Colour tunable photoluminescence studies of Dy3+/Eu3+ co-doped BaPbAlFB glasses for epoxy-free w-LED applications

Barium lead alumino fluoroborate (BaPbAlFB) glasses singly and co-doped with Dy3+ and Eu3+ ions were prepared by melt quench method and studied their photoluminescence (PL) properties along with energy migration. Singly doped Dy3+/Eu3+ ions in BaPbAlFB glasses under 365 and 393 nm excitation wavelengths emit the PL bands through Dy: (4F9/2 → 6H15/2 and 4F9/2 → 6H13/2) and Eu: (5D0→7F2) transitions located at (Dy: 483 and 575 nm) and (Eu: 613 nm) respectively. Further, tunability of PL emissions is achieved by exciting the Dy3+/Eu3+ co-doped BaPbAlFB glasses at various wavelengths. The obtained CIE (0.34, 0.33) and CCT value (5080.4 K) of Dy3+/Eu3+ co-doped BaPbAlFB glass produced white light emission under 365 nm excitation wavelength. The corresponding CCT values of Dy3+/Eu3+ co-doped BaPbAlFB glasses varied from 1580 to 5080 K and their corresponding color emission changes from intense red to white light regime quite suitable for fabrication of epoxy-free w-LED devices.

Passive anti-leakage of blue light for phosphor-converted white LEDs with crystal nanocellulose materials

A phenomenon known as "blue-light leakage" caused by overheating pcW-LEDs has recently been identified, and it poses a risk to users. This study focuses on investigating and optimizing a solution to address this issue. To tackle the problem of overheating and blue light leakage, we explored the application of a specific thermochromic material called crystal nano cellulose (CNC). We introduced CNC inside the epoxy lens of white LEDs. Importantly, under standard conditions, CNC has a negligible impact on the optical properties of the output white light. However, when overheating conditions arise, leading to blue light leakage, the temperature increase triggers a darkening effect in CNC. This thermochromic behavior of CNC allows it to strongly absorb the blue light, resulting in a significant suppression of the output luminous flux. As a result, the lamp dims, which not only prevents the user's eyes from being exposed to harmful bluish light but also serves as an indicator of aging in the pcW-LED. By implementing CNC as a responsive material in the design of white LEDs, this study offers a practical and effective solution to mitigate the negative effects of blue-light leakage caused by overheating. This improvement enhances the safety and comfort of users while also providing an early warning system for the aging of pcW-LEDs.

An Efficient Method to Determine the Thermal Behavior of Composite Material with Loading High Thermal Conductivity Fillers

Improvement of the thermal conductivity of encapsulant material using doping filler is an important requirement for electronic device packaging. We proposed a simple method for determining the thermal characteristics of composite material that can help save time, increase research performance, and reduce the cost of buying testing equipment. Based on the theory of Fourier law, a general 3D model is simplified into a 2D model, which can then be applied to calculate the thermal conductivity of the tested sample. The temperature distribution inside the sample is simulated by the finite element method using MATLAB software; this is a simple and useful option for researchers who conduct studies on thermal conduction. In addition, an experimental setup is proposed to help determine the extent of thermal conductivity improvement in a sample with doping filler compared to a bare sample. This method is helpful for research on optoelectronics packaging, which relates to the enhancement of thermal conductivity composite material.

Active thermal-fuse for stopping blue light leakage of white light-emitting diodes driven by constant current

In this study, we proposed and demonstrated a circuit design for solving problems related to blue light leakage (e.g., eye damage) when phosphor-converted white light-emitting diodes (pcW-LEDs) overheat. This circuit only needs a positive thermal coefficient thermistor, resistor, and diodes in series and parallel; thus, it can easily be integrated into components. Simulations and corresponding experimental results show that this method can accurately suppress the overheating component’s injection current and allow for LEDs to work normally after returning to the operating temperature. It thus allows the user's eyes to be actively protected, e.g., to avoid exposure to the bluish light when overheating occurs. In addition, the quenching of luminous flux is a signal to remind the user to replace the LED. The proposed method is low-cost, effective, simple, and useful for increasing the quality of LED lighting and biological safety.

Methods of Fast Analysis of DC–DC Converters—A Review

The paper discusses the methods of fast analysis of DC–DC converters dedicated to computer programmes. Literature methods of such an analysis are presented, which enable determination of the characteristics of the considered converters in the steady state and in the transient states. The simplifications adopted at the stage of developing these methods are discussed, and their influence on the accuracy of computations is indicated. Particular attention is paid to the methods of fast analysis of DC–DC converters, taking into account thermal phenomena in semiconductor devices. The sample results of computations of the DC–DC boost type converter obtained with the use of the selected methods are presented. The scope of application of particular computation methods and their duration times are discussed. Computations were performed with the use of SPICE and PLECS.

Perovskite White Light Emitting Diodes: Progress, Challenges, and Opportunities

As global warming, energy shortages, and environment pollution have intensified, low-carbon and energy-saving lighting technology has attracted great attention worldwide. Light emitting diodes (LEDs) have been around for decades and are considered to be the most ideal lighting technology currently due to their high luminescence efficiency (LE) and long lifespan. Besides, along with the development of modern technology, lighting technologies with higher performance and more functions are desired. Perovskite based LEDs (PeLEDs) have recently emerged as ideal candidates for lighting technology owing to the extraordinary photoelectric properties of perovskite, such as high photoluminescence quantum yields (PLQYs), easy wavelength tuning, and low-cost synthesis. Herein, we open this review by introducing the background of white LEDs (WLEDs), including their light-emitting mechanism, typical characteristics, and key indicators in applications. Then, four main approaches to fabricate WLEDs are discussed and compared. After that, in accordance with the four categories, we focus on the recent progress of white PeLEDs (Pe-WLEDs), followed by the challenges and opportunities for Pe-WLEDs in practical application. Meanwhile, some pertinent countermeasures to their challenges are put forward. Finally, the development promise of Pe-WLEDs is explored.

Compact Modelling of Electrical, Optical and Thermal Properties of Multi-Colour Power LEDs Operating on a Common PCB

This paper concerns the problem of modelling electrical, thermal and optical properties of multi-colour power light-emitting diodes (LEDs) situated on a common PCB (Printed Circuit Board). A new form of electro-thermo-optical model of such power LEDs is proposed in the form of a subcircuit for SPICE (Simulation Program with Integrated Circuits Emphasis). With the use of this model, the currents and voltages of the considered devices, their junction temperature and selected radiometric parameters can be calculated, taking into account self-heating phenomena in each LED and mutual thermal couplings between each pair of the considered devices. The form of the formulated model is described, and a manner of parameter estimation is also proposed. The correctness and usefulness of the proposed model are verified experimentally for six power LEDs emitting light of different colours and mounted on an experimental PCB prepared by the producer of the investigated devices. Verification was performed for the investigated diodes operating alone and together. Good agreement between the results of measurements and computations was obtained. It was also proved that the main thermal and optical parameters of the investigated LEDs depend on a dominant wavelength of the emitted light.

Influence of Selected Factors on Thermal Parameters of the Components of Forced Cooling Systems of Electronic Devices

The paper presents some investigation results on the properties of forced cooling systems dedicated to electronic devices. Different structures of such systems, including Peltier modules, heat sinks, fans, and thermal interfaces, are considered. Compact thermal models of such systems are formulated. These models take into account a multipath heat transfer and make it possible to compute waveforms of the device’s internal temperature at selected values of the power dissipated in the device. The analytical formulas describing the dependences of the thermal resistance of electronic devices co-operating with the considered cooling systems on the power dissipated in the cooled electronic device and the power feeding the Peltier module and the speed of airflow caused by a fan are proposed. The correctness of the proposed models is verified experimentally in a wide range of powers dissipated in electronic devices operating in different configurations of the used cooling system.

Factors influencing the working temperature of quantum dot light-emitting diodes

Quantum dot light-emitting diodes (QLEDs) possess huge potential in display due to their outstanding optoelectronic performance; however, serve degradation during operation blocks their practical applications. High temperature is regarded as one of major factors causing degradation. Therefore, a systematical study on the working temperature of QLEDs is very essential and urgent for the development of high stable QLEDs. In this work, different influence factors such as the electro-optic conversion efficiency (EOCE), voltage, current density, active area, substrate size, substrate type and sample contact are discussed in detail on the working temperature of QLEDs. The research results show that the working temperature of general QLEDs under normal operation conditions is usually smaller than 75 °C when the ambient temperature is 25 °C. However, temperature of QLEDs working under extreme conditions, such as high power or small substrate size, will exceed 100 °C, resulting in irreversible damage to the devices. Moreover, some effective measures to reduce the working temperature are also proposed. The analysis and discussion of various influencing factors in this work will provide guidance for the design of stable QLEDs and help them work at a safer temperature.

Thermal, Photometric and Radiometric Properties of Multi-Color LEDs Situated on the Common PCB

This paper presents the results of experimental investigations illustrating the influence of the spectra of the light emitted by power LEDs on their thermal, photometric and radiometric parameters. The investigations were performed for six diodes emitting white or monochromatic light of different spectra. Each of these diodes was produced by the same manufacturer, mounted in the same package and the tested devices were soldered to the common PCB. In the paper, the manner and set-ups making possible measurements of self and transfer transient thermal impedances, illuminance and the surface power density of the light emitted by the tested devices are described. Selected results of measurements are shown and discussed. These results prove that the spectra of the emitted light influence self-transient thermal impedances of the considered devices and transfer transient thermal impedances between some pairs of these devices. Additionally, it is proved that thermal couplings between the tested diodes strongly influence their junction temperature and the surface power density of the emitted radiation.

Influence of a Thermal Pad on Selected Parameters of Power LEDs

This paper is devoted to the analysis of the influence of thermal pads on electric, optical, and thermal parameters of power LEDs. Measurements of parameters, such as thermal resistance, optical efficiency, and optical power, were performed for selected types of power LEDs operating with a thermal pad and without it at different values of the diode forward current and temperature of the cold plate. First, the measurement set-up used in the paper is described in detail. Then, the measurement results obtained for both considered manners of power LED assembly are compared. Some characteristics that illustrate the influence of forward current and temperature of the cold plate on electric, thermal, and optical properties of the tested devices are presented and discussed. It is shown that the use of the thermal pad makes it possible to achieve more advantageous values of operating parameters of the considered semiconductor devices at lower values of their junction temperature, which guarantees an increase in their lifetime.

Compact Thermal Modeling of Modules Containing Multiple Power LEDs

Temperature is an essential factor affecting the operation of light-emitting diodes (LEDs), which are often used in circuits containing multiple devices influencing each other. Therefore, the thermal models of such circuits should take into account not only the self-heating effects, but also the mutual thermal influences among devices. This problem is illustrated here based on the example of a module containing six LEDs forming on the substrate a hexagon. This module is supposed to operate without any heat sink in the natural convection cooling conditions, hence it has been proposed to increase the thermal pad area in order to lower the device-operating temperature. In the experimental part of the paper, the recorded diode-heating curves are processed using the network identification by deconvolution method. This allows for the computation of the thermal time constant spectra and the generation of device-compact thermal models. Moreover, the influence of the thermal pad surface area on the device temperature and the thermal coupling between LEDs is investigated.

Electrothermal Averaged Model of a Diode-Transistor Switch Including IGBT and a Rapid Switching Diode

This study proposes an electrothermal averaged model of the diode–transistor switch including insulated gate bipolar transistor (IGBT) and a rapid switching diode. The presented model has the form of subcircuits dedicated for simulation program with integrated circuit emphasis (SPICE) and it makes it possible to compute characteristics of DC–DC converters at the steady state considering self-heating phenomena, both in the diode and in IGBT. This kind of model allows computations of voltages, currents and internal temperatures of all used semiconductor devices at the steady state. The formulas used in this model are adequate for both: continuous conducting mode (CCM) and discontinuous conducting mode (DCM). Correctness of the proposed model is verified experimentally for a boost converter including IGBT. Good accuracy in modeling these converter characteristics is obtained.

Compact Thermal Model of the Pulse Transformer Taking into Account Nonlinearity of Heat Transfer

This paper presents a compact nonlinear thermal model of pulse transformers. The proposed model takes into account differentiation in values of the temperatures of a ferromagnetic core and each winding. The model is formulated in the form of an electric network realising electrothermal analogy. It consists of current sources representing power dissipated in the core and in each of the windings, capacitors representing thermal capacitances and controlled current sources modelling the influence of dissipated power on the thermal resistances in the proposed model. Both self-heating phenomena in each component of the transformer and mutual thermal couplings between each pair of these components are taken into account. A description of the elaborated model is presented, and the process to estimate the model parameters is proposed. The proposed model was verified experimentally for different transformers. Good agreement between the calculated and measured waveforms of each component temperature of the tested pulse transformers was obtained. Differences between the results of measurements and calculations did not exceed 9% for transformers with a toroidal core and 13% for planar transformers.

Enhancing light conversion efficiency of YAG:Ce phosphor substrate using nanoimprinted functional structures

A phosphor substrate converts a moderate amount of blue light to green light to produce white light. In this study, we have successfully demonstrated the enhancement of the light extraction efficiency of YAG:Ce phosphor substrate using a simple imprint process. Spin-on-glass materials were used to fabricate a pattern on the surface of a phosphor substrate, and nano- and micro-scale patterns were formed to test the performance according to the size of pattern. The light extraction efficiency of the phosphor substrate with a micro-cone pattern increased by 33.2% compared with the flat phosphor substrate.

Development of high luminous efficacy red-emitting phosphor-in-glass for high-power LED lighting systems using our original low Tg and Ts glass

Red-emitting phosphor-in-glass (PiG) materials with a high luminescence efficiency were developed by co-firing commercial red-emitting (Sr,Ca)AlSiN₃:Eu²⁺ (SCASN) phosphor and our original glass frits with a composition of Li₂O-Na₂O-ZnO-Al₂O₃-B₂O₃-P₂O₅, which has a low Tg (271°C) and Ts (380°C) temperatures. By optimizing the sintering temperature and the content of the phosphor, the highest luminous efficacy was obtained for the 3 wt.% SCASN-based PiG sintered at 400°C, which showed a luminous efficacy of 25 lm/W rad by combining the PiG materials with a blue light-emitting diode (LED) chip-on-board (COB, incident power = 100 mA). The internal quantum efficiency of this sample under excitation at 450 nm was 53%. The PiG-based LED maintained a high luminous efficacy when the incident power of the blue COB increased up to 5.5 W (47 lm).

Implementation of a Dimmable LED Driver with Extendable Parallel Structure and Capacitive Current Sharing

A dimmable LED driver along with an extendable series structure and interleaved capacitive current sharing is presented herein, the LED connection of which is changed from the traditional series structure to the proposed parallel structure. The number of LED strings can be extended. As the number of LED strings is increased, the output voltage of this LED driver and the voltage stress on the main switch are ideally not influenced. Moreover, only one current sensor is needed to achieve current control and dimming. In this paper, the basic operating principle of the proposed LED driver is described and analyzed. Finally, the effectiveness of this LED driver is demonstrated by experiment based on the field-programmable gate array (FPGA).

Circuit Topology Analysis for LED Lighting and its Formulation Development

Light emitted diode (LED) is becoming more popular in the illumination field, and the design of LED lighting is generally made to provide illumination at lower power usage, helping save energy. A power electronic converter is needed to provide the power conversion for these LEDs to meet high efficiency, reduce components, and have low voltage ripple magnitude. The power supply for LED is revisited in this paper. The LEDs connected in series with diode, transistor, or inductor paths are examined. The formulation for each of the cases is described, including the classical converters of buck, boost, buck–boost, and Ćuk. The circuit reductions of the classic circuit, circuit without the capacitor, and without a freewheeling diode are studied. Using LED to replace freewheeling diodes is proposed for circuit component reduction. General equations for different connection paths have been developed. The efficiency and output ripple amplitude of the proposed power converters are investigated. Analytical study shows that the efficiency of proposed circuits can be high and voltage ripple magnitude of proposed circuits can be low. The results show that the proposed circuit topologies can be easily adapted to design LED lighting, which can meet the criteria of high efficiency, minimum components, and low-voltage ripple magnitude at the same time.

Enhancement of Heat Dissipation in Ultraviolet Light-Emitting Diodes by a Vertically Oriented Graphene Nanowall Buffer Layer

For III-nitride-based devices, such as high-brightness light-emitting diodes (LEDs), the poor heat dissipation of the sapphire substrate is deleterious to the energy efficiency and restricts many of their applications. Herein, the role of vertically oriented graphene (VG) nanowalls as a buffer layer for improving the heat dissipation in AlN films on sapphire substrates is studied. It is found that VG nanowalls can effectively enhance the heat dissipation between an AlN film and a sapphire substrate in the longitudinal direction because of their unique vertical structure and good thermal conductivity. Thus, an LED fabricated on a VG-sapphire substrate shows a 37% improved light output power under a high injection current (350 mA) with an effective 3.8% temperature reduction. Moreover, the introduction of VG nanowalls does not degrade the quality of the AlN film, but instead promotes AlN nucleation and significantly reduces the epilayer strain that is generated during the cooling process. These findings suggest that the VG nanowalls can be a good buffer layer candidate in III-nitride semiconductor devices, especially for improving the heat dissipation in high-brightness LEDs.

Optimization of a Series Converter for Low-Frequency Ripple Cancellation of an LED Driver

In this paper, the optimization of the power and control stages of a previously proposed topology for an off-line LED electronic driver is presented. The full system avoids the use of electrolytic capacitors at the D C link, therefore increasing the lifespan and reliability of the driver. As a consequence of having a relatively small capacitance, the D C link operates with a large Low-Frequency ( L F ) voltage ripple. This work presents a design optimization for the power and control stages of a current-fed bidirectional buck converter, operating as the LED current control stage. As this block processes only the A C power arising from the L F voltage ripple, it can increase the system efficiency against the typical two-stage solution. In the original proposal, the main drawback was the high inductor losses due to the resulting large inductor currents and large inductance value. The proposed optimization ensures an enhanced design of the inductor while keeping a constant current through the LEDs. A new optimization methodology is proposed and the theoretical results have been validated in a built prototype for a 40 W LED lamp.

Two-dimensional temperature distribution measurement of light-emitting diodes by micro-hyperspectral imaging-based reflected light method

We propose a non-contact measurement method for determining two-dimensional (2D) temperature distribution of light-emitting diodes (LEDs). This method is based on both micro-hyperspectral imaging technology and reflected light method, owning merits of both high efficiency and high spatial resolution. Blue and green bare LEDs are used as LED under test, while red and near-infrared LEDs provide incident light to avoid spectral overlapping so as to reduce measurement error. During data processing, the convolution linear filtering algorithm is employed to improve the measurement accuracy. This proposed method is compared with the micro-thermocouple and infrared thermal imaging, with their respective comparison results in fairly good agreements. For spatial resolution of 2D temperature distribution, this method increases at least one order of magnitude compared with the thermal imaging method.

On the development of an effective image acquisition system for diamond quality grading

In this paper, an effective diamond image acquisition system is proposed for capturing images of the diamond sample under inspection separately with transmitted and reflected light. The proposed system consists of two parts: a lighting system and an imaging system. The lighting system is a newly designed integrating sphere that can control the light distribution in the image plane and provide uniform illumination to the diamond sample while it is being inspected under white LED lamps with a specific spectral power distribution. The imaging system comprises industrial-grade lenses, cameras, and a sample loading plate. One set of lenses and a camera is installed at the port of the lower hemisphere. This arrangement ensures that all light rays illuminated to the image plane are transmitted light. On the other hand, all light captured by the other set of lenses and a camera installed on the upper hemisphere is reflected light. The proposed design utilizes the ray tracing processing method based on the physical properties of materials and adjusts the parameters of each optical component. When properly designed, the signal-to-noise ratio of captured diamond images is significantly improved, so that imperfections inside the diamond or on its surface can be clearly distinguished. The proposed image acquisition system is cost-effective and compact. The experimental results agree well with the theoretical analysis and verify the effectiveness of the proposed image acquisition system. The proposed system will facilitate, improve, and simplify automated diamond quality grading.

Optical Signatures of Transiently Disordered Semiconductor Nanocrystals

The optoelectronic properties of semiconductor nanocrystals (NCs) have led to efforts to integrate them as the active material in light-emitting diodes, solid-state lighting, and lasers. Understanding related high carrier injection conditions is therefore critical as resultant thermal effects can impact optical properties. The physical integrity of NCs is indeed questionable as recent transient X-ray diffraction studies have suggested that nanoscopic particles reversibly lose crystalline order, or melt, under high fluence photoexcitation. Informed by such studies, here, we examine CdSe NCs under elevated fluences to determine the impact of lattice disordering on optical properties. To this end, we implement intensity-dependent transient absorption using both one- and two-pump methods where the latter effectively subtracts out the NC optical signatures associated with lower fluence photoexcitation, especially band-edge features. At elevated fluences, we observe a long-lived induced absorption at a lower energy than the crystalline-NC bandgap across a wide range of sizes that follows power-dependent trends and kinetics consistent with the prior transient X-ray measurements. NC photoluminescence studies provide further evidence that melting influences optical properties. These methods of characterizing bandgap narrowing caused by lattice disordering could facilitate routes to improved optical amplification and band-edge emission at high excitation density.

Low-Cost Nanostructures from Nanoparticle-Assisted Large-Scale Lithography Significantly Enhance Thermal Energy Transport across Solid Interfaces

Enhancing thermal energy transport across solid interfaces is of critical importance to a wide variety of applications ranging from energy systems and lighting devices to electronics. Nanoscale surface roughness is usually considered detrimental to interfacial thermal transport because of its role in phonon scattering. In this study, however, we demonstrate significant thermal conductance enhancements across metal-semiconductor interfaces by as much as 90% higher than that of the planar interfaces using engineered nanostructures fabricated by Au nanoparticle (NP)-assisted lithography, where self-assembled Au NPs are used as an efficient etching mask to pattern solid substrates over large surface areas. The enlarged interfacial contact area due to the presence of nanostructures is the main reason for the significantly enhanced thermal transport. It is further demonstrated that the conductance can be systematically tuned over a wide range through the use of the Au NP self-assembly process that is regulated by a sacrificial Sb layer whose thickness determines the size and density of the nanostructures produced. This strategy is tested on two technologically important semiconductors, Si and GaN, and their interfacial thermal conductance with Al being measured using the time-domain thermoreflectance technique. Moreover, the nanostructured interfaces can maintain the enhanced conductance for a temperature range of 30-110 °C-the operating temperatures commonly experienced by energy, lighting, and electronic devices. Our results could provide a wafer-scale and low-cost strategy for improving the thermal management of these devices.

Al₂O₃-Cu Substrate with Co-Continuous Phases Made by Powder Sintering Process

Ceramic-Al substrates with co-continuous ceramic and metal phases, which exhibit high thermal conductivity and compatible coefficient of thermal expansion (CTE), have been widely investigated through the process of die-casting. In this research, a kind of powder sintering process was proposed for fabricating ceramic-Cu composite substrates with co-continuous phases. Copper fiber (Cu_f) has excellent thermal conductivity and large aspect ratio, making it an ideal material to form bridging network structures in the ceramic-Cu composite. To maintain the large aspect ratio of Cu_f, and densify the composite substrate, ZnO-SiO₂-CaO glass was introduced as a sintering additive. Both Al₂O₃/glass/Cu_f and Al₂O₃/30glass/Cu_p composite substrates were hot-pressed at 850 °C under 25 MPa. Experimental results showed that the thermal conductivity of Al₂O₃/30glass/30Cu_f composite substrate was as high as 38.9 W/mK, which was about 6 times that of Al₂O₃/30glass; in contrast, the thermal conductivity of Al₂O₃/30glass/30Cu_p composite substrate was only 25.9 W/mK. Microstructure observation showed that, influenced by hot press and corrosion of molten ZnO-SiO₂-CaO glass, the copper fibers were deformed under hot-pressing, and some local melting-like phenomena occurred on the surface of copper fiber at 850 °C under 25 MPa. The molten phase originating from surface of Cu_f welded the overlapping node of copper fibers during cooling process. Finally, the interconnecting metal bridging in ceramic matrix was formed and behaved as a rapid heat-dissipating channel, which is similar to substrates prepared through die-casting process by porous ceramic and melted Al.

Experimental Study on Horizontal Cylinders with Triangular Fins under Natural Convection

In this study, thermal resistances of horizontal cylinders with triangular fins were measured in regard to fin numbers, fins heights, and temperature differences. Thereafter, an empirical correlation was proposed and validated for predicting the Nusselt numbers under the following conditions: Rayleigh number, 200,000–1,000,000; fin aspect ratio, 1.6–5.0; and fin number, 9–72. Finally, with the proposed correlation, the effects of fin numbers, fins heights, and fin thicknesses on the thermal resistances of the horizontal cylinders with triangular fins were investigated. It was shown that the thermal resistance generally increases as the fin number, fin height, and fin thickness increase. It is expected that horizontal cylinders for various cooling devices with triangular fins can be designed based on the findings of the present study.

Physical Limitations of Phosphor layer thickness and concentration for White LEDs

Increasing phosphor layer thickness and concentration can enhance the lumen flux of white LED (W-LED). In this work, we found that increasing the phosphor layer thickness and concentration can increase its temperature, and there is also a maximum thickness and concentration beyond which their increase will not lead to lumen increase, but only temperature increase. Higher thickness and higher concentration also results in warm light instead of White light. The maximum thickness and concentration are found to be limited by the scattering of light rays with higher % decrease of blue light rays than the yellow light rays. The results obtained in this work can also be used to compute the temperature and thermo-mechanical stress distribution of an encapsulated LED, demonstrating its usefulness to the design of encapsulated LED packages. Simulation software like ANSYS and TracePro are used extensively to verify the root cause mechanisms.

Lumen degradation analysis of LED lamps based on the subsystem isolation method

The lumen degradation of LED lamps undergoing an accelerated aging test is investigated. The entire LED lamp is divided into three subsystems, namely, driver, lampshade, and LED light source. The parameters of output power [Watts (W)], transmittance (%), and lumen flux (lm) are adopted in the analysis of the degradation of the driver, lampshade, and LED light source, respectively. Two groups of LED lamps are aged under the ambient temperatures of 25°C and 85°C, respectively, with the aging time of 2000 h. The lumen degradation of the lamps is from 3.8% to 4.9% for the group under a temperature of 25°C and from 10.6% to 12.7% for the group under a temperature of 85°C. The LED light source is the most aggressive part of the three subsystems, which accounts for 70.5% of the lumen degradation of the LED lamp on average. The lampshade is the second degradation source, which causes 21.5% of the total amount on average. The driver is the third degradation source, which causes 6.5% under 25°C and 2.8% under 85°C of the total amount on average.

Catadioptric freeform optical system design for LED off-axis road illumination applications

The aim of this paper is to develop a new composite structure of catadioptric optical system containing both freeform refractive surface and freeform total internal reflective (TIR) surface for LED road illumination applications. The role of freeform refractive part is to generate the shifted general rectangular illumination pattern to optimally match the shape of the road surface. The application of TIR mechanism is aimed to control the stray light in the sidewalk direction of the road luminaire and maximize the efficient energy efficiency. In this paper, we use the "double pole" ray mapping technique to design the refractive optical surface and the θ-φ coordinate ray mapping technique to derive the freeform TIR surface. The simulation shows that the novel catadioptric design has relatively high collection efficiency, thus high average illuminance level inside the effective illumination area. This lens also has good control of stray light on the backside of the road luminaire.

Noncontact and instant detection of phosphor temperature in phosphor-converted white LEDs

Phosphor-converted white light-emitting diodes (pc-WLEDs) have become a major light source in general lighting. To stabilize the photometric characteristics of pc-WLEDs, much effort has been made to manage the heat dissipation of the LED dies. The thermal problems of the phosphor parts, a critical reliability concern for pc-WLEDs, have recently attracted academic interest. This study proposed a practical approach for measuring phosphor temperature in an operating pc-WLED using a noncontact, instant detection method to remotely monitor the emission spectrum. Conventionally, an infrared camera or thermocouples have been used to measure temperature. An IR camera requires good calibration on the emissivity and is usually blocked by the lens or other components covered on the phosphors. Moreover, a thermocouple requires time to reach the thermal equivalence between the detector and the sample under testing, and this approach is destructive when used for inner detection. Our approach has advantages over the conventional methods because it is noninvasive, noncontact, and instant, and inner detection. The approach is also independent of the peak wavelength of pumping lights, the concentration and thickness of phosphor, and correlated color temperatures.

White Light Emission from Fluorescent SiC with Porous Surface

ABSTARCT

We report for the first time a NUV light to white light conversion in a N-B co-doped 6H-SiC (fluorescent SiC) layer containing a hybrid structure. The surface of fluorescent SiC sample contains porous structures fabricated by anodic oxidation method. After passivation by 20 nm thick Al₂O₃, the photoluminescence intensity from the porous layer was significant enhanced by a factor of more than 12. Using a porous layer of moderate thickness (~10 µm), high-quality white light emission was realized by combining the independent emissions of blue-green emission from the porous layer and yellow emission from the bulk fluorescent SiC layer. A high color rendering index of 81.1 has been achieved. Photoluminescence spectra in porous layers fabricated in both commercial n-type and lab grown N-B co-doped 6H-SiC show two emission peaks centered approximately at 460 nm and 530 nm. Such blue-green emission phenomenon can be attributed to neutral oxygen vacancies and interface C-related surface defects generated dring anodic oxidation process. Porous fluorescent SiC can offer a great flexibility in color rendering by changing the thickness of porous layer and bulk fluorescent layer. Such a novel approach opens a new perspective for the development of high performance and rare-earth element free white light emitting materials.

High Thermal Dissipation of Al Heat Sink When Inserting Ceramic Powders by Ultrasonic Mechanical Coating and Armoring

Aluminum alloys, which serve as heat sink in light-emitting diode (LED) lighting, are often inherent with a high thermal conductivity, but poor thermal total emissivity. Thus, high emissive coatings on the Al substrate can enhance the thermal dissipation efficiency of radiation. In this study, the ultrasonic mechanical coating and armoring (UMCA) technique was used to insert various ceramic combinations, such as Al₂O₃, SiO₂, or graphite, to enhance thermal dissipation. Analytic models have been established to couple the thermal radiation and convection on the sample surface through heat flow equations. A promising match has been reached between the theoretical predictions and experimental measurements. With the adequate insertion of ceramic powders, the temperature of the Al heat sinks can be lowered by 5–11 °C, which is highly favorable for applications requiring cooling components.

Disparity between online and offline tests in accelerated aging tests of LED lamps under electric stress

The accelerated aging tests under electric stress for one type of LED lamp are conducted, and the differences between online and offline tests of the degradation of luminous flux are studied in this paper. The transformation of the two test modes is achieved with an adjustable AC voltage stabilized power source. Experimental results show that the exponential fitting of the luminous flux degradation in online tests possesses a higher fitting degree for most lamps, and the degradation rate of the luminous flux by online tests is always lower than that by offline tests. Bayes estimation and Weibull distribution are used to calculate the failure probabilities under the accelerated voltages, and then the reliability of the lamps under rated voltage of 220 V is estimated by use of the inverse power law model. Results show that the relative error of the lifetime estimation by offline tests increases as the failure probability decreases, and it cannot be neglected when the failure probability is less than 1%. The relative errors of lifetime estimation are 7.9%, 5.8%, 4.2%, and 3.5%, at the failure probabilities of 0.1%, 1%, 5%, and 10%, respectively.

Design methodology for micro-discrete planar optics with minimum illumination loss for an extended source

Recently, studies have examined techniques for modeling the light distribution of light-emitting diodes (LEDs) for various applications owing to their low power consumption, longevity, and light weight. The energy mapping technique, a design method that matches the energy distributions of an LED light source and target area, has been the focus of active research because of its design efficiency and accuracy. However, these studies have not considered the effects of the emitting area of the LED source. Therefore, there are limitations to the design accuracy for small, high-power applications with a short distance between the light source and optical system. A design method for compensating for the light distribution of an extended source after the initial optics design based on a point source was proposed to overcome such limits, but its time-consuming process and limited design accuracy with multiple iterations raised the need for a new design method that considers an extended source in the initial design stage. This study proposed a method for designing discrete planar optics that controls the light distribution and minimizes the optical loss with an extended source and verified the proposed method experimentally. First, the extended source was modeled theoretically, and a design method for discrete planar optics with the optimum groove angle through energy mapping was proposed. To verify the design method, design for the discrete planar optics was achieved for applications in illumination for LED flash. In addition, discrete planar optics for LED illuminance were designed and fabricated to create a uniform illuminance distribution. Optical characterization of these structures showed that the design was optimal; i.e., we plotted the optical losses as a function of the groove angle, and found a clear minimum. Simulations and measurements showed that an efficient optical design was achieved for an extended source.

Highly improved reliability of amber light emitting diode with Ca -α-SiAlON phosphor in glass formed by gas pressure sintering for automotive applications

Phosphor in glass (PiG) with 40 wt% of Ca-α-SiAlON phosphor and 60 wt% of Pb-free silicate glass was synthesized and mounted on a high-power blue LED to make an amber LED for automotive applications. Gas pressure sintering was applied after the conventional sintering process was used to achieve fully dense PiG plates. Changes in photoluminescence spectra and color coordination were inspected by varying the thickness of the plates that were mounted after optical polishing and machining. A trade-off between luminous flux and color purity was observed. The commercial feasibility of amber PiG packaged LED, which can satisfy international regulations for automotive components, was successfully demonstrated by examining the practical reliability under 85% humidity at an 85°C condition.

Light-Emitting Diodes with Hierarchical and Multifunctional Surface Structures for High Light Extraction and an Antifouling Effect

Bioinspired hierarchical structures on the surface of vertical light-emitting diodes (VLEDs) are demonstrated by combining a self-assembled dip-coating process and nanopatterning transfer method using thermal release tape. This versatile surface structure can efficiently reduce the total internal reflection and add functions, such as superhydrophobicity and high oleophobicity, to achieve an antifouling effect for VLEDs.

Wet-chemical preparation of barium magnesium orthophosphate, Ba2Mg(PO4)2:Eu2+, nanorod phosphor with enhanced optical and photoluminescence properties

We report a wet-chemical synthesis of yellow-emitting nanorod phosphors based on a Eu²⁺-doped Ba₂Mg(PO₄)₂ (BMP:Eu) orthophosphate compound, and its enhanced optical and photoluminescence properties.

A complete inorganic colour converter based on quantum-dot-embedded silicate glasses for white light-emitting-diodes

A complete inorganic quantum dot color converter for a white LED is achieved using silicate-based quantum-dot-embedded glasses (QDEGs).

Study of temperature distributions in pc-WLEDs with different phosphor packages

Phosphor converted white LEDs (pc-WLED) with different geometries, namely remote-dome, remote-plate, half-dome, and conformal-coating packages, were studied using simulations to obtain the thermal power and the temperature distribution. The results of an experiment carried out to measure the temperature distribution in these pc-WLEDs validate the simulation results. A hotspot always occurs in the phosphor region. The conformal-coating package is the most feasible choice to reduce the hotspot temperature comparing with the other geometries.

Comparison of online and offline tests in LED accelerated reliability tests under temperature stress

Accelerated aging tests are the main method used in the evaluation of LED reliability, and can be performed in either online or offline modes. The goal of this study is to provide the difference between the two test modes. In the experiments, the sample is attached to different heat sinks to acquire the optical parameters under different junction temperatures of LEDs. By measuring the junction temperature in the aging process (Tj1), and the junction temperature in the testing process (Tj2), we achieve consistency with an online test of Tj1 and Tj2 and a difference with an offline test of Tj1 and Tj2. Experimental results show that the degradation rate of the luminous flux rises as Tj2 increases, which yields a difference of projected life L(70%) of 8% to 13%. For color shifts over 5000 h of aging, the online test shows a larger variation of the distance from the Planckian locus, about 40% to 50% more than the normal test at an ambient temperature of 25°C.

Three-Dimensional Porous Copper-Graphene Heterostructures with Durability and High Heat Dissipation Performance

Porous materials have historically been of interest for a wide range of applications in thermal management, for example, in heat exchangers and thermal barriers. Rapid progress in electronic and optoelectronic technology necessitates more efficient spreading and dissipation of the heat generated in these devices, calling for the development of new thermal management materials. Here, we report an effective technique for the synthesis of porous Cu-graphene heterostructures with pores of about 30 μm and a porosity of 35%. Graphene layers were grown on the surfaces of porous Cu, which was formed via the coalescence of molten Cu microparticles. The surface passivation with graphene layers resulted in a thermal conductivity higher than that of porous Cu, especially at high temperatures (approximately 40% at 1173 K). The improved heat dissipation properties of the porous structures were demonstrated by analysis of the thermal resistance and temperature distribution of LED chips mounted on the structures. The effective combination of the structural and material properties of porous Cu-graphene heterostructures provides a new material for effective thermal management of high-power electronic and optoelectronic devices.

Ultraviolet light stable and transparent sol-gel methyl siloxane hybrid material for UV light-emitting diode (UV LED) encapsulant

An ultraviolet (UV) transparent and stable methyl-siloxane hybrid material was prepared by a facile sol-gel method. The transparency and stability of a UV-LED encapsulant is an important issue because it affects UV light extraction efficiency and long-term reliability. We introduced a novel concept for UV-LED encapsulation using a thermally curable oligosiloxane resin. The encapsulant was fabricated by a hydrosilylation of hydrogen-methyl oligosiloxane resin and vinyl-methyl siloxane resin, and showed a comparable transmittance to polydimethylsiloxane (PDMS) in the UVB (∼300 nm) region. Most remarkably, the methyl-siloxane hybrid materials exhibited long-term UV stability under light soaking in UVB (∼300 nm) for 1000 h.

Facile synthesis and optical properties of Ce:YAG polycrystalline ceramics with different SiO2 content

According to the FESEM micrographs of the as-prepared samples, they show the texture of polycrystalline materials and better uniformity appears in sample A.