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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.

Recently, two Science papers tackled the challenge of harvesting low-grade heat via liquid thermocells. One combines TGC and TDC to significantly enhance thermopower by an order of magnitude, and the other reveals the selectively thermosensitive crystallization/dissolution at two electrodes. These intriguing and inspiring advances in terms of materials, mechanisms, and performances are briefly introduced, and some remaining challenges and future directions are outlined for further thermopower improvement and practical implementation.

Due to the difference of local power in semiconductor chips, the chip temperature is usually non-uniform with hot spots, which may cause problems like thermal stress and interfacial delamination, thus deteriorating the reliability and decreasing the electrical performance. Therefore, uniform chip temperature is demanded and pursued both by both industry and academics. The most commonly used single-phase microchannel liquid cooling is efficient but suffered from severe temperature non-uniformity, which may be tackled by tuning local channel densities. However, the existing one-dimensional model is not enough for the channel density design of microchannel with non-uniform heat flux, for it's lack of the description of heat spreading effect. In this study, we presented a bi-layer compact thermal model with considering the three-dimensional heat spreading and thermal conduction for the junction temperature gradient prediction in microchannels with non-uniform heat source. This analytical model accurately predicts the junction temperature gradient with error within 4.8% compared with COMSOL simulation results. Combining the present model with genetic algorithm, we designed a microchannel with optimized local channel widths to achieve the temperature uniformity as high as ∼90%. The maximum junction surface temperature difference is reduced from 45 °C to 13 °C with the existence of hot-spot with heat flux density of 200W/cm2. The present model and design strategy can be used to diminish the hot spot and achieve uniform temperature in high-power chips for extensive applications like integrated circuits, IGBT, HEMT, etc.

Laser-excited remote phosphor (LERP) has been reported to be an effective approach to produce high-luminance white light based on laser diodes (LDs). However, the local phosphor temperature may easily reach thermal quenching point due to the local high light power density, resulting in a significant drop/deterioration of efficiency, reliability and lifetime. In this paper, we focused on the phosphor thermal quenching and developed an optical-thermal coupling model to predict the high phosphor temperature of LERP. From this model, both accurate phosphor heating and temperature can be obtained by iteration. For validation, experiments were performed to verify the model and good agreement was observed between the measurements and the theoretical predictions. Based on the validated model, the critical incident power against thermal quenching under various factors was systematically studied. It was found in the experiments that when a 680 mW laser spot with a diameter of 1.0 mm was projected onto a phosphor layer, the phosphor temperature was as high as 549.0 °C, which would result in severe thermal quenching and even silicone carbonization. It was also found that increasing pump spot from 0.5 mm to 3.0 mm can dramatically enhance critical power by 19 times. The effect of decreasing phosphor layer thickness on critical power enhancement was explained by the model. Some suggestions were also provided to prevent thermal quenching and improve the optical/thermal performance of LERP.

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.

Light-emitting diode (LED) is regarded as the green lighting source in 21st century due to its advantages of high efficiency, energy saving, environmental protection and compact size, etc. With the popularization and application of LED in more and more lighting occasions and the improvement of people’s requirement for the quality of light source, more and more attention has been paid to the light quality of LED. The main indexes for evaluating the light quality of LED are the efficiency, angular color uniformity (ACU) and color rendering index (CRI), which are closely related to LED packaging. LED packaging is a process of assembling the chip and other packaging materials together to form the final lighting products, it plays a key role in mechanical protection, external signals and electrical connections, heat dissipation and light quality control. This paper makes a systematic exposition of the control technology of efficiency, ACU and CRI from the view of LED packaging, mainly involves three key packaging technologies: 1) thermal management. For LED products, about 60% of the input electrical power is converted to heat, therefore, the failure of the effectively heat dissipation would cause sharply increase of the temperature, which leads to the decrease of efficiency and induces reliability problems. 2) Optical design. The LED light is generate by the electron-hole recombination from the active layer of the chip, then it across the chip, phosphor layer, encapsulant layer, lens and other packaging materials. Due to scattering, refraction and absorption of the packaging materials, the light propagation direction and path would be changed, and part of the light is absorbed and converted into heat. Therefore, the optical design not only can control the LED light energy distribution, but also can reduce the light absorption in the packaging materials to improve the efficiency. 3) Phosphor coating. Compared to the multi-color LED chips packaging method, coating the monochromatic or polychromatic fluorescent powder on the LED chip to achieve white light is more popular and widely applied in the LED industrial due to its correlated color temperature (CCT) adjustability and process flexibility. For the phosphor-converted white LED, the phosphor coating plays a key role in deciding the ACU and CRI.

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%.

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.

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.

Color performance is an important parameter for high-quality light-emitting diode (LED) lighting. Color-rendering index (CRI) and color quality scale (CQS) are two independent parameters to assess the color performance, but high CRI does not correspond to high CQS, and vice versa. Therefore, it's urgent to find a comprehensive and effective metric for assessing the color performance of LEDs that can simultaneously exhibit high color-rendering index (CRI) and high color quality scale (CQS) values. In this study, a genetic algorithm with a penalty function was proposed for realizing spectral optimization by boosting the maximum attainable luminous efficacy of radiation (LER) of spectra while constraining both high CRI and CQS. By simulations, white spectra from LEDs with CRI≥95 and CQS≥95 were achieved at different correlated color temperatures (CCTs) from 2020 K to 7929 K. Further, a real spectra-tunable LED module consisting of four LEDs is fabricated, and high LER (344 lm/W) and color performance (CRI≥90, CQS=90) was realized by tuning driving currents.

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.

In this study, we realized a cylindrical tuber silicone layer for improving the light efficiency of chip-on-board light-emitting diodes (COB-LEDs) by fabricating patterned LED substrate with both silicone-wetting and silicone-repellency surfaces. To realize silicone-repellency surface, low surface energy modified nanosilica particles were prepared and deposited on the LED substrate to form porous hierarchical structure. Light efficiency enhancement for blue light COB-LEDs with pure cylindrical tuber silicone layer and white light COB-LEDs with phosphor–silicone composite layer was studied. The results show that for blue light COB-LEDs with pure cylindrical tuber silicone layer, the light efficiency increases with the contact angle and a highest light efficiency enhancement of 62.6% was achieved at 90 deg when compared to the flat silicone layer. For white light COB-LEDs at correlated color temperature (CCT) of ∼5500 K, the cylindrical tuber silicone layer enhances the light efficiency by 13.6% when compared to the conventional flat phosphor layer.

Light Emitting Diodes (LEDs) are getting popular due to their wide applicability in various domestic and industrial applications. However, the advent of high-power LEDs is accompanied with the critical issue of higher heat power management, coupled with enhanced flux levels. Natural and forced convection techniques with air as the working fluid are unable to provide thermal management at required operating safe temperatures in several upcoming compact LED array designs. Hence, there is substantial advancement in the development of thermal management solutions for the packaging of high-power LEDs. Liquid jets and sprays are potential candidates which need further exploration, especially from a point of view of packaging with LED modules. Both configurations can operate in single-phase and two-phase boiling regimes. While considerable literature is available on single-phase liquid jets, liquid sprays pose several challenges in terms of flow parameter management and associated transport physics. In this paper, we review nuances of the latter technology, i.e., spray cooling technique, crucial for thermal management of LEDs. A comprehensive overview of flow distribution and heat transfer during impingement of liquid sprays on heated surfaces is presented from the viewpoint of temperature control of high power LED sources. Additionally, the data presented will help in developing a configuration design of a liquid spray-based thermal management system integrated with high heat flux devices.

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.

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 (

In this letter, remote quantum dot phosphor-converted light-emitting diodes (QD-LEDs) with air encapsulation, silicone lens, and silicone encapsulation were fabricated. The effects of different packaging methods on the optical and thermal performances of QD-LEDs were evaluated based on the experimental tests and simulation. Optical efficiency and spectral stability were tested by experiment, and the temperature was assessed by finite-element simulation and infrared thermal imager tests. It was found that the silicone encapsulation type could convert more blue light into QDs emission light due to the reabsorption of backward reflected blue light. The silicone encapsulation type showed only a 6.2% decrease in QDs emission peak intensity when the driving current varied from 50 to 500 mA, while the silicone lens type dropped by 20.4% and the air encapsulation dropped by 36.8%. It was also confirmed that the QDs temperature in silicone encapsulation was 24 °C lower than those in the air encapsulation type and the silicone lens type at driving current of 300 mA.

The diffusive nature of heat flow lays a formidable obstacle for directional heat manipulation, not akin to the wave-governed electromagnetics that can be well controlled in intensity and direction. By modulating the near-field radiative heat transfer among graphene/SiC core-shell (GSCS) nanoparticles, we propose the concept of thermal routing to address the directional heat manipulation in a particular many-body setup. The graphene shell introduces a minor polarizability peak and remarkably modifies the localized surface resonance of the particle, which plays a significant role in the radiative heat transfer within the many-body system consisting of GSCS nanoparticles. Consequently, Fermi levels of graphene shells matching allows directional radiative heat flow, thus enabling thermal routing manifested by variant designated temperature distributions. The proposed thermal routing could be used to dynamically tune heat flow in integrated nano-objects for thermal manipulation, and also opens avenues for exploiting novel thermal functionalities via radiative heat transfer at the nanoscale.

In this study, we proposed a flexible method to realize a cylindrical tuber encapsulant layer. Firstly, patterned LED substrate with silicone-wetting and silicone-repellency surfaces was prepared by depositing low surface energy modified nanosilica particles. Secondly, coating phosphor gel onto the LED chip. Thirdly, coating the encapsulant (silicone OE6550A/B) onto the phosphor gel layer. The silicone stops spreading at the border of silicone-wetting and silicone-repellency surfaces and its final contact angle can be adjusted to any value between the contact angle of silicone-wetting and silicone-repellency surface, so it is very easy to realize dome-shape silicone layer with contact angle of ∼90°. For COB-LEDs with LED array in series, the final silicone layer geometry is similar to the cylindrical tuber. The results show that compared to the conventional flat encapsulant layer, the proposed encapsulant layer can improve the light efficiency by > 60% for pure blue light and 13.6% for white light at correlated color temperature (CCT) of ∼5500K.

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.

In the phosphor coating process of the phosphor-converted light-emitting diodes (pc-LEDs) packaging, the phosphors settle spontaneously due to the density difference between the phosphor particles (YAG-04, about 4800 kg/m3) and the silicone matrix (OE6550 A/B, about 1120 kg/m3). The phosphor sedimentation affects the phosphor distribution and changes the optical performance of pc-LEDs, such as the luminous efficiency (LE) and correlative color temperature (CCT). Therefore, a deep understanding of the phosphor sedimentation effect on the optical performance of pc-LEDs is urgent needed. In this study, we applied an online testing experimental setup to investigate effect of the substrate temperature on the phosphor sedimentation of pc-LEDs. LED substrate was heated to a pre-defined temperature before the phosphor gel been coated, a range of substrate temperature from 30 °C to 100 °C with a step of 10 °C was investigated. The results show that the substrate temperature can accelerate or diminish the phosphor sedimentation, when temperature increases from 30 °C to 100 °C, the CCT variation decreases from 39.62% to 0.27%, and the LE variation decreases from 4.08% to 0.21%.

In phosphor-converted light-emitting diodes (pc-LEDs), phosphor heating can lead to decrease of luminous efficiency or even thermal quenching due to local high phosphor temperature. Phosphor scattering model has been used to calculate the phosphor heat generation. Using this model, there are two methods for heating power estimation, one is the subtractive method and the other is the integral method. In this work, we conducted comparative study of these two methods. We found the heating power of integral method is lower than that of the subtractive method. Phosphor heat generation measurement was performed and results showed that the experimental data agreed well with the subtractive method. Finite element model (FEM) was used to evaluate the phosphor temperature by inputting the calculated heating power. Higher phosphor temperature was obtained by the subtractive method due to relative higher heat load. Infrared thermal test verified that the measured phosphor temperature was close to that of the subtractive method. Therefore, the phosphor scattering model using the subtractive method can be applied for accurate phosphor heating and temperature estimation.

Human comfort has become one of the most important criteria in modern lighting architecture. Here, we proposed a tuning strategy to enhance the non-image forming photobiological effect on the human circadian rhythm based on quantum-dots-converted white light-emitting diodes (QDs-WLEDs). We introduced the limiting variability of the circadian action factor (CAF), defined as the ratio of circadian efficiency and luminous efficiency of radiation. The CAF was deeply discussed and was found to be a function of constraining the color rendering index (CRI) and correlated color temperatures. The maximum CAF variability of QDs-WLEDs was found to be dependent on the QDs' peak wavelength and full width at half maximum. With the optimized parameters, the packaging materials were synthesized and WLEDs were packaged. Experimental results show that at CRI > 90, the maximum CAF variability can be tuned by 3.83 times (from 0.251 at 2700 K to 0.961 at 6500 K), which implies that our approach could reduce the number of tunable channels, and could achieve wider CAF variability.

In this study, genetic algorithm (GA) was utilized to optimize the phosphor distribution to decrease the junction temperature of white phosphor-converted light-emitting diodes (pc-LEDs). The key steps of the GA were introduced, including selection, crossover, and mutation. Both the junction temperature and the entransy dissipation of each evolution were calculated. It was found that with evolutions, the phosphor particles tend to build a “thermal bridge” between the chip and the convective boundary and spread along the convective boundary. The junction temperature decreases from ∼157.5 °C to ∼150 °C and the entransy dissipation decreases from ∼18 W K to ∼6 W K. The least entransy dissipation principle was demonstrated to be the rule that governs the optimization processes.

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 0.34 g/cm 3 , 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 2 μm, which is denoted as the critical size.

Acquiring the junction temperature of light emitting diode (LED) is essential for performance evaluation. But it is hard to get in the multipleLED applications. In this paper, an engineering method is presented to estimate the junction temperatures of LEDs in multiple LED applications. This method is mainly based on an analytical model, and it can be easily applied with some simple measurements. Simulations and experiments were conducted to prove the feasibility of the method, and the deviations among the results obtained by the present method with those by simulation as well as experiments are less than 2% and 3%, respectively. In the final part of this study, the engineering method was used to analyze the thermal resistances of a street lamp. The material of leadframe was found to affect the system thermal resistance mostly, and the choice of solder material strongly depended on the material of the leadframe.

This paper reports a new way to conduct fracture mechanics experiments of welded thin-walled structure for aircraft fuselage applications made of the Al 6156 ally using lock-in infrared thermography. The heat wave, generated by both the themo-mechanical coupling and intrinsic energy dissipation was detected by an infrared camera. The crack growth rate measured by the lock-in infrared thermography is found to be consistent with that measured by conventional method. It is observed that the crack can either grow underneath the stiffener or grow into the stiffener web with the crack length measured at the same time. In addition, the crack tip plastic zone was observed before the specimen was about to be destroyed.

GaN-based light-emitting diodes (LEDs) with graded-thickness quantum wells and barriers (GMQW-LEDs) are fabricated and researched in this letter. The light power and carrier distribution of GMQW-LEDs are compared with those of LEDs with original uniform MQW (OR-LEDs), graded-thickness quantum wells (GQW-LEDs), and graded-thickness quantum barriers (GQB-LEDs) through numerical simulation, respectively. The experimental results show that light power of GMQW-LEDs is enhanced significantly compared with that of OR-LEDs. The simulation results reveal that GMQW-LEDs show light output power enhancements of 25.7%, 14.3%, and 9.2% compared with OR-LEDs, GQW-LEDs, and GQB-LEDs at current density of 100 A/cm2, respectively. This is due to the superior hole distribution in quantum wells, which inhibits the electron leakage and enhances the radiative recombination.

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.

In this letter, we modeled the bi-layer phosphors in light-emitting diode packages and optimized the concentration gradient to enhance the light extraction efficiency (LEE). The modeling processes were presented in detail. Based on our bi-layer model, we changed the concentration from 0.05 to 0.4 g/cm3 in both the upper and lower phosphor layers. It was found that when the concentration gradient between the upper layer and lower layer increases from 0.05:0.4 to 0.4:0.05, the LEE would be enhanced as much as 17.63%. The trends were validated by experiments. Some potential ways were proposed to realize this kind of phosphor configuration where the concentration of the upper layer was larger than that of the lower layer.

For light-emitting diode (LED) packages, temperatures, especially junction and phosphor temperature, can greatly affect the optical performance. Thus, measuring the temperatures accurately is of significance for the performance estimation. Among the methods of measuring temperature, physical contact method, represented by thermocouple method, is most convenient and economical. In this paper, we measured the junction and phosphor temperature of LED packages by thermocouple. In the experiments to measure junction temperature of a blue LED module, we simplified the judgment by measuring the air temperature above the bare chip and it was found that the test result was much higher than that obtained by another recognized method. The causes were ascribed to two factors, i.e. thermal resistance of the air gap and the light energy absorption. In the experiments to measure phosphor temperature, plug-in measurement was adopted to eliminate the influence of air gap's thermal resistance. However, the tested value turned out to be even higher. Then the main cause of the inaccurate measurement by thermocouple was attributed to the light energy absorption and conversion. And an experiment was carried out to support the explanation. As a whole, to accurately measure the LED's temperatures by thermocouple, the light absorption effect needs to be considered.

Recent years, semiconductor quantum dots (QDs) have attracted tremendous attentions for their unique characteristics for solid-state lighting (SSL) and thin-film display applications. The pure and tunable spectra of QDs make it possible to simultaneously achieve excellent color-rendering properties and high luminous efficiency (LE) when combining colloidal QDs with light-emitting diodes (LEDs). Due to its solution-based synthetic route, QDs are impractical for fabrication of LED. QDs have to be incorporated into polymer matrix, and the mixture is dispensed into the LED mold or placed onto the LED to fabricate the QD–LEDs, which is known as the packaging process. In this process, the compatibility of QDs' surface ligands with the polymer matrix should be ensured, otherwise the poor compatibility can lead to agglomeration or surface damage of QDs. Besides, combination of QDs–polymer with LED chip is a key step that converts part of blue light into other wavelengths (WLs) of light, so as to generate white light in the end. Since QD-LEDs consist of three or more kinds of QDs, the spectra distribution should be optimized to achieve a high color-rendering ability. This requires both theoretical spectra optimization and experimental validation. In addition, to prolong the reliability and lifetime of QD-LEDs, QDs have to be protected from oxygen and moisture penetration. And the heat generation inside the package should be well controlled because high temperature results in QDs' thermal quenching, consequently deteriorates QD-LEDs' performance greatly. Overall, QD-LEDs' packaging and applications present the above-mentioned technical challenges. A profound and comprehensive understanding of these problems enables the advancements of QD-LEDs' packaging processes and designs. In this review, we summarized the recent progress in the packaging of QD-LEDs. The wide applications of QD-LEDs in lighting and display were overviewed, followed by the challenges and the corresponding progresses for the QD-LEDs' packaging. 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 QD-LEDs' packaging and application technologies.

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.

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.

Metamaterials have attracted sufficient attentions in exploring new phenomena and functions, improving performance and efficiency, and extending to other physical fields. But how to detect the metamaterial structures is still lacking though it is significantly demanded in imaging analysis or military application. Here, we successfully attempt this problem by flying laser point to detect the edge of the thermal metamaterial structures. The ideal thermal cloak, concentrator and rotator are taken as examples with brief introduction of the design process. The dynamic hot spot behaviors, corresponding to the evanescent dissipation of thermal energy, of the flying laser point are analyzed with full description of the specific features and successful identification of the three thermal structures on the plane. The proposed method can be used for edge detection, and help developing some transient imaging analyzer or anti-function structures.

In this paper, a self-heating model was developed based on Run Hu's light scattering model to calculate the heat generation and an integrated thermal resistance model was established to predict the temperature of phosphor silicone. It is found that the trends of highest temperature along with the phosphor concentration and particles diameters are both non-monotonic with a wave crest. While the temperature varies monotonically with the changes of thermal conductivity of matrix and thickness of phosphor silicone, separately. Then the physical mechanism behind these phenomena are discussed respectively.

In this study, a brightness-enhancement-film- (BEF) adaptive method is proposed to design freeform lenses for enhancing brightness performance in a direct-lit light-emitting diode (LED) backlight system. A detailed design algorithm is presented based on the analysis of the output optical properties of the BEF. By introducing a constriction factor, we can control the light intensity distribution curve at will to adapt to the characteristics of the BEF and make more light transmit through the BEF. Compared with an LED backlight system without a freeform lens, the BEF-adaptive lens method can improve axial luminance by 20.67% and output efficiency by 6.02%.

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%.

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 μm when taking LEE, CCT, and ACU into consideration.

The phosphor-converted light emitting diodes (LEDs) are the most frequently used white light LED. Due to the Stoke's loss, the silicone/phosphor composites generate a considerable amount of heat. However, as a result of different sedimentation times or coating processes, the phosphor particles present different distributions in the silicone matrix, which has been proved to strongly influence the performance and the reliability of LEDs. A lattice Boltzmann model with heat source is established to investigate the effect of phosphor particle distributions on the local temperature in the phosphor layer. The phosphor heat generation was calculated by a modified Kubelka-Munk theory. Simulations were conducted under four different phosphor particle distributions which correspond to four cases: homogeneous distribution, partial sedimentation, complete sedimentation, and remote coating, respectively. Then the temperature fields of the phosphor layer under these conditions were obtained. The minimum and maximum temperatures in the phosphor layer were analyzed to investigate the effect of phosphor self-heating under different phosphor particle distributions. According to the simulation results, different phosphor particle distributions lead to the hotspot location shift and different temperatures. The minimum temperatures were hardly influenced under different phosphor particle distributions, while the maximum temperatures have a difference over 26 K between these different cases. The remote coating brings the highest temperature which reaches 374.049 K and the phosphor sedimentation can decrease the maximum to a certain extent. The phosphor self-heating with different phosphor particle distributions brings obvious influence on LED thermal management.

Thermal management has always been crucial for the improving of LED reliability. Most researches focus on the thermal management outside the package like fin optimization, microchannel, microjet cooling, but few refer to the thermal management inside the package. In this paper, a contrast experiment was carried out by an online testing method to explore the LED's optical degradation under conditions with/without thermal management inside the LED package. Results show that different LED degradation characteristics appear in the experiment and the sample without package-inside thermal management exhibits carbonization phenomenon. Thermal management inside the package should be further reinforced to enhance the reliability of LED package.

Thermal management for white light-emitting diode (LED) packages is of great importance. Conventional thermal management methods mainly focus on the outside of LED package, but we, in this study, focused on the thermal management inside the LED package. Besides the heat generated in LED chips, we also consider the phosphor heat generation, which is calculated by Kubelka-Munk theory. We analyzed the effect of phosphor particle distribution on the thermal behaviors of LED packages. Two kinds of phosphor sizes, i.e. 5 μm and 7 μm, were applied for configuration optimization. Finite-element method (FEM) was adopted to simulate three cases. It is found that when the upper phosphor layer has larger concentration, the phosphor temperature is higher but the light extraction efficiency (LEE) is lower; when the lower phosphor layer has larger concentration, the phosphor temperature is lower but the LEE is higher. The phosphor temperature uniformity can be enhanced by 0.22% when the concentration of the lower phosphor layer is larger. Phosphor sedimentation, although deteriorates the optical performance, benefits the thermal behaviors on the contrary. There is a compromise that the thermal behaviors and optical performance seem to be inconsistent for different phosphor configurations.

The effective thermal conductivity (ETC) of silicone matrix embedded with particulate phosphors is of great importance in the thermal management of white light-emitting diode (LED) packages, not only because the mixture provides the heat dissipation path but also because the phosphors may generate heat as well. In this paper, a 2D/3D unit cell method with considering interface resistance was proposed to calculate the ETC of the silicone matrix embedded with particulate phosphors. To make the method more general, cubic, spherical and cylindrical particulate phosphors were calculated and discussed respectively. In order to validate the method, hot wire method was used to measure the thermal conductivity of phosphor silicone matrix. It is found that when the volume concentration increases from 0.038 to 0.25, the relative error between the experiments and calculations is within 5%. The ETC calculated by 2D model is slightly smaller than that by 3D model. The 3D model has relatively smaller critical concentration because the probability of forming continuous conductive chain by phosphor particles is lager in the 3D model than the 2D model. The shape of particle affect the ETC and the effect becomes more obvious at large concentration. The heat generated by phosphor particles is found to have no effect on the ETC.

Current crowding effects (CCEs) on light extraction efficiency (LEE) of conventional GaN-based light-emitting diodes (LEDs) are analyzed through Monte Carlo ray-tracing simulation. The non-uniform radiative power distribution of the active layer of the Monte Carlo model is obtained based on the current spreading theory and rate equation. The simulation results illustrate that CCE around n-pad (n-CCE) has little effect on LEE, while CCE around p-pad (p-CCE) results in a notable LEE droop due to the significant absorption of photons emitted under p-pad. LEE droop is alleviated by a SiO2 current blocking layer (CBL) and reflective p-pad. Compared to the conventional LEDs without CBL, the simulated LEE of LEDs with CBL at 20 A/cm2 and 70 A/cm2 is enhanced by 7.7% and 19.0%, respectively. It is further enhanced by 7.6% and 11.4% after employing a reflective p-pad due to decreased absorption. These enhancements are in accordance with the experimental results. Output power of LEDs with CBL is enhanced by 8.7% and 18.2% at 20 A/cm2 and 70 A/cm2, respectively. And the reflective p-pad results in a further enhancement of 8.9% and 12.7%.

Experimental investigations were carried out on the light extraction efficiency (LEE) of the phosphor-converted white light-emitting diodes (LEDs) with bi-layer phosphor configuration. Phosphor modeling process was briefly introduced. It is found that when the upper phosphor's concentration is 0.4 g/cm3 and the lower layer's concentration is 0.05 g/cm3, the LEE was enhanced as much as 23.0%. It was concluded that when upper phosphor layer has larger concentration than the lower layer, the LEE would be enhanced. The mechanisms behind the phenomena were discussed.

In the conventional freeform lens design, people usually assume the inner surface as hemispherical and only the outer surface is designed. An important design degree of freedom on the inner surface is abandoned. In this study, by distributing the deviation angle, we proposed a method to design the inner and outer surfaces of freeform lens simultaneously. Detailed design method was presented and several examples were designed to verify the method. It was found when the distribution ratio is 0.75, the simulated illumination is the most uniform and its Fresnel light loss is the rather smaller than that conventional freeform lens. By overall consideration of the illumination uniformity and the Fresnel loss, the distribution ratio of 0.75 is the optimal extra condition and corresponds to best performance.

White light emitting diodes (LEDs) with microstructure array on top surface of the phosphor silicone layer are demonstrated to have enhanced the light extraction efficiency (LEE) compared with white LEDs with flat phosphor layer. The effects of microstructure array on the LEE of LEDs are investigated by ray tracing simulations and experiments. Simulation results reveal that spherical cap microstructure array can increase the LEE by 12%. Experimental results also show that spherical cap microstructure array fabricated by a molding method can increase the luminous efficiency by 10.1%.

Secondary optics design in the past research only considered illuminance uniformity and ignored luminance uniformity and glare issue, which may result in serious security problem for vehicle drivers. A novel un-symmetric freeform LED lens designed for roadway lighting is proposed. Optical performance of un-symmetric lens will be compared with symmetric lens by optical software. The simulation results demonstrate that light intensity distribution curve (LIDC) of LED luminaire becomes un-symmetric and the overall roadway surface luminance uniformity with un-symmetric lens increases to 0.6, the longitudinal road surface luminance uniformity increases to 0.8, and light power density (LPD) drops to 0.7 W/m2.