Warm White Light-emitting Diodes Research Articles

Color-Adjustable Na5y(Moo4)4: Dy3+, Eu3+ Phosphors For Cold and Warm White Leds for Different Activity Scenarios

Color-Adjustable Na5y(Moo4)4: Dy3+, Eu3+ Phosphors For Cold and Warm White Leds for Different Activity Scenarios

Color tunable emission from Eu3+ and Tm3+ co-doped CsPbBr3 quantum dot glass nanocomposites.

Cesium lead bromide (CsPbBr3) quantum dots (QDs) have shown great potential in the field of luminescent materials owing to their superior optical and electrical properties. However, instability and lack of multicolor emissions resulting from the intrinsic nature of CsPbBr3 QDs are still the major challenge for their commercialization. Herein, Eu3+ and Tm3+ co-doped CsPbBr3 QD glass nanocomposites (GNCs) are successfully synthesized via traditional melt-quenching followed by a heat-treatment route to obtain tunable emission in a durable host material. Tm3+ ions are doped to blue-shift the main emission peak of CsPbBr3 QDs, while Eu3+ ions are incorporated to compensate for the red deficiency. Accordingly, a tunable color emission spanning the entire visible spectrum is achieved from GNCs with a fixed composition. The incorporation of Eu3+ and Tm3+ ions promotes the crystallization of CsPbBr3 QDs in the glass host resulting in ∼100% photoluminescence quantum yield (PLQY) using a dilution method. The selected glass host has also been proven to effectively protect CsPbBr3 QDs against chemical, thermal and photo degradation. Interestingly, the selected Eu3+/Tm3+ co-doped CsPbBr3 QD GNC shows warm-white light with a low color temperature of 3692 K without utilizing any commercial phosphors. This indicates that the produced GNCs have the potential to be used as light convertor materials in multi-color LED or warm white LED applications due to their robust stability and extremely pure and tunable emission colors.

Color tunable SrZrO3:Sm3+ nanopowders with satisfactory photoluminescent, band engineering properties for warm white LEDs and advanced forensic applications

In the present investigation, color tunable samarium ions incorporated SrZrO3 nanopowders fabricated by simple solution combustion technique by using Aloe vera gel as fuel studied. The thorough characterization of the synthesised SrZrO3 nano materials by different analytical techniques has been explored. The powder X-ray diffraction profiles authorizes the orthorhombic phase of SrZrO3. The band gap energies were evaluated by diffuse reflectance spectra profiles and it was found that the values were observed between 4.42-4.81 eV. The dumb-bell shaped morphology was confirmed by the scanning electron microscope images. The size of the particles was estimated by means of Transmission electron microscope and it was found to be around 30 nm. The photoluminescence properties were studied by means of emission spectra at room temperature. The three prominent peaks of Sm ions were observed at 555 (4G5/2→6H5/2), 609 (4G5/2→6H7/2) and 645 nm (4G5/2→6H9/2) respectively under 403 nm (6H5/2→4F7/2) excitation wavelength. The highest PL emission was observed for 5 mol% of Sm incorporated SrZrO3. The photoluminescence emission decreases beyond 5 mol% of dopant concentrations owing to the well-known concentration quenching mechanism. Commission international de I'Eclairage diagram unveils orange color due to (0.6362, 0.3633) lines and the average color purity of the fabricated powders was found to be ∼ 90%. Further, the average internal quantum efficiency was estimated and it was observed to be 36.46%. In addition to this, the optimized powders were utilized to detect the latent fingerprint on non-porous surface. The found results indicated that, the powders can simulate the clear ridge details including the 3 types such as sweat pores without any background hindrance. The achieved results recommend the prepared phosphors can be excellently employed in the production of the white light emitting diode applications as orange-red component and also used for the advanced forensic applications.

Anchoring of red perovskite nanocrystals on YAG:Ce phosphor for high color rendering index WLEDs

Widespread application of YAG:Ce-based white light-emitting diodes (WLEDs) is being challenged for their poor color rendering index (CRI) and high correlated color temperature (CCT) due to the lack of red emission. Organic-inorganic hybrid perovskite nanocrystals (MAPbX3 PNCs, X = Cl, Br, I or mixture of them), presenting high photoluminescence quantum yield, tunable emission color and simple fabrication process have been recognized as promising red-emitting component for warm WLEDs. However, additional protection process for PNCs always needed due to their poor stability, which complicates the fabrication procedures of WLEDs. Besides, nano-sized PNCs and micron YAG:Ce powder display different sinking behavior in silicone, which also hard to ensure the uniform distribution of the two components and subsequently stable optical characteristics of the device. Herein, a one-pot anchoring strategy is implemented to realize the enhanced thermal and photo-stabilities of in-situ generated red-emitting MAPbBr0.5I2.5 PNCs and simultaneously strong interaction between the PNCs and YAG:Ce to form PNCs@A-YAG composite, which greatly simplifies the processes, and also avoids the uneven distribution of phosphor and PNCs in silicone. Furthermore, the WLED, fabricated by PNCs@A-YAG and a commercial blue chip, exhibits high CRI of 92, low CCT of 4755 K, and super high saturated red color R9 of 95, which are of vital importance in achieving warm white light for indoor lighting applications. This work could promote the practical application of perovskite nanocrystals in the field of high color rendering index WLEDs.

Emerging near-unity internal quantum efficiency and color purity from red-emitting phosphors for warm white LED with enhanced color rendition

A robust and bright red emitter is recognized as a key enabler for high-quality warm white light-emitting diode (w-LED). Herein, triple molybdate scheelite-type NaCaLa1-x(MoO4)3:xEu3+ (NCLM:xEu3+) phosphors derived from NaCaLa(MoO4)3 host by the equivalent-valent substitution strategy are successfully synthesized and systematically studied. Impressively, the NCLM:Eu3+ phosphors (λem = 615 nm) can be effectively stimulated by 395 and 466 nm light, exhibiting an ultra-high color purity and quantum efficiency close to 100%. The structure-related emission characteristics were analyzed based on the Judd-Oflet theory. The packaged white LED device used tri-color phosphors displays a superior color rendering index of 86.0 and low correlated color temperature of 2797 K. Moreover, the NCLM-based polydimethylsiloxane film maintains an impressive luminescent performance when it is folded or bent, demonstrating the potential applications in the field of flexible display and other light-emitting devices.

Wireless Controlled Wide Range Tuneable CCT Lighting System Using Three Component Colour Mixing

This paper proposes a novel approach of wireless controlled, tenable correlated colour temperature (CCT) lighting system with high colour rendering index (CRI) using the concept of colour mixing. Using Grassmann’s colour mixing theory, phosphor coated warm white LED source (CCT 2800 K) is mixed with the blue and green LED light sources to meet the requirement of high CRI and high value of red object identification parameter R9. A detailed mathematical formulation is derived for easy controlling of variable CCT and illuminance and a flowchart of wireless control scheme is incorporated and experimentally validated in this article. As a result, the locus of the blended light source is very close to the Planckian locus on CIE1931 chromaticity diagram with a minimum uv D value of +0.0005. This proposed system provides a very wide range CCT starting from 2800 K to 15000 K with a high CRI value of 94 at CCT 5000 K. Another feature of this blended light source is the true colour identification parameter of red object (R9), which is also increased to 93 at CCT 8000 K. This system has very wide application in industries like textile, food processing etc., in museum, art gallery, and medical application like surgical lighting for its high CRI and R9 value with wide range of CCTs.

A facile surface passivation strategy for Na2SiF6:Mn4+,Li+ phosphors to achieve high moisture resistance and luminescent thermal stability

Recently, Mn4+-doped fluoride red phosphors become widely used as red emitting components of WLEDs (white light-emitting diodes), but such phosphors are easy to hydrolyze in humidity environment and it limits their application. Herein, LNSF:Mn4+-CA (LNSF: Mn4+ = Na2SiF6: 0.066Mn4+,0.06 Li+, CA = passivation with citric acid solution) with high water resistance, luminescent thermal stability and luminescent intensity was synthesized though the H2O2-free hydrothermal method and surface passivation process. The water resistance, luminescent thermal stability and luminescent intensity are obviously enhanced by surface passivation. The high thermal stability is caused by strong negative thermal quenching (NTQ). The mechanism of the strong NTQ is attributed to the increase of electron traps after surface passivation. Furthermore, mechanism for improvement of water resistance is suggested as that a homogeneous Mn4+-rare surface protective layer is in situ formed on surface of the sample after surface passivation. The layer isolates external moisture and then makes the sample resistive to interaction with water. Improvement of water resistance is demonstrated as that after immersing in water for 6 h (0.25 day), luminescent intensity of the un-passivation sample drops to 42.73% of the initial one, while that of the passivation sample remains to have 92.33% of its initial one. The prototype WLEDs with good luminescent performances (CCT = 3916 K and Ra = 89.6) was assembled by using the optimal coated sample. The results suggest that the optimal passivated sample is potential red emitting phosphor for the application in blue-based warm WLEDs.

Enhancement of the luminescent thermal stability and water resistance of K2SiF6:Mn4+,Na+ by double coating of GQDs and K2SiF6

Nowadays, it is still a big challenge to synthesize Mn4+-doped fluoride red phosphors with high water resistance, luminescent intensity and thermal stability simultaneously. Therefore, KSF:M,N@GQDs@KSF (K2SiF6:0.06Mn,0.10Na@GQDs (12 mg/mol)@K2SiF6(0.2 wt), KSF: K2SiF6, GQDS: OH-contained graphene quantum dots) with high luminescent intensity, high luminescent thermal stability and water resistance have been synthesized via a combination of H2O2-free room temperature reaction method, hydrothermal coating method and room temperature surface coating process. Experimental results show that enhancement of negative thermal quenching (NTQ) effect and water resistance can be synergistically induced by double coating of GQDs and KSF. After being immersed in deionized water for 360 min, emission intensities of the KSF:M,N@GQDs and KSF:M,N@GQDs@KSF are decreased from 100% to 70.57% and 91.63%, respectively, indicating that water resistance of the latter is obviously improved by coating of KSF. The improved water resistance is attributed to preventing hydrolysis of [MnF6]2- on sample’s surface by insoluble coating shell of KSF. A prototype WLEDs (white light-emitting diodes) emitted warm white light (CCT = 4546 K and Ra = 91.3, at a driving current of 20 mA) has been assembled by coating mixture of KSF:M,N@GQDs@KSF, yellow emitting phosphor (YAG:Ce3+) and epoxy resin on blue light InGaN chips. The good performances of the WLEDs suggest that the KSF:M,N@GQDs@KSF has potential applications in blue-based warm WLEDs.

Ultra-wideband warm white light emission from self-trapped excitons in CsAgCl2

The materials with self-trapped exciton (STE) emission are considered to have great prospects in application of lighting, due to the large Stokes shift and wide emission band. Here we report a novel STE emission material CsAgCl2 that can emit warm white light with a spectrum covering almost the entire visible range. At room temperature, the peak wavelength of its emission is about 625 nm, and it exhibits a broad half peak width up to 235 nm. A warm white LED was prepared using a 268 nm UV LED as the excitation source. It shows a correlated color temperature (CCT) of 4062 K and a color rendering index (CRI) of 93. The results indicate that CsAgCl2 is an ideal material for high color rendering lighting.

A red phosphor of Ba3In2F12:Mn4+ with enhanced moisture stability for warm WLED application

Although fluoride phosphors have distinguishing feature in luminescence spectra such as narrow band red light emission and broad band blue light excitation, the instability of [MnF6]2- limits their commercial applications. In this regard, we use weak reducing agents such as citric acid (CA) and oxalic acid (OA) for surface modification of a novel red phosphor Ba3In2F12:Mn4+ (BIF:Mn). A protective homogeneous passivation layer is formed on the surface of the phosphor particles with reduced Mn4+ content to improve the moisture resistance of Mn4+ doped fluoride phosphor BIF:Mn. In addition, their crystal structures, morphologies, photoluminescence characteristics, and the mechanism of luminescence enhancement are also described in details. By using near-ultraviolet or blue light-emitting diodes (LEDs) to excite it, an efficient red emission has been achieved with a maximum peak at 631 nm. In order to verify their application value, a warm white LED (WLED) with a color rendering index (CRI = 81.4) and a correlated color temperature (CCT = 4779 K) has fabricated with the as-prepared BIF:Mn and commercial yellow phosphor (YAG:Ce) on blue GaInN chip.

Full-visible-spectrum lighting enabled by site-selective occupation in the high efficient and thermal stable (Rb, K)2CaPO4F: Eu2+ solid-solution phosphors

Tuning the luminescent properties of phosphors by crystal-site engineering is a good and efficient strategy for developing high-quality white light-emitting diodes (WLEDs). However, it is still challenging to modulate the cyan to red spectral emission in a single matrix and activator phosphor to obtain multicolor output and achieve full-visible-spectrum lighting. Herein, the strategy to manipulate site-selective occupation of Eu2+ activators by substituting the cation Rb+ with K+ is reported to obtain a versatile color output. The large-scale redshift regulation of emission spectrum from cyan to red was achieved in fluor-phosphate Rb2-yKyCaPO4F: 0.0085Eu2+ (R2-yKyCPOF: 0.0085Eu2+) (0 ≤ y ≤ 1.8) phosphors, which is designed by using Rb2CaPO4F: 0.0085Eu2+ (RCPOF: 0.0085Eu2+) cyan-emitting phosphor. The redistribution of Eu2+ ions in Rb1, Rb2, and Ca sites is achieved by introducing the cation K+ to substitute Rb+, and R2-yKyCPOF: 0.0085Eu2+ (0 ≤ y ≤ 1.8) is kept in the pure phase. Furthermore, the mixed phosphors of RCPOF: 0.0085Eu2+, RKCPOF: 0.0085Eu2+, and R0.2K1.8CPOF: 0.0085Eu2+ show excellent optical performance and good thermal stability. Full-visible spectrum lighting is successfully achieved with the combination of a 395 nm NUV chip and the blend of phosphor-converted WLEDs. The K+ substitution reported in this paper can support a project for warm WLEDs by manipulating the distribution of Eu2+ activator among different cation sites.

Design of smart light source based on bi-color LED with single duty cycle for correlated color temperature adjustment

Color-adjustable light sources facilitate both mood lighting and daylight harvesting. A single duty cycle can be used by a bi-color LED to adjust the correlated color temperature (CCT) by associating it with the duty cycle of the pulse width modulation dimming signal of the cool and warm white LEDs. The one-to-one mapping relationship between the single duty cycle and the CCT is based on the color mixing theory of bi-color LEDs. A method to correlate the dimming signals for bi-color LEDs is presented. The influence of the time characteristics of the two basic signals on dimming and CCT adjustment is analyzed. The dimming system of bi-color LEDs is designed, and the method used to adjust the CCT with a single duty cycle is verified. The experiment showed that the CCT can be accurately adjusted by the proposed method.

Ionic liquid assisted pure blue emission CsPbBr3 quantum dots with improved optical properties and alkyl chain regulated stability

Stability issue of blue emitting CsPbX3 quantum dots (QDs) remains a great challenge for their application in optoelectronic devices. In this work, hydrophobic ionic liquids (ILs) 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (NTf2) were used to obtain blue emission CsPbBr3 QDs with high color purity featured with a 16 nm full width at half-maximum (FWHM). Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) results elucidated ILs formed effective passivation on QDs through stronger interaction with surface Pb2+ compared to OA/OAm. This passivation effect led to the enhancement of photoluminescence quantum yields (PLQYs) from 33% for the pristine sample to 51% for QDs modified by ILs. And the stability of the modified QDs was enhanced by keeping the blue emitting even after 32 h UV irradiation. More interestingly, there is an alkyl chain effect existed that QDs with longer alkyl chain ILs exhibit better stability than the ones with shorter alkyl chains. It may relate to the higher hydrophobicity originated from long alkyl chain. Resultantly, Warm white light emitting diode (WLED) based on the IL-contained QDs was fabricated which qualified with a Commission Internationale de L'Eclairage (CIE) color coordinates of (0.33,0.35), Correlated color temperature (CCT) of 5416 K and maximum luminance of 9062.9 cd/m2. The demonstrated ILs passivation strategy might be an effective method for preparing blue emitting perovskite QDs with high PLQYs and stability.

Cost-Effective Strategy for the Synthesis of Air-Stable CH3NH3PbX3 (X = Cl, Br, and I) Quantum Dots with Bright Emission.

Lead halide perovskite quantum dots (QDs) are known as prospective optoelectronic device materials because of their excellent luminescence, extraordinary photoelectric performance, and specific octahedron framework. Herein, we report a cost-effective approach for synthesizing highly stable CH3NH3PbBr3 QDs in low-polarity binary solvents without nitrogen protection. The CH3NH3PbBr3 QDs are tunable from 1.2 to 4.2 nm by adjusting the proportion of oleic acid and oleylamine as capping ligands. The photoluminescence quantum yield of CH3NH3PbBr3 QDs can reach 87.4%. The fluorescence can maintain over 80% of its earliest emission intensity under the atmosphere after 5 days, which is much better than that (∼10%) of QDs with ligand-assisted reprecipitation. The possible reaction mechanism of preparing CH3NH3PbBr3 QDs was also addressed. Notably, such a strategy can be applied extensively in the preparation of other lead halide perovskite QDs. Furthermore, the as-prepared thick PMMA-coated CH3NH3PbBr3 QDs were further conjoined with a red luminescence powder on a blue InGaN chip to obtain a powerful efficiency (45.4 lm W-1) warm white light-emitting diode.

Luminescence properties of Mn4+-doped CaAl2O4as a red emitting phosphor for white LEDs

A series of Mn4+-activated CaAl2O4 (CAO) compounds were synthesized by co-precipitation to seek a candidate for a red-emitting phosphor to be employed in a white LED. The crystal structure, morphology, and fluorescence properties of the as-obtained phosphors were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and photoluminescence (PL). The deep red luminescence of Mn4+ in CaAl2O4 is reported and discussed. The excitation spectrum exhibited broadband emission between 260 and 550 nm with three peaks dominating at 320 nm due to the transition of 4A24T1. Theemission spectra between 600 to 720 nm displays an overwhelming emission peak at 654 nm owing to the 2E4A2 transition of Mn4+ ion.This research demonstrates the great promise of CaAl2O4:Mn4+ as a commercial red phosphor in warm white LEDs and opens up new avenues for the exploration of novel non-rare-earth red-emitting phosphors.

Synthesis and Waterproofness Improvement of K3AlF6:Mn4+ Phosphor for Warm White Light-emitting Diodes

In this study, we synthesized K3AlF6:Mn4+ phosphor by co-precipitation method. The crystal structure and photoluminescence properties were investigated. By surface modification of K3AlF6:Mn4+ using K3AlF6, the moisture resistance performances of the phosphor can be significantly improved. It was found that the luminescence performance of K3AlF6:Mn4+@K3AlF6, which was dispersed in water for 2 h, is unchanged but the uncoated sample reduces dramatically. White light emitting diodes (WLEDs) based on the phosphor combined with commercial YAG:Ce3+ coated on a blue LED showed significantly improved of performance with color correlated temperature (CCT) from 5307 K down to 3528 K and color rendering index (CRI) from 64 up to 87. The results exhibit the potential for the application of K3AlF6:Mn4+@K3AlF6 as a red phosphor in warm WLEDs.

Structural, luminescence, energy transfer mechanism, and photometric properties of Sm3+ -doped KYBO phosphors

Potassium yttrium borate (KY(1-x)SmxBO3) referred as KYBO:Sm3+ phosphors with the varying of Sm3+ ions (x = 0.005, 0.01, 0.05, 0.1, 0.2, 0.3, 0.5 mol%) were fabricated via the solid-state reaction process. The structural and optical studies were systematically investigated via X-ray diffractometer (XRD), excitation (PLE), emission (PL) spectra, and decay curves. The structure of KYBO:Sm3+ phosphors is attributed to hexagonal with an average particle size of 34 nm. Scanning electron microscopy (SEM) image and energy dispersive X-ray analysis (EDAX) spectrum were also studied. The dominant excitation wavelength at 406 nm is ascribed to 6H5/2 → 4F7/2 transition and the emission spectra exhibits three peaks at 573, 609 and 656 nm belonging to 4G5/2 → 6H5/2, 7/2, 9/2 transitions of Sm3+ ions, respectively. Critical distance (Rc) and energy transfer rates (Arad) were also calculated. The energy transfer (ET) mechanism from luminescence spectra was proved by Dexter and Schulman theory. The decay of 4G5/2 level for KYBO:Sm3+ phosphors exhibited non-exponential nature by shortening the lifetime from 1.85 to 0.3 ms. Moreover, the photometric parameters such as color coordinates (0.557, 0.440), correlated color temperatures (CCT:1784–2552 K), and color purity (CP:93–96%) were also obtained for KYBO:Sm3+ phosphors. These results considered to be in the orange-red emission region for warm white light-emitting diodes.

NUV-pumped luminescence of thermally stable samarium-activated alkali metal borophosphate phosphor

Exploring outstanding rare-earth activated inorganic phosphors with good thermostability has always been a research focus for high-power white light-emitting diodes (LEDs). In this study, we report a Sm3+-activated KNa4B2P3O13 (KNBP) powder phase. Its particle morphology, photoluminescence properties, concentration quenching mechanism, thermal quenching mechanism, and chromatic properties are demonstrated. Upon the near-ultraviolet (NUV) irradiation of 402 nm, the powder phase exhibits orange-red visible luminescence performance, originating from typical 4G5/2→6HJ/2 (J = 5, 7, 9) transitions of Sm3+. Importantly, the photoluminescence performance has good thermostability, low correlated color temperature (CCT), and high color purity (CP), indicating its promising application in the NUV-pumped warm white LEDs.

Green synthesis, luminescent properties and application for WLED of flower-like K2LiAlF6:Mn4+ phosphor

Chrysanthemum-like K 2 LiAlF 6 :Mn 4+ phosphors were synthesized by simple co-precipitation method without using HF. The K 2 LiAlF 6 :Mn 4+ phosphor can be effectively excited through blue light at 465 nm, then produces sharp red-light emission near 634 nm. The luminescence quenching of Mn 4+ is caused by the d-d interaction between Mn 4+ ions. The effects of surfactant and lithium source on the luminescent properties of K 2 LiAlF 6 :Mn 4+ phosphors were discussed. Mn 4+ is located in a strong crystal field environment in the K 2 LiAlF 6 matrix and its emission mechanism is discussed. Warm white light emitting diode (WLED) device, which is fabricated by coating a certain proportion mixture of yellow YAG:Ce 3+ phosphor and red K 2 LiAlF 6 :5%Mn 4+ phosphor on the blue GaN chip, has low correlation color temperature (CCT = 3588 K), high color rendering index (R a = 90.8) and good electroluminescence stability under different currents. These properties illustrate that K 2 LiAlF 6 :5%Mn 4+ phosphor is a promising red light emitting material. • Chrysanthemum-like K 2 LiAlF 6 :Mn 4+ were prepared by using low-toxic NH 4 F and HCl instead of highly toxic HF. • The effects of reaction conditions on luminous characteristics of K 2 LiAlF 6 :Mn 4+ were discussed. • K 2 LiAlF 6 :Mn 4+ phosphors have excellent photoelectric performance for LED application.

Small molecule organic solar cells toward improved stability and performance for Indoor Light Harvesting Application

In this work, we fabricate, characterize and present a stability study of p-DTS(FBTTh 2 ) 2 :PC 70 BM inverted organic solar cells (iOSCs) with two different solution-processed electron transport layers (ETLs), PFN and ZnO, under indoor light and AM 1.5 G illumination spectrum. A warm white color light-emitting diode was used as the indoor light source. Under the luminance of 1000 lux, a maximum power conversion efficiency (PCE) of 10.85% and maximum power density (MPP) of 45.1 μW/cm 2 were obtained for cells with PFN as ETL. After 1536 h of constant indoor light illumination, the PCE remained above 70% of the original value. The best stability of encapsulated devices, under AM 1.5 illumination spectrum was also observed for these cells, which still maintained a PCE above 60% after 6000 h. Performance of small-molecule organic solar cells for Indoor Light Harvesting Application. PFN as electron transport layer for inverted solution-processed small-molecule organic solar cells (SM-iOSC). 1.5 times longer lifetime of encapsulated SM-iOSC versus currently used inverted polymer solar cells (iPSC). Reduced S-shape deformation in the JV curves of HIZO-iOSC vs. PFN-iOSC. Degradation analysis by the solar cells equivalent circuit model.