Ziquan Guo

Optimization Studies of Two-Phosphor-Coated White Light-Emitting Diodes

Three-hump InGaN-based white light-emitting diodes (LEDs) precoated with traditional yellow/green phosphors and red-emitting quantum dots (QDs), have been numerically investigated. Under variations of eight correlated color temperatures (CCTs), three wavelengths, two bandwidths, and two peak heights, optimal results of luminous efficacy radiation (LER) and color rendering index (CRI) are identified and retained through filtering off billions of unqualified candidates. These results include LER = 390 lm/W and CRI = 90 [chromaticity difference (Duv)<;0.0054] at CCT = 3000 K. In addition, our photometric and colorimetric sensitivity studies provide the dependence of LER, CRI, CCT, and Duv on LED spectral parameters affected by operating temperatures. Finally, we have discovered that higher instabilities may be induced for cool white LEDs (CCT = 6500 K) than for warm white LEDs (CCT = 3000 K) within the analysis of CCT versus spectral parameters.

Study on the correlations between color rendering indices and the spectral power distribution

The intrinsic spectrally resolved sensitivity (ISRS) of color rendering indices (CRIs) is investigated by using spectral loss simulations. It is demonstrated that R(a) exhibits large sensitivities around 444, 480, 564, and 622 nm, while for R(9) the sensitivity peaks are around 461, 581 and 630 nm, which all shift slightly with the correlated color temperature. If considering the ISRS as a bridge between the spectral power distribution of LED and its CRI, one could obtain a high CRI by minimizing the deviation between the shapes of the illuminant spectrum and the reference spectrum, both after modulations by the ISRS as a weighting function. This approach, recommended as a guideline for the spectra design aiming at a high CRI, is described and justified in depth via a mathematical model. This method is spectra-oriented and could largely facilitate the spectra design.

Studies of Scotopic/Photopic Ratios for Color-Tunable White Light-Emitting Diodes

Three- and four-hump InGaN-based white light-emitting diodes (LEDs) have been computationally investigated. The investigation includes three-hump LEDs precoated with green- and red-emitting quantum dots (QDs) and four-hump LEDs precoated with green-, yellow-, and red-emitting QDs. Results show scotopic/photopic (S/P) ratios 3.80 and color rendering indices (CRIs) ≥ 70 for three-hump LEDs and S/P ratios 3.90 and CRIs ≥ 70 for four-hump LEDs under a correlated color temperature (CCT) of 45 000 K. Furthermore, for both three- and four-hump LEDs under the condition of CCT 5000 K, optimal results in this paper exhibit capabilities of exceeding S/P = 2.50 and simultaneously satisfying the criterion of CRI ≥ 70. Our simulation procedure is conducted under variations of nine CCTs, several wavelengths, and peak heights. Optimized results of S/P ratios and CRIs are identified and retained through filtering off a great number of unqualified candidates. Finally, present findings may help improve the quality of eye visions pertaining to LED illumination and help reduce the electrical energy consumption related to LED usage.

Maximal rectification ratios for idealized bi-segment thermal rectifiers

Thermal rectifiers whose forward heat fluxes are greater than reverse counterparts have been extensively studied. Here we have discovered, idealized, and derived the ultimate limit of such rectification ratios, which are partially validated by numerical simulations, experiments, and micro-scale Hamiltonian-oscillator analyses. For rectifiers whose thermal conductivities (κ) are linear with the temperature, this limit is simply a numerical value of 3. For those whose conductivities are nonlinear with temperatures, the maxima equal κmax/κmin, where two extremes denote values of the solid segment materials that can be possibly found or fabricated within a reasonable temperature range. Recommendations for manufacturing high-ratio rectifiers are also given with examples. Under idealized assumptions, these proposed rectification limits cannot be defied by any bi-segment thermal rectifiers.

Improvements on Optical and Chromatic Uniformities of Light-Emitting Diodes With Microscale-Roughness-Controlled Surfaces

In the light-emitting-diode (LED) industry, it is a great challenge to simultaneously achieve high efficiency values and excellent light quality in practical applications. In this paper, we have designed microscale-roughness-controlled-surface modules to optimize rectified mellowness index, quantum efficiency, correlated color temperature, and color rendering index; we have also discovered a mechanism that reduces the total internal reflection trapped within the diffuser, maximizes phosphor fluorescence probabilities, and randomizes photon emissions. The first effect subsequently leads to the photon-propagation-direction rectification, such that incident photons are allowed to escape from the top but are discouraged to reversely exit from the bottom. In addition to the discovery of this mechanism, we have proposed a previously undefined criterion that evaluates the uniformity of various optical parameters exemplified by wavelengths, solid angles, and spatial distributions. Current findings offer a guide on future research for human-eye-comfort-related uniformities.

Multi-function indoor light sources based on light-emitting diodes-a solution for healthy lighting.

A solution for multi-functional indoor light sources is proposed to achieve the new concept of healthy lighting. A remotely controllable light source that embodies a quadruple-chip light-emitting diode and driven by pulse-width-modulation currents is designed. Therefore, spectral power distributions (SPDs) of the light source can be readily controlled. An algorithm, namely the optical power ratio algorithm, is developed to select all suitable SPDs adapted for various applications. Principles of selection are based on those traditional visual indices, as well as on some non-visual parameters such as circadian action factor, circadian efficacy of radiation and circadian illuminance. We investigate in detail the correlation among these parameters and provide SPDs with both decent visual and non-visual performances for three typical cases. The study suggests some fundamental principles for designing healthy light sources, and can be regarded as a guide for designing indoor light sources of the next generation.

Determining Junction Temperature in InGaN Light-Emitting Diodes Using Low Forward Currents

We propose an experimental method that determines junction temperatures in light-emitting diodes by measuring currents while holding the low forward voltages constant. In this procedure, we first calibrate current-temperature-rela- tionship parameters under the condition of negligible thermal generation. With one of the two parametric values, we discover the existence of a forward voltage peak that yields most sensitive measurements of the junction temperature. Results show a nearly linear relationship between the algorithmic currents and temperature reciprocals with high testing precision.

Study on color-tunable phosphor-coated white light-emitting diodes with high S/P ratios

In this study, we have investigated the trade-off between the color rendering index (CRI, Ra) and the scotopic/photopic ratio (S/P) for color-tunable phosphor-coated white light-emitting diodes (LEDs) at two CRI limitations (Ra ≥ 70 and Ra ≥ 96). First, luminescent spectra measurements have been conducted to determine experimental results of Ra and S/P under various correlated color temperatures (CCTs). Then, a nonlinear programming method has been adopted for the optimization of Ra and S/P by varying spectral shapes through adjusting spectral parameters, such as peak wavelengths, full-width at half-maxima, and relative intensities. Therefore, polynomial curves of optimal S/P versus CCT at two Ra limitations have been discovered, enabling users to obtain optimal S/P under arbitrary CCTs within [2700 K, 6500 K]. In addition, a comparison study between the present work and our previous work has also been conducted at Ra = 70, and a fair agreement of optimal S/P has been observed.

Defects dynamics during ageing cycles of InGaN blue light-emitting diodes revealed by evolution of external quantum efficiency--current dependence.

We report in detail the defect dynamics in the active region by monitoring the external quantum efficiency (EQE) - injection current curves, I-V curves, and electroluminescence spectra during the ageing test, under a forward current of 850 mA (85 A/cm2), room temperature. We apply a two-level model to analyze the EQE curves and the electroluminescence spectra. The results suggest that high injection density during the ageing may reduce the density of the Shockley-Reed-Hall nonradiative recombination centers and enhance the carrier mobility and diffusion length. The former effect would directly lead to initial surge of EQE, whereas the latter would enhance the effect of extended defects which leads to reduction in peak EQE and increase in EQE droop rate.

Optical Degradation Mechanisms of Indium Gallium Nitride-Based White Light Emitting Diodes by High-Temperature Aging Tests

We have measured five major mechanisms that govern light-emitting diode (LED) degradation, calculated respective percentages, and identified the most influential mechanism during the time evolution of aging. After 480 h, due to the reduction of phosphor-conversion efficiency, the increase of non-radiative centers, the deterioration of thermal properties, the darkening of silicone resin lenses, and the degradation of Ag reflective layers, approximately 42% of the average optical power has decayed. Within this 42%, Ag degradation contributes approximately 28% (with the remaining 14% contributed by the degradation of silicone resin lenses), primarily because other chemical elements have deposited on the surface of Ag reflective layers, thus lowering the reflectivity of these layers. The present work can help draw the communitys attention to lowering LED junction temperatures, and to the optimization of packaging designs.