Warm White Light-emitting Diodes Research Articles

Efficiently Luminescent and Stable Lead‐free Cs3Cu2Cl5@Silica Nanocrystals for White Light‐Emitting Diodes and Communication

AbstractThe poor stability of lead‐free Cs3Cu2Cl5nanocrystals (NCs) is the main impediment for their broad practical applications. Here, a sol‐gel reaction using tetraethoxysilane (TEOS) as the silica coating precursor to prepare Cs3Cu2Cl5@SiOxNCs is proposed. Due to the efficient passivation and robust protection of SiOxshells on the NCs surface, Cs3Cu2Cl5@SiOxNCs solution and films exhibit a high photoluminescence quantum yield of 98% and 76%, respectively, and improved stability against water and heat. Cs3Cu2Cl5@SiOxNCs are then used to fabricate warm white light‐emitting diodes (WLEDs), which show a high color‐rendering index of 94 and a proper correlated color temperature of 5049 K, and maintain an excellent thermal (265–360 K) and prolonged operating (380 h) stability. In addition, the good white light emission facilitates the application in visible light communication (VLC). The VLC light source demonstrates a −3 dB bandwidth of 420 kHz, and the achieved data rate of the VLC system reaches 2.65 Mbps using orthogonal frequency division multiplexing modulation with a bit loading. As a result, lead‐free Cs3Cu2Cl5@SiOxNCs with an efficient optical performance and an excellent stability might promote the development of stable NCs and extend the applications in optoelectronics.

Investigation of the efficiency of BiOI/BiOCl composite photocatalysts using UV, cool and warm white LED light sources - Photon efficiency, toxicity, reusability, matrix effect, and energy consumption

BiOI, BiOCl, and their composites (BiOI:BiOCl) with molar ratios from 95:5 to 5:95 were synthesized and tested in the transformation of methyl orange (MO) and sulfamethoxypyridazine (SMP) antibiotic, using three various LED light sources: UV LEDs (398 nm), cool and warm white LEDs (400–700 nm). The 80:20 BiOI:BiOCl photocatalyst showed the best adsorption capacity for MO and enhanced activity compared to BiOI and BiOCl. The apparent quantum yield (Φapp) of the MO and SMP transformation for cool and warm white light was slightly lower than for 398 nm UV radiation. The effect of methanol and 1,4-benzoquinone proved that the transformation is initiated mainly via direct charge transfer, resulting in the demethylation of MO and SO2 extrusion from SMP. The change of photocatalytic efficiency was followed during three cycles. After the first one, the transformation rates decreased, but there was no significant difference between the second and third cycles. The decreased efficiency is most probably caused by the intermediates, whose continuous accumulation was observed during the cycles. Ecotoxicity measurements confirmed that no toxic substances were leached from the catalyst, but the transformation of both MO and SMP results in toxic intermediates. Using 80:20 BiOI:BiOCl and LED light source, the energy requirement of the removal is about half of the value determined using TiO2 and a mercury vapor lamp. The effect of some components of wastewater (Cl−, HCO3− and humic acids), pH, and two matrices on the composite photocatalysts' efficiency and stability were also investigated.

Pink-emitting RbBaBP2O8:Sm phosphor with diamondoid structure and high thermostability

Microsized borophosphate-based phosphors of RbBaBP2O8:Sm were successfully prepared in this research. X-ray diffraction investigation proves that all phosphors are single phases belonging to the RbBaBP2O8-type diamondoid structure. According to the XPS, luminescence, and optical decay time investigations, there are the co-existence of the divalent and trivalent samarium ions in the RBBP matrix. Upon near-ultraviolet (401 nm) irradiation, the characteristic transitions of both Sm3+ and Sm2+ ions contribute to pink light emission. The content quenching and thermal quenching mechanisms are studied. The RbBaBP2O8:Sm phosphor shows high thermal stability, good color stability, and low correlated color temperature, indicating its promising application in warm white LEDs.

Solution-processed efficient and bright lead-free warm-white LEDs based on cesium copper halides

Solution-processed efficient and bright lead-free warm-white LEDs based on cesium copper halides

Carbon‐Dot‐Based White‐Light‐Emitting Diodes with Adjustable Correlated Color Temperature Guided by Machine Learning

AbstractHere, we show the fabrication of the carbon dots (CDs) with green and orange emissions from PTCDA (perylene‐3,4,9,10‐tetracarboxylic dianhydride). Using these CDs as emitters, the orange (or green) CDs LEDs were fabricated, which show electroluminescence (EL) spectra centered at 560 nm (or 498 nm) with an external quantum efficiency (EQE) of 1.98 % (1.76 %) adhering a luminescence of 626 cd m−2 (or 519 cd m−2). The machine learning was successfully used to predict PL CCT value. With the model, the white photoluminescence (PL) emission with adjustable correlated color temperature (CCT) from 3093 to 11018 K via combining blue, green, and orange CDs was achieved. Then, we obtained the warm white CDs LEDs with CCT of 3107, 4071 and 4548 K, and cold white CDs LEDs with CCT of 5632 (CIE coordinates of (0.33, 0.33), EQE: 1.18 %, luminescence: 598 cd m−2) and 6034 K accurately.

Carbon-Dot-Based White-Light-Emitting Diodes with Adjustable Correlated Color Temperature Guided by Machine Learning.

Here, we show the fabrication of the carbon dots (CDs) with green and orange emissions from PTCDA (perylene-3,4,9,10-tetracarboxylic dianhydride). Using these CDs as emitters, the orange (or green) CDs LEDs were fabricated, which show electroluminescence (EL) spectra centered at 560 nm (or 498 nm) with an external quantum efficiency (EQE) of 1.98 % (1.76 %) adhering a luminescence of 626 cd m-2 (or 519 cd m-2 ). The machine learning was successfully used to predict PL CCT value. With the model, the white photoluminescence (PL) emission with adjustable correlated color temperature (CCT) from 3093 to 11018 K via combining blue, green, and orange CDs was achieved. Then, we obtained the warm white CDs LEDs with CCT of 3107, 4071 and 4548 K, and cold white CDs LEDs with CCT of 5632 (CIE coordinates of (0.33, 0.33), EQE: 1.18 %, luminescence: 598 cd m-2 ) and 6034 K accurately.

Engineering white light-emitting diodes with high color rendering index from biomass carbonized polymer dots

Carbonized polymer dots (CPDs) have promise in the fields of sensing, bioimaging, and optoelectronic devices due to their excellent optical properties, favorable biocompatibility, and superior stability. Biomass CPDs present greater advantages in terms of their lack of toxicity, low cost, easy preparation, and feasibility in terms of luminescence-related applications. Here, two kinds of fluorescent CPDs were obtained through the simple hydrothermal method using biomass avocado peel (CPDs-P) and sarcocarp (CPDs-S) as carbon sources. Interestingly, these two biomass CPDs have excellent applications in ion detection and light-emitting diodes (LEDs). Analysis and results show that CPDs-P possess better sensitivity to Fe3+ because they have more oxygen-containing functional groups. After mixing with epoxy resin, warm and cold white LEDs with CIE (Commission Internationale de L 'Eclairage) coordinates (0.38, 0.39) and (0.29, 0.34) were constructed successfully from extremely stable CPDs-P and CPDs-S. The high color rendering index of the prepared white LEDs are 90.47 and 84.54. This study shows that these biomass CPDs are promising materials in sensing and white LEDs illumination.

Highly stable CsPbBr3 quantum dots by silica-coating and ligand modification for white light-emitting diodes and visible light communication

Owing to their superior optical and electronic properties, all inorganic metal halide CsPbX3 (X = Cl, Br, and I) perovskite quantum dots (QDs) are regarded as excellent candidates for various optoelectronic applications. However, the instability of such materials greatly hampers their practical applications. In this work, silica-coated didodecyldimethylammonium bromide (DDAB) capped CsPbBr3 QDs are prepared via a facile method at room temperature. The as-prepared DDAB-CsPbBr3/SiO2 QDs composites demonstrate an effectively improved photoluminescence quantum yield (PLQY) and stability against ethanol and heat. Moreover, the green DDAB-CsPbBr3/SiO2 QDs composites and red InAgZnS QDs are applied as color-converting layers on a blue LED chip for warm white light-emitting diodes (WLEDs). Such WLEDs exhibit an excellent luminescent performance with a color rendering index (CRI) of 88, a color coordinate of (0.41, 0.38), a correlated color temperature (CCT) of 3209 K, and a high power efficiency of 63.4 lm W−1. Besides, such WLEDs are used for visible light communication (VLC), exhibiting a typical low-pass frequency response, with a corresponding −3 dB bandwidth of about 1.5 MHz. By applying orthogonal frequency division multiplexing (OFDM) with a bit loading, a maximum achievable rate of the VLC system reaches 5.9 Mbps, which is almost four times of their measured −3 dB bandwidth. These results demonstrate the potential of prepared DDAB-CsPbBr3/SiO2 QDs composites not only in high-performance WLEDs, but also as an excitation light source to achieve VLC.

High color rendering index composite phosphor-in-glass for high-power white laser lighting

The aim of this study is to investigate a thermally robust white color converter for high-power solid-state lighting, especially laser lighting. Lu3Al5O12:Ce3+/CaAlSiN3:Eu2+ phosphor-in-glass samples (LuAG&CASN-PiGs) were synthesized via a low temperature co-sintering technique. The prepared LuAG&CASN-PiGs exhibited remarkably high internal quantum efficiency of 87 %. Tunable warm white light-emitting diodes (wLED) were acquired by tailoring the sample thicknesses and phosphor contents. The optimized sample showed a high luminous flux of 183.68 lm under a blue laser diodes (LDs). In addition, the chromaticity of white LDs based on the LuAG&CASN-PiGs shifted from cool to warm white by changing the sample thicknesses. High quality white light in wLDs was achieved (Ra＝95). More importantly, the constructed LuAG&CASN-PiG converted LDs with a heat sink exhibited the luminous efficiency of 216.79 lm W−1. The results revealed that the prepared LuAG&CASN-PiG had great potential for application in solid-state laser light sources.

Regulation of the Photon Spectrum on Growth and Nutritional Attributes of Baby-Leaf Lettuce at Harvest and during Postharvest Storage.

The photon flux density (PFD) and spectrum regulate the growth, quality attributes, and postharvest physiology of leafy vegetables grown indoors. However, limited information is available on how a photon spectrum enriched with a broad range of different wavebands regulates these factors. To determine this, we grew baby-leaf lettuce ‘Rouxai’ under a PFD of 200 µmol m−2 s−1 provided by warm-white (WW; control) light-emitting diodes (LEDs) supplemented with either 30 µmol m−2 s−1 of ultraviolet-A (+UV30) or 50 µmol m−2 s−1 of blue (+B50), green (+G50), red (+R50), or WW (+WW50) light. We then quantified growth attributes and accumulated secondary metabolites at harvest and during storage in darkness at 5 °C. Additional +G50 light increased shoot fresh and dry weight by 53% and 59% compared to the control. Relative chlorophyll concentration increased under +UV30, +G50, and especially +B50. At harvest, +B50 increased total phenolic content (TPC) by 25% and anthocyanin content (TAC) by 2.0-fold. Additionally, +G50 increased antiradical activity (DPPH) by 29%. After each day of storage, TPC decreased by 2.9 to 7.1% and DPPH by 3.0 to 6.2%, while TAC degradation was less pronounced. Principal component analysis indicated a distinct effect of +G50 on the lettuce at harvest. However, concentrations of metabolites before and during storage were usually greatest under the +B50 and +R50 treatments.

High Quantum Yield Gd4.67Si3O13:Eu3+ Red-Emitting Phosphor for Tunable White Light-Emitting Devices Driven by UV or Blue LED

Eu3+-activated oxide components appear as a kind of rare-earth red phosphors that can be used for warm white light-emitting diode (LEDs). At present, it is still a challenge to synthesize oxide-component phosphors with high photoluminescence quantum yields compared to those of nitride and fluoride red phosphors. Herein, we propose a kind of Eu3+-activated Gd4.67Si3O13 rare-earth silicate phosphor by the convenient high-temperature solid reaction method. These designed phosphors can be effectively excited by either 394 nm near-ultraviolet light or 465 nm blue light and then emit red light at a dominant wavelength of 615 nm. Particularly, for the optimal Eu3+-concentration-activated Gd3.67EuSi3O13 phosphor, photoluminescence quantum yields are measured to be above 80%, which is superior to that of the general rare-earth oxide salt phosphor and comparable to that of commercial rare-earth nitride red phosphors such as Ca-α-SiAlON:Eu2+ (QY = 70.5%) and CaAlSiN3:Ce3+ (QY = 80%). Packaged with commercial yellow phosphor Ce3+:YAG and UV or blue LED chip, our red component serves to improve the CRI and CCT for the tunable white light-emitting devices. Moreover, the phosphor has good thermal cycling stability with a thermal-quenching activation energy of 0.28912 eV. Finally, the temperature-dependent properties make the Gd4.67Si3O13:Eu3+ phosphors have potential application in optical temperature measurement systems. The maximum relative sensitivity Sr is found to be 1.05 × 10–2 K–1 at 456 K.

Tuning luminescence of Ba3Si6O12N2:Eu2+ phosphor for full-spectrum warm white LED lighting

• A cyan emission was achieved via the replacement of Si-N with Al-O in Ba 3 Si 6 O 12 N 2 :0.05Eu 2+ . • The emission peak could be shifted from 512 nm to 495 nm. • The emission intensity of the optimal cyan phosphor is 36% higher than Ba 3 Si 6 O 12 N 2 :0.05Eu 2+ . • The CCT of wLED decreases from 4407 K to 4038 K and the Ra increase from 85.8 to 90.8. Developing cyan phosphors excited by near ultraviolet (n-UV) or blue LED chips is extremely significant for making up for the cyan cavity of the existing white light emitting diodes (wLEDs). In this research, a cyan emission was successfully achieved via the replacement of Si-N with Al-O in Ba 2.85 Si 6 O 12 N 2 :0.15Eu 2+ green phosphor. The emission peak could be shifted from 512 nm to 495 nm. The mechanism of spectral blue shift can be interpreted by the decrease of the crystal field splitting strength, which is induced by the square of the bond length and the decrease of centroid shift of 5d level, which is induced by the spectroscopic polarizability and 6th power of the bond length. The optimal cyan Ba 2.85 Si 5.65 Al 0.35 O 12.35 N 1.65 :0.15Eu 2+ phosphor possess a broad excitation and emission band at the same time, whose emission intensity is 36% higher than that of Ba 2.85 Si 6 O 12 N 2 :0.15Eu 2+ . The CCT of the wLED with Ba 2.85 Si 5.65 Al 0.35 O 12.35 N 1.65 :0.15Eu 2+ phosphor realizes from 4407 K to 4038 K and the Ra values achieve from 85.8 to 90.8. The results indicate that Ba 2.85 Si 6- y Al y O 12+ y N 2- y :0.15Eu 2+ phosphor can serve as a cyan component for application in full-spectrum warm white lighting.

Efficient and bright warm-white electroluminescence from lead-free metal halides

Solution-processed metal-halide perovskites are emerging as one of the most promising materials for displays, lighting and energy generation. Currently, the best-performing perovskite optoelectronic devices are based on lead halides and the lead toxicity severely restricts their practical applications. Moreover, efficient white electroluminescence from broadband-emission metal halides remains a challenge. Here we demonstrate efficient and bright lead-free LEDs based on cesium copper halides enabled by introducing an organic additive (Tween, polyethylene glycol sorbitan monooleate) into the precursor solutions. We find the additive can reduce the trap states, enhancing the photoluminescence quantum efficiency of the metal halide films, and increase the surface potential, facilitating the hole injection and transport in the LEDs. Consequently, we achieve warm-white LEDs reaching an external quantum efficiency of 3.1% and a luminance of 1570 cd m−2 at a low voltage of 5.4 V, showing great promise of lead-free metal halides for solution-processed white LED applications.

Solid solution of novel LixByGdEu (WO4)2 (B=Na, K) red phosphors: Influence of Na/K substitution on microstructures, Judd-Ofelt and luminescence properties for WLED applications

In the present investigation, new materials of the type LixByGdEu(WO4)2 (B= Na, K) have been synthesized by solid state reaction. The influence of Li/K and Li/Na ions on the photoluminescence properties and structures are presented in detail. Rietveld refinement indicates the crystalline phase of the compounds and incorporation of doping leads to change in the interatomic distance. The SEM observations shows that all the prepared phosphors have spheroid, bi-pyramidal shape with different grain size. An Optical energy bandgap of 3.0 and 3.2 eV were observed for LixByGdEu(WO4)2 (B= Na, K) phosphors. The Photoluminescence spectra shows that the phosphors can be excited by near UV light which is relevant for lighting technology. Luminescence emission spectrum reveals the two characteristic emissions located at 613 nm (electric dipole) and 586 nm (magnetic dipole). The 5D0→7F2 electric dipole transitions are dominant, indicating that Eu3+ ion occupies the single crystallographic site without the stark splitting in the host matrix. Interestingly, doping of Na, K shows 40% enhancement of photoluminescence intensity than reported LiGdEu(WO4)2 phosphor. The critical distance of energy transfer is found be 10 Å indicating the multipole-multipole interactions. Judd Ofelt (J-O) theory is implemented to explore the luminescence behavior and local structure around activator with respect to Eu3+ ion. The JO intensity parameter trend i.e, Ω2 > Ω4 indicate that the asymmetric environment around the activator. In addition, the Cathodoluminesence study also suggests the intense red emission of phosphors at low voltage and current. The chromaticity coordinates of the phosphors are much closer to NTSC trichromatic coordinates indicating the promising red phosphors with excellent luminescence properties. The CCT values around 3500K reveals that the LixByGdEu(WO4)2 (B= Na, K) red phosphors can be used for warm white LED applications.

Novel yellow–orange-emitting Ba3Lu4O9: Sm3+ phosphors with good thermal stability and high color purity for solid state lighting

A series of novel Sm3+ activated yellow–orange-emitting phosphors have been synthesized successfully by the conventional solid-state method. Their phase composition and luminescence properties have been investigated and discussed systematically. Upon excitation at 410 nm, all the studied phosphors showed intense narrow emissions with a prominent peak centered at around 610 nm, corresponding to 4G5/2—6H7/2 transition. The critical concentration of Sm3+ ions was determined to be 0.02, and the concentration quenching mechanism was verified as dipole–dipole interaction. The investigation of thermal stability revealed that the photoluminescence intensity of Ba3Lu3.92O9:0.08Sm3+ at 425 K remained around 81.4% of that at 298 K. Besides, when excited at 410 nm, the studied phosphor exhibited high color purity of 99.5% with the CIE chromaticity coordinates of (0.5974, 0.4010) and a low Correlation Color Temperature of 2217 K. Consequently, it could be expected that the as-synthesized Ba3Lu4O9:Sm3+ phosphors would be a promising candidate of yellow–orange component for warm white or amber LEDs.

K2SiF6:Mn4+@K2SiF6 phosphor with remarkable negative thermal quenching and high water resistance for warm white LEDs

Poor water resistance and luminescent thermal stability remain great challenges for commercial narrow red emitting Mn4+-doped fluoride phosphors for white light emitting diodes (WLEDs). In this work, a simple coating method is developed to synthesize K2SiF6:0.06Mn4+@0.10K2SiF6 red emitting composite phosphor with excellent water resistance, luminescent thermal stability and high efficiency. After immersing into water for 300 min, the optimal sample maintains 88% of the initial integrated PL intensity, whereas that of uncoated control sample steeply drops down to 1%. A remarkable negative-thermal-quenching effect is observed that the integrated PL intensities at 120, 150, 180 and 210 °C are 176, 198, 214 and 213% of that initial one at 30 °C. The mechanism of above behaviors are discussed and proposed: The K2SiF6 shell layer not only prevents the hydrolysis of Mn4+ by water and but also cuts off the transfer of energy to surface defects, leads to the increase of radiation transition probability faster than that of non-radiation transition with the increase of temperature. These findings not only illustrate the multiple effects of coating K2SiF6 on the luminescent performances of K2SiF6:Mn4+ red emitting phosphor, but also provide a facile strategy to improve the water resistance as well as luminescent thermal stability of Mn4+-doped fluoride phosphors for warm WLEDs in the future.

Voltage distortion in LED street lighting installations

The paper considers two power quality issues: voltage distortion and voltage drop along the pole lines of different LED street (road) lighting installations. The research includes three parts: (1) development of a novel model and the corresponding software for calculating voltage harmonics in the three-phase LED street lighting installations, where the load is nonsymmetrical and nonlinear. The model is based on the application of symmetrical components and the impedance per phase model. (2) Experiments on two sections of a pilot street lighting installation, one with 6 poles with neutral white and the other with 5 poles with warm white LED luminaires (the former were with conventional, and the latter with improved drivers regarding harmonic emissions). Their results were used to define harmonic characteristics of the LED luminaires and validate the model. (3) The developed software was applied to four typical street (road) lighting installations, two with a single-sided and two with a central pole layout. The major outcome was the determination of the maximum permissible number of the LED luminaires which can be connected to the feeding cable without exceeding any of the limits related to voltage drop, total voltage harmonic distortion and individual voltage harmonics in either the full or reduced street lighting regimes.

Propellar shaped triple bond rigidified D-A-π-A triphenylamine dye as back electron interceptor in iodine and cobalt electrolyte DSSCs under full sun and indoor light

In the present manuscript, we report the synthesis and characterization of a new molecularly engineered triple bond rigidified D-A-π-A propeller shaped triphenylamine dye having benzothiadiazole as auxiliary acceptor (TPAA8). We compared the behavior of TPAA8 with the parent dye TPAA1 in D-π-A architecture without auxiliary acceptor, which was reported previously from our group (Vinayak et al., 2016). TPAA8 was found to be efficient in harvesting light and was having a favourable energetics suitable to realize forward electron transfer process in fabricated devices. TPAA8 with the introduction of auxiliary benzothiadiazole acceptor proved to be a promising candidate in preventing recombination with conventional iodine and alternate cobalt based redox electrolytes which was studied in detail employing various optical and electrical perturbation techniques. With alternate cobalt electrolyte ([Co(bpy)3]2+/3+) TPAA8 delivered an improved performance of 6.40 ± 0.05%, which is an improvement of 124% in power conversion efficiency compared to the parent TPAA1 sensitizer devoid of auxiliary acceptor having an efficiency of 2.85%. TPAA8 also proved to be an efficient candidate for indoor light harvesting delivering a power conversion efficiency of 11.03 ± 1.7% employing [Co(bpy)3]2+/3+ redox mediator under 1000 lux warm white LED illumination.

Inorganic lead-free cesium copper chlorine nanocrystal for highly efficient and stable warm white light-emitting diodes

Inorganic cesium lead halide ( CsPbX 3 , X = Cl , Br, I) nanocrystals (NCs) attract extensive attention because of their excellent optoelectronic performance. However, the classic CsPbX 3 NCs suffer from toxicity and instability, which impede their further applications in commercial fields. Here the inorganic lead-free cesium copper chlorine NCs are synthesized by a facile hot-injection method. The blue-emission 3D CsCu 2 Cl 3 and green-emission 0D Cs 3 Cu 2 Cl 5 NCs are prepared at 70°C and 120°C, respectively, suggesting that the reaction temperature may account for the final components. Owing to the self-trapped exciton effect, the unique optical properties, such as high photoluminescence (PL) quantum yield, broadband emission, large Stokes shift, and long PL decay time, are demonstrated for both cesium copper chlorine NCs. Moreover, highly efficient and stable warm white light-emitting diodes are fabricated with CsCu 2 Cl 3 and Cs 3 Cu 2 Cl 5 NCs. The study highlights the promising potential for lead-free cesium copper chlorine nanocrystals in nontoxic solid-state lighting applications.

Low cost, high color rendition, CCT variable lighting system based on W-G-B LED

This article proposes a novel approach of tunable correlated color temperature (CCT) lighting system with high color rendition using three component color mixing concept. Using Grassmann’s color mixing theory, phosphor coated warm white LED source (CCT 2800 K) is mixed with the blue (467 nm) and green (527 nm) LED light sources and a detailed mathematical formulation is derived for easy controlling of variable CCT and experimentally validated in this article. As a result, the locus of the blended light source is very close to the Planckian locus on Commission Internationale de l’Eclairage (CIE) 1931 chromaticity diagram. The generated color of the light sources is evaluated by both CIE color rendering index (CRI) method and Illuminating Engineering Society (IES) TM-30-18 method. This proposed system provides a wide range of CCTs starting from 2800 K to 7500 K with a very high CRI (CIE Ra) value of 94, color fidelity index (IES Rf) score of around 85 and color gamut index (IES Rg) of 95. As this system uses only three color component, the cost of this system is comparatively low with respect to other high CRI, tunable CCT lighting system available in market.