http://ijece.iaescore.com The study of convex-dual-layer remote phosphor geometry in upgrading WLEDs color rendering

Info 2021 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


INTRODUCTION
Over the past few years, the demand of high-efficient lighting source has been increasing. There are some requirements that a qualified lighting source must meet to be applied in advanced illuminating applications such as high durability, power saving, robustness, waste-free, and small size. The white-light light-emitting diode (LED) is an extremely potential option that fully meets those conditions [1]. With the fast-switching advancement, LEDs got more and more attention for manufacturers and have been applied in many state-of-the-art technologies, including smart lighting and auto lighting. Besides, in the aspect of biomedical imaging, LEDs were noticed as a light source for high-resolution full-field optical coherence microscopy (FF-OCM) [2], [3]. However, due to the low illuminating performance, their applications have been restricted in many fields, which probably is the main force for researchers and manufacturers to improve the lighting performances of WLEDs. To assess the lighting performance of the LED we need to rely on the color rendering index (CRI) to compare the light radiation from LED to a natural light source, such as sunlight, or black-body radiation. If the phosphor's emission spectrum in the LED matches the blackbody radiation's broad continuous spectrum, the CRI is high and so is the light quality [4]- [6]. There are many ways to construct a white-light emitting structure with LED chips and phosphor; however, the configuration containing one blue chip and yellow phosphor is the most common because of cheap production cost [7], [8]. Although this structure is most commonly used, the constant usage gradually revealed that this structure is not optimal for lighting efficiency. This is concluded after considering the low CRI results despite many enhancing efforts and high deterioration rate of this structure. According to recent researches, the color rendering index can reach higher values when adjusting the emission spectra to desired value by combining appropriate phosphor materials and blue LED chips [9]- [12]. In order to correct the issue with light color and CRI, we conducted a dissection of LED and phosphor spectrum with empirical and mathematical computations for more information. We have also known that the biggest obstacle for customized white LEDs having adjustable correlated color temperatures (CCTs) is to achieve high CRI and maintain high lumen efficacy simultaneously. The package with one blue LED chip and two layers of phosphor can result in good CRI but show low lighting performance because of Stokes shift [13]- [15]. On the other hand, the package of blue and red LED chips combined with a phosphor film can help both CRI and efficacy but is unable to adjust the color like the two phosphors package [16]- [19]. Thus, employing more LED chips and phosphor materials seems to be the solution for advancing WLED CRI, lumen efficiency and color tunability. Therefore, we decided to use 9 LED chips and two phosphors to fabricate the WLED configuration and analyzed the effects of the phosphors on the optical properties of the WLED.
In this study, the color design based on the Beer-Lambert law and linear conversion was investigated in relation to the fabrication demands of subtle chromatic discrepancy for white-light LEDs. The white-light LED packages in the experiments are comprised of yttrium aluminum garnet (YAG), blue chips, and a layer of red phosphor Ca5B2SiO10:Eu3+. The different phosphor proportions and densities are used to create a dual-layer remote phosphor structure with convex shape and to conduct a thorough analysis of the performance of LEDs. The results show that the proposed convex-dual-layer design is easy to fabricate and yields high efficiency. Furthermore, the information this study provided is crucial to the further development of WLEDs, especially when high-illuminating WLEDs are seen as a potential for being applied in imaging techniques such as optical coherence tomography (OCT) [20].

PREPARATION AND SIMULATION 2.1. Preparation
The first step to prepare for a successful experiment is to understand the nature of Ca5B2SiO10:Eu 3+ through its ingredients: 100 g of CaCO3 (with a very large majority of Ca), 8.8 gram of Eu2O3 (with one in twenty of Eu), 13.2 g of SiO2 (with one in fifth of Si) and 31 g of H3BO3. Secondly, it is essential to accurately execute the process of creating Ca5B2SiO10:Eu 3+ . The first step is to mix the substances by dry milling or grinding. Next, fire the combination in open quartz boats in the air and at a temperature of 1100 degrees Celsius for 1 hour; after milling, we will get the mixture powderized. Moving on to the second step, let's take the previous combination and fire in the same condition but at 1200 degrees Celsius. Afterwards, we will also powderize the result of the second process. Finally, fire them again at 1300 degrees Celsius. The fired product is powderized for the last time and stored. The process of creating Ca5B2SiO10:Eu 3+ is seen as successful when the combination can acquire the following optical properties. During the emission, the compound has a red color. In addition, its emission peaks belong to 2.03 eV. Moreover, -(4.88 eV) and -(3.40 eV) are the indices of excitation efficiency by UV.

Simulation
In this report, both the LightTools program and the Mie-theory are utilized to create the simulation of the LED and for experiments and results verification [21], [22]. After successfully fabricating the red phosphor Ca5B2SiO10:Eu 3+ , the WLED dual-layer remote phosphor structure can be simulated with the correlated color temperature of 5600 K. Then, the experimented can be carried out to investigate the influences of this red phosphor on the lighting performances. Figure 1 demonstrates how the Ca5B2SiO10:Eu 3+ and YAG:Ce 3+ phosphor layers are organized in the in-cup phosphor configuration of WLEDs. As can be seen, the model in Figure 1 includes blue chips, a reflector cup, phosphor layers, and a silicone layer. The order of the phosphor layers is Ca5B2SiO10:Eu 3+ phosphor, the yellow YAG:Ce 3+ phosphor, and the silicone. The reflector here is used to cover the LED chips to prevent them from environmental impacts. The size of this reflector is 2. 07 8 9.85 in depth x bottom length x top surface length. Each blue chip has an emission power of 1.16 W at the peak wavelength of 453 nm. The refractive index of Ca5B2SiO10:Eu 3+ phosphor is 1.85 and YAG:Ce 3+ is 1.83. The concentration of YAG:Ce 3+ needed to be adjusted according to Ca5B2SiO10:Eu 3+ concentration to keep the average CCTs.

RESULTS AND DISCUSSION
The variations of YAG:Ce 3+ phosphor to keep the average CCTs when Ca5B2SiO10:Eu 3+ phosphor increases are shown in Figure 2. In particular, with the concentration of Ca5B2SiO10:Eu 3+ rising from 2% to 26% wt., the concentration of YAG:Ce 3+ phosphor needs to be decreased to keep the average CCTs. The variation based on these results also affects scattering and absorption properties, thus, it can lead to potential improvements in WLEDs color quality and light output. Therefore, choosing the right concentration of Ca5B2SiO10:Eu 3+ is a crucial requirement to ensure the performance of phosphor in WLEDs.
In Figure 3, the effects of red phosphor Ca5B2SiO10:Eu 3+ in the concentration range of 2%-24% are described on emission spectra of WLEDs. As can be seen, there are three spectral regions at which the growth in red-light components was observed when the Ca5B2SiO10:Eu 3+ concentration increases. In particular, the most notable spectral region is 648-738 nm, and the other two are 420-480 nm and 500-640 nm. The blue light falls into the 420-480 nm of the emission spectrum, thus, also benefits from the spectrum enhancement and achieve better quality. Additionally, the higher color temperature resulted in greater emission spectra. The results confirm that the presence of Ca5B2SiO10:Eu 3+ at low color temperature (6600 K) and high color temperature (7700 K) will be very helpful for the color quality of WLEDs. This is a crucial finding that influences the selection of the correct concentration to manufacture an optimal LEDs device. Yet, the reduction luminous efficiency is also noticed when the color quality of the WLED is improved. Figure 4 demonstrated the benefits of using Ca5B2SiO10:Eu 3+ red phosphor in improving the color rendering index (CRI) of WLED. This enhancement can be attributed to Ca5B2SiO10:Eu 3+ absorption feature. Particularly, the red phosphor Ca5B2SiO10:Eu 3+ also absorbs yellow lights but the blue-light absorbance is more dominant. As a result, the red light components in WLEDs increase when the package contains the phosphor Ca5B2SiO10:Eu 3+ , which is beneficial to the CRI. Color rendering index is one of the valuable indicators that can evaluate the quality of WLEDs. The WLEDs with high CRI are considered to be higher quality; however, it also leads to higher production cost. Nevertheless, utilizing Ca5B2SiO10:Eu 3+ phosphor can lower the cost; thus, this material turns out to be one of the suitable candidates for large-scale production of WLEDs with higher color quality.
In recent years, CRI is not the only index to color quality of WLEDs as it is not a very in-depth indicator. Instead, the color quality scale (CQS) has been investigated and considered as a superior to CRI because CQS can examine not only CRI but also the viewer's preference and color coordinates.  shows the influences of the red phosphor Ca5B2SiO10:Eu 3+ on the CQS that helps enhance the quality of color. The ability of enhancing color quality for WLEDs of Ca5B2SiO10:Eu 3+ phosphor has been verified yet there is a drawback that should be considered: the reduction in the amount of emitted light. The mathematical formulas for computing the transmitted blue light and the converted yellow light in the convex-dual-layer remote phosphor structure, a potential part that can enhance the efficiency of LEDs, are presented in the next part.  (1) Here, σ depends on the peak wavelength λpeak, and the calculation of FWHM Δλ can be: (2) The white-light LED's SPD in connection with the utilized yellow YAG phosphors and blue LED chips can hypothetically be viewed as the aggregate for the spectra of the blue and yellow lights. Practically speaking, nonetheless, the supposed yellow phosphor radiates lights in not only yellow but also green spectra. Given that the blue range and yellow range are selected, it is possible to use the green range to demonstrate the contrast between the essentially estimated SPD and twofold shading (blue and yellow shading) range model. In this way, a green range can be added to the twofold range model, leading to the investigation using trispectrum (B-G-Y) model which is displayed in (3) and after that is replaced altered by the expressions in (4).   λpeak Peak wavelength (nm).  Δλ Full-width at half-maximum (FWHM) (nm).  η Ratio of specific spectra to white spectrum, dimensionless.  Popt_b, Popt_g, Popt_y, and Popt_total Optical power (W) for the blue, green, yellow, and white spectra, respectively.  λpeak_b, λpeak_g, and λpeak_y Peak wavelengths (nm) for the blue, green, and yellow spectra, respectively.  σb, σg, and σy FWHM-related coefficients (nm) for the blue, green, and yellow spectra, respectively.  ηb, ηg, and ηy Ratios of blue-green-yellow (B-G-Y) spectra to white spectrum, respectively, dimensionless.  λ1, λ2 Wavelengths at half of the peak intensity.
Hence, a tricolor spectrum can be used to describe the SPD modeling for the phosphor-coated white LED, and this can be viewed as the extended Gaussian structure. The scattering investigation of Ca5B2SiO10:Eu 3+ particles is conducted with the Mie-scattering theory. Moreover, this theory contributes to computing the scattering cross-section Csca for spherical particles [25], [26]. Besides that, based on the Beer-Lambert law, the computation for transmitted light power can be expressed as [27], [28]: I0, µext and L are the incident light power, the extinction coefficient and the thickness of phosphor layer (mm), respectively. Besides, µext can be expressed as: µext=Nr.Cext, with Nr is the number density distribution of particles (mm -3 ), and Cext (mm 2 ) indicates the phosphor particle's extinction cross-section. By applying expression (5), the outcome shows that dual-layer remote phosphor structure has higher luminous flux than the single-layer phosphor structure. Therefore, the dual-layer phosphor structure actually works better and enables the improvement of luminous flux in WLEDs. Moreover, the calculated results indicated that the concentration of Ca5B2SiO10:Eu 3+ greatly and directly affects the color quality and luminescence performance of WLED dual-layer remote phosphor configuration. In particular, the extinction coefficient µext, demonstrated in the Lambert-Beer law, and the concentration of Ca5B2SiO10:Eu 3+ increase together; however, the light emission energy increase in an inverse direction to the extinction coefficient µext. Consequently, the degradation in lumen output occurs as the phosphor layer is thicken (the concentration of Ca5B2SiO10:Eu 3+ is increased). This point was also described in Figure 6, where the luminous intensity decreases at all CCTs with the rise of Ca5B2SiO10:Eu 3+ concentration, from 0% to 26%.
Generally, dual-layer phosphor structure with red phosphor Ca5B2SiO10:Eu 3+ already created much better luminous flux than the single-layer. In addition, Ca5B2SiO10:Eu 3+ also gives CRI and CQS many benefits. Therefore, a slight reduction in luminous flux is acceptable considering the overall light output and color quality. Thus, the final decision of an appropriate Ca5B2SiO10:Eu 3+ concentration is made by the manufacturers after them considering their goals of WLED products.

CONCLUSION
This research has successfully applied new technology to create WLEDs with high lighting performance and a changeable emission spectrum. A color design of the WLEDs model is form by applying the Lambert-Beer law and the linear conversion, from which the WLED can adjust its white light to be close to the requirement of manufacturers. The outcomes of the research conducted on the dual-layer phosphor package demonstrate better light output in comparison to the result from the single-layer package. Moreover, the increase in Ca5B2SiO10:Eu 3+ concentration helps the chromatic performance of WLED improve due to the corresponding development of the CRI and CQS. However, the excessive concentration of Ca5B2SiO10:Eu 3+ particles (over 24%) results in degraded lumen efficiency. With the information from this paper, the concentration of Ca5B2SiO10:Eu 3+ phosphor layer can be determined by manufacturers to address the issues in their production of high-quality WLEDs.