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Photoluminescent composites play a critical role of light converters in light-emitting devices, especially in high-power light-emitting diodes and laser diodes, while the nonradiative Stokes loss in photoluminescent particles not only generates thermal phonons with temperature rise but also degrades their photonic/electronic properties. Moreover, these micro/nanoscale heat sources are usually dispersed in a low-thermal-conductivity polymer matrix, which makes it tough to dissipate heat out efficiently, resulting in significant thermal quenching. Reinforcing the heat dissipation of photoluminescent composites is considerably important and challenging since their optical performance will be easily damaged by the thermal reinforcement processes. In this Perspective, we briefly introduce the heat generation and transportation mechanisms in photoluminescent composites and then emphasize the recent progresses in heat manipulation of photoluminescent composites. Finally, we outline some challenges and possible solutions for addressing the thermal management of photoluminescent composites as well as some future directions in this field. [ABSTRACT FROM AUTHOR]

Quantum dots (QDs) are facing significant photoluminescence degradation in moisture environment. In QDs-silicone composites, the poor water resistance of silicone matrix makes it easy for water and oxygen molecules to erode QDs. To tackle this issue, we proposed a new QDs protection strategy by introducing short-chain silica precursors onto the QDs’ surface, so that a dense silica passivation layer could be formed onto the QDs nanoparticles. Sol-gel method based on 3-aminopropyl triethoxysilane (APTES), 3-mercaptopropyl trimethoxysilane (MPTMS), and 3-mercaptopropyl triethoxysilane (MPTES) were adopted to prepare the uniform and crack-free QDs-silica glass (QD-glass). Because of the crosslinking of short-chain precursors, the formed silica glass possesses 38.6% smaller pore width and 68.6% lower pore volume than silicone, indicating its denser cross-linked network surrounding QDs. After 360 h water immersion, the QDs-glass demonstrated a 6% enhancement in red-light peak intensity, and maintained a stable full width at half maximum (FWHM) and peak wavelength, proving its excellent water-resistant ability. However, the conventional QDs-silicone composites not only showed a decrease of 75.3% in red-light peak intensity, but also a broadened FWHM and a redshifted peak wavelength after water immersion. QDs-glass also showed superior photostability after 132 h exposure to blue light. Red-light peak intensity of QDs-glass remained 87.3% of the initial while that of QDs-silicone decreased to 19.8%. And the intensity of QDs-glass dropped to 62.3% of that under 20 °C after thermal treatment of 160 °C. Besides, under increasing driving currents, the light conversion efficiency drop of QDs-glass is only one fifth that of QDs-silicone. Based on the QDs-glass, the white light-emitting diodes was achieved with a high luminous efficiency of 126.5 lm Wâˆ'1 and a high color rendering index of 95.4. Thus, the newly proposed QD-glass has great significance in guaranteeing the working reliability of QDs-converted devices against moisture and high-power environment. [ABSTRACT FROM AUTHOR]

In this work, QDs/hBNs/silicone annular fins (QDs-AF) are proposed as a new type of packaging structure to establish rapid heat dissipation pathways between QDs and heat sink. The QDs-AF structure was firstly designed based on commercial LEDs modules. Then, the annular fin materials including QDs, thermally conductive hBN platelets (hBNs), and silicone were determined by balancing the tradeoff of optical and thermal performances. Finally, the optimal structure of QDs-AF was obtained through thermal simulation by changing the values of fin thicknesses of different initial QDs-AF structures. According to the optimal QDs-AF structure with 4 fins and fin thickness of 0.28 mm, we fabricated the QDs-AF and its WLEDs. Compared with QDs-WLEDs, the maximum surface working temperature of QDs-AF-WLEDs was reduced by 20 °C at 1000 mA. In addition, the QDs-AF-WLEDs showed an excellent optical performance with a high color rendering index of 90.3 and a high luminous efficiency of 124.1 lm $\text{W}^{-1}$. The application of QDs-AF in WLEDs is a new and significant method to achieve a lower working temperature and protect QDs from thermal quenching in WLEDs. [ABSTRACT FROM AUTHOR]

Quantum dots (QDs)/phosphor hybrid white light emitting diodes (QDs-LEDs) have attracted extensive attention in recent years due to their advantages of high light efficiency and high color rendering index (CRI). However, the QDs present poor thermal and moisture stability, which hinders the commercialization of QDs-LEDs. The layered packaging structure with QDs/silicone film upon the phosphors/silicone film was proposed to improve the thermal stability of QDs-LEDs, while the moisture stability of this packaging structure has not been verified so far. In this study, the moisture stability of QDs-LEDs with layered packaging structure in a high moisture environment with relative humidity of ~99.8% was experimentally investigated. The results show that the normalized light peak intensity degradations of QDs/silicone and phosphor/silicone films are 12.5% and 54.0% after aging for 178h, which indicates that the moisture stability of QDs is the key factor for determining the moisture stability of QDs-LEDs. The red light peak intensity degradations of QDs-LEDs with QDs/silicone film upon and below the phosphor/silicone film are 21.1% and 8.0% after aging for 178h, indicating that layered packaging structure with QDs/silicone film at the down layer can enhance the moisture stability of QDs-LEDs. [ABSTRACT FROM AUTHOR]

The conventional chip-on-board light-emitting diodes (COB-LEDs) with flat encapsulant layer present low light efficiency due to the total internal reflection (TIR) happens at the encapsulant-air interface. In this study, microlens array was realized on the flat encapsulant layer of COB-LEDs to diminish the TIR at the encapsulant-air interface. The microlens array was fabricated by selectively exposing the UV glue film in UV-light through a pre-designed photomask. Micro-pillars form after the UV-light exposure and microlens array forms spontaneously due to the wetting process of uncured UV glue on the micro-pillars. The geometry of the microlens array was experimentally optimized, and the light efficiency enhancement of COB-LEDs by the microlens array was analyzed. The results show that optimal geometry of microlens array with curvature of ~41° and base diameter of $\sim 87~\mu \text{m}$ can be realized by adjusting the UV-light exposure time as >40 s and UV glue film thickness as $> 80~\mu \text{m}$. Compared to the traditional flat encapsulant layer, the microlens array with optimal geometry increases the light efficiency of blue and ~5000 K white COB-LEDs by 50.9% and 9.31%. [ABSTRACT FROM AUTHOR]

The chip-on-board light-emitting diodes (COB-LEDs) present low light efficiency due to the total internal reflection (TIR) at the flat encapsulant–air interface. In this study, a simple method based on complementary printing was proposed to realize the millilens array on the flat encapsulant layer to diminish the TIR effect, and thereby improve the light efficiency of the COB-LEDs. Experiments and optical simulation based on the Monte Carlo ray-tracing method were conducted to investigate the effect of the lens curvature θ, lens radius R, and lens spacing D on the light efficiency of the COB-LEDs. The results show that the light efficiency of the COB-LEDs increases with R and decreases with D, and COB-LEDs with hemispherical lens arrays (θ = 90°) have the highest light efficiency when R and D are fixed. Compared with the conventional COB-LEDs with a flat encapsulant, closely arranged hemispherical lens arrays with an R of 0.5, 1, and 1.5 mm enhance the light efficiency of the blue COB-LEDs by 87.56%, 90.62%, and 95.73% and enhance the light efficiency of ~6000-K white COB-LEDs by 17.24%, 18.86%, and 20.33%, respectively. [ABSTRACT FROM AUTHOR]

In this study, we studied the phosphor sedimentation effect in white phosphor-converted light-emitting diode packages by modeling the multi-layer phosphors with gradient concentrations. The essence of phosphor sedimentation can attribute to the variation of phosphor concentrations. By modifying the Kubelka-Munk theory, we built a multi-layer phosphor model with considering the light scattering, light absorption, and light conversion process simultaneously. With a brief review of Kubelka-Munk theory, multi-layer phosphors were modeled on the basis of single-layer phosphor model. The phosphor sedimentation effect was characterized by modeling multi-layer phosphors with gradient concentrations, whereas keeping the total amount of phosphors at the same level. It is found from the five calculation cases that phosphor sedimentation will cause the drop of light extraction efficiency (LEE) by 13.04%. Furthermore, the phosphor layer with inverse-gradient concentrations will enhance the LEE 16.56%. To figure out the reasons, the light losses were calculated, and it is proved that the light loss is enhanced when phosphor sedimentation happens. [ABSTRACT FROM AUTHOR]

Quantum dots-light-emitting diodes (QDs-LEDs) have attracted extensive attention in recent years due to their outstanding performance of color-rendering index (CRI). QDs were found to undergo severe fluorescence quenching in moisture environments, while rare solutions were proposed to solve this issue. In this article, 1H,1H,2H,2H-perfluorooctyltrichlorosilane (FOTS)-modified superhydrophobic nanosilica (FOTS-NS) particles were synthesized and coated on the QDs/polymer films to prevent the moisture from penetrating the films. The light efficiency of the FOTS-NS-coated LEDs was investigated, and the moisture stability of the FOTS-NS-coated QDs/polymer films was tested in an extreme moisture environment (50 °C, 99.6% relative humidity) for 270 h. The results show that compared with the uncoated LEDs, the FOTS-NS layer increases the light efficiency by ~10% for the blue LEDs and ~3.8% for the white LEDs (correlated color temperature of ~5000 K and Ra of ~91) by optimizing the particle deposition density of the FOTS-NS. Besides, the normalized peak intensity degradation of QDs embedded in the coated and uncoated films after aging for 270 h is ~35.68% and ~77.66%, showing the capacity of FOTS-NS layer on enhancing the moisture stability of the QDs-LEDs. [ABSTRACT FROM AUTHOR]

Recently, quantum dots-converted white light-emitting diodes (QDs-WLEDs) are attracting numerous attention due to their high luminous efficiency and excellent color quality. As for color conversion material, the quantum dots (QDs) are commonly embedded into a low-thermal-conductivity polymer matrix. In this case, their generated heat during the photoluminescence process can hardly be dissipated into the heat sink, leading to a high working temperature and reduced lifetime. Adding particles with high thermal conductivity to the QDs layer can enhance its thermal conductivity, and thus reduce QDs’ working temperature. At the same time, these particles may affect the optical properties of QDs. However, this problem has still not been deeply studied. In this article, we systematically investigated the effects of the highly thermal-conductive hexagonal boron nitride sheets (hBNSs) on the optothermal performances of QDs/phosphor film in white light-emitting diodes (WLEDs). The thermal conductivity of QDs/phosphor film was significantly increased by 24% after adding 5wt% of 45- $\mu \text{m}$ -diameter hBNS. As for the optical performance, the transparency of the silicone gel film with 45- $\mu \text{m}$ -diameter hBNS was much better than that with 6–9- $\mu \text{m}$ -diameter hBNS under the same weight fraction. Furthermore, the scattering effect of hBNS plays a more important role in enhancing the light conversion performance of QDs than that of phosphor. At last, a color stability test showed the increasing rate of correlated color temperature (IRCCT) of hBNS-added WLEDs are 21% smaller than that of common WLEDs after working 153 h, meaning a better QDs stability in hBNS-added WLEDs. [ABSTRACT FROM AUTHOR]

Phosphor-in-glass (PiG)-based white light-emitting diodes (WLEDs) are regarded as promising solid-state lighting sources because of their high efficiency, excellent thermal stability, and facile packaging process. In this letter, we investigated the optical and thermal performances of PiG-based WLEDs with different packaging structures. The PiG converter was prepared by printing and sintering a phosphor glass film on a sapphire plate. Three types of PiG-based WLEDs were fabricated, PiG on chip (PiG-1), PiG with air gap (PiG-2), and PiG with silicone encapsulation (PiG-3). The experimental results show that the PiG-1 sample achieves a highest luminous efficacy (LE) of 110.5 lm/W, a correlated color temperature (CCT) of 6080 K, and a color rendering index (CRI) of 70.5. In addition, the PiG-2 sample displays a lowest working temperature under various currents. [ABSTRACT FROM AUTHOR]

Quantum dots (QDs) are considered as promising color converters in the next-generation white light-emitting diodes (WLEDs) due to their tunable wavelength, high quantum yield, and excellent color rendering. However, the practical applications of QDs and QD composite films are severely hindered by their intrinsical drawbacks of thermal quenching and degradation. In this paper, we proposed an efficient heat dissipation approach to solve this issue by stacking a QD film on a heat-conducting phosphor-sapphire composite (PSC) plate. The PSC plate was prepared by printing and sintering YAG phosphor glass film on a sapphire substrate, which plays the roles of yellow converter and heat sink for the QD film. The luminescent performances of WLEDs were controlled by adjusting the QD contents. The high heat-conducting PSC plate provides an effective heat transfer channel for the QD film, which can sufficiently dissipate the heat to ambient air. Compared with the QD film stacked on the traditional phosphor-in-glass (PiG) and phosphor-in-silicone (PiS), the working temperatures of QD-PSC are reduced by 37 °C and 65 °C at 1000 mA, respectively. The QD-PSC-based WLEDs achieve stable luminescent performances for various operation times. The results demonstrate that the proposed approach can enhance the heat dissipation of QDs and realize stable white lighting. [ABSTRACT FROM AUTHOR]

Abstract: Although quantum dots (QDs) show excellent advantages in flexible wavelength‐tuning and high color rendering capability in white light‐emitting diodes (WLEDs) lighting and display applications, the less‐than‐one quantum efficiency inevitably gives rise to a non‐negligible heat generation problem, which induces high‐temperature quenching issues of QDs and severely hinders their potential applications. Efficient heat dissipation for these nanoscale QDs is challenging since these nanoparticle “heat sources” are usually embedded in a low‐thermal conductivity polymer matrix. In this work, this problem is attempted by targeting cooling of the QDs in the silicone matrix by electrostatically bonding the hexagonal boron nitride (hBN) platelets onto QDs without sacrificing the optical performance of WLEDs. The red‐emissive QDs/hBN composites are mixed with yellow‐emissive phosphor to fabricate QDs/hBN‐WLEDs. Due to the effective heat transfer channels established by the QDs/hBN in the silicone, the heat could be dissipated efficiently to ambient air, and the working temperature of WLEDs is reduced by 22.7 °C at 300 mA. The QDs/hBN‐WLEDs still maintain a high luminous efficiency of 108.5 lm W−1 and a high color rendering index of Ra > 95, R9 > 90, showing that the present strategy can improve heat dissipation without sacrificing the optical performance. [ABSTRACT FROM AUTHOR]

Light efficiency of chip-on-board light-emitting diodes (COB-LEDs) is much lower than the single-chip packaging LEDs due to its flat phosphor layer, and hemispherical phosphor layer realization is a great challenge in COB-LEDs packaging due to the low surface tension of the phosphor gel. In this paper, we demonstrated a facile method to fabricate patterned surfaces to deal with this challenge. First, nanosilica (NS) particles with average diameter of 70 nm were fabricated by hydrolyzing the tetraethoxysilane and further modified by 1H,1H,2H,2H-perfluorooctyl-trichlorosilane, then patterned surfaces were fabricated by introducing a tailored template into the NS coating process. The results show that the NS coated surfaces display repellency to the water and phosphor gel with porous lotus leaf-like hierarchical structure, when the particle deposition density (PDD) of the NS particles increases from 0 to 6 g/m2, the contact angle (CA) of water increases from 34° to 161°, and the CA of phosphor gel increases from 22° to 145°. Hemispherical phosphor layer was achieved with the patterned surfaces when the PDD is 1.5 g/m2. Compared to the conventional flat phosphor layer, the hemispherical phosphor layer enhances the light efficiency by 11.74% and 14.52% for 4000 and 5000 K COB-LEDs. [ABSTRACT FROM PUBLISHER]

In this letter, we investigated the dynamic phosphor sedimentation effect on the optical performance of phosphor-converted light-emitting diodes (pcLEDs) with dispensed phosphor layer. To analyze the phosphor sedimentation effect, we realized two packaging structures to sediment phosphor particles above/outside LED chip separately. The phosphor sedimentation effect on the luminous efficiency and correlated color temperature (CCT) were tested by experiments, followed by a quantitative exploration to the mechanism by evaluating the angular color uniformity and light intensity distribution-based Monte Carlo ray-tracing simulations. Results show that phosphor sedimentation happened above LED chip will decrease CCT by 33.19%, and happened outside LED chip will increase CCT by 269%. For the conventional packaging structure, phosphor sedimentation will lead to the decreasing CCT at first and the increasing CCT soon afterwards, and the CCT variation is 39.62%. [ABSTRACT FROM PUBLISHER]

Phosphor temperature in high-power white light-emitting diodes can greatly affect the optical properties, reliability, and lifetime. Accurate estimation of phosphor temperature is the first step for enhancing thermal management inside the package. In this paper, the phosphor temperature was measured by a plug-in method with thermocouple. The thermocouple’s bead was inserted into the phosphor layer, and then the silicone matrix got cured. The phosphor temperature was measured under various input currents. Under 350 mA, the temperature difference between the measured value and the junction temperature was about 17 °C higher than those in the references. The reason for this overestimation was attributed to light energy absorption and conversion of the bead, which was confirmed by thermal simulations. [ABSTRACT FROM PUBLISHER]

The silicone/phosphor composite is widely used in light emitting diode (LED) packaging. The composite thermal properties, especially the effective thermal conductivity, strongly influence the LED performance. In this paper, a lattice Boltzmann model was presented to predict the silicone/phosphor composite effective thermal conductivity. Based on the present lattice Boltzmann model, a random generation method was established to describe the phosphor particle distribution in composite. Benchmarks were conducted by comparing the simulation results with theoretical solutions for simple cases. Then the model was applied to analyze the effective thermal conductivity of the silicone/phosphor composite and its size effect. The deviations between simulation and experimental results are <7 %, when the phosphor volume fraction varies from 0.038 to 0.45. The simulation results also indicate that effective thermal conductivity of the composite with larger particles is higher than that with small particles at the same volume fraction. While mixing these two sizes of phosphor particles provides an extra enhancement for the effective thermal conductivity. [ABSTRACT FROM AUTHOR]

Photoluminescence quantum dots (QDs) have been considered as a kind of promising light converting materials with high luminous efficiency, tunable spectrum, and narrow emission. However, the current light conversion efficiency (LCE) of QD films is at a relative low level, which will result in many problems, such as inferior luminous performance, severe self-heating, etc. To enhance the LCE of QD films, SiO2 particles were doped into QD films by physical blending, for their remarkable light scattering effect. The LCE enhancement after adding SiO2 particles were studied by experiments. Experimental results showed that the SiO2 modified QD films improved the LCE up to 63.45%, which was 103.88% higher than conventional QD films. Besides, as the SiO2 particles diameters increased, the optimal mass fractions of SiO2 particles for top LCE would decrease. [ABSTRACT FROM PUBLISHER]

Light-emitting diodes (LEDs) are widely used in our daily lives. Both light and heat are generated from LED chips and then transmitted or conducted through multiple packaging materials and interfaces. Part of the transmitted light converts into heat along the light propagation; in return, the accumulation of heat leads to the degradation of light output. The accumulated heat negatively influences the reliability and longevity of LEDs, and thus thermal management is critical for LED packaging and applications. On the other hand, in LED packaging processes, many fluid flow problems exist, such as phosphor coating, silicone injection, chip bonding, solder reflow, etc. Among them, phosphor coating is the most important process which is essential for LED performance. Phosphor gel is a kind of non-Newton fluid and its coating process is a typical fluid-flow problem. Overall, since LED packaging and applications present many heat and fluid flow problems, obtaining a full understanding of these problems enables advancements in the development of LED processes and designs. In this review, the emphasis is placed on heat generation in chips, heat flow in packages and application products, fluid flow in phosphor coating process, etc. This is a domain in which significant progress has been achieved in the last decade, and reporting on these advances will facilitate state-of-the-art LED packaging and application technologies. [ABSTRACT FROM AUTHOR]

In this letter, we proposed a remote phosphor coating method by lens wetting for enhancing the angular color uniformity of phosphor-converted white light-emitting diodes. Simulations based on the volume of fluid (VOF) method were applied to investigate the geometry evolution of a phosphor gel droplet after being dropped onto the lens, and the stable geometry of the phosphor gel was obtained. Monte Carlo ray tracing was used to study the optical performance of LED samples with the stable phosphor geometries. Besides, experiments were conducted to verify the fabrication flexibility of the proposed method. The VOF simulation and experimental results show that the proposed method can realize the hemispherical remote phosphor layers with uniform or non-uniform thickness by adjusting the coating volume. The optical simulations and the experimental measurements show that the LED samples with non-uniform thickness remote phosphor geometry achieves smaller correlated color temperature deviation (<200 K at 5000 K) than the uniform thickness remote phosphor geometry. [ABSTRACT FROM PUBLISHER]

Low light efficiency and poor angular color uniformity (ACU) are the key challenges of chip-on-board packaging light-emitting diodes (LEDs). In this paper, we demonstrate a phosphor geometry, and its controlling method for enhancing the optical performance of chip-on-board packaging LEDs, its fabrication flexibility, and availability are validated by experiments, and its effect on the optical performance is analyzed by optical simulations and experiments. The simulation results show that compared with the conventional flat geometry, the cylindrical tuber geometry can effectively improve the light efficiency and ACU, and the light efficiency enhancement increases with correlated color temperature. Experimental results show that for the case that the encapsulation layer only consists of silicone, the proposed geometry enhances the light efficiency up to 63.1%. In addition, for another case, in which the encapsulation layer consists of silicone and phosphor particles, when the average correlated color temperature (CCT) is about 5000 K, the proposed geometry increases the light efficiency by 11.7%, and the angular CCT deviations for the flat geometry and the proposed geometry are 1805 and 475 K, respectively. [ABSTRACT FROM PUBLISHER]

We demonstrated a thermal remote phosphor coating method for realizing high angular color uniformity (ACU) and high efficiency of phosphor-converted white-light-emitting diodes based on thermal control. The proposed phosphor-coating method can fabricate remote phosphor layer geometries through a simple package process. Experimental results show that compared with those samples packaged by conventional dispensing coating, this method can efficiently improve the ACU. Angular color-correlated temperature (CCT) deviation of the test samples by the present method can reduce from 1100 to 90 K for an average CCT of 4300 K from −90° to +90° view angles, and the CCT distributions are 150 and 250 K for average CCTs of 5300 and 6300 K, respectively. In addition, this method can improve the lumen efficiency by 4.45% for an average CCT of about 4300 K, and increased by 4.96% and 5.45% for average CCTs of 5300 and 6300 K, respectively. [ABSTRACT FROM PUBLISHER]

In this letter, we studied the effect of chip offset on the optical performance of bare chip and white light-emitting diode (LED) package through a series of comparative experiments. LED modules with different chip offset distances were packaged and measured. The results show that chip position offset has a great influence on white LED’s performance. When the offset distance reaches 1 mm, the luminous efficiency decreases by 22.9% and the angular color distribution is deteriorated. Two methods are recommended to avoid this effect. [ABSTRACT FROM PUBLISHER]

Settling of the heavy phosphor particles in the silicone encapsulation is a root-cause for variation of the correlated color temperature causing yield reduction in the fabrication of white light-emitting diodes (LEDs). When prefabricated particles consisting of the phosphor and the silicone are used, settling can be significantly reduced and a dramatic enhancement of the consistency of LED performance can be achieved. A reduction of the change in correlated color temperature after 60 min of settling on the chip from >900 K for traditional phosphor particles to only 8 K for particles with a weight ratio of phosphor and silicone of 1:1 in an otherwise identical coating procedure is demonstrated. [ABSTRACT FROM PUBLISHER]

Abstract: Phosphor heat generation in phosphor-converted light-emitting diodes (pc-LEDs) plays an important role in affecting the optical and thermal performance of LEDs, but it is hard to measure directly. In this paper, we developed a method to calculate the phosphor heat generation by modifying the Kubelka–Munk theory. With full consideration of light scattering, absorption and conversion inside the phosphor layer simultaneously, we calculated the phosphor heat generation and analyzed its trend with changing the phosphor quantum efficiency and phosphor parameters (concentration, thickness, particle size). Experiments were also conducted to validate the calculations. It was found that with the increase of phosphor concentration from 0.11 to 0.34g/cm3, the total energy loss increases by 54.11%. The phosphor concentration and thickness played similar roles in affecting the phosphor heat generation. The maximum phosphor heat generation corresponded to the phosphor particle size of around 2μm, which is denoted as the critical size. [Copyright &y& Elsevier]

To better understand the reliability problem in high-power light-emitting diodes (LEDs), an online test is critical under an accelerated life test. In this paper, an experimental equipment was proposed for in situ measurement to evaluate the degradation of LED's optical properties. Signals such as illuminance and correlated color temperature were transmitted by the heat-resistant optical cable and were finally collected by detectors with high precision. The multichannel outputs were provided to simultaneously measure several samples. The measuring distance optimization was conducted by a ray tracing method, and the characteristics of optical output were listed. Accelerated life tests also indicate that the system error is less than 0.2%, and the measuring uncertainty of equipment can be controlled within 2%. A comparison experiment according to LM-80 measurement standard also shows a good agreement between the experimental data and the reference value. [ABSTRACT FROM PUBLISHER]

Abstract: Thermocouple is often used to measure surface temperature of light emitting diode (LED). However, it was found that the method was not accurate in this work. An experiment was conducted to prove this in the first part. In the experiment, the air temperature above a lit LED chip was measured by a thermocouple, and it was found the measurement temperatures were higher than the LED junction temperatures. When the drive current was 1000mA, the deviation reached as much as 148.61%, which proved that the temperature measured by the thermocouple was not accurate. The reason for the inaccuracy was supposed to be the absorption of the optical energy by the thermocouple. Theoretical derivation was conducted and it was found that the measurement temperature was related with the local luminance around the thermocouple and the optical absorption coefficient of the thermocouple. Comparison experiments were conducted and qualitatively validated the supposition. [Copyright &y& Elsevier]

In this study, we modeled the transmitted and reflected light of a phosphor layer in light-emitting diode packages by coupling the revised Kubelka-Munk and Mie-Lorenz theories. Through analyzing the transmitted and reflected light separately, it is found that the transmitted and reflected blue light vary monotonically with the changes of phosphor particle size, concentration and thickness. While the trends of the transmitted and reflected yellow light are non-monotonic, which are influenced by the interactions among the phosphor particle size, concentration and thickness. The light extraction efficiency through a phosphor layer was discussed. The reasons behind these phenomena were also presented. [ABSTRACT FROM PUBLISHER]

Abstract: Silicone carbonizations were observed in the multiple high power LEDs modules. This phenomenon was explained to be caused by the effect of phosphor self-heating. Comparative experiments were conducted, and the results validated the existence of phosphor self-heating, which caused the local high temperature. A thermal model of LED packaging based on phosphor particles under stair-like thermal load was built, and simulations were carried out to analyze the thermal performance of the phosphor layer. It is found that the highest temperature of the phosphor particles can reach 315.9°C, which would result in the phosphor quenching. As the phosphor quenching happens, the temperature can gradually increase to 540.16°C, which is highly enough to cause the silicone carbonization. One method to decrease temperature of the phosphor layer was proposed in the final part of this paper. [Copyright &y& Elsevier]

Conformal phosphor distribution is a well known way to improve optical performance for white light emitting diode (LED) packaging. However, it is difficult to be realized by mature and cheap dispensing coating methods. In this paper, with consideration given to the effect of boundary constraint on liquid wetting morphology, a method to achieve phosphor conformal distribution is presented by conventional dispensing coating with controlling the slanting angle of reflector cups. The numerical simulations and experiments on phosphor dispensed into reflector cups with different slanting angles are conducted. The results show that uniform phosphor distribution can be obtained when the slanting angle is the same as the contact angle between the phosphor slurry and the reflector cup side wall. The rule can be used to guide the design of the reflector cup and a simple change of reflector cup can realize phosphor conformal coating for white LEDs by the low cost traditional dispensing coating process. [ABSTRACT FROM PUBLISHER]

Abstract: Composites of silicone matrix filled with phosphor are widely used in light emitting diodes (LEDs) packaging. Its thermal conductivity is one of the important properties for LED thermal analysis. In this paper, a unit cell model (UCM) that includes interface resistance between phosphor and silicone matrix was built and applied to predict the thermal conductivity of uncured silicone/phosphor composites at different phosphor volume fractions. The thermal conductivities of seven uncured silicone/phosphor composites samples were measured for comparison with modeling results. The comparison shows that the model result matches to the experimental data within ±6% at the low volume fraction from 3.8% to 25%. Both the experiment and modeling results show that thermal conductivity of the composite has a sudden increase when volume fraction increases to near 40%, which can be explained by the percolation theory. For higher volume fraction, the effect of the interface resistance Rb between the phosphor fillers and silicone matrix cannot be neglected. [Copyright &y& Elsevier]

In this study, a model for calculating the bidirectional scattering properties of phosphor layer was established by considering the light absorption and light conversion of phosphor particle simultaneously. Based on the present model, the light extraction efficiency was calculated and validated by comparing with the existing experimental data and models. It is demonstrated that the present model predicts better. The transmission and reflection intensities of blue light and yellow light were found to be the functions of phosphor thickness. There exists a close critical phosphor thickness for reflected blue light and transmitted yellow light. The critical phosphor thickness is relative to the phosphor concentration. [ABSTRACT FROM PUBLISHER]

Abstract: Phosphor settling phenomena exists during the phosphor coating process in the light emitting diode (LED) packaging industry. It is perceived that phosphor settling will affect the concentration of the phosphor, and consequently the concentration will influence the optical performance of phosphor-converted white LED light source. In this paper, an experiment based on the real packaging process was conducted to investigate the phosphor settling phenomena. It was found that the concentration variation of the phosphor embedded in the silicone matrix was very small (less than 1%). Based on the observation of the experiments, the effect of the phosphor settling in the silicone matrix on light extraction efficiency (LEE), correlated color temperature (CCT), angular color uniformity (ACU) and light intensity distribution curve (LIDC) was investigated and discussed by the three dimensional Monte Carlo ray-tracing simulations. It was discovered that the effect of the phosphor settling on the optical performance could be neglected when using the present packaging process. [Copyright &y& Elsevier]

Junction temperature of high power light emitting diodes (LEDs), which is crucial for the thermal management of solid-state lighting, needs to be measured accurately. In this paper, a dynamic junction temperature measurement system for LEDs was proposed and the calibration including instrument calibration and factor K calibration were presented. The influence of the fast switch time in dynamic junction temperature test was analyzed and measurement errors caused by sampling delay were quantified. To prove the accuracy of the present system, comparison experiment was conducted. It shows a good agreement between the experimental data and reference value. Experiments also show that the measurement accuracy of the instrument can be up to 0.1 °C, and the standard error of temperature measurement can be controlled within 1%. [ABSTRACT FROM AUTHOR]

Abstract: Multiple-chip packaging becomes common in LEDs packaging community. For such type of packaging, thermal spreading resistance is an important factor to affect the total thermal performance of LEDs. In this study, a general analytical solution is used to study the whole temperature field of LED packaging substrate, this solution is based on the method of variable separation for thermal spreading resistances of eccentric heat sources on a rectangular flux channel. The feasibility of the analytical method used in LEDs packaging has been proven by the temperature comparison with existing experimental and numerical results of an 80W LED street lamp. By changing the chips arrangement on the substrate, temperature field optimization is conducted with maximal temperature difference of the substrate as the target function. The results show that spreading resistance plays a significant role to affect temperature field. When the LED distributions are effectively designed, the highest temperature on the substrate goes lower and the lowest temperature on the board goes higher, the temperature field becomes uniform, its spreading resistance becomes lower. [Copyright &y& Elsevier]

The optical extraction efficiency and reliability of light emitting diodes (LEDs) relies heavily on successful thermal management due to their inherit dependence on the low junction temperature of LED chips. In this paper, a microjet-based cooling system is proposed for the thermal management of high power LEDs. Experimental and numerical investigations on such an active cooling system were conducted. Thermocouples were packaged with LED chips to conduct an online measurement of the temperature and evaluate the cooling performance of the proposed system. The experimental results demonstrate that the microjet-based cooling system has good cooling performance. For a 2 × 2 LED chip array, when the input power is 5.6 W and the environmental temperature is 28°C, the temperature of the 2 × 2 LED chip array reaches 72°C within 2 minutes and continues to increase sharply if no active cooling technique is applied. By using the proposed cooling system to cool the LEDs, however, the maximum LED temperature measured by thermocouples will remain stable at about 36.7°C, when the flow rate of the micropump is 9.7 mL/s. With consideration of the experimental difficulty, a numerical investigation was conducted on flow and temperature distribution in the microjet device. The feasibility of the numerical model was proven by comparison with experimental results. The numerical results showed that at a flow rate of 3.2 mL/s, the heat transfer coefficient of the impinging jets in the proposed system was about 5523 W/m2·K, and the pressure drop in the microjet device was about 1368 Pa. [ABSTRACT FROM AUTHOR]

Owing to the strong light scattering effect of TiO2 nanoparticle and silicone composite, a method for improving optical performances of chip-on-board (COB) packaging LEDs was proposed through introducing a thin auxiliary encapsulation layer with high-concentration TiO2 nanoparticle and silicone composite below the main encapsulation layer. Its optical performance enhancement effect was examined by experiments. Results show that for the main encapsulation layer only consisting silicone, the proposed packaging method enhances the light extraction efficiency (LEE) up to 65%. However, for another case in which the main encapsulation layer is phosphor and silicone composite, the LEE enhancement effect is correlated to the phosphor concentration. When the phosphor concentration reduces from 0.12 to 0.035 g/cm \(^{3}\) , the LEE enhancement effect increases from 6% to 24%. Meanwhile, the angular correlated color temperature (CCT) deviation is reduced from 900 to 470K between viewing angles from −90° to 90°, when the average CCT is \(\sim 5500\) K. [ABSTRACT FROM AUTHOR]

The light extraction efficiency (LEE), correlated color temperature (CCT), and angular color uniformity (ACU) of the conformal coating phosphor silicone gel were investigated by Monte Carlo ray-tracing simulations. Relative to the conventional dispensing method, the conformal coating method can make the light-emitting diode (LED) module obtain better optical properties. It was found that the thickness of the phosphor layer in conformal coating was a key factor determining the optical performance. The thickness could be optimized for good optical properties. The optimal thickness is around 50–70 \mum when taking LEE, CCT, and ACU into consideration. [ABSTRACT FROM PUBLISHER]

Abstract The light propagation properties within the phosphor layer in phosphor-converted light-emitting diodes (pc-LEDs) have been extensively described by the two-wavelength model, in which two individual excitation and emission wavelengths are selected. The ignorance of the wavelength-dependence may cause deviation and cannot return the spectrum. In this paper, we extended the two-wavelength model to a multi-wavelength model based on fluorescent radiative transfer equation (FRTE) to characterize the overall light propagation behavior in the phosphor layer. In addition to the light absorption, forward scattering, and fluorescence characteristics, the re-absorption effect was further considered. The spectral radiance at any spatial location and angular direction for each wavelength was iteratively solved using the spectral element method (SEM). The model was validated by comparing the calculated spectrum and angular correlated color temperature (CCT) with experiments and good agreement was achieved with the corresponding maximum deviations of 7.6% and 3.6%, respectively. We also conducted a comparison between the multi-wavelength model with the two-wavelength model. We found that the two-wavelength model may underestimate or overestimate the optical power in the blue region and yellow region, respectively. It was attributed to assuming the peak absorption coefficient and quantum efficiency for all excitation wavelengths in the two-wavelength model. [ABSTRACT FROM AUTHOR]

The development of high-power light-emitting diode (LED) devices has been bedeviled by the reliability problems. And most reliability issues are caused by the packaging materials rather than the chips. However, which packaging material is the most influential remains unrevealed. To answer this question, a statistical method was introduced in this paper. Optical simulations were conducted to calculate the optical output power of LED package according to the orthogonal experimental design. Range and variance analyses were carried out to determine the significance of the relevant factors on the LED's light output. The results showed that the dome lens among the non-luminescent packaging materials had the most significance in affecting the light output. It is concluded that this method is useful in detecting the most significant part of LED packaging materials during the development of new packaging structures and is beneficial for enhancing the whole reliability of LED package effectively. [ABSTRACT FROM AUTHOR]

Due to the huge food demand of the population, the luminescent materials with excellent performance applied for indoor plant cultivation have been extensively studied. In this work, a series of Mn4+-doped LaBaZnTaO 6 phosphors in were synthesized and its luminescence mechanism has been researched in detail. Due to their broad absorption band, they could be effectively excited by the NUV to blue LED chip. Importantly, suitable emission in the red region overlapped with the absorption of plant pigments making it possible to be used in indoor plant cultivation. On the other hand, high-sensitivity non-contact optical temperature sensor is the focus of current research, so the optical thermometer-based lifetime model also was designed and investigated in this work. Interestingly, the maximum relative sensitivity was 1.38 %K−1@473 K, suggesting a good development potential of the sample in optical thermometry. Finally, the warm WLED device was fabricated and achieved a superior CRI value of 90.95 and a good CCT of 3582 K. All the obtained results suggest that it could become a promising red component to be applied in optical thermometer and lighting sources. • Novel double-perovskite LaBaZnTa 1- x Mn x O 6 phosphors were successfully prepared. • The crystal structure and photoluminescence properties were investigated. • The packaged LED devices were suggested for dual-functional lighting sources. • The optical thermometer-based lifetime model was introduced. [ABSTRACT FROM AUTHOR]

Abstract: In this study, we demonstrated a double freeform-surface lens to realize uniform illumination and minimum Fresnel losses for light-emitting diode (LED). In the present design, the inner surface and outer surface of the freeform lens were designed simultaneously, thus the light path can be controlled more flexibly. The detailed calculation and design process were presented. Monte Carlo ray-tracing simulation and Fresnel losses calculation were conducted to validate the present freeform lens. The simulation and calculation results indicated that the present freeform lens could enhance the illumination uniformity greatly and realize minimum Fresnel losses as low as 7.67%. [Copyright &y& Elsevier]

Abstract Phosphor-in-glass (PiG) has become as a promising resin-free color converter for high-power light-emitting diodes (LEDs). However, the challenge in luminescent performances of PiG still remain due to the deficiency of red emission spectrum. Herein, a stable red-emitting PiG was prepared for high-power lighting by introducing 3.5MgO·0.5MgF 2 ·GeO 2 :Mn4+ (MFG:Mn4+) phosphor particles into a tellurite glass matrix. The microstructures and luminescent performances of prepared PiGs were investigated. PiG-based LEDs were fabricated by combining the prepared PiGs and ultraviolet (UV)-LED chips and their optical performances were adjusted by altering phosphor content. Consequently, the introduced phosphor crystals are chemically stable under sintering process and maintain intact in the glass matrix. With the increase of phosphor content, the red emission intensity is increased. Furthermore, the prepared PiG weakens the thermal quenching of MFG:Mn4+ phosphor and exhibits excellent thermal stability under high temperature operation. The results demonstrate that the prepared PiG is a stable and efficient red-emitting converter for high-power lighting. Highlights • The PiG was prepared by introducing MFG:Mn4+ phosphor into tellurite glass matrix. • The microstructures and luminescent performances of PiG were investigated. • The prepared PiG exhibits excellent thermal stability. • The PiG is a stable and efficient red-emitting converter for high-power lighting. [ABSTRACT FROM AUTHOR]

A series of reddish orange light-emitting NaBaLa 2 (PO 4 ) 3 : Sm 3+ (NBLP:Sm 3+ ) phosphors are designed and synthesized in this work for the first time. The crystal structure of NaBaLa 2 (PO 4 ) 3 (NBLP) has been studied by the Rietveld refinement analysis, which shows that the NBLP belongs to eulytite-type orthophosphate. Luminescence properties of NBLP:Sm 3+ phosphors were investigated in detail, including excitation and emission spectra, concentration quenching, fluorescence decay, Inokuti-Hirayama (I-H) model analysis, etc. The Commission Internationale de L'Eclairage (CIE) coordinates exhibited a shift slightly from reddish-orange (0.5782, 0.4171) to orange (0.5605, 0.4312) with the increase of Sm 3+ concentration. In addition, the NBLP:Sm 3+ phosphor shows good thermal stability and acceptable quantum efficiency. From the obtained results, the prepared phosphor NBLP: Sm 3+ can be demonstrated as a potential orange-red luminescent material. [ABSTRACT FROM AUTHOR]

Besides the junction temperature, phosphor temperature is another key parameter to characterize the thermal behavior of phosphor-converted light-emitting diodes (pc-LEDs). However, the measurement of phosphor temperature remains a challenge. In this paper, we proposed a modified bidirectional thermal resistance model for the junction and phosphor temperature estimation. Compared with the conventional thermal resistance model, both the heat generation of the phosphor layer and the heat flow through the phosphor layer were further considered in this model. Three LED packaging structures were fabricated and measured to complete the model. The heat generation of the chip and phosphor layer was measured. With varying driving current from 0.05 A to 0.65 A with an increment of 0.1 A, the maximum deviation of the predicted and measured junction and phosphor temperature is less than 1% and 9.2%, respectively, which proves the feasibility of the proposed model for the junction and phosphor temperature estimation. [ABSTRACT FROM AUTHOR]