Zhuguang Liu

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.

Optical enhancement brought by doping Gd(3+) ions into Ce: YAG ceramics for indoor white light-emitting diodes.

We dope Gd(3+) ions into Ce: YAG ceramics to induce red-shift in the photoluminescence, of which the degree is proportional to the Gd(3+) doping concentration. This kind of ceramic, when collaborating with InGaN blue chips, proves to be a promising fluorescent material of white light-emitting diodes, for not only its high in-line transmittance and decent quantum yield, but also the improvement in color rendering ability brought by the red-shift, which enhances the ratio of red portion in spectra. We demonstrate that 10% is the optimized value of Gd(3+) doping concentration, as it provides the maximum color rendering index of ~78 with luminous efficiency as high as 128 lm/W.

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.

Temperature dependent carrier localization in AlGaInN near-ultraviolet light-emitting diodes

We investigate the carrier localization effect in the low-indium ultraviolet AlGaInN light-emitting diodes with a 365 nm peak and a wide yellow luminous band over the visible range. Temperature-dependent electroluminescence spectra (EL) are measured under a wide range of temperature. We found that carrier localization effect relies on the carrier mobility and manifests itself by altering several macroscopic quantities, such as ELs and electrical resistance of the device. Under moderate injection densities, plots of EL peak energy vs. temperatures exhibits S-shapes. At low temperatures, line-width broadening in EL spectra and irregular humps in I-V curves were observed at similar level of injection densities. Both phenomena diminish as temperature increases and eventually disappear at room temperature. All the results stem from carrier localization and following delocalization effect. It suggests that the carrier mobility determine the degree of carrier localization effect - inactive carriers tend to be localized at low temperature but escape at high temperature from bindings of localization centers. As a result, carrier localization is intense only at low temperature for low-indium devices.

Evolution of crystal imperfections during current-stress ageing tests of green InGaN light-emitting diodes

We perform ageing tests under high current on several green InGaN light-emitting diodes and compare the luminous homogeneities of chip surfaces, shapes of external quantum efficiency (EQE) curves, and electroluminescence spectra during different ageing stages. By curve fittings to the EQE curves, with the ABC and two-level models, we discover that a high injection current density can modify the defect configuration in quantum wells even at room temperature, as high-temperature annealing can. For In-rich devices, the removal of localization centers is another origin of luminous intensity decay in addition to the formation of point defects.

Optimization of light efficacy and angular color uniformity by hybrid phosphor particle size for white light-emitting diode

For phosphor involved wLED, it is reported that phosphor with a larger-particle-diameter brings a relatively higher light efficacy, whereas phosphor with a smaller-particle-diameter provides a better angular color spatial uniformity. This study mixed small- and large-particle-diameter phosphor powders together (hybrid phosphors) to balance light efficacy and angular color uniformity and pursue optimal results. Phosphor with small-particle-diameter of 4 µm was employed and it was mixed into each large-particle-diameter phosphor of 10, 16, 22, and 26 µm, at mass percentage from 0 % to 50 % with an interval of 10 %, respectively. Remote phosphor package was adopted and overall phosphor concentration was kept constant for better comparison. Moreover, absorption coefficient μ abs , scattering coefficient μ sca and extinction coefficient μ ext of each hybrid phosphors were calculated based on Mie theory to further discuss the experiment results. Results show that, the introduction of small-particle-diameter phosphor to large one can highly improve angular color uniformity while only slightly reduce light efficacy. The optimal performance with angular color uniformity of 91.6% as well as normalized light efficacy of 95.7% was achieved in the wLED with hybrid phosphors consisting of 60 wt. % powder of 22 µm and 40 wt. %powder of 4 µm.