Weicheng Shu

Precise optical modeling of quantum dots for white light-emitting diodes

Quantum dots (QDs)-based white light-emitting diodes (QDs-WLEDs) have been attracting numerous attentions in lighting and flat panel display applications, by virtue of their high luminous efficacy and excellent color rendering ability. However, QDs’ key optical parameters including scattering, absorption and anisotropy coefficients for optical modeling are still unclear, which are severely against the design and optimization of QDs-WLEDs. In this work, we proposed a new precise optical modeling approach towards QDs. Optical properties of QDs-polymer film were obtained for the first time, by combining double integrating sphere (DIS) system measurement with inverse adding doubling (IAD) algorithm calculation. The measured results show that the typical scattering, absorption and anisotropy coefficients of red emissive QDs are 2.9382 mm−1, 3.7000 mm−1 and 0.4918 for blue light, respectively, and 1.2490 mm−1, 0.6062 mm−1 and 0.5038 for red light, respectively. A Monte-Carlo ray-tracing model was set-up for validation. With a maximum deviation of 1.16%, the simulated values quantitatively agree with the experimental results. Therefore, our approach provides an effective way for optical properties measurement and precise optical modeling of QDs for QDs-WLEDs.

A Facile Approach to Fabricate Patterned Surfaces for Enhancing Light Efficiency of COB-LEDs

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.

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.

White light-emitting diodes with enhanced luminous efficiency and high color rendering using separated quantum dots@silica/phosphor structure

White light-emitting diodes (WLEDs) composed of blue LED chip, yellow phosphor and red quantum dots (QDs) are considered as potential alternative for next generation artificial light source, with their high luminous efficiency (LE) and color rendering index (CRI). However, the poor compatibility of QDs/silicone and the optical reabsorption effect between the mixed QDs/phosphor particles severely hinder the wide utilization of QDs-WLEDs. Therefore, in this letter, a separated QDs/phosphor structure was proposed to eliminate the reabsorption energy losses, and a silica shell was coated onto the QDs surface to solve the compatibility problem between QDs and silicone. With CRI > 90 and R9 > 90, the newly proposed QDs@silica nanoparticles based WLEDs (QSNs-WLEDs) demonstrated a LE enhancement of 14.6 % over conventional mixed type WLEDs. Benefit from the silica shell coating, the QSNs-WLEDs also show high stability under different driving current.

Structural design of LED packaging in terms of lumen reliability by a statistical method

The structural design for LED packaging is of great importance. S o far, the methods for LED packaging are mostly from the perspective of enhancing the emitted lights optical performance. In this paper, we proposed a statistical method for the structural design of LED packaging in terms of lumen reliability. Orthogonal experimental design was used in the determination of the experimental scheme. Optical simulations were conducted to calculate the optical power of LED package according to the scheme. Two traditional structural models were selected as objectives. Range and variance analyses were applied to investigate the significance of structural factors on the LEDs light output before guiding the structural design. According to the analyses, it is suggested that the lens and silicone encapsulant can be combined as one and the quality of the heat slugs top surface should be enhanced pointedly. It is concluded that this method is useful for the development and design of new packaging structures from the perspective of reliability.

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.