High-power White Light-emitting Diodes Research Articles

Site occupancy and photoluminescence tuning of La3Si6−xAlxN11−x/3:Ce3+phosphors for high power white light-emitting diodes

A series of blue light excitable yellow-emitting La3Si6−xAlxN11−x/3:Ce3+ (x = 0, 0.02, 0.04, 0.06, 0.08, and 0.1) phosphors were prepared by a high temperature solid state reactions method. The variations in micro-structure and luminescent properties induced by substituting the Al3+/Si4+ ratio in La3Si6−xAlxN11−x/3:Ce3+ and the related mechanism were investigated. Interestingly, the emission spectrum displayed a red-shift by strengthening the longer wavelength component of the spectrum instead of shifting the emission peak, which was verified to be ascribed to the changed site occupancy of Ce3+. The inducement of redistribution of Ce3+ by introduction of Al3+ as well as its influence on local structures were explored. A tunable spectrum could be obtained through controlling the entering rate of Ce3+ at two different sites. The color rendering index of this phosphor system could be greatly improved with sufficient emission efficiency and thermal stability for applications in high power white light-emitting diodes.

Lock-in Thermography for the Development of New Materials

Thermal management is one of the crucial concerns for mid- and high-power white light-emitting diodes (LEDs). Most of the currently available white LEDs on the market consist of a blue LED chip coated with a yellow phosphor-polymer composite. Besides the heat-induced degradation of the phosphor-polymer composite, the so-called thermal quenching, i.e. the decrease in light output upon increasing temperature, is also a pressing issue. Both result in an efficiency decrease and in a color change of the LED. In this work, a method to determine the thermal diffusivity and the thermal conductivity of phosphors is presented using lanthanide-doped glasses as test samples. In the quest for temperature-stable phosphors with a high thermal conductivity, such luminescent glasses might represent an attractive alternative. Thermal waves are introduced to the samples by periodical heating with a laser. Lock-in infrared thermography, a non-destructive method, is used to monitor even small changes in surface temperature. The phase delay of the thermal wave passing through the sample is analyzed at different lock-in frequencies and a mathematical fitting of the phase data then provides the coefficients required to determine the thermal diffusivity. Subsequently, the thermal conductivity is calculated from the thermal diffusivity, the mass density, and the specific heat capacity.

Application of Particle Filter to On-board life estimation of LED lights

Light-emitting diode (LED) based lights used in safety critical applications need continuous monitoring of their light output since lumen degradation is one of many failure modes. Airport ground lighting is one such application where LED lights are replacing traditional halogen lamp-based lights in the approach, runways, and taxiways. The question that still remains to be addressed is the prediction of the remaining useful life (RUL) of LED light any time during the service life cycle of the light. Life prediction methods based on reliability analysis and filtering algorithms proposed in the past were predominantly based on accelerated life tests and cannot be readily implemented for on-board diagnosis. Here, we present a model-based prognostic approach that uses particle filtering (PF) to predict the RUL for high power white LEDs that can be readily implemented on-board the lighting system. Lumen maintenance data at different test conditions are used in the PF method to calculate the $L_{70}$ , i.e., time at which lumen output reaches 70% of the initial value. A junction temperature model is developed by mapping the lumen degradation data to the junction temperature. $L_{70}$ calculated from PF is compared with a standard method recommended by the Illumination Engineering Society (IES). Finally, the RUL obtained using both the methods were analyzed.

Single Stage–Single Switch High Power Factor Long Lifetime AC-DC Driver for High-power White Light-emitting Diodes in Streetlight Applications

To achieve the main advantages of individual high-power white light-emitting diodes, light-emitting diode drivers should feature a high power factor, high efficiency, and a long lifetime. In streetlight applications, due to the inconvenience in reaching out to light-emitting diode fixtures for replacing out-of-order drivers, the durability (long lifetime) of drivers plays an important part in designing lighting products. In this article, a new integrated, single-stage single-switch AC-DC driver for high-power white light-emitting diodes featuring high efficiency, a high power factor, and a long lifetime for streetlight applications is proposed. The single power switch of the proposed driver operates in a non-floating manner without a need for any snubber circuits due to proper inherent operation, which makes the driver simple and reliable. To verify accurate operation of the driver as well as validate the analysis and design procedure discussed herein, a 35-watt light-emitting diode fixture is first simulated in PSIM software (PowerSIM Inc., USA), and then a laboratory prototype is implemented. Simulation and experimental results verify proper performance, high input power factor, and validity of the analysis and the design procedure adopted in the process.

Red-emitting CaLa4(SiO4)3O:Eu3+ phosphor with superior thermal stability and high quantum efficiency for warm w-LEDs

To accelerate the next generation of high power white light-emitting diodes, the development of efficient and stable red-emitting phosphors is urgent. In this regard, a series of CaLa4(SiO4)3O:Eu3+ phosphors with red-emitting band centered at 612 nm are prepared by a high temperature solid state reaction. X-ray diffraction patterns and Rietveld refinement confirm that all the samples are oxyapatite structure with space group of P63/m. Impressively, the quantum efficiency of the prepared red phosphor reaches as high as 97.5% under blue light excitation, and its emission intensity at 423 K remains 82.4% of that at room temperature, which are comparable to those of the commercial Y2O3:Eu3+ red phosphor. As a consequence, a warm white light-emitting diode with a luminous efficiency of 84.9 lm/W, a correlated color temperature of 4012 K and a color rendering index of 84.4 is realized by blending red-emitting CaLa4(SiO4)3O:Eu3+ phosphors and yellow-emitting Y3Al5O12:Ce3+ ones with a blue LED chip. It is expected that the explored CaLa4(SiO4)3O:Eu3+ red phosphor can be a good candidate for application in solid-state lighting.

Facile preparation of patterned phosphor-in-glass with excellent luminous properties through screen-printing for high-power white light-emitting diodes

In order to improve the luminous properties of high-power white light-emitting diodes (WLEDs), we proposed a facile preparation of patterned phosphor-in-glass (PiG) in which yellow Y3Al5O12:Ce3+ (YAG:Ce3+) and red CaAlSiN3:Eu2+ (CASN:Eu2+) phosphor parts are separated. The patterned PiGs with sector piece and concentric ring phosphor geometries were prepared by screen-printing and low-temperature sintering. Experimental results indicated that LED modules packaged by the patterned PiGs yield high luminous efficacy and excellent color quality owing to the reducing reabsorption of yellow emission by red phosphor. In addition, the angular color uniformity (ACU) of LED modules with the ring patterned PiGs are better than those of LED modules with the piece patterned PiGs. It is also discovered that the luminous efficacy and corresponding correlated color temperature (CCT) of LED module packaged by the piece/ring patterned PiG are decreased with the increasing of piece/ring number.

Facile Atmospheric Pressure Synthesis of High Thermal Stability and Narrow-Band Red-Emitting SrLiAl3N4:Eu(2+) Phosphor for High Color Rendering Index White Light-Emitting Diodes.

Red phosphors (e.g., SrLiAl3N4:Eu(2+)) with high thermal stability and narrow-band properties are urgently explored to meet the next-generation high-power white light-emitting diodes (LEDs). However, to date, synthesis of such phosphors remains an arduous task. Herein, we report, for the first time, a facile method to synthesize SrLiAl3N4:Eu(2+) through Sr3N2, Li3N, Al, and EuN under atmospheric pressure. The as-synthesized narrow-band red-emitting phosphor exhibits excellent thermal stability, including small chromaticity shift and low thermal quenching. Intriguingly, the title phosphor shows an anomalous increase in theoretical lumen equivalent with the increase of temperature as a result of blue shift and band broadening of the emission band, which is crucial for high-power white LEDs. Utilizing the title phosphor, commercial YAG:Ce(3+), and InGaN-based blue LED chip, a proof-of-concept warm white LEDs with a color rendering index (CRI) of 91.1 and R9 = 68 is achieved. Therefore, our results highlight that this method, which is based on atmospheric pressure synthesis, may open a new means to explore narrow-band-emitting nitride phosphor. In addition, the underlying requirements to design Eu(2+)-doped narrow-band-emitting phosphors were also summarized.

Facile fabrication and luminescence characteristics of Ce:YAG phosphor glass thick films coated on a glass substrate for white LEDs

A pivotal step in providing a better fluorescent material that has high luminous efficacy and excellent thermal stability is to utilize inexpensive phosphors for white light-emitting diodes (W-LEDs). Herein, we demonstrate a feasible tape-casting technique for creating phosphor thick films that consist of Ce: YAG phosphor embedded in relatively low melting point glass frits on an ultrathin glass substrate with controllable film thickness. The glass matrix has ideal densification and interfaces with the glass substrate at a relatively low temperature of 580°C. Subsequently, the structure and optical properties of the phosphor layer are investigated. In addition, the effect of the phosphor concentration, thick film thickness and location (top or bottom) of the phosphor layer on the photoluminescence properties and chromaticity are also discussed with respect to use in W-LEDs. Significantly, this promising structure has excellent thermal stability and the potential to overcome current limitations of phosphors in high-power W-LEDs. Finally, a high-performance W-LED based on the planar phosphor glass exhibits a luminous efficiency of 108.45lmW−1, a correlated color temperature of 5408K and a color rendering index of 76.

Prognostics-based qualification of high-power white LEDs using Lévy process approach

Due to their versatility in a variety of applications and the growing market demand, high-power white light-emitting diodes (LEDs) have attracted considerable attention. Reliability qualification testing is an essential part of the product development process to ensure the reliability of a new LED product before its release. However, the widely used IES-TM-21 method does not provide comprehensive reliability information. For more accurate and effective qualification, this paper presents a novel method based on prognostics techniques. Prognostics is an engineering technology predicting the future reliability or determining the remaining useful lifetime (RUL) of a product by assessing the extent of deviation or degradation from its expected normal operating conditions. A Lévy subordinator of a mixed Gamma and compound Poisson process is used to describe the actual degradation process of LEDs characterized by random sporadic small jumps of degradation degree, and the reliability function is derived for qualification with different distribution forms of jump sizes. The IES LM-80 test results reported by different LED vendors are used to develop and validate the qualification methodology. This study will be helpful for LED manufacturers to reduce the total test time and cost required to qualify the reliability of an LED product.

Preparation and luminescent properties of the coating of phosphor in lead-free glass by multilayer screen-printing

The coating of phosphor in glass (PiG) for white light emitting diodes (WLEDs) has been prepared by multilayer screen-printing and low sintering-temperature. The lead-free and low-melting glass was synthesized, and the main compositions were P2O5-ZnO-B2O3 (PZB). The microstructure, luminescence spectra and lifetime behaviours of coating were investigated. The results of microstructure revealed that decreasing content of phosphor and increasing sintering temperatures were benefit for the coating because of better sintering. Based on the luminescence spectra, the suitable content of phosphor was 25% and sintering temperature was 600 °C. Under blue light (460 nm) exciting, the broad yellow band around 550 nm was obtained. It’s indicated that the white light can be achieved. The coating showed longer lifetime with increasing content of phosphor and lower sintering temperature. The obtained coating of PiG is expected to be an excellent material instead of organic resin for high-power WLEDs packaging.

Stable and chromaticity-tunable phosphor-in-glass inorganic color converter for high-power warm white light-emitting diode

White light-emitting diodes (w-LEDs) have captured much attention due to their superior properties of high luminous efficiency, low energy consumption and long lifetime. However, current white lighting devices suffer from several drawbacks such as the deficiency of red component and the poor thermal stability of silicone. In this work, a chromaticity-tunable phosphor-in-glass (PiG) inorganic color converter has been successfully fabricated for high-power warm w-LEDs. At first, a series of La2(1−x)Eu2xTi2O7 (x=0.05–0.5) red phosphors with excellent thermal stability and high quantum efficiency have been synthesized. Importantly, the as-synthesized La2Ti2O7:Eu3+ and commercial YAG:Ce3+ phosphors were incorporated into a specially low-melting precursor glass to form PiG composite. Finally, the high-power warm w-LEDs were constructed by coupling a PiG plate on an InGaN blue chip, and the w-LED showed an optimal luminous efficacy of 105lm/W, a CCT of 4809K and a CRI of 83.5.

Applying grey model to predict the useful lifetime for high-power white LEDs

An accelerated degradation test has been widely used to analyze the useful lifetime of high-power white light-emitting diodes (HPWLEDs). A two-stage method is proposed to predict the useful lifetime of HPWLEDs. In the first stage, an exponential model first fits the degradation data. In the second stage, a response model based on an inverse power (exponential) law under different stresses is derived for predicting the useful lifetime under normal operating conditions. To improve the fitting accuracy for the degradation data, we consider the grey model. The useful lifetimes based on the exponential and grey models using the two-stage method are obtained as 192,317 and 184,538 h, respectively.

Effective Heat Dissipation from Color-Converting Plates in High-Power White Light Emitting Diodes by Transparent Graphene Wrapping

We have developed a hybrid phosphor-in-glass plate (PGP) for application in a remote phosphor configuration of high-power white light emitting diodes (WLEDs), in which single-layer graphene was used to modulate the thermal characteristics of the PGP. The degradation of luminescence in the PGP following an increase in temperature could be prevented by applying single-layer graphene. First, it was observed that the emission intensity of the PGP was enhanced by about 20% with graphene wrapping. Notably, the surface temperature of the graphene-wrapped PGP (G-PGP) was found to be higher than that of the bare PGP, implying that the graphene layer effectively acted as a heat dissipation medium on the PGP surface to reduce the thermal quenching of the constituent phosphors. Moreover, these experimental observations were clearly verified through a two-dimensional cellular automata simulation technique and the underlying mechanisms were analyzed. As a result, the proposed G-PGP was found to be efficient in maintaining the luminescence properties of the WLED, and is a promising development in high power WLED applications. This research could be further extended to generate a new class of optical or optoelectronic materials with possible uses in a variety of applications.

Optical and thermal properties of phosphors based on lead-silicate glass for high-power white LEDs

A study is reported of the properties of a new phosphor material based on a highly refractive leadsilicate glass and microparticles of yttrium-aluminum garnet doped with cerium ions (YAG: Ce3+). The mass percentage fraction of YAG: Ce3+ microparticles in the material was varied from 50 to 90%. The optical properties of the phosphor were examined, as well as its thermal properties when used as a primary optical material in high-power light-emitting diode (LED) units. The results obtained reveal problems of excess heat removal from an LED structure, which appear on passing from phosphor binders based on silicone elastomers to glasses, and demonstrate that the phosphor developed in the study is promising for obtaining warm white light.

Analysis and Modeling of High-Power Phosphor-Coated White Light-Emitting Diodes With a Large Surface Area

Modern high-power white light-emitting diodes (LEDs) composed of multiple blue LED chips and yellow phosphor coatings have been successfully commercialized because of their high-luminous efficacy. The multiple-chip LED packages usually come with flat structures that have large surface areas for considerable heat loss. Starting from the analysis and modeling of the blue LED chip, this paper introduces the thermal path through the phosphor layer to form the white phosphor-coated (PC) white LED device model for photometric, electric, and thermal performance analysis. The power distribution of the blue LED chip and that of the PC white LED device are compared. Based on this new analysis, the increase in the heat dissipation coefficient, equivalent thermal resistance, and power loss caused by the phosphor coating can be quantified. New equations suitable for device manufacturers to qualify their devices and design engineers to optimize LED system designs are derived. The analytical results are in good agreement with the practical mesurements.

Optical design of a light-emitting diode lamp for a maritime lighthouse

Traffic signaling is an emerging field for light-emitting diode (LED) applications. This sustainable power-saving illumination technology can be used in maritime signaling thanks to the recently updated norms, where the possibility to utilize LED sources is explicitly cited, and to the availability of high-power white LEDs that, combined with suitable lenses, permit us to obtain well-collimated beams. This paper describes the optical design of a LED-based lamp that can replace a traditional lamp in an authentic marine lighthouse. This source recombines multiple separated LEDs realizing a quasi-punctual localized source. Advantages can be lower energy consumption, higher efficiency, longer life, fewer faults, slower aging, and minor maintenance costs. The proposed LED source allows us to keep and to utilize the old Fresnel lenses of the lighthouse, which very often have historical value.

Hybrid 2D photonic crystal-assisted Lu3Al5O12:Ce ceramic-plate phosphor and free-standing red film phosphor for white LEDs with high color-rendering index.

This paper reports the combined optical effects of a two-dimensional (2D) SiNx photonic crystal layer (PCL)-assisted Lu3Al5O12:Ce (LuAG:Ce) green ceramic-plate phosphor (CPP) and a free-standing (Sr,Ca)AlSiN3:Eu red film phosphor to enhance luminous efficacy, color rendering index (CRI), and special CRI (R9) of LuAG:Ce CPP-capped white light-emitting diodes (LEDs) for high-power white LEDs at 350 mA. By introducing the 2D SiNx PCL, the luminous efficacy was improved by a factor of 1.25 and 1.15 compared to that of the conventional flat CPP-capped LED and the thickness-increased CPP-capped LED (with a thickness of 0.15 mm), respectively, while maintaining low color-rendering properties. The combining of the free-standing red film phosphor in the flat CPP-capped, the 2D PCL-assisted CPP-capped, and the thickness-increased CPP-capped LEDs led to enhancement of the CRI and the special CRI (R9); it also led to a decrease of the correlated color temperature (CCT) due to broad wavelength coverage via the addition of red emission. High CRI (94), natural white CCT (4450 K), and acceptable luminous efficacy (71.1 lm/W) were attained from the 2D PCL-assisted LuAG:Ce CPP/free-standing red film phosphor-based LED using a red phosphor concentration of 7.5 wt %. It is expected that the combination of the 2D PCL and the free-standing red film phosphor will be a good candidate for achieving a high-power white CPP-capped LED with excellent CRI.

Advances in transparent glass–ceramic phosphors for white light-emitting diodes—A review

Abstract Currently, the major commercial white light-emitting diode is the phosphor converted LED made of blue-emitting chip and Y 3 Al 5 O 12 :Ce 3+ yellow phosphor dispersed in organic silicone. However, the organic binder in high-power device ages easily and turns yellow due to accumulated heat emitted from chip, which adversely affects the device properties such as luminous efficacy and color coordination, and therefore reduces its long-term reliability as well as lifetime. In this mini-review article, we provide an overview of recent progresses in developing transparent inorganic glass–ceramics phosphors excitable by blue chip, as an alternative to conventional polymer-based phosphor converter, for construction of high-power white light-emitting diodes. Two kinds of synthesis routes, glass crystallization and low-temperature co-sintering, are discussed in detail. Afterwards, the materials design, structure/property optimization as well as glass–ceramic-based WLED devices construction are summarized. Finally, challenges and future advances for the realization of transparent glass–ceramics in commercial applications will be presented.

Various nanofabrication approaches towards two-dimensional photonic crystals for ceramic plate phosphor-capped white light-emitting diodes

This paper compares four different nanofabrication processes towards various two-dimensional (2D) photonic crystals (PCs) on a Y3Al5O12:Ce3+ (YAG:Ce) yellow ceramic plate phosphor (CPP) in order to enhance the luminous efficacy of YAG:Ce CPP-capped high-power white light-emitting diodes (LEDs). Compared with conventional flat CPP-capped LEDs, every type of 2D PC-assisted YAG CPP-capped LED improves the outcoupling efficiency, current and temperature stability, angular stability of color temperature, and thermal conductivity. Careful evaluation of the optical properties of 2D PC-assisted CPP LEDs shows that the highest luminous efficacy (∼142.7 lm W−1) is attained from a 2D TiO2 nanobowl PC-CPP LED sample, which was fabricated by nanosphere-based template growth and the dry etching process, and this evaluation indicates that 2D PC-assisted CPP-capped white LEDs will lead to further technological advancements for applications in highly efficient and thermally stable high-power white LEDs.

Useful lifetime analysis for high-power white LEDs

An accelerated degradation test is used to analyze the useful lifetime of high-power white light-emitting diodes (HPWLEDs) as the point at which the light output declines to 70% of the initial flux in lumens, called L70 In this study, the degradation-data-driven method (DDDM), including the approximation method, the analytical method, and the two-staged method, is used to analyze the useful lifetime of HPWLEDs. A response model based on an inverse power (exponential) law under different stresses is used to predict the useful lifetime under operating conditions. However, the degradation model for each HPWLED is usually fitted to an exponential function. In order to improve the fit accuracy, we present a bi-exponential model for the degradation curve of HPWLEDs. The estimation of the model parameters are easily obtained by using the nonlinear least square method. Through numerical examples, the results show that the bi-exponential model performs better than the exponential model based on the two-staged method. The extrapolation algorithm for L70 should be fitted to a bi-exponential extrapolation model and two-staged method.