Performance Of White Light-emitting Diodes Research Articles

Controllable morphology, excellent stability and non-equivalent doping-induced optical properties of Mn4+-activated K2NaScF6 red phosphor for high-quality warm WLED

Currently, Mn4+ activated fluoride red phosphor has been widely studied to improve the optical performance of white light-emitting diode (WLED) in the field of high-efficiency lighting and wide color gamut backlight display. In this paper, a series of red-emitting K2NaScF6:Mn4+ phosphors are synthesized by a simple co-precipitation method with a non-equivalent substitution of Mn4+ for Sc3+ at room temperature. The crystal structure, morphology and elemental composition of K2NaScF6:Mn4+ phosphors prepared using different reaction conditions are systematically investigated. Interestingly, the K2NaScF6:Mn4+ phosphors exhibit narrowband red emission with low correlated color temperature (CCT), high color purity and short fluorescence lifetime. Compared to equivalent doping, the mechanism of short fluorescence lifetime caused by non-equivalent doping has also been systematically discussed. The optimal doping concentration is determined by setting a series of Mn4+ doping concentrations and the concentration quenching mechanism is analyzed. The crystal field strength (Dq), Racah parameters (B and C) and nephelauxetic ratio (β1) are calculated in detail to evaluate the crystal field environment of Mn4+ in K2NaScF6. The good water resistance of the as-prepared sample K2NaScF6:Mn4+ is verified by comparing with the commercially available phosphor K2SiF6:Mn4+. Moreover, the thermal stability is also analyzed by calculating the activation energy (Ea), chromaticity shift (ΔE) and chromaticity coordinate variation (CCV). Importantly, the as-packaged warm WLED (InGaN chip + YAG:Ce3+ + K2NaScF6:Mn4+) has excellent optical properties (CCT = 4082 K, color rendering index (CRI, Ra = 91.2) and luminous efficiency (LE) = 78.86 lm/W), and it exhibits good output stability at high drive currents. In summary, our work demonstrates that K2NaScF6:Mn4+ red phosphors possess a great potential application in warm WLED for the indoor lighting.

Extensive emission tuning and characterization of highly efficient CuInS2 quantum dots for white light-emitting diodes.

Whole visible range emitting CuInS2/ZnS QDs were obtained with broad band-width and high luminous efficiency by altering the Cu/In ratio and coating ZnS layer. 1-Dodecanethiol (DDT) as a sulfur source in the ZnS coating process can inhibit the lattice defects caused by Zn2+ inter-diffusion, thus increasing the photoluminescence quantum yield (PL QY). Then the stability and lighting performance of white light-emitting diodes (WLEDs) based on these CuInS2/ZnS QDs were characterized. The optimized WLED device exhibited a moderate luminous efficacy (LE) (70.33 lm·W-1) and ultrahigh color qualities (CRI Ra = 92.7, R9 = 95.9, R13 = 96.3) with warm white at a correlated color temperature (CCT) of 4052 K.

Improving the quality of red light emitting from Eu3+ by Co-doping Tb3+ in LuBa3(BO3)3

The red light emitting from phosphors activated with Eu3+ is of great significance for optimizing the comprehensive performances of white light-emitting diodes (WLEDs) for solid-state lighting. Herein, LuBa3(BO3)3:Eu3+ and LuBa3(BO3)3:Tb3+,Eu3+ phosphors are synthesized via the high temperature solid-state reaction to investigate the effects of the doping concentration of Eu3+ and the co-doping of Tb3+ on the quality of red emission from Eu3+ in the host. Under the excitation of 392 nm light, the dominant emission peak of LuBa3(BO3)3:xEu3+ gradually shifts from 593 nm to 610 nm when the doping concentration of Eu3+ is increased from 0.02 to 0.10, and the quality of red light emitting from Eu3+ in the host is found to be further improved with increasing the co-doping concentration of Tb3+, both of which are explained with the Judd-Ofelt theory. The calculated results indicate that the enhancement of red emission is attributed to the decrease of the environment symmetry of Eu3+ by increasing the concentration of Eu3+ in LuBa3(BO3)3:Eu3+ or co-doping Tb3+ ions. The color purity of the emission from LuBa3(BO3)3:0.10 Tb3+,0.10Eu3+ is 94.11%, and the luminescent intensity of the phosphor at 425 K is 69% of that at room temperature. Moreover, a WLED device with the high color rendering index of 91.1 is obtained by fabricating LuBa3(BO3)3:0.10 Tb3+,0.10Eu3+ and other phosphors with a 365 nm LED chip. The good comprehensive performances suggest that LuBa3(BO3)3:Tb3+,Eu3+ has a great application prospect as a red phosphor in the field of high-quality solid lighting.

Novel Mn4+-Activated K2Nb1-xMoxF7 (0 ≤ x ≤ 0.15) Solid Solution Red Phosphors with Superior Moisture Resistance and Good Thermal Stability.

Nowadays, Mn4+-activated fluoride red phosphors with excellent luminescence properties have triggered tremendous attentions for enhancing the performance of white light-emitting diodes (WLEDs). Nonetheless, the poor moisture resistance of these phosphors impedes their commercialization. Herein, we proposed the dual strategies of "solid solution design" and "charge compensation" to design K2Nb1-xMoxF7 novel fluoride solid solution system, and synthesized the Mn4+-activated K2Nb1-xMoxF7 (0 ≤ x ≤ 0.15, x represents the mol % of Mo6+ in the initial solution) red phosphors via co-precipitation method. The doping of Mo6+ not only significantly improve the moisture resistance of the K2NbF7: Mn4+ phosphor without any passivation and surface coating, but also effectively enhance the luminescence properties and thermal stability. In particular, the obtained K2Nb1-xMoxF7: Mn4+ (x = 0.05) phosphor possesses the quantum yield of 47.22% and retains 69.95% of its initial emission intensity at 353 K. Notably, the normalized intensity of the red emission peak (627 nm) for the K2Nb1-xMoxF7: Mn4+ (x = 0.05) phosphor is 86.37% of its initial intensity after immersion for 1440 min, prominently higher than that of the K2NbF7: Mn4+ phosphor. Moreover, a high-performance WLED with high CRI of 88 and low CCT of 3979 K is fabricated by combining blue chip (InGaN), yellow phosphor (Y3Al5O12: Ce3+) and the K2Nb1-xMoxF7: Mn4+ (x = 0.05) red phosphor. Our findings convincingly demonstrate that the K2Nb1-xMoxF7: Mn4+ phosphors have a good practical application in WLEDs.

Modeling and Monte Carlo simulation of photon transmission in glass-packaged WLEDs

Luminous efficiency is a key factor affecting the luminous performance of white light-emitting diodes (WLEDs). However, the mechanism and photon transport process in WLEDs encapsulated with multilayer films have not been elucidated. In this work, the effect of five different encapsulated inorganic glasses on the luminescence efficiency of WLEDs was investigated by using the Monte Carlo method. Compared with the conformal coating method, the away-from-coating method is more beneficial to improve the light output of WLEDs. The results show that the optical property of WLEDs is closely related to the concentration of phosphor and the thickness of the fluorescent glass. In addition, compared with other fluorescent glasses, fluorophosphate fluorescent glass has higher blue and yellow light transmittance and lower yellow light loss rate, which is suitable to be used as packaging materials to effectively improve the light output efficiency of WLEDs.

Influence of Phosphor Particle Size on Visual and Non-Visual Performances of White Light-Emitting Diodes for General Lighting

Influence of Phosphor Particle Size on Visual and Non-Visual Performances of White Light-Emitting Diodes for General Lighting

Unraveling the Ultrafast Self-assembly and Photoluminescence in Zero-Dimensional Mn2+-Based Halides with Narrow-Band Green Emissions

The discovery of narrow-band luminescent materials remains an immense challenge to optimize the performance of white light-emitting diodes (LEDs). So far, the zero-dimensional (0D) Mn2+-based halid...

Effects of NaB<sub>2</sub>O<sub>4</sub>:Mn<sup>2+</sup> and Ba<sub>2</sub>Li<sub>2</sub>Si<sub>2</sub>O<sub>7</sub>:Sn<sup>3+</sup>,Mn<sup>2+</sup> phosphors and remote structure organizations on the white light-emitting diodes with quantum dots and phosphors

In order to increase the optical features of white light-emitting diodes (WLEDs), quantum dots (QDs) and phosphor materials have been proposed because of outstanding performance. The configuration of WLEDs with QDs layer and phosphor-silicone layer suggested placing these components separately to limit light loss, and enhance consistency at contact surface of QDs. In this research, the effects of QDs and phosphor on the performance of WLEDs are concluded through experiments. The emitted light and PL spectra were examined thoroughly, and infrared thermal imagers were applied to simulate the heat generation of an actual WLED device. The results show that with the configuration of 60 mA energy source, WLEDs which has the QDs-on-phosphor form attained luminous efficiency (LE) of 110 lm/W, with color rendering index (CRI) of Ra=92 and R9=80, whereas the WLEDs which has the phosphor-on-QDs form only has 68 lm/W in LE, Ra=57 and R9=24. Furthermore, WLEDs which has the QDs-on-phosphor form has less high temperature generated at the components’ conjunction in comparison to the counterpart, the peak generated heat in QDs-on-phosphor WLEDs is also lower and the heating capacity gap between 2 structures can go up to 12.3°C.

(Y,Gd)BO<sub>3</sub>:Eu red phosphor for dual-layer phosphor structure to enhance the optical performance of white light-emitting diodes

Among the structures using for fabricating white light-emitting diodes (WLEDs) such as the conformal coating or in-cup geometries, the remote phosphor structure gives the highest luminous efficacy. However, in terms of color quality, its performance is not as good as the others. The red-light compensation has been reported as the effective solution for enhancing the color quality of WLEDs. Hence, this study adopted the idea and applied to the dual-layer phosphor structure. The phosphor used to boost the red color in light formation is (Y,Gd)BO3:Eu particle. The dual-layer remote phosphor structure was simulated with the red (Y,Gd)BO3:Eu phosphor layer above the original yellow phosphor YAG:Ce3+ one. The WLEDs with different correlated color temperatures of 5600 K, 6600 K and 7700K were experimented. Mie-theory and Lambert-Beer law were applied to examine the results. The growth in color rendering index (CRI) and color quality scale (CQS) with the increase of (Y,Gd)BO3:Eu phosphor concentration was observed. Nevertheless, the lumen efficacy would be degraded if the concentration was over a certain number. The information provided in this article is useful for the development of high-power WLED production with greater color quality.

Investigation on the application of ZnO nanostructures to improve the optical performance of white light-emitting diodes

Though combining blue LED chips with yellow phosphor has been the most common method in white light-emitting diode (WLED) production, the attained angular correlated color temperature (CCT) uniformity is still poor. Thus, this article proposes to add ZnO nanostructures to WLED packages to promote the color uniformity of the WLEDs. The outcomes of the research demonstrate that utilizing ZnO at different amount can affect the scattering energy and the CCT deviations in WLEDs packages in different extents. Particularly, adding the node-like (N-ZnO), sheet-like (S-ZnO), and rod-like (R-ZnO) leads to the corresponding decreases of CCT deviations from 3455.49 K to 96.30 K, 40.03 K, and 60.09 K, respectively. Meanwhile, with 0.25% N-ZnO, 0.75% S-ZnO, and 0.25 % R-ZnO, WLED devices can achieve both better CCT homogeneity and lower reduction in luminous flux. The results of this article can be a valuable document for the manufacturer to use as reference in improving their WLED products.

Study of the Optical Properties of Multi-Particle Phosphors by the FDTD and Ray Tracing Combined Method

It is well known that the optical properties of multi-particle phosphor are crucial to the light performance of white light-emitting diodes (LEDs). Note that the optical properties including scattering or absorption properties for a single particle are easy to be calculated. However, due to the large computation considering the complicated re-scattering and re-absorption, it is difficult to calculate the scattering behaviors of the multi-particles. A common method to reduce the computation, which can cause unknown deviations, is to replace the multi-particle scattering properties by using the average scattering data of single particles. In this work, a cluster of multi-phosphor particles are directly simulated by the finite-difference time-domain (FDTD) method. The total scattering data of the cluster was processed as a bulk scattering parameter and imported to the Monte-Carlo ray-tracing (RT) method to realize a large-scale multi-particle scattering calculation. A polynomial mathematical model was built according to the multi-particle scattering data. An experiment was carried out for verifying the accuracy of the method in this work. The mean absolute percentages of the previous method are 1.68, 2.06, and 1.22 times larger than the multi-particle method compared with the experimental curves, respectively.

Rapid synthesis of Ce3+:YAG via CO2 laser irradiation combustion method: Influence of Ce doping and thickness of phosphor ceramic on the performance of a white LED device

A new, rapid method was reported to produce Y3Al5O12 phosphors doped with different concentrations of Ce3+ (YAG:Ce3+) within 1.5 ​min via continuous-wave CO2 laser irradiation, in which a CO2 laser is utilised as the combustion source. X-ray powder diffraction confirmed the formation of pure cubic phase in the as-synthesised powder. Thus, laser irradiation plays an important role in optimising reaction kinetics to obtain high-quality crystal with high luminescence intensity. Furthermore, the incorporation of Ce3+ ions into the YAG matrix can be enhanced through sintering powders at 1050 ​°C for 5 ​h. Optical properties were examined comparatively using photoluminescence emission. Additionally, the performance of white light-emitting diode (LED) can be regulated easily by altering the Ce3+ doping concentration and YAG:Ce3+ layer thickness on blue InGaN LED chip. Excellent white light, which has a CIE coordinate of (0.31, 0.36) that approached the standard WLED values, a colour rendering index of about 69.4, a correlated colour temperature of about 6299 ​K and a luminous radiation efficiency exceeding 169 lm/W, was obtained for YAG doped with 2 ​mol% Ce concentration with a coating layer thickness of 1 ​mm.

Moisture resistance, luminescence enhancement, energy transfer and tunable color of novel core-shell structure BaGeF6:Mn4+ phosphor

Recently, Mn4+-doped narrow band red-emitting fluoride phosphors with exceptional luminescence properties have been widely studied for improving the performance of white light-emitting diodes (WLEDs). However, the poor water stability and single luminescent color limit their wider application. Herein, the design idea of new core-shell structure is proposed to improve the properties of BaGeF6:Mn4+ red phosphors by coating poly propylene glycol (PPG) and NaGdF4:Dy3+ nanoparticles. The phase purity, morphology and elemental composition are confirmed via X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), respectively. The luminescence properties of all phosphors are researched in detail with excitation spectra, emission spectra and fluorescence lifetime curves. PPG coating layer can not only modify the surface defects of the BaGeF6:Mn4+ phosphor to enhance the luminous intensity, but also successfully improve the water resistance of the phosphor. Therefore, BaGeF6:Mn4+@PPG phosphor as red component is more effective for packaging warm WLED. In addition, PPG is considered as the adsorption media layer to connect BaGeF6:Mn4+ phosphors and NaGdF4:Dy3+ nanoparticles. The as-synthesized BaGeF6:Mn4+@PPG-NaGdF4:Dy3+ composite phosphors can emit blue, yellow and red light simultaneously under the excitation of ultraviolet light. The energy transfer processes from Gd3+ to Dy3+ and from Dy3+ to Mn4+ are systematically discussed. By changing the doping concentration of Mn4+ and excitation wavelengths, the photochromic properties of the composite phosphor are successfully realized and the warm white light emission is obtained under the excitation of ultraviolet light. These findings demonstrate that the BaGeF6:Mn4+@PPG-NaGdF4:Dy3+ composite phosphors have more value for practical application.

A red-emitting Mn4+ activated phosphor with controlled morphology and two-dimensional luminescence nanofiber film: Synthesis and application for high-performance warm white light-emitting diodes (WLEDs)

Mn4+ activated fluoride red phosphors, Na2GeF6:Mn4+, with controllable morphology and size, were successfully synthesized through simple coprecipitation method. Using the hollow Na2GeF6:Mn4+ particles, we successfully fabricated two-dimensional red-emitting nanofiber film via electrospinning technology. The phase purity, morphology and composition of the samples were confirmed via X-ray powder diffraction (XRD), scanning electron microscope (SEM) and energy-dispersive X-ray spectrometer (EDS), respectively. Different surfactants were introduced into the experimental process, which successfully controlled the sample morphology. The broad excitation band from 300 nm to 550 nm matches well with commercial blue light emitting diodes, and narrow band emission red light can be observed in the range of 575–675 nm because of spin-forbidden 2Eg - 4A2g energy level transition of Mn4+ ion. The effect of doping concentration on luminescence and the mechanism of concentration quenching were discussed. On the basis of the experimental excitation (PLE) and emission (PL) spectra, the crystal field strength of the Mn4+ occupation environment was estimated and the nephelauxetic effect was researched for the Mn4+ ions in the Na2GeF6 host. Temperature-dependent properties of Na2GeF6:Mn4+ particles and nanofiber film were systematically analysed. Moreover, the Na2GeF6:Mn4+ particles and nanofiber film can significantly change the performance of white light-emitting diodes (WLEDs), the nanofiber film is a more appropriate candidate than Na2GeF6:Mn4+ particle for warm WLED applications.

The enhancement of emission intensity and enlargement of color gamut by a simple local structure substitution with highly thermal stability preserved

The local crystal structure engineering becomes an important strategy to design new phosphors with enhanced optical and thermal performance of white light-emitting diodes. Herein, a series of Na3Sc2(PO4)3: Eu2+ and KyNa2.97-ySc2(PO4)3: 0.03Eu2+ phosphors were synthesized via traditional high temperature solid-state reaction method. X-ray powder diffraction analysis and Rietveld refinement provide insight in the detailed crystal structure. Furthermore, Eu2+ doped Na3Sc2(PO4)3 exhibits bright blue emission in 400–540 nm spectral range with a maximum value at ~ 460 nm under n-UV light excitation. The concentration quenching mechanism of Eu2+ in Na3Sc2(PO4)3 is certified to be a dipole-dipole interaction. Additionally, crystal structure tailoring is a potential strategy to design new phosphors for particular applications. Therefore, the effects of K+ substitution on the structure and photoluminescence of Eu2+ activated Na3Sc2(PO4)3 is presented in detail. Rietveld refinement data revealed that unit cell volume and Na/K–O band length increase when K+ occupy the Na+ sites. This sensitive local structure resulted in a considerable enhancement of the photoluminescence intensity of Eu2+. Incorporation of K+ in the crystal structure is a feasible route to realize fine-tuning of emission color and broaden the color gamut. In the meantime, Na2.7K0.27Sc2(PO4)3: 0.03Eu2+ phosphor exhibits excellent thermal stability at high temperature over a significant radiative recombination of energy transfer from traps to Eu2+. These results confirm that Na2.7K0.27Sc2(PO4)3: 0.03Eu2+ phosphor might be used as a blue component in n-UV chip activated white light-emitting diodes for the next-generation of indoor solid-state lighting applications.

Effect of Quantum Dot Scattering and Absorption on the Optical Performance of White Light-Emitting Diodes

Quantum dots (QDs) show a great potential for light-emitting diodes (LEDs) packaging, which still face great challenges compared with the matured phosphor downconversion materials. These are probably caused by the unique scattering and absorption properties of QDs, which are extremely different to the traditional phosphor due to their several nanometers size, while their effect on QDs-converted LEDs (QCLEDs) is barely studied. In this paper, we have experimentally and theoretically investigated the effect of scattering and absorption of CdSe/ZnS QDs on the optical performance for QCLEDs by comparing with the traditional yttrium aluminum garnet phosphor. Results indicate that the strong absorption (reabsorption) of QDs causes low radiant efficacy and stability for QCLEDs; their weak scattering also leads to a low color uniformity. It demonstrates that their unique scattering and absorption properties are key factors leading to low optical performance of QCLEDs compared with the traditional phosphor-converted LEDs. For purpose of gaining the white LED with high efficiency and stability, it is highly suggested to use a low QD concentration to reduce the reabsorption loss and the total internal reflection loss. We believe that this paper can provide a better understanding of improving the optical performance for QCLEDs from the prospective of scattering and absorption. In the future, it is important to use low QD concentration to gain high-performance white LEDs with high downconversion efficiency by optimizing packaging structures.

Enhanced optical and thermal performance of white light-emitting diodes with horizontally layered quantum dots phosphor nanocomposites

Due to their good color rendering ability, white light-emitting diodes (WLEDs) with conventional phosphor and quantum dots (QDs) are gaining increasing attention. However, their optical and thermal performances are still limited especially for the ones with QDs-phosphor mixed nanocomposites. In this work, we propose a novel packaging scheme with horizontally layered QDs-phosphor nanocomposites to obtain an enhanced optical and thermal performance for WLEDs. Three different WLEDs, including QDs-phosphor mixed type, QDs-outside type, and QDs-inside type, were fabricated and compared. With 30 wt. % phosphor and 0.15 wt. % QDs nanocomposite, the QDs-outside type WLED shows a 21.8% increase of luminous efficiency, better color rendering ability, and a 27.0% decrease of the maximum nanocomposite temperature at 400 mA, compared with the mixed-type WLED. The reduced re-absorption between phosphor and QDs is responsible for the performance enhancement when they are separated. However, such reduced absorption can be traded off by the improper layered configuration, which is demonstrated by the worst performance of the QDs-inside type. Further, we demonstrate that the higher energy transfer efficiency between excitation light and nanocomposite in the QDs-outside type WLED is the key reason for its enhanced optical and thermal performance.

Novel lighting properties of white LEDs with two-layered remote phosphor package using red-emitting α-SrO·3B2O3:Sm2+ phosphor

Abstract This paper investigates a method for improving the lighting performance of white light-emitting diodes (WLEDs), packaged using two separating remote phosphor layers, yellow-emitting YAG:Ce phosphor layer and red-emitting α-SrO·3B2O3:Sm2+ phosphor layer. The thicknesses of these two layers are 800 μm and 200 μm, respectively. Both of them have been examined at average correlated color temperatures (CCT) of 7700 K and 8500 K. For this two-layer model, the concentration of red phosphor has been varied from 2 % to 30 % in the upper layer, while in the lower layer the yellow phosphor concentration was kept at 15 %. It was found interesting that the lighting properties, such as color rendering index (CRI) and luminous flux, are enhanced significantly, while the color uniformity is maintained at a level relatively close to the level in one-layer configuration (measured at the same correlated color temperature). Besides, the transmitted and reflected light of each phosphor layer have been revised by combining Kubelka-Munk and Mie-Lorenz theories. Through the analysis, it is demonstrated that the packaging configuration of two-layered remote phosphor that contains red-emitting α-SrO·3B2O3:Sm2+ phosphor particles provides a practical solution to general WLEDs lighting.

Perovskite CsPbBr1.2I1.8 quantum dot alloying for application in white light-emitting diodes with excellent color rendering index

Abstract Halide perovskites as novel optoelectronic materials have been considered to be good candidates as efficient luminescence converters for light-emitting diodes. In this work, the alloying CsPbBr 1.2 I 1.8 quantum dots (QDs) and perovskite CsPbI 3 QDs were synthesized, and their phase structure, morphology and luminescence toward humidity were investigated. Compared with CsPbI 3 QDs, the alloying CsPbBr 1.2 I 1.8 QDs displayed more stable cubic phase and better resistance to moisture. In addition, the enhanced phase-stability has been explained. Furtherly, the CsPbBr 1.2 I 1.8 QDs were used as red-emitting phosphors to improve the performance of white light-emitting diodes (WLED), and the as-fabricated WLED showed high luminous efficiency and excellent color rendering index (Ra), which indicated that the CsPbBr 1.2 I 1.8 QDs were promising red-emitting materials. Significantly, the values of Ra and correlated color temperature (CCT) were basically stable with the variation of operation current, implying the excellent color stability of CsPbBr 1.2 I 1.8 QDs-modified WLED.

Novel red-emitting phosphors A2HfF6:Mn4+ (A = Rb+, Cs+) for solid-state lighting.

New red-emitting phosphors A2HfF6:Mn4+ (A = Rb+, Cs+) with a single phase have been successfully synthesized via a simple ion exchange method, and their structures and luminescence properties were investigated. It was found that Mn4+ ions in Rb2HfF6 and Cs2HfF6 who share wide band gaps can possess broad excitation bands in the blue regions and intense red emission with internal quantum yields of 0.556 and 0.652, respectively. Meanwhile, these red phosphors exhibit high chemical and thermal stabilities. A series of LED devices with a tunable color rendering index and color temperature were fabricated with these samples which can remarkably optimize the optical performances of white light-emitting diodes (w-LEDs). These results indicate that A2HfF6:Mn4+ phosphors can be promising red phosphors in w-LEDs.