|Phuc Dang Huu||Institute of Applied Technology, Thu Dau Mot University, Binh Duong Province, Vietnam|
|Dieu An Nguyen Thi||Faculty of Electrical Engineering Technology, Industrial University of Ho Chi Minh City, Ho Chi Minh City, Vietnam|
The hue output for remote phosphor layout is still a concern for researchers and producers working on developing WLED devices since said layout proves inferior to the conformal or in-cup phosphor package. However, the luminous efficiency in the layout showed significant improvements. Thus, this study aims to achieve higher color quality accompanied by high lumen output for remote phosphor structures by using additional phosphor layers. Specifically, the study introduces the double-film and triple-film configurations in which the red and green phosphors were added. The best structure for WLED applications is determined by comparing these structures. The influences of each phosphor structure are investigated and examined on optical parameters of WLEDs, which have different correlated hue temperatures between 5600 K and 8500 K. The optical features are CRI, CQS, color deviation (CD), and photoluminescence (PL). The results demonstrate that the triple-layer structure improved the color quality more effectively than the dual-layer structure due to higher attained CRI and CQS and smaller DC figures. The hypothesis of Mie-scattering indicates that the enhancement of scattering in the triple-layer is responsible for these achievements. Thus, the findings of this study are reliable and valuable for manufacturers.
Color quality; Color quality scale; Color rendering index; Luminous efficiency; Remote phosphor structure; Triple-layer phosphor structure; WLEDs
Light-emitting diodes (LEDs) have laid the concrete foundation for their widespread usage in the solid-state lighting (SSL) industry (Sulistiyanto et al., 2014; Widiyati et al., 2018). They can especially be served as an alternative solution to halogen, incandescent, and fluorescent illuminating systems since they are safer and sturdier. Researchers also pointed out that LEDs potentially have better energy conversion efficiency leading to less heat release, longer lifetime, and more stable brightness performance (Nguyen et al., 2021a; Luo et al., 2020). A LED package was conventionally fabricated utilizing the synthetic of a blue chip and a yellow phosphor material. Thus, the generated light of LED was comprised of blue and yellow lights from these two elements (Anh et al., 2017b). The freely dispersed phosphor coating is one of the most simple and massively applied techniques for manufacturing WLEDs. The mixture of phosphor particles and silicone was spread freely above the bare blue chip.
The advantages of using said method were adjustable phosphor layer thickness and low cost, yet the disadvantage is the poor optical performance of the LED package. Therefore, it is inappropriate to produce high-power WLEDs (Jiang et al., 2017; Yang et al., 2019b; Yuce et al., 2019). The conformal phosphor coating was then introduced to improve the quality of LEDs. Even though this method effectively enhanced the color uniformity due to its ability to provide uniform color distribution, the backscattering in the package caused degradation in light extraction efficiency (Yang et al., 2019a; Ding et al., 2018; Gao et al., 2018). Additionally, these methods directly placed the phosphor layer onto the LED chip surface, stimulating calefaction at the joint and reducing the phosphor's efficiency and the lifetime of the LED device. Hence, researchers devised the idea of creating a gap or adding another film between the main phosphor-silicone encapsulation and the LED chip to improve light extraction and minimize heat generation (Anh et al., 2020; Zhang et al., 2016). This structure was called the remote phosphor structure, and it fastly became a popular topic in various studies of enhancements for WLEDs’ optical properties. A thermal-isolated encapsulation was designed to increase the light output power by controlling heat generation during the high operation time of a WLED (Li et al., 2016). The light extraction efficiency of the remote phosphor package was improved using various approaches, including internal reflection enhancement by employing the innerphosphor-coated polymer hemispherical shell lens, downward-light reflection by the airgap embedded structure, and backscattered-light reduction by the ring remote phosphor structure (Ge et al., 2015). To create a high-grade WLED, aside from the essential light extraction efficiency, another decisive factor is color quality, including CRI and CQS. The study introduced the layout with two sheets to improve chromatic adequacy and luminous efficiency. It was shown that by using the remote phosphor structure with two phosphor layers, the WLED package could improve its lumen output by 5% and color uniformity by offering a significant reduction in the angular color deviation (Thon et al., 2021). However, the performance of a dual-layer phosphor package is usually determined by the phosphor layer that has been added. Notably, with a red phosphor layer, CRI figures can be improved, but the luminous flux is degraded, and vice versa in the case of using green phosphor. Besides, according to previous studies, the growth in phosphor concentration led to a higher reabsorption loss in the package, which decreased the luminous flux (Cheng et al., 2019; Anh et al., 2017a). Then, the concept of using one more phosphor layer can be potential consideration. The triple-film distant phosphor configuration was proposed with the participation of the yellow, green, and red phosphor films. The first film is the yellow phosphor, which is closest to the LED chip, the second film is the green phosphor, and the third film is the red phosphor. In this study, the authors concentrate on the performance of triple-layer remote phosphor film on the illumination effectiveness of the WLED. Phosphor concentrations are also adjusted to observe and analyze changes in the packages of light extraction, emission spectra, color deviation, CRI, and CQS (Davis & Ohno, 2010). Furthermore, the comparison among the single-layer, two-sheet, and three-sheet remote layouts is presented to prove that using the three-film remote design is more beneficial to the quality of LED. The phosphors used to fabricate the phosphor packages in the article are yellow phosphor YAG: Ce3+, red phosphor CaO: Eu3+, and green phosphor Zn2SiO4:Mn2+. In addition, the authors apply the hypothesis of Mie scattering and the law of Lambert-Beer to the measurements. Thus, the outcomes in this study are more reliable and can assist manufacturers in determining the most suitable remote phosphor package to accomplish their goals in high-power WLED production.
The comparison of lighting performance among the triple-layer (YGR), dual-layer (YG and YR), and single-layer (Y) are provided in this article to figure out the most suitable remote phosphor structure for further WLED developments. WLED models are simulated using green Zn2SiO4:Mn2+ and red CaO:Eu3+. Mie's hypothesis and Beer's law would be utilized to investigate and validate the measured outcomes. The findings indicate that the YR is advantageous to the CRI of the WLED as the red phosphor helps to increase the red components in the white-light spectral region. At the same time, the YG is suitable for the luminous flux due to the rise in the green spectral intensity. Despite ranking second in terms of CRI, the YGR has the highest value in CQS and luminous efficacy regardless of CCTs. Moreover, the color deviation in YGR is the smallest, implying that the color uniformity in YGR is the highest. Therefore, YGR is the superior remote structure that can be used for higher quality WLED creation.
This research is supported by the Industrial University of Ho Chi Minh City (IUH) under grant number 121/HD-DHCN.
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