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

Received Feb 25, 2021 Revised May 20, 2021 Accepted Jul 10, 2021 The concept of the analysis is to put a CaAl2O4:Mn green phosphor layer on top of the YAG:Ce yellow phosphor layer. After that, find the added CaAl2O4:Mn 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:Mn brings great benefits to increase not only the luminous flux but also the color homogeneity. Especially, the higher the CaAl2O4:Mn 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:Mn 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.


INTRODUCTION
The fourth potential illumination source generation used to replace the traditional one, phosphor transformed white light emitting diodes (pc-WLEDs), which has a wide range of prospects in lighting solutions, as can be shown in [1]- [5]. The implementation of white light emitting diodes has become increasingly popular in various areas of everyday routine of people, such as landscape, street lighting, backlighting, while the efficiency of illumination harness and the consistency of angles of the associated chromatic temperature of the white LEDs are still the demanding factors that limit its enhancement [6]- [8]. Future advancements in luminescent performance and color consistency are critical, given the steady growth in consumer demand and implementations [9], [10]. A combination of the light of blue from the reverse red phosphorus with the light of yellow from the LED chip is the most common method for producing white light today. While this idea seems to be very common, it cannot be denied that the layout of LEDs and the layout of phosphorous layers, especially the color rendering index, play a significant role in assessing the luminous quality [11]- [14]. Several common methods of phosphorus coating have been suggested in order to manufacture LEDs, such as dispensing and conformal coatings [15]- [18]. Still, these configurations do not have good lighting chromaticity due to the deterioration of the illuminating transformation of phosphorus matter caused by the yellow emitted phosphorus directly contacting the LED chips, leads to a temperature rise at the juncture of the LED and phosphorus layers. Minimizing the influence of heat will therefore increase the phosphor's efficiency and keep the phosphorus from being irremediably harmed. A lot of earlier experiments have shown that the heating effect is reduced with the remote structure of phosphor layers that are located distant from the heat origin (LED chip). With an adequate space set between the phosphor compounding and the LED chip, the backscatter and dissemination of inner lighting could be restricted. This approach is the optimal solution for LED heat control, so the luminous efficiency and color consistency of LEDs can be increased [19], [20]. The distant phosphor configuration, on the other hand, is adequately suited for regular illumination but does not meet other demands for many other lighting implementations, which is perhaps why developing the upcoming generation of LEDs is critical. Several new remote phosphorus structures were proposed for future developments, with the goal of minimizing phosphorus backscatter to the chip and improving luminescent efficiency. Some other research found that a reversed cone lens encapsulant and a distant phosphorus layer covering the ring would redirect light beams from the blue chip to the LED surface, decreasing internal reflection losses within the LED [21]. A patterned distant phosphorus configuration with a clear area in the perimeter zone and no coating phosphorus on the outer surface could achieve optimum dependence of angles associated color temperature homogeneity and chromatic stabilization [22]. In contrast, the patterned sapphire substrate applied to the remote phosphorus in a far-field pattern could have even greater homogeneity of the associated color temperature than a standard template [23], [24]. To enhance the illumination performance of LEDs, distant phosphorus with a double-layer packet is suggested. The above-mentioned researches did concentrate on enhancing the uniformity of color and luminous flux performance of phosphorus remote structure WLEDs, yet investigated primarily on single-chip WLEDs and models with low color temperature. In the meantime, it is difficult to develop the optical parameters for high color temperature WLEDs.
This article suggests a remote phosphorous double-layer layout to increase the color standard of WLEDs with varying color temperatures from 5600 K to 8500 K. The novel concept of the article is to utilize the CaAl2O4:Mn 2+ green phosphor layer to raise the composition of green light in WLEDs, leading to better luminous flux (LF) and color homogeneity (CH). The article reports in detail the combination of chemical elements of CaAl2O4:Mn 2+ affecting the visual properties of WLEDs as well. The results of the article show that LF and CH significantly improve with the addition of phosphor layers CaAl2O4:Mn 2+ . However, to prevent a deep fall in the color rendering ability (CRI) and color quality ratio (CQS) when the green phosphorus concentration rises extremely, the adequate CaAl2O4:Mn 2+ concentration should be determined. Three notable points can be reported with the additional layer of green phosphor on top of the layer of yellow phosphor YAG:Ce 3+ are: the increased green light component leading to heightened the spectrum of white light emitting light, better emitted flux, and higher dispersion and transmittance of WLED illumination with the risen CaAl2O4:Mn 2+ concentration. The selection of proper concentration of CaAl2O4:Mn 2+ is therefore essential for the WLEDs' balance of illuminating beam and color quality.

COMPUTATIONAL SIMULATION 2.1. Provision of green-emitting CaAl2O4:Mn 2+ phosphor
CaAl2O4:Mn 2+ particles are forms of yellow-green phosphorus and become more popular owing to their peak emission of 2.28 eV and a lot of excellent features such as optimal quantum yield and high temperature permanence. The particle sizes and concentration of CaAl2O4:Mn 2+ phosphors may have a significant impact on their illumination characteristics. The ingredients consist of CaCO3, Al2O3, MnCO3, CaF2 and Mn 2+ ion, all of which, as seen in Table 1, are used as raw materials. In addition, CaAl2O4:Mn 2+ is used especially for fluorescent lamps with very high-loading and a long lifetime. As a result, it is amongst the most popular phosphors of commercially produced oxide. Overall, the CaAl2O4:Mn 2+ manufacturing phase is being as: First of all, to be combined together, we slurry all the ingredients in the water. They are then ball-milled into small particles in the water. After that, when they are dried in the air, the materials will be powdered. Next, this powder will be fired with CO at 1300°C within an hour in covered alumina crucibles and powderized by dry milling. Then, with CO at 1200°C within an hour, they are fired again in open quartz boats. Eventually, we wash the product in a liter of water in a solution of NH4Cl and then several times in plain water.

Simulation of MC-WLEDs
The layers of phosphors in actual MCW-LEDs are modeled with clear silicone sheets utilizing the simulating software of LightTools 9.0. This simulation procedure is carried out over two distinct time periods; (1) it is critical to identify and construct the configuration models and optic features of MCW-LEDs; (2) the optic impacts of phosphor layers are then well regulated by the CaAl2O4:Mn 2+ concentration variation. To evaluate the effect of YAG:Ce 3+ and CaAl2O4:Mn 2+ phosphor compounding on the performance of MCW-LED lamps, some comparisons must be made. Particularly, effects of the green-yellow dual-layer remote phosphorus model, with mean CCTs of 5600 K-8500 K, are stated and explained. Figure 1 (a), (b), (c), (d) depicts a detailed overview of a MCW-LED lamp with conformal phosphor compounding and a mean CCT of 8500 K. Apart from the green phosphor CaAl2O4:Mn 2+ , the specifications of other WLED elements are also expressed. The reflector's base length is 8 mm, its height is 2.07 mm, and its top surface length is 9.85 mm. The conformal phosphor compounding coats nine chips with a thickness of 0.08 mm. Each LED chip is attached to the reflector space through a squared ground region of 1.14 mm2 and 0.15-mm height. The reflective radiation of a blue LED chip is 1.16 W, with a wavelength that peaks at 453 nm.

RESULTS AND ANALYSIS
The reverse mutation among the concentration of green CaAl2O4:Mn 2+ and yellow YAG:Ce 3+ films can be seen in Figure 2. There are two meanings of this change: one is to preserve mean CCTs, the other is to impact the absorption and scattering in WLEDs of two phosphor layers. The color quality and the performance of luminous flux of WLEDs are ultimately influenced by this. Thus, the selection of CaAl2O4:Mn 2+ concentration defines the color standard of WLEDs. Whenever the CaAl2O4:Mn 2+ goes from 2% wt. to 20% wt., the concentration of yellow YAG:Ce 3+ declines to preserve the mean CCTs. There is a similarity with WLEDs with varying CCTs in the range of 5600 K -8500 K. Figures 3-7 show the influence of changing concentration of CaAl2O4:Mn 2+ green phosphor on the transmittance range of WLEDs. It is possible to make a decision based on the manufacturer's specifications. WLEDs that need high color performance can minimize illuminated beam by a minor fraction. White light is the combination of the spectral field displayed in Figures 3-7. The five figures depict spectra at 5600 K, 6600 K, 7000 K, 7700 K, and 8500 K. Clearly, the intensity trend increases with concentration in two regions of the lighting spectral range: 420 nm -480 nm and 500 nm -640 nm. This increase in the two-band emission spectra demonstrates an increase in the output luminous flux. Moreover, the blue-light scattering in WLED is increased, which means the scattering in the phosphorous layer and in WLEDs is increased, resulting in color uniformity becoming favored. This is a significant finding when CaAl2O4:Mn 2+ is used. Controlling the color uniformity of the high temperature remote phosphor structure, in particular, is hard. This study concluded   This section will illustrate and show the statistical structure of transferred blue lighting and transformed yellow lighting in the dual-layer phosphorus structure, which can result in a significant improvement in LED performance [25], [26]. For a single-layer distant phosphorus packet with the thickness of phosphorus layer set at 2h, the transferred blue lighting and transformed yellow lighting are as shown in (1) and (2) For the double-layer distant phosphor packet, the transferred light of blue and the transformed light of yellow, with h is the phosphor-layer thickness, are defined as (3), (4): Where h indicates each phosphor-layer thickness. To define the one-and two-layered distant phosphor packages, the "1" and "2" subscriptions are used. β indicates the transfer factor for the light of blue transforming to the light of yellow. γ is the backscatter factor of yellow light. The lighting intensity of the blue LED shown by PB0 is the blue-lighting intensity (PB) and the yellow-lighting intensity (PY). αB and αY explain the sections of the losing energy of the blue and yellow beams through their spreading of the phosphorus layer respectively. As compared to a single-layer configuration, the illumination performance of pc-LEDs with a dual-layer phosphorous configuration is significantly higher: (5) In (5) shows that dual-layer distant phosphorus WLEDs have a higher luminescent quality than single-layer phosphorus WLEDs. Thus, the paper demonstrated the efficacy of the released illuminating beam of the green-yellow double-layer distant phosphor model. The findings in Figure 8 demonstrate that the illuminating beam released increases considerably as the CaAl2O4:Mn 2+ concentration increases from 2% wt. to 20% wt. The color variance is greatly decreased with the phosphor CaAl2O4:Mn 2+ concentration in all three average CCTs, according to the findings of Figure 9. These achievements should be attributed to the green phosphor film's absorption feature. As the CaAl2O4:Mn 2+ phosphor absorbs the light of blue from the LED chip; these green phosphor particles convert it to the light of green. The CaAl2O4:Mn 2+ particles absorb yellow lighting in addition to the blue lighting from the chip. However, the absorption properties of the substrate contribute to getting stronger blue light absorption, compared to the other absorption event. As a result of the inclusion of CaAl2O4:Mn 2+ , the green light content in WLEDs increases, resulting in an improvement in the color uniformity indicator. Color homogeneity is a vital parameter of modern WLED lamp parameters. Undoubtedly, the better the color homogeneity coefficient, the more expensive WLED white light. The benefit of using CaAl2O4:Mn 2+ is its least expensive application. CaAl2O4:Mn 2+ can therefore be commonly used. Color conformity is one of the notable factors to consider when assessing the color consistency of WLEDs. Yet, color consistency cannot be said to be decent with a high color homogeneity index. Therefore, recent studies provided an index for color rendering and a scale for color quality. When the color rendering 11  indicator is illuminated by an illumination, it assesses the real color of a thing. The unbalance of color is caused by the excessive amount of green light among the three major colors: blue, yellow and green. This influences the chromatic standard of WLEDs, leading to the decline of color fidelity of a WLED. With the use of CaAl2O4:Mn 2+ remote phosphor layer, the findings in Figure 10 indicate a slight reduction in CRI. Nonetheless, these are appropriate because CRI is merely a shortcoming in CQS. When drawing a comparison between CRI and CQS, CQS is more significant and more difficult to accomplish. CQS is a three-factor indicator that is calculated by three factors: the color rendering index, the viewer's selection, and color coordinate. For these three important variables, CQS is almost a real total indicator of color consistency. Figure 11 depicts the enhancement of CQS in the existence of the distant phosphor CaAl2O4:Mn 2+ layer. Moreover, as the concentration of CaAl2O4:Mn 2+ is increased, CQS does not change significantly if the concentration of CaAl2O4:Mn 2+ is less than 10% wt. When the CaAl2O4:Mn 2+ concentration is higher than 10% wt., both CRI and CQS are greatly decreased due to extreme color loss when green is predominant. As a consequence, when using green phosphor CaAl2O4:Mn 2+ , proper concentration selection is critical.

CONCLUSION
The paper represents the influence of CaAl2O4:Mn 2+ green phosphorus on the optic characteristics of the configuration of double-layer phosphorus. The research showed that CaAl2O4:Mn 2+ is a suitable choice for improving color uniformity using Monte Carlo computational simulations. Furthermore, this green phosphor is applicable not only to WLEDs with low color temperatures, for example 5600 K, but also to those with a color temperature greater than 7700 K, such as 8500 K in our study. The findings of this analysis have thus accomplished the objective of enhancing the standard of color and luminous flux, which is very complicated with the remote configuration of phosphorus. However, one minor disadvantage occurs for CRI and CQS. When the concentration of CaAl2O4:Mn 2+ is increased excessively, either CRI or CQS decrease significantly. It is therefore necessary to select the proper concentration, based on the objectives of the manufacturer. In relation to producing greater color uniformity and luminous flux WLEDs, the paper has given a great amount of essential information for reference.