3:Eu2+ red phosphor as well as a 450 nm LED chip, which demonstrates a bright warm-white light emission with a high CRI (Ra = 92.6), a low correlated color temperature (3660 K), and good CIE chromaticity coordinates of (0.3993, 0.3939) as well as a high luminous efficacy of 45 lmW−1. Therefore, this newly discovered efficient cyan-emitting CLHSA:1%Ce3+ phosphor is promising for application in blue-chip-pumped full-visible-spectrum warm-white LEDs with high CRI values.

A novel scheelite‐type LiCaGd(WO4)3:Eu3+ red phosphors with prominent thermal stability and high quantum efficiency

AbstractA series of LiCaGd(WO4)3: xEu3+ (0 ≤ x ≤ 1.0) red phosphors with tetragonal scheelite structure were synthesized via the conventional solid‐state reaction. Their crystal structure, photoluminescence excitation (PLE), and photoluminescence (PL) spectra, thermal stability and quantum efficiency were investigated. The phosphors exhibit a typical red light upon 395 nm near ultraviolet excitation, and the strongest emission peak at 617 nm is dominated by the 5D0→7F2 transition of Eu3+ ions. The PL intensity of the phosphors gradually increases with the increase of Eu3+ doping concentration, and the concentration quenching phenomenon is hardly observed. The quantum efficiency and the color purity of the phosphor reach maximum values of about 94.2 and 96.6% at x = 1.0, respectively. More importantly, LiCaGd(WO4)3:xEu3+ phosphors have prominent thermal stability. The temperature‐dependent PL intensity of the phosphors at 423 K is only reduced to 89.1% of the PL intensity at 303 K, which is superior to that of commercial red phosphors Y2O3:Eu3+. Finally, LiCaGd(WO4)3:Eu3+ phosphor is packaged with near ultraviolet InGaN chips to fabricate white light emitting diodes, which has a low color temperature (CCT = 4622 K) and a high color rendering index (CRI= 89.6).

Luminescence properties of multicolor-tunable emitting LiYGeO4: Tb3+, Eu3+ phosphors with excellent thermal stability for WLEDs

In order to investigate the impact of rare earth (Tb3+/Eu3+) ion doping on the luminescent properties of LiYGeO4, a series of LiYGeO4: 30%Tb3+, yEu3+ (0.0% ≤ y ≤ 1.0%) phosphors were successfully prepared using the solid-state reacthion in this work. The analysis revealed that Tb3+/Eu3+ ions were successfully doped into the LiYGeO4 lattice, confirmed by of X-ray diffractometer (XRD) patterns, Rietveld refinement, and element distribution results. The LiYGeO4: 30%Tb3+, yEu3+ samples exhibit efficient excitation by both ultraviolet (378 nm) and blue light (482 nm). Furthermore, the luminescence intensity, quantum yield, and thermal stability under 378 nm excitation were found to be greater than those under 482 nm excitation. Increasing the concentration of Eu3+ ion doping resulted in an energy transfer from the green region to the yellow region and finally to the orange red region, as evidenced by the Commission Internationale de L′Eclairage (CIE) coordinates of LiYGeO4: 30%Tb3+, yEu3+ phosphors. The white light emitting diodes (WLEDs) prepared utilizing LiYGeO4: 30%Tb3+, 0.75%Eu3+ phosphor shows a high color rendering index (Ra) of ∼93.2, a low correlated color temperature (CCT) of 5134 K, emitting bright natural light. This study helps to understand the mechanism of energy transfer, improves the luminescent material system of LiYGeO4, and provides new ideas and options for designing a high-performing lighting sources.

Towards efficient blue Eu2+ activated pyrophosphate phosphor for warm white LED applications

A blue-emitting LiCsBaP2O7:Eu2+(LCBPO:Eu2+) pyrophosphate phosphor prepared by solid-state synthesis that emits bright blue light with a first-rate internal quantum efficiency and high color purity is reported. Furthermore, when heated from 298 K to 473 K, LCBPO:0.015Eu2+ phosphor exhibits excellent color stability. Impressively, LCBPO:0.015Eu2+ blue phosphor exhibits exceptional stability against air exposure over a month. Fabricating a near-ultraviolet (n-UV) pumped pc-wLED using LCBPO:0.015Eu2+ along with green (Sr,Ba)2SiO4:Eu2+ and red (Ca,Sr)AlSiN3:Eu2+ phosphors demonstrates a well distributed warm white light with a high color-rendering index (Ra) of 91.7, a low correlated color temperature (CCT) of 3382 K and a high luminous efficiency (LE) of 26.1 l m/W. The high-performance optical characteristics, straightforward synthesis procedures and economical initial materials cost of LCBPO:Eu2+ support the potential of this material for usage in warm white LED-based lighting systems.

Realizing thermal and moisture stability in the K2Ge1-xTixF6:Mn4+ red phosphors with high efficiency for WLEDs

Developing red light-emitting phosphors with high performances is an imperative issue for achieving high-quality warm white light-emitting diodes. Here, red light-emitting K2Ge1-xTixF6:Mn4+ phosphors with high luminescence performances are obtained. Benefiting from the Ti4+-doping in K2GeF6, a high internal quantum yield of 96% can be obtained. The emission intensity at 423 K keeps 83% of the initial value at 298 K in K2Ge0.6Ti0·4F6:Mn4+. The moisture resistance property of K2Ge0.6Ti0·4F6:Mn4+ is significantly enhanced and can keep 71% of the original luminescence intensity after immersing in deionized water for 180 min. A warm white light-emitting diode with high color rendering index of 92.4, low correlated color temperatures of 3113 K, and high luminous efficiency of 123.1 l m/W is simultaneously obtained by encapsulating K2Ge0.6Ti0·4F6:Mn4+ with commercial yellow phosphor on blue chips. This work proves the prospects of K2Ge1-xTixF6:Mn4+ as promising red phosphors for potentially viable application in warm white light-emitting diodes.

전면 카메라로 촬영한 얼굴 사진의 선호도 분석 - 삼성전자 Galaxy S22 Ultra와 Apple iPhone 13 ProMax를 중심으로

The use of the front camera, or the ‘selfie camera’ is increasing among the young generation. The act of taking self-photos is not only to capture the moment, but also to enhance ones’ self-esteem and even as a way of self-branding. The current camera manufacturers are aiming for high specification and high performance of the camera. However, accurate color reproduction of the camera may not correlate with high satisfaction of the users. This study focuses on the selfie camera of the two representative smartphone brands, Samsung Galaxy S22 Ultra and Apple iPhone 13 Pro Max. The study asked the participants to take selfies under four different lighting environments with three colored backgrounds using the two smartphones. Participants evaluated preference scores for each portrait on the two smartphones in the range of 7, where 1 is the least preferred and 7 for the most preferred. The results for the preference of the selfie camera showed that the average preference score was higher for Samsung Galaxy S22 Ultra than Apple iPhone 13 Pro Max by 0.44. The preference for the two displays were very similar with difference of only 0.01. For the background color, light pink and light yellow was preferred for Samsung Galaxy S22 Ultra but dark blue was preferred for Apple iPhone 13 Pro Max. Relatively dark environment with low color temperature was referred for both smartphones. The interview revealed that the users preferred Samsung Galaxy S22 Ultra for the skin color and less detailed expression for the skin texture. Users who preferred Apple iPhone 13 Pro Max liked the captured atmosphere of the photo, especially at low luminance low color temperature environment. This study places the significance on understanding how the users evaluate the photos taken by the selfie camera.

Cation substitution induced highly symmetric crystal structure in cyan-green-emitting Ca2La1-xLuxHf2Al3O12:Ce3+ solid-solution phosphors with enhanced photoluminescence emission and thermal stability: Toward full-visible-spectrum white LEDs

Highly efficient cyan-green-emitting phosphors are urgently needed for high-color-quality full-visible-spectrum white light-emitting diodes (LEDs). Herein, we report on efficient near-ultraviolet-excited cyan-green-emitting Ca2La1-xLuxHf2Al3O12:Ce3+ (CLa1-xLuxHA:Ce3+) garnet phosphors, which are developed based on the solid-solution design of chemical cation substitution. All phosphor samples present a garnet crystal structure with the Ia3‾d space group; with increasing Lu3+ ions content, the lattice could be gradually contracted, along with a high symmetry as well as rigidity. Furthermore, under the 408 nm excitation, the cyan-green emissions of CLa1-xLuxHA:Ce3+ phosphors are progressively enhanced with a slight blue-shift, due to crystal field splitting and Stokes shift. Importantly, quantum efficiency and thermal stability performances are greatly improved by introducing smaller Lu3+ ions to replace the bigger La3+ ions, owing to the high rigidity structure with high symmetry. The optimized CLa0.5Lu0.5HA:Ce3+ phosphor exhibits an intense cyan-green emission band with high internal quantum efficiency of 76.4% and external quantum efficiency of 54.4%. Through employing the as-prepared CLa0.5Lu0.5HA:Ce3+ cyan-green phosphor, we have successfully fabricated a full-visible-spectrum LED device, demonstrating a bright warm-white light emission with an extremely high color rendering index (Ra = 98.0, R9 = 95.9, and R12 = 94.3) and low correlated color temperature (3497 K) under 100 mA driving current. This study not only reveals the correlation between the crystal structure evolution and luminescent properties of phosphor materials, but also provides new strategies for the design and development of novel efficient luminescent materials for high-quality full-visible-spectrum LED lighting.

Structure and photoluminescence studies of Rb3GdSi2O7: Eu2+ phosphor used for NUV converted white LEDs

In pursuit of healthy lighting, the exploitation of high-quality Eu2+-activated phosphors available for near-ultraviolet light-emitting diode (NUV-LED) chip-based white LED (WLED) has fetched growing interest. Herein, a novel NUV-light excitable Rb3GdSi2O7: Eu2+ phosphor is successfully synthesized via a high-temperature solid-state reaction method. The crystal structure of Rb3GdSi2O7 is found to be hexagonal, belonging to space group P63/mmc. The Rb3GdSi2O7: Eu2+ phosphor exhibits a broad excitation band in the 300–450 nm wavelength range, and it emits strong yellow-green light with peak at 543 nm upon 365 nm excitation. Benefitting from the high structure rigidity of host matrix, Rb3GdSi2O7: Eu2+ also possesses a desired luminescence thermal stability (423 K, 85.6%). The WLED device fabricated using a 365 nm chip and a blend of the as-prepared Rb3GdSi2O7: Eu2+ yellow-green phosphor and commercial BaMgAl10O17: Eu2+ blue phosphor displays an excellent color-rendering index of 95.9 at a low correlated color temperature of 4373 K. This work confirms the applicative potential of Rb3GdSi2O7: Eu2+ in NUV-converted warm WLEDs, which may promote the development of high-color-quality healthy lighting.