Review of Low-Frequency Noise Properties of High-Power White LEDs during Long-Term Aging

Low-frequency noise investigation is a highly sensitive and very informative method for characterization of white nitride-based light-emitting diodes (LEDs) as well as for the evaluation of their degradation. We present a review of quality and reliability investigations of high-power (1 W and 3 W) white light-emitting diodes during long-term aging at the maximum permissible forward current at room temperature. The research was centered on the investigation of blue InGaN and AlInGaN quantum wells (QWs) LEDs covered by a YAG:Ce3+ phosphor layer for white light emission. The current-voltage, light output power, and low-frequency noise characteristics were measured. A broadband silicon photodetector and two-color (blue and red) selective silicon photodetectors were used for the LED output power detection, which makes it possible to separate physical processes related to the initial blue light radiation and the phosphor luminescence. Particular attention was paid to the measurement and interpretation of the simultaneous cross-correlation coefficient between electrical and optical fluctuations. The presented method enables to determine which part of fluctuations originates in the quantum well layer of the LED. The technique using the two-color selective photodetector enables investigation of changes in the noise properties of the main blue light source and the phosphor layer during the long-term aging.

Improvement of double-layer phosphor structure WLEDS in color homogeneity and luminous flux

The concept of the analysis is to put a CaAl2O4:Mn2+ green phosphor layer on top of the YAG:Ce3+ yellow phosphor layer. After that, find the added CaAl2O4:Mn2+ concentration appropriate for the highest luminous flux (LF) and color homogeneity (CH). In this analysis, five equivalent WLEDs were applied but with distinct color temperatures, including 5600 K - 8500 K. The findings showed that CaAl2O4:Mn2+ brings great benefits to increase not only the luminous flux but also the color homogeneity. Especially, the higher the CaAl2O4:Mn2+ concentration, the more the luminous flux released by WLEDs, owing to the risen content of the light of green in WLEDs. Nevertheless, as the CaAl2O4:Mn2+ concentration raised significantly, a small reduction in the color rendering metric (CRI) and color quality scale (CQS) occurred. This is supported by simulation and calculation according to the theory of Monte Carlo. The paper results are the crucial contribution to the manufacture of WLEDs with better optical performance and color homogeneity of remote phosphor configurations.

Enhancing light scattering effect of white LEDs with ZnO nanostructures

Pc-LEDs, the lighting method that blends blue LED light with yellow light from phosphor to discharge white radiation, is one of the most advance known for high lumen output. However, pc-LEDs has inferior due to angular CCT deviation, which prevent pc-LEDs from reaching better performance. As a result, this research is conducted to address the need of pc-LEDs development by introducing a configuration doped with ZnO nanoparticles. The finite-difference time-domain (FDTD) method and the phosphor layer containing ZnO were applied in the experiments. The effect of ZnO-filled on the performance of color quality pc-LEDs is confirmed through calculated results. In particular, the uniformity of scattered light is improved with the presence of ZnO. In addition, ZnO particles also minimize the deviation of color temperature and enhance the color quality. Although there is a small decline in lumen output to achieve better color temperature uniformity, however, with suitable concentrations such as 0.25% N-ZnO, 0.25% S-ZnO, and 0. 75% R-ZnO, the decline is acceptable. The research on ZnO pc-LEDs demonstrates that this affordable and simple configuration can improve lighting properties and create other directions to enhance white light

The study of convex-dual-layer remote phosphor geometry in upgrading WLEDs color rendering index

<span>The white-light light-emitting diode (LED) is a semiconductor light source that usually has one chip and one phosphor layer. Because of that simple structure, the color rendering index (CRI) is really poor. Therefore, structure with double layer of phosphor and multiple chips has been studied with the phosphorus proportions and densities in the silicone are constantly changed to find the best option to improve optical properties. In research, we use red phosphor Ca5B2SiO10:Eu3+ layer to place above the yellow phosphor one, and both of them have a convex design. Then, the experiments and measurements are carried out to figure out the effects of this red phosphor as well as the convex-double-layer remote phosphor design on the LED’s performances. The measured results reveal that the light output is enhanced significantly when using convex-dual-layer structure instead of the single-layer design. Additionally, the Ca5B2SiO10:Eu3+ concentration benefits CRI and CQS at around 6600 K and 7700 K correlated color temperature (CCT). Yet, the lumen output shows a slight decline as this red phosphor concentration surpass 26% wt. Through the experiments, it is found that a double layer of chip and double phosphorus is the best structure which could support the quality of CRI and luminous flux.</span>

(Y,Gd)BO3:Eu red phosphor for dual-layer phosphor structure to enhance the optical performance of white light-emitting diodes

Among the structures using for fabricating white light-emitting diodes (WLEDs) such as the conformal coating or in-cup geometries, the remote phosphor structure gives the highest luminous efficacy. However, in terms of color quality, its performance is not as good as the others. The red-light compensation has been reported as the effective solution for enhancing the color quality of WLEDs. Hence, this study adopted the idea and applied to the dual-layer phosphor structure. The phosphor used to boost the red color in light formation is (Y,Gd)BO3:Eu particle. The dual-layer remote phosphor structure was simulated with the red (Y,Gd)BO3:Eu phosphor layer above the original yellow phosphor YAG:Ce3+ one. The WLEDs with different correlated color temperatures of 5600 K, 6600 K and 7700K were experimented. Mie-theory and Lambert-Beer law were applied to examine the results. The growth in color rendering index (CRI) and color quality scale (CQS) with the increase of (Y,Gd)BO3:Eu phosphor concentration was observed. Nevertheless, the lumen efficacy would be degraded if the concentration was over a certain number. The information provided in this article is useful for the development of high-power WLED production with greater color quality.

Applying SiO2 nano-particles for improving optical properties of WLED conformal and in-cup structures

This article is the analysis of SiO2 nano-particles’ influences on the luminous efficiency and the color temperature uniformity of a remote phosphor structure in a WLED. The purpose of integrating SiO2 into the silicone layer in the remote phosphor structure is to significantly promote the scattering occurrences. Particularly, with an appropriate proportion of SiO2, there could be more blue lights generated at large angles, leading to reducing the angular-dependent color temperature deviation. The luminous flux also can get benefits from SiO2 addition owing to a proper air-phosphor layer refractive index ratio provided by this SiO2/silicone compound. The attained experimental results were compared with optical values of a non-SiO2 remote phosphor configuration and showed a notable enhancement. The color deviation was reduced by approximately 600 K in the angles from -700 ­to 700. Additionally, the lumen efficiency was improved by 2.25% at 120 mA driving current. Hence, SiO2 can be used to boost both color uniformity and luminous efficacy for remote-phosphor WLED.

Improving the optical efficiency of white light-emitting diodes based on phosphor-in-glass by a dual-layer remote phosphorus structure with the application of LiLaO2:Eu3+ and CaSO4:Ce3+, Mn2+

While the remote phosphor structure is not an appropriate solution for WLED color uniformity, it is more advantageous for the luminous output of WLED than the conformal phosphor or in-cup phosphor structure. Acknowledging the ability of the remote phosphor structure, many studies have been carried out to surmount the color quality disadvantage of this structure. A dual-layer remote phosphor configuration is proposed in this research paper to acquire better color quality for WLEDs through heightening the color rendering index (CRI) and the color quality scale (CQS). The color temperature of the WLED packages this study is 8500 K. By inserting a layer of green CaSO4:Ce3+,Mn2+ or red LiLaO2:Eu3+ phosphor on the yellow YAG:Ce3+ phosphor layer, the phosphor structure configuration can be constructed. Then, to get the best color quality, the concentration of added phosphor LiLaO2:Eu3+ would be changed. The findings showed the rise of CRI and CQS along with the LiLaO2:Eu3+, which implies the influence of LiLaO2:Eu3+ to the growth of red light components within WLEDs packages. The greater the concentration of LiLaO2:Eu3+ is, the more the CRI and CQS increase. Meanwhile, the luminous flux gains from the green phosphor CaSO4:Ce3+,Mn2+. Nevertheless, the luminous flux and color quality would decrease if the concentrations of both red LiLaO2:Eu3+ and green CaSO4:Ce3+,Mn2+ phosphors reach a certain corresponding level. Centered on the Mie-scattering theory and the law of Lambert-Beer, this result is illustrated. The findings in this research are vital references for manufacturing WLEDs with higher white light performance.

Triple-layer remote phosphor structure: a selection of the higher color quality and lumen efficiency for WLEDs

To enhance color quality of glass-based phosphor-converted white light-emitting diodes (pc-WLEDs) with multi-layer remote phosphor layer structures, two phosphors, green CdS:In and red ZnS:Te,Mn, are integrated into the glass matrix and applied to the dual-layer and triple-layer WLED packages. The attained results were examined with Mie-scattering theory and Lambert-Beer law. The dual-layer showed significant enhancement in color rendering index (CRI), in the range of approximate 80-90. Meanwhile, CRI in the triple-layer was lower and stayed around 66. In terms of color quality scale (CQS), a more overall color evaluating index, triple-layer structure helps the glass-based WLED achieve higher value than the dual-layer. The triple-layer is also beneficial to the luminous efficacy, according to the experimented results. Thus, the triple-layer structure can be used to strengthen the benefit of the glass matrix used in WLED products.

Color-Tunable White LEDs with Single Chip Realized through Phosphor Pattern and Thermal-Modulating Optical Film

In this paper, a new method to regulate the correlated color temperature (CCT) of white light-emitting diodes (LEDs) is proposed for the single-chip packaging structure, in which the blue light distribution emitted from the chip in the red/yellow phosphor layer was modulated through changing the paraffin-polydimethylsiloxane (PDMS) film transparence and haze. The results show that the transmittance of the paraffin-PDMS film can be modulated from 49.76% to 97.64%, while the haze of that ranges from 88.19% to 63.10%. When the thickness of paraffin-PDMS film is 0.6 mm, and the paraffin-PDMS film concentration is 30 wt%, the CCT of white LED decreases from 15177 K to 3615 K with the increase of thermal load in the paraffin-PDMS film. The modulating range of its CCT reaches 11562 K. The maximum CCT variation at the same test condition is only 536 K in the repeated experiments within one week.