Xingjian Yu

Thermal Remote Phosphor Coating for Phosphor-Converted White-Light-Emitting Diodes

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.

Effect of Packaging Method on Performance of Light-Emitting Diodes With Quantum Dot Phosphor

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.

A Cylindrical Tuber Encapsulant Geometry for Enhancing Optical Performance of Chip-on-Board Packaging Light-Emitting Diodes

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.

Angular Color Uniformity Enhancement of White LEDs by Lens Wetting Phosphor Coating

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.

Structural optimization for remote white light-emitting diodes with quantum dots and phosphor: packaging sequence matters

White light-emitting diodes (WLEDs) with quantum dots (QDs) and phosphor have attracted tremendous attentions due to their excellent color rendering ability. In the packaging process, QDs layer and phosphor-silicone layer tend to be separated to reduce the reabsorption losses, and to maintain the stability of QDs surface ligands. This study investigated the packaging sequence between QDs and phosphor on the optical and thermal performances of WLEDs. The output optical power and PL spectra were measured and analyzed, and the temperature fields were simulated and validated experimentally by infrared thermal imager. It was found that when driven by 60 mA, the QDs-on-phosphor type WLEDs achieved luminous efficiency (LE) of 110 lm/W, with color rendering index (CRI) of Ra = 92 and R9 = 80, while the phosphor-on-QDs type WLEDs demonstrated lower LE of 68 lm/W, with Ra = 57 and R9 = 24. Moreover, the QDs-on-phosphor type WLEDs generated less heat than that of another, consequently the highest temperature in the QDs-on-phosphor type was lower than another, and the temperature difference can reach 12.3°C. Therefore, in terms of packaging sequence, the QDs-on-phosphor type is an optimal packaging architecture for higher optical efficiency, better color rendering ability and lower device temperature.

Examination of the Thermal Cloaking Effectiveness with Layered Engineering Materials

The concentrically layered thermal cloaks with isotropic materials could realize the equivalent thermal cloaking effect with Pendrys cloak, while the effectiveness is scarcely investigated quantitatively. Here we examine the cloaking effectiveness quantitatively by evaluating the standard deviation of the temperature difference between the simulated plane with the layered thermal cloak and Pendrys thermal cloak. The design rules for the isotropic materials in terms of thermal conductivity and layer thickness are presented. The present method could quantitatively evaluate the cloaking effectiveness, and could open avenues for analyzing the cloaking effect, detecting the (anti-) cloaks, etc.

Design of a brightness-enhancement-film-adaptive freeform lens to enhance overall performance in direct-lit light-emitting diode backlighting.

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%.

Exploring the proper experimental conditions in 2D thermal cloaking demonstration

Although thermal cloak has been studied extensively, the specific discussions on the proper experimental conditions to successfully observe the thermal cloaking effect are lacking. In this study, we focus on exploring the proper experimental conditions for 2D thermal cloaking demonstration. A mathematical model is established and detailed discussions are presented based on the model. The proper experimental conditions are suggested and verified with finite element simulations.

Phosphor Temperature Overestimation in High-Power Light-Emitting Diode by Thermocouple

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.

Energy-Saving Light Source Spectrum Optimization by Considering Object's Reflectance

When an object is illuminated, part of the incident light is absorbed by the object that does not contribute to the illumination. If the light sources spectrum can be optimized by decreasing the light energy along the absorbed wavelengths, substantial energy can be saved. In this study, a spectrum optimization method by means of a genetic algorithm to minimize the light energy along the absorbed wavelengths while maintaining the light quality of the light source was proposed. Under the constraint that the color differences of the object illuminated by the optimized illuminant and the reference illuminant do not exceed specified levels, maximum energy saving ratio (ESR) is achieved for various reference illuminants. It is found that through the proposed method effectively saving the energy, for a monochromatic object, the ESR can reach 49.5% with barely recognizable color difference, whereas for a polychromatic object, the ESR can reach 55.2% with an acceptable color difference for each patch of the object.