Yupu Ma

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.

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.

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.

Dynamic Phosphor Sedimentation Effect on the Optical Performance of White LEDs

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%.

Directional heat transport through thermal reflection meta-device

Directional heat transfer may be hard to realize due to the fact that heat transfer is diffusive. In this paper, we try to take one step forward based on the transformation thermodynamics. A special structure and meta-device is proposed to “reflect” the heat flow directionally–just like the mirror to the light beam, in which the heat flow just one-time changes the direction rather than gradually changing the directions in isotropic materials. The benefits of such thermal reflection meta-device are discussed by comparing the corresponding thermal resistance with the same structures of isotropic materials. The proposed meta-device is verified to possess the low thermal resistance and high heat transfer ability with least energy loss, and can be made by nature-existing isotropic materials with specific structures.

Investigation on dip-transfer phosphor coating for light-emitting diodes: Experiments and VOF simulations

Dip-transfer phosphor coating method and its benefit on enhancing angular color uniformity (ACU) of white light-emitting diodes (LEDs) were previously reported, however, for applying this method in mass production, its fluid transfer mechanism and packaging consistency needs to be further investigated. The dip-transfer process is divided into two process, they are dipping process and transfer process. In our previous study, the dipping process were studied with experiments and simulations. In this study, we further studied the transfer process with numerical simulations based on combination of the volume of fluid (VOF) method and the dynamic mesh model, four parameters include post radius, withdrawal velocity, transfer height and phosphor gel viscosity were investigated. Besides, the packaging consistency of the dip-transfer phosphor coating method was studied with experiments. The simulated results show that the transfer volume decreases with the post radius, phosphor withdrawal velocity and phosphor gel viscosity, while keep the same with the transfer height. The experimental results show that the packaging consistency is highly rely on the transfer volume, with transfer volume varies from 0.71 μl to 6.12 ul, the maximum transfer volume deviation (MTVD) changes from 6.98% to 2.31%.

Silica doped quantum dots film with enhanced light conversion efficiency for white light emitting diodes

Quantum dots (QDs) have broad prospect in illumination and display in the future with their high quantum efficiency, tunable color in the entire visible range, and excellent color rendering ability. QDs are generally prepared as remote films in white light-emitting diodes(WLED), which gives rise to low light conversion efficiency (LCE). In order to enhance the LCE of QD-converted WLED (QCWLED), a variety of mass fraction of micrometer-scale silica particles (SPs) were doped in QDs-PMMA films (QPFs) to prepare silica-QDs-PMMA films (SQPFs). Excited by identical blue light, the optical properties including optical power, reflectance, transmittance and collimating transmittance of the SQPFs and the QPF were measured by a double integrating sphere system, and their absorption coefficient, scattering coefficient and anisotropic coefficient were calculated by the inverse adding doubling algorithm (IADA). The effect of the SPs concentration on the optical properties of SQPFs was discussed. Results suggested that SQPFs possess higher LCE than the QPF and the LCE of the SQPFs increased gradually with the rise of the SPs mass fraction in the range from 0 % to 50 %. The absorption coefficient, scattering coefficient and anisotropy coefficient of the SQPFs were significantly related to the concentration of the SPs. Finally, the mechanism about how SPs work was analyzed.

Effect of the substrate temperature on the phosphor sedimentation of phosphor-converted LEDs

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%.

A comparative study of phosphor scattering model for phosphor-converted light-emitting diodes

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.