Yanhua Cheng

Realization of wide circadian variability by quantum dots-luminescent mesoporous silica-based white light-emitting diodes

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 CRIxa0>xa090, 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.

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 2.9382u2009mm−1, 3.7000u2009mm−1 and 0.4918 for blue light, respectively, and 1.2490u2009mm−1, 0.6062u2009mm−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.

Thermal analysis of white light-emitting diodes structures with hybrid quantum dots/phosphor layer

This study quantitatively analyzed the optical and thermal performances of remote quantum dots (QDs)-based white light-emitting diodes (QDs-WLEDs) with the same spectra power distribution (SPD) but reverse packaging structures. The output optical power and PL spectra were measured and analyzed by an integrating sphere system, and the temperature fields were simulated by combing optical measurement with thermal simulation, finally the temperature fields were validated by infrared thermal imager. It was found that when achieved identical SPD, the QDs-on-phosphor type achieved LE of 112.2 lm/W, while the phosphor-on-QDs type demonstrated lower LE of 101.4 lm/W. Moreover, the QDs-on-phosphor type 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 11.2°C at driving current of 160 mA. Therefore, the QDs-on-phosphor type is an optimal packaging architecture for higher optical efficiency and lower device temperature.

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.

Design of a hydro-dynamically levitated centrifugal micro-pump for the active liquid cooling system

The active liquid cooling system is urgently required for heat dissipation of high-power electronics. However, the key part of the system, micro-pump, is still challenged by the bearing failure problem caused by the mechanical friction. We designed a hydro-dynamically levitated centrifugal micro-pump, providing a 1.0 L/min flow rate and a 45 KPa pressure head at 20000 rpm with the size as small as 23mm×23mm×25mm. By utilizing radial and axial hydro-dynamically levitated bearings, the rotating units can be successfully levitated and realize no contact and no friction. The designed micro-pump has large power density, high rotating speed, long-life and high-reliability and we believe that it may have extensive application in active liquid cooling systems.

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.

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.

Optical modeling based on mean free path calculations for quantum dot phosphors applied to optoelectronic devices: comment

A recent publication by Shin et al. [Opt. Express25, A113 (2017)] contains an optical simulation model for the quantum dots (QDs) nanophosphor based on the mean free path concept. We show that their measured scattering pattern of QDs is misleading, which would result in a fatal deviation between simulated and actual values.