Phosphor-converted LEDs Research Articles

Thermal Quenching Mechanism of Mn4+ in Na2SiF6, NaKSiF6, and K2SiF6 Phosphors: Insights from the First-Principles Analysis.

This study aims to identify the key factors governing the thermal quenching of Mn4+ ion luminescence in fluoride-based phosphor materials used as red emitters in modern-day phosphor-converted LED devices. Here, we employ first-principles calculations for Mn4+-doped Na2SiF6, NaKSiF6, and K2SiF6 hosts to explore how host properties and local coordination environments influence thermal quenching behavior. The ΔSCF method was used to model the geometric structures of the Mn4+4A2 (ground) and 2E, 4T2 (excited) states and the energies of the optical transitions between these states. Our results reveal that thermal quenching in Na2SiF6 and K2SiF6 phosphors occurs through thermally activated 2E → 4T2 → 4A2 crossover. In contrast, thermal quenching in NaKSiF6 is due to other nonradiative decay pathways. Investigations of the mechanical stability of these fluorides show that NaKSiF6 is mechanically unstable. We suggest that this property of the host limits the luminescence efficiency of the embedded Mn4+ ions. We also determined the reason for the difference in the intensity of the 2E → 4A2 emission transition (ZPL) in the systems. These findings advance our fundamental understanding of the thermal quenching mechanism of Mn4+ ion luminescence in fluorides, and the results can aid future discoveries of technologically useful phosphors through high-throughput design methodologies.

Reconstructing the Surface of K2SiF6:Mn4+ Phosphors toward Enhanced Moisture Resistance for White LED Applications.

The K2SiF6:Mn4+ (KSFM) phosphor featuring efficient ultranarrow red emissions is an outstanding candidate for white light-emitting diode (WLED) applications. However, poor moisture resistance seriously affects its application performance. In this study, a two-step surface reconstruction strategy is proposed to dramatically enhance the moisture resistance of commercially available KSFM phosphors, involving treatment with H2NbF7 and subsequent hydrothermal treatment. The modified KSFM phosphor exhibits a high internal quantum efficiency (IQE) of 98.9% after the two-step surface treatment. Meanwhile, nearly 100% of the initial emission intensity is retained for the modified KSFM phosphor even after aging in high temperature (85 °C) and high relative humidity (85% RH) environments for 6 days, in sharp contrast to only 18.6% retention for the original KSFM phosphor. The relative emission intensity of the modified KSFM remains at 98.9% even after being immersed in water for 6 h. Additionally, the phosphor-converted LED fabricated with the modified KSFM phosphor demonstrated excellent long-term stability, retaining up to 97.9% of initial luminous efficacy after aging under 85 °C and 85% RH conditions for 500 h. The moisture-resistance mechanism is elucidated on the basis of spectroscopic analysis as well as structural and compositional characterization of the phosphor surface layer, which can be attributed to the formation of a robust Mn4+-rare shell with high crystalline quality following this two-step surface treatment. The findings contribute to the performance improvements of KSFM phosphors for industrial applications.

The spectral mismatch correction factor estimation using broadband photometer measurements and catalog parameters for tested white LED sources

Using a modeled look-up chart developed in this work, we show that accurate photometric measurements of characteristics like illuminance or luminous intensity of test LED sources can be measured in one step. This is a simple broadband measurement that may substitute for complicated and costly spectral measurements currently in place. To develop the look-up chart, the typical minimum and maximum of the Spectral Mismatch Correction Factor (SMCF) for a given photometers was estimated in relation to catalog parameters of the LEDs such as correlated color temperature (CCT) and melanopic daylight efficacy ratio (mDER). This research was based on the unique and large dataset of real photometers spectral response srel(λ) collected and measured at accredited laboratories located at America, Asia and Europe and modeled LED's spectral power distribution (SPD) data. Independent look-up tables were developed for color-mixed LEDs (cm-LEDs) and white phosphor-converted LEDs (pc-LEDs), two common LED types.

Studying the optical and electronic properties of Eu-doped CdSe phosphors using first principles calculations incorporating the spin orbit coupling method of DFT

We studied the electronic and optical properties of pure and Eu-doped CdSe phosphor materials using the first-principles calculation method, which is a feature of Density Functional Theory. The FP-LAPW method was employed for the study as incorporated in the Wien2k code. The GGA + U + SOC method of DFT was used for the first time in our study to investigate the electronic as well as optical characteristics of Eu-doped CdSe materials. Eu doping was done in two different ratios to study the effects of altered dopant concentration. The doping increased the band gap of the pure CdSe parental material. The calculated band gaps in our study were 0.42 eV for pure CdSe, 0.67 eV for a single Eu atom doped in CdSe, and 1.64 eV for two Eu atoms doped in CdSe. The measured electronic and optical characteristics of undoped i.e, pure CdSe and Eu-doped CdSe showed that the doping significantly changed the optical behavior of CdSe, which may be useful for applications in phosphor-converted LEDs.

A novel Cr3+-doped MgGaBO4 phosphor: Expanding the horizons in plant illumination

Near-infrared (NIR) spectroscopy technology has demonstrated unparalleled efficacy in night vision, iris authentication, plant illumination, food inspection, and various other domains. Nonetheless, the quest for novel phosphors exhibiting superior luminescent efficiency and enduring thermal stability persists as a formidable obstacle. In this study, phosphors MgGaBO4: Cr3+, Al3+ were successfully synthesized by high-temperature solid-phase reaction method. Firstly, starting from MgGaBO4: Cr3+, the crystal field environment around Cr3+ ions was modulated by replacing Ga3+ with Al3+ to further improve the luminescence performance of the phosphor. The emission peak wavelength of MgGaBO4: 0.0075Cr3+,0.005Al3+ phosphor was about 756 nm, the full width at half-maximum (FWHM) was about 174 nm under the blue light excitation of 440 nm. In addition, the luminescence intensity at 100 °C remains at 77.41 % of that at room temperature. Finally, the excellent moisture resistance of this phosphor was demonstrated. This work not only introduces a novel broadband NIR phosphor with promising application potential in advanced agricultural lighting but also proposes a synergistic enhancement strategy for effectively improving the performance of broadband NIR phosphor-converted LED light sources.

Harnessing the inherent photoluminescence of Ba2MgTeO6 double perovskite phosphor for visible to near infrared pc-LED applications

Phosphor-converted LEDs (pc-LEDs) emitting in the visible region based on undoped tellurate double perovskites have not been explored till now. A cyan-emitting pc-LED is fabricated using the host Ba2MgTeO6 double perovskite for the first time. The as-fabricated cyan LED emits light in the visible region with the maximum emission at 484 nm. The obtained CIE coordinates of (0.26, 0.37) ensure a cyan light, and LED exhibits superior color stability even at higher input drive current. An attempt to develop a white emitting phosphor was done by substituting Eu3+ ions into the Ba2MgTeO6 matrix. Followed by this, a phosphor converted LED emitting in the bluish white region was fabricated by combining near UV chip and BMTO: 0.02 Eu3+ phosphor. Further, an inherent near-infrared (NIR) luminescence in Ba2MgTeO6 is also discovered and it originates from the 3T1u, 3A1u – 1A1g electronic transitions within the Te4+ ions. Upon 367 nm excitation, Ba2MgTeO6 exhibits strong broadband NIR emission, which spans from 780 nm to 1150 nm with a maximum emission at 889 nm and full width at half maximum (FWHM) of about 115 nm. Finally, an efficient pc-LED emitting in the NIR region is fabricated using the intrinsic near-infrared luminescence observed in the Ba2MgTeO6 phosphor. The pc-LED covering the near infrared region can be potentially used for various applications, including plant cultivation, biosensors, night vision cameras, etc.

Investigating the influence of varied ratios of red and far-red light on lettuce (Lactuca sativa): effects on growth, photosynthetic characteristics and chlorophyll fluorescence.

Far red photon flux accelerates photosynthetic electron transfer rates through photosynthetic pigments, influencing various biological processes. In this study, we investigated the impact of differing red and far-red light ratios on plant growth using LED lamps with different wavelengths and Ca1.8Mg1.2Al2Ge3O12:0.03Cr3+ phosphor materials. The control group (CK) consisted of a plant growth special lamp with 450 nm blue light + 650 nm red light. Four treatments were established: F1 (650 nm red light), F2 (CK + 730 nm far-red light in a 3:2 ratio), F3 (650 nm red light + 730 nm far-red light in a 3:2 ratio), and F4 (CK + phosphor-converted far-red LED in a 3:2 ratio). The study assessed changes in red and far-red light ratios and their impact on the growth morphology, photosynthetic characteristics, fluorescence characteristics, stomatal status, and nutritional quality of cream lettuce. The results revealed that the F3 light treatment exhibited superior growth characteristics and quality compared to the CK treatment. Notably, leaf area, aboveground fresh weight, vitamin C content, and total soluble sugar significantly increased. Additionally, the addition of far-red light resulted in an increase in stomatal density and size, and the F3 treatments were accompanied by increases in net photosynthetic rate (Pn), transpiration rate (Tr), intercellular CO2 concentration (Ci), and stomatal conductance (Gs). The results demonstrated that the F3 treatment, with its optimal red-to-far-red light ratio, promoted plant growth and photosynthetic characteristics. This indicates its suitability for supplementing artificial light sources in plant factories and greenhouses.

Co-training machine learning enables interpretable discovery of near-infrared phosphors with high performance

Near-infrared (NIR) phosphors based on Cr3+ doped garnets present great potential in the next generation of NIR light sources. Nevertheless, the huge searching space for the garnet composition makes the rapid discovery of NIR phosphors with high performance remain a great challenge for the scientific community. Herein, a generalizable machine learning (ML) strategy is designed to accelerate the exploration of innovative NIR phosphors via establishing the relationship between key parameters and emission peak wavelength (EPW). We propose a semi-supervised co-training model based on kernel ridge regression (KRR) and support vector regression (SVR), which successfully establishes an expanded dataset with unlabeled dataset (previously unidentified garnets), addressing the overfitting issue resulted from a small dataset and greatly improving the model generalization capability. The model is then interpreted to extract valuable insights into the contribution originated from different features. And a new type NIR luminescent material of Lu3Y2Ga3O12: Cr3+ (EPW~750 nm) is efficiently screened, which demonstrates a high internal (external) quantum efficiency of 97.1% (38.8%) and good thermal stability, particularly exhibiting promising application in the NIR phosphor-converted LEDs (pc-LED). These results suggest the strategy proposed in this work could provide new viewpoint and direction for developing NIR luminescence materials.

Synthesis, structure and photoluminescent properties of far-red Y3Ga5O12:Cr3+ phosphors

Far-red (FR) photosensitive pigment (PFR) is vital for plant photomorphogenesis. Phosphor-converted (pc) LEDs are the next-generation FR light devices. How to obtain FR-emitting phosphors with a good quantum efficiency, suitable photoluminescence and high thermal stability is still difficult. Herein, we optimize the Y3Ga5O12:Cr3+ FR phosphor, which has an emission peak at 711 nm and a full width at half maximum of 74 nm, closing to the PFR absorption band. By adopting the solid-state sintering technology, the internal quantum efficiency reaches 85.5 %, more than 1.5 times higher than that reported by the liquid reaction (55 %). Furthermore, the external quantum efficiency reaches as high as 33.1 %, indicating the promising application in FR-LEDs.

Effect of the correlated color temperature of a phosphor-converted LED on indoor visible light communication

Correlated color temperature (CCT) is an important parameter of the phosphor-converted LED (PC-LED); international standards specify its range in different scenarios. However, little has been done to investigate the effect of CCT on the performance of visible light communication (VLC). In this paper, we propose an analytical model for the transmission performance of an LED VLC system based on the dual components of blue and yellow light from the luminescence mechanism of the PC-LED. On this basis, the effects of CCT on the PC-LED’s input current and output optical power between the dual components are analyzed. The simulation results show that the increase of CCT expands the modulation bandwidth of the PC-LED, and the trend of the BER with CCT is also related to the transmission data rate. In addition, the maximum data rate of the PC-LED with CCT from 2700 K to 6500 K under a certain transmission distance is also simulated in this paper.

Broadband near infrared emission in Cr3+,Yb3+, Li+ tri-doped In2TeO6

Cr3+ near infrared (NIR) emission is witnessed in tellurite compound for the first time. Broadband emission that peaks at 840 nm and has 157 nm of full width at half maxima (FWHM) appears in the Cr3+ doped In2TeO6 phosphor when excited at 455 nm. Further introduction of Li+ ions into the In2TeO6 can distort the octahedral environment of the doped Cr3+ ions, resulting in 4 times enhancement in luminescence intensity and a 1.75-fold enhancement in internal quantum efficiency (IQE). Moreover, the FWHM increases to 282 nm in the In2TeO6: Cr3+,Li+,Yb3+ phosphor induced by the Cr3+ to Yb3+ energy transfer. LED device utilizing this NIR phosphor has been constructed. At 150 mA drive current, the optoelectrical efficiency is 11.189 % and the NIR output power is 50.72 mW. The In2TeO6: Cr3+,Li+,Yb3+phosphor exhibits enormous potential in NIR phosphor-converted LED (pc-LED).

Control of a Semi-closed Plant Factory with Artificial Lighting-based on Two Different LED with NB-IoT

Plant factory artificial light (PFAL) is an effective technique for producing large amounts of crops per area and high-quality plant growth. This work aims to construct a semi-closed PFAL growth system based on NB-IoT using two types of LED arrays: phosphor-converted LED (pc-LED) and RB-LED. Next, while examining the features of the artificial light spectrum, compare the Curry leaf kale and Chinese kale in seedlings under various LED light sources. An NB-IoT module with the MAGELLAN platform monitored and controlled the temperature, humidity, and illumination of the semi-closed PFAL growing system. The results indicate that cos lettuce cultivated with PCLEDs is likely more photosynthesis-capable than cos lettuce grown with RBLEDs. Compared to RB-LED, the average fresh weight of the cos lettuce from PC-LED was significantly higher. The data gathered from the cloud system under the MAGELLAN platform during the 7-day trial, the control of lighting and watering in the semi-closed PFAL system, and the measurement results of environmental factors were all accurately completed. Organic veggies could be grown in a home or school using the semi-PFAL growing technique.

Broadband NIR luminescence of Cr3+-doped Mg4Nb2O9 phosphors toward LED light source applications for NIR spectrometry

Abstract Herein, we report the photoluminescence (PL) properties of Cr3+-doped Mg4Nb2O9 phosphors for applications in NIR phosphor-converted LEDs. Trigonal Mg4Nb2O9 crystals with Cr3+ substitution at three sites were synthesized as the main phase using a solid-state reaction. X-ray photoelectron spectrometry and diffuse reflectance spectroscopy indicated that only trivalent chromium ions were present in the samples. Broadband NIR emission of Cr3+ suitable for combination with Si photodetectors was confirmed at approximately 850 nm. The maximum PL quantum yield (PLQY) was 0.62, and concentration quenching explained the Cr concentration dependence of the PLQY and PL decay time. Low-temperature PL measurements suggested that the PL spectrum comprised three distinct emission bands. The results of the phonon line simulation, low-temperature PL excitation measurements, and crystal-field parameter estimation clarified the correlation between these emission bands and the Cr3+ substitution sites.

Enhancing the inherent NIR photoluminescence in SrLaLiTeO6 through Cr3+-Yb3+ co-substitution for high performance pc-LEDs.

Until now, double perovskite tellurates have not been reported to exhibit inherent NIR photoluminescence. Therefore, the current study's revelation of inherent NIR luminescence in SrLaLiTeO6 double perovskite centered at 970 nm under 340 nm excitation is particularly intriguing. This phenomenon is attributed to the photoluminescence of Te4+ ions. This study also examines the NIR luminescence of Cr3+-Yb3+ co-doped SrLaLiTeO6. The host and SrLaLiTeO6:3%Cr3+, y%Yb3+ (y = 1, 2, 4, 6, 8, 10 mol%) were synthesized using the solid-state ceramic route. The successful incorporation of Cr3+ and Yb3+ ions into the host lattice was confirmed through XRD, Raman, and diffuse reflectance spectral analyses. Under 270 nm excitation, the photoluminescence (PL) of SrLaLiTeO6:Cr3+ exhibits a blueshift of the PL band to 965 nm due to the 4T2(4F) → 4A2g(4F) emission component of Cr3+ ions. The excited-state lifetime of SrLaLiTeO6:0.5%Cr3+ was measured at 36 μs, but this decreased to 26 μs as the Cr3+ concentration reached 10 mol%, primarily due to the enhancement of non-radiative energy transfer between Cr3+ ions. Incorporating Yb3+ into the system results in additional spectral lines with an enhanced intensity in the range of 970 nm to 1125 nm when excited at 270 nm. These emission lines correspond to the 2F5/2 → 2F7/2 transitions of Yb3+ ions, indicating an efficient energy transfer from Cr3+ to Yb3+. Furthermore, the study also reveals that Yb3+ emission is observed even without Cr3+ ions in SrLaLiTeO6 under 340 nm excitation, suggesting the possibility of energy transfer from the host to Yb3+ ions. The thermal stability and crystal field parameters of the synthesized phosphors are also explained in detail. To explore the potential of these phosphors in practical applications, phosphor-converted NIR LEDs were fabricated using SrLaLiTeO6, SrLaLiTeO6:3% Cr3+, and SrLaLiTeO6:3% Cr3+, 1%Yb3+.

Comparative Study of the Photoelectric and Thermal Performance Between Phosphor-Converted and Phosphor-Free Golden Light LED

Comparative Study of the Photoelectric and Thermal Performance Between Phosphor-Converted and Phosphor-Free Golden Light LED

CaLaLiTeO6: Mn4+, Tm3+ phosphors with multiband near-infrared emission towards multifunctional applications

Near-infrared phosphors have garnered considerable interest for various detection technologies. However, effective blue light excitable phosphors emitting in the NIR range are still lacking in these devices. In this work, we synthesized a monoclinic perovskite phosphor, CaLaLiTeO6: Mn4+, Tm3+, which emits NIR light at 700 nm, 800 nm and 1450 nm under 480 nm excitation. Through systematic study of the fluorescence lifetime of doped Mn4+ ions, we determined this arises from energy transfer from Mn to Tm, enhancing Tm luminescence at 800 nm and 1450 nm. The measured internal quantum efficiency (IQE) of the phosphors can reach 66.36 %. Additionally, photoluminescence properties were examined under 808 nm laser diode excitation. Finally, we utilized a 480 nm blue light chip to excite the CaLaLiTeO6: 0.6 % Mn4+, 1.5 % Tm3+ phosphor and verified its functionality as a NIR light source. Results indicate phosphor-converted LEDs based on this material can be successfully used in a variety of disciplines, including medical infrared thermal imaging, enhanced night vision, solution concentration analysis. This information serves as a crucial reference for developing multifunctional phosphors.

Study of morphological, elemental, optical and excitation wavelength dependent red photoluminescence in Eu3+ doped Li2SrSiO4 for solid state lighting

A comprehensive investigation of Eu3+ doped Li2SrSiO4 phosphors synthesized via the solid-state reaction method was conducted. XRD confirmed the crystalline phase and SEM along with and its 3D projection's observations confirmed that the phosphor particles were in the micro meter size range, indicating their suitability for applications in primary phosphor-converted LEDs. FTIR spectroscopy elucidated vibrational characteristics, while TGA, EDX and XPS analysed thermal stability, elemental composition and valence states respectively. Doping-induced band gap increase was attributed to the Moss-Burstein effect. Photoluminescence excitation studies revealed enhanced absorption at 394 nm compared to the charge transfer transition band (CTB) at 254 nm, aligning with industry-standard near-ultraviolet chips. XPS suggested two distinct Eu3+ sites in the LSS crystal, corroborated by excitation wavelength-dependent PL emission. As a result of excitation wavelengths, the magnetic-dipole transition (5D0 → 7F1) at 592 nm, the electric-dipole transition (5D0 → 7F2) at 615 nm and the induced electric-dipole transition ((5D0 → 7F3,4) at 650 and 700 nm arose. Additionally, the phosphor gave an improved conversion efficiency at 2% Eu doping. Low synthesis temperature, micro-sized particles, thermal stable, short lifetime, excitation bands at near UV to blue region, colour tunability, lower CCT, LER and similar CIE to available red phosphors are the key factors for the Li2SrSiO4:Eu3+ proving reliable red phosphor for solid state lighting.

Novel YAGG: Cr garnet phosphor ceramic with broadband near infrared (NIR) emission and luminescence thermal stability for plant growth

In recent years, NIR (Near-Infrared) phosphor-converted LEDs (pc-LEDs) have played an indispensable role in the field of plant lighting. This study presents the development of a novel garnet-based phosphor ceramic, YAGG:0.035Cr3+, obtained through a high-temperature solid-state method. Through optical and structural performance analysis, it was found that the sample can be effectively excited by a commercial 450 nm blue LED, emitting near-infrared light in the range of 650–850 nm. This emission spectrum overlaps with approximately 74% of PFR spectrum absorbed by plant growth, and it achieves QY of around 40%. Notably, this ceramic demonstrates excellent thermal luminescence properties, maintaining approximately 96% of its integrated emission intensity at 423 K compared to room temperature. To explain this outstanding thermal performance, we analyzed that the gradual substitution of the 4T2 level for the 2E level leads to the dominance of the 4T2-4A2 transition process in PL. Furthermore, the YAGG:0.035Cr3+ phosphor ceramic was packaged with a 450 nm LED, and the emitted light intensity exhibited a linear relationship with the driving current. Even under high-power driving conditions of 1000 mA, the luminescence did not reach saturation. These results effectively demonstrate the potential application of YAGG:0.035Cr3+ phosphor ceramic in the field of plant lighting.

Mg4Nb2O9:Cr3+: broadband near-infrared luminescence

The exploration of broadband Near infrared (NIR) phosphors is one of the research priorities to enable the advancement of NIR devices. In this work, Mg4Nb2O9:Cr3+ phosphors were prepared by high-temperature solid-state reaction method. Mg4Nb2O9:Cr3+ can emit ultra-broadband NIR centred at 880 nm with an extremely full width at half maximum (FWHM) of near 200 nm, and NIR phosphor-converted LED (pc-LED) has been fabricated by combining a 450 nm blue LED with MNO:0.5%Cr to demonstrate its application prospect in night vision.

Broadband near-infrared luminescence in the novel Li3Cs2Sr2B3P6O24:Cr3+ phosphor for NIR pc-LED

Broadband phosphors materials with near-infrared emission have recently gained significant attention for their applications in bio-medication and bio-imaging, phototherapy, night vision, non-destructive diagnostic solution, and quality/risk control of food. This study introduces a novel phosphor material with a broad NIR emission spanning 700–1100 nm. Under 450 nm excitation, this material features a wide full width at half maximum (FWHM) of 170 nm. This broad-spectrum emission results from Cr3+ ion occupancy at Sr2+ sites. A promising quantum yield (PLQY) of 34.7 % was measured. The emission intensity at 423 K maintains 31.3 % of that at 298 K, which needs to be improved. Moreover, successful fabrication of packaged near-infrared phosphor-converted LEDs (pc-LEDs) displaying a broad emission centered on 890 nm further underscores the exceptional potential of Li3Cs2Sr2B3P6O24:Cr3+ phosphors in the realm of broadband NIR pc-LED applications.