We investigate divergent behaviors in the emission of blue and green light-emitting diodes (LEDs) when they are subjected to the current-stress aging. Microscopic hyperspectral imaging is introduced to measure spatially resolved mappings of the peak intensity, the peak energy, and the full width at half maximum. Also, plots of external quantum efficiency versus injection current are measured to determine the extent of the Shockley-Read-Hall (SRH) nonradiative recombination rate based on the two-level model. As a result, a phenomenological model is proposed to account for the competitive mechanism between the carrier localization and the SRH recombination. In the blue LED case, the increase in the radiative recombination within localization states cancels the SRH nonradiative recombination within point defects. While in the green LED case, the enhancement of SRH recombination and the remission of carrier localization jointly kill the radiative recombination.

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 κ_max/κ_min, 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.

In this paper, we perform experiments by varying temperature and electrical current, and carry out the theoretical spectral optimization primarily on one-phosphor-coated three-hump white light-emitting diode (LED) for healthy indoor or outdoor lighting by using the genetic algorithm (GA). We obtain low circadian action factor (CAF) or high CAF according to different application occasions (such as bedroom lighting or office lighting), good CIE (Commission Internationale de L-Eclairage) color rendering index (CRI, Ra) or IES (Illumination Engineering Society of North America) color fidelity index (CFI, Rf) and color gamut index (CGI, Rg) or other color performances, and possibly high luminous efficacy of radiation (LER, K) at various correlated color temperatures (CCTs) from 2700 K to 6500 K. For instance, in the case of one-phosphor-coated three-hump white LEDs at CCT=3000 K, the low CAF as 0.264 (Ra =80, Rf =77, Rg=103, K=350.0 lm/W) can be achieved, while that of standard light at the same CCT is 0.407. In addition, we also compare four types of three-hump white LEDs in performances of circadian action, luminous efficacy of radiation, and color rendering. This work can help both industry and academia understand the application of three-hump white LEDs in the healthy indoor or outdoor lighting.

In this work, the genetic algorithm is employed to optimize both circadian action factor (CAF) and color quality of laser-based illuminants (LBIs) with three, four, and five spectral bands to disclose its possible use in two common white lighting applications, i.e. bedroom lighting and office lighting. Comparing all LBIs at a correlated color temperature (CCT) of 3000 K and a color rendering index of 80, the CAF of four-band LBIs reaches a minimum of 0.238 and maintains at a possibly highest luminous efficacy of radiation (LER) of 422 lm/W among all cases. The performances of white LBIs are also compared with those of white light-emitting diodes (LEDs). The results demonstrate that, under the same conditions of color rendering and color temperature, both four-band LBIs and four-band LEDs exhibit the largest circadian tunability of about 4.7, while four-band LBIs possess much higher LER at the same time compared with four-band LEDs. In addition, for the display application, the investigation on the optimal circadian tunability as a function of color gamut at two CCTs (3000 K and 6500 K) is also performed. We believe that this study can serve as a useful guidance for the application of LBIs in both the healthy general lighting and display.

We investigate temperature-dependent carrier transfer and efficiency droop on AlGaN-based deep ultraviolet light-emitting diodes. The Shockley-Read-Hall (SRH) recombination and carrier leakage are highly associated with the poor thermal stability. The existence of Auger recombination and carrier leakage is identified by the m-power method. A modified ABC model with an additional term f(n) related to carrier leakage is employed to analyze the evolution of multiple recombination mechanisms. The SRH process strongly suppresses both Auger recombination and carrier leakage at low currents. At high currents, the latter two processes are responsible for the efficiency droop and exhibit an anti-correlation upon temperature.