Exellent Color Quality and Luminous Flux of Wleds Using Triple-Layer Remote Phosphor Configuration

This study proposed a triple-layer remote phosphor (TRP) structure to improve the color and luminous ux of white LEDs (WLEDs). TRP structure consists of 3 di erent phosphor layers: yellow YAG:Ce layer below, red CaMgSi2O6:Eu ,Mn phosphor on top and green layer Ba2Li2Si2O7:Sn ,Mn phosphor in the middle. Using red CaMgSi2O6:Eu ,Mn to control the red light component leads to the increase in color rendering index (CRI). Utilizing the green CaMgSi2O6:Eu ,Mn phosphor to control the green light component results in the increase in luminous e cacy (LE) of WLEDs. Furthermore, when the concentration of these two phosphors increased, yellow layer YAG:Ce concentration decreased to maintain average correlated color temperatures (ACCTs) in the range from 6000K to 8500K. Besides CRI and LE, color quality scale (CQS) is also analyzed through the control of green and red phosphors concentrations. The research results show that the higher the concentration of CaMgSi2O6:Eu ,Mn is, the better the CRI becomes. In contrast, CRI decreased signi cantly when increasing the concentration of Ba2Li2Si2O7:Sn ,Mn . Meanwhile, CQS achieved notable enhancement in the concentration range of 10% -14% CaMgSi2O6:Eu ,Mn, regardless of Ba2Li2Si2O7:Sn ,Mn concentration. LE, in particular, can also increase by more than 40% along with the improvement of CRI and CQS due to the reduction of the backscattered light and the addition of green light. Research results are a valuable reference for producers who wish to improve the color quality and enhance the luminous ux of WLEDs.

Abstract. This study proposed a triple-layer remote phosphor (TRP) structure to improve the color and luminous ux of white LEDs (WLEDs). TRP structure consists of 3 dierent phosphor layers: yellow YAG:Ce 3+ layer below, red CaMgSi 2 O 6 :Eu 2+ ,Mn 2+ phosphor on top and green layer Ba 2 Li 2 Si 2 O 7 :Sn 2+ ,Mn 2+ phosphor in the middle.
Using red CaMgSi 2 O 6 :Eu 2+ ,Mn 2+ to control the red light component leads to the increase in color rendering index (CRI). Utilizing the green CaMgSi 2 O 6 :Eu 2+ ,Mn 2+ phosphor to control the green light component results in the increase in luminous ecacy (LE) of WLEDs. Furthermore, when the concentration of these two phosphors increased, yellow layer YAG:Ce 3+ concentration decreased to maintain average correlated color temperatures (ACCTs) in the range from 6000K to 8500K. Besides CRI and LE, color quality scale (CQS) is also analyzed through the control of green and red phosphors concentrations. The research results show that the higher the concentration of CaMgSi 2 O 6 :Eu 2+ ,Mn 2+ is, the better the CRI becomes. In contrast, CRI decreased signicantly when increasing the concentration of Ba 2 Li 2 Si 2 O 7 :Sn 2+ ,Mn 2+ . Meanwhile, CQS achieved notable enhance-ment in the concentration range of 10% -14% CaMgSi 2 O 6 :Eu 2+ ,Mn 2+ , regardless of Ba 2 Li 2 Si 2 O 7 :Sn 2+ ,Mn 2+ concentration. LE, in particular, can also increase by more than 40% along with the improvement of CRI and CQS due to the reduction of the backscattered light and the addition of green light. Research results are a valuable reference for producers who wish to improve the color quality and enhance the luminous ux of WLEDs.

Introduction
Phosphor-converted white light-emitting diodes (WLEDs) with many outstanding features such as smallness, energy saving, cost eciency and cohesion in color have been perceived as a new and improved light source [1]- [4]. The complementary principle of colors is applied in WLEDs as blue light from the blue chip and yellow light from the phosphor layer merge in the congu-74 c 2020 Journal of Advanced Engineering and Computation (JAEC) ration [5]. It is expected that WLEDs will be used for solid-state lighting system; however, the luminous eciency must be improved in order to be used for the aforementioned purpose [6]- [9]. In order to produce the white light, using the freely dispersed coating method is the most well-known one. The transparent encapsulated resin and the phosphor powder are mixed and then dispersed on the phosphor package to fabricate the white light in the process. This procedure may allow better control over phosphor layer thickness and signicantly lower the expenses; however, it cannot support the production of high-quality WLEDs [10]- [12]. Therefore, a method that helps to distribute the color homogeneously and has angular homogeneity of correlated color temperature (CCT) such as the conformal coating method is used as a substitution [13]. The luminous eciency of the conformal phosphor structure, however, decreases due to the backscattering eect this structure has. The idea of separating the chip and the phosphor layer in remote phosphor structures are presented in previous studies [14]- [16]. The extraction eciency benets from the polymer hemispherical shell lens with an interior phosphor coating that enhances the light extraction inside of the reection structure. Moreover, the luminous eciency is also improved because the air-gap embedded structure reects the light downward.
In 2018, Nhan's team used the red-emitting α−SrO·3B 2 O 3 :Sm 2+ for increasing the optical properties of single-remote phosphor. By varying α−SrO·3B 2 O 3 :Sm 2+ concentration from 2% to 24%, the obtained results proposed that color uniformity, color rendering index (CRI), color quality scale (CQS), and luminous ecacy could be improved signicantly [17]. With the target of the improvement of CRI and CQS, in 2019, Lee's team has applied the red-emitting Mg 2 TiO 4 :Mn 4+ phosphor in the dual-layer remote geometry [18,19]. However, the luminous ux is a disadvantage in these studies.
The concentration of phosphor, along with the package structure, is also a critical element that aects the luminous ux. When the phosphor concentration increases, it will cause the re-absorption loss in the phosphor layer to rise.
The luminous eciency of the device will be lowered as a result, especially at lower CCTs. Thus, the improvements in the blue and yellow light emission and the reduction in light loss from backscattering and reection are desirable targets.
The triple-layer remote phosphor structure WLEDs with color temperatures from 6000K to 8500K are proposed in this study. The TRP structure consists of three different phosphor layers with green phosphor layer Ba 2 Li 2 Si 2 O 7 :Sn 2+ ,Mn 2+ between yellow phosphor layer YAG:Ce 3+ and red CaMgSi 2 O 6 :Eu 2+ ,Mn 2+ phosphor layer. The green phosphor layer adds green light components to improve the luminous ux emitted while the red light component is supplemented by red phosphor layer to improve color quality. The results show that when there is a balance between 3 colors of yellow, green, and red the color quality can reach the highest value, and the luminous ux of WLEDs is reduced only by an insignicant amount. these steps are critical and based on the step(s) before them. The rst step is mixing the materials by dipping into methanol with a few cubic centimeters of water. Second, let it dry in a condition of air. After the materials are dried, re them in the capped quartz tubes and fused with N 2 at the condition of 1000 0 C for 1 hour and then continue to fry the powdery products in capped quartz tubes but with CO instead of N 2 in an hour at a temperature of 1150 0 C. The next step is to pick up the product and wash it several times with water. Finally, leave them to dry and we will have the We start by mixing BaCO 3 + Li 2 CO 3 + SiO 2 using the dry grinding or milling method. After the rst step, we continue to re the mixture in open boats under 850 0 C for an hour while adding in H 2 . We will then proceed with SnO + MnCO 3 + NH 4 Br by soaking them in methanol and stirring the mixture until it reaches unifor-mity. Then dry the mixture in the condition of air to reduce it to powder form. The powder will be fried with N 2 in capped quartz tubes for 1 hour. After that, turning the product to powder and put it through the ring process one more time in open quartz boats under 850 0 C temperature but for about 16 hours (overnight). Once the previous step is done, store the nal powder product in a well-closed container.

2.2.
Simulation of TRP  overlays on 9 LED chips with the measurements of 1.14 mm bottom square and 0.15 mm height that are embedded in the gaps on the reector. These blue chips emit a radiant ux of 1.16 W at 455 nm wavelength. Even though the concentration of phosphor particles are constantly changing from 2% to 24%, the control over YAG:Ce 3+ wt keeps the average CCT values remain static in their cases. Furthermore, the spectra values of YAG:Ce 3+ including absorption spectrum and emission spectrum are presented in Fig. 1(e). Meanwhile, the excitation spectrum and emission spectrum of CaMgSi 2 O 6 :Eu 2+ ,Mn 2+ are displayed in Fig. 1(f). 3.
Results and discussion Figure 2 shows the CRI values varying with the concentration of red phosphor and green phosphor from 2% to 20%. The CRI gradually increases with the addition of red phosphor concentration and reaches the maximum value at 20% concentration. On the other hand, the increase in green phosphor does not benet CRI, due to the fact that when the concentration of green phosphor rises from 2% to 20%, CRI continuously decreases regardless of the improvement in red phosphor or the changes in average correlated color temperature (ACCT). From the results of Fig. 2, it is clear that the red light component in WLEDs, which comes from the red phosphor layer CaMgSi 2 O 6 :Eu 2+ ,Mn 2+ , needs improvement in order to boost the value of CRI. When green phosphor Ba 2 Li 2 Si 2 O 7 :Sn 2+ ,Mn 2+ concentration increases, the green light component prevails, and that is a disadvantage for CRI because the energy conversion in red phosphorous layer decreases as the concentration of green phosphor increases. According to TRP structure, the green phosphor layer is below the red phosphor layer, which means the light reaches the green phosphor layer rst, before going through the red layer. So, green phosphor more correctly in the human eyes when there is a lighting eect. However, besides the true color of objects, the preference of the viewers and the color coordi-nates are two important criteria that CRI does not have access to.
However, color quality scale (CQS) can evaluate the combination of three factor: CRI, the preference of the viewer and the color coordinates for white light. Hence, in a comparison between CRI and CQS, CQS value stands out as a more important and dicult target to achieve. The remaining question is how to improve the CQS value of WLEDs? Does it only require the enhancement in the red light component to improve the CRI? To nd answers to these questions, CQS values are also presented in Fig. 3. In general, CQS increases with red CaMgSi 2 O 6 :Eu 2+ ,Mn 2+ phosphor.
However, unlike the CRI, the CQS experiences a small change when the concentration of the green phosphor layer Ba 2 Li 2 Si 2 O 7 :Sn 2+ ,Mn 2+ varies. From the results shown in Fig. 3, it is possible to conrm that both the green phosphor and the red phosphor contribute to the improvement of CQS. The balance between 3 colors: yellow, green and red is the key to enhance CQS. When the concentration of red phosphor or green phosphor increases, yellow phosphor concentration decreases to maintain the ACCT.
The reduced yellow phosphor concentration causes the yellow light component to decrease, and this has two benets. The rst one is reducing the amount of backscattered light to the LED chip so that the luminous ux improves signicantly. Another benet of reducing yellow phosphor concentration is to lower the yellow light component and replace the yellow light component with the red and green light components. Gaining control over CQS is the key to manage these 3 color components. CQS increases gradually when the green phosphor Ba 2 Li 2 Si 2 O 7 :Sn 2+ ,Mn 2+ concentration moves from 2% to 10% and then gradually decreases. The highest CQS values are obtained when Ba 2 Li 2 Si 2 O 7 :Sn 2+ ,Mn 2+ is from 10% to 14%. When the green phosphor concentration is low (2% to 10%), the yellow light component still dominates, therefore, the light transmission energy is lost due to backscattering, which leads to CQS not reaching its maximum. When the green phosphor concentration is between 10% and 14%, the green light component is enough for CQS to reach the highest level. However, if the concentration of Ba 2 Li 2 Si 2 O 7 :Sn 2+ ,Mn 2+ keep going up, the green light components become excessive, causing a color imbalance among the 3 primary colors green, red and yellow. Therefore, the increase in green phosphor concentration from that point onward will cause the CQS to decrease.
Controlling the color quality of remote phosphor structures is more complex than that of conformal phosphor or in-cup phosphor structures. It is even more dicult with WLEDs that have ACCTs from 7000K -8500K. Nonetheless, the results showed that with the TRP structure the higher the ACCTs, the greater the CQS. In addition to reducing the amount of backscattered light, the TRP structure also supports the scattering of light inside WLEDs. This enhancement in scattering is benecial to the mixing of light components, resulting in a high-quality white light. However, does this enhancement in the scattering process reduces the light transmission energy?
The focus of the next part is the mathematical model used to calculate the transmitted blue light and converted yellow light in the doublelayer phosphor structure, which is an area that can generate important changes for the LED efciency. The formulas for transmitted blue light and converted yellow light in single layer remote phosphor package with the phosphor layer thickness of 2h are as follows: The transmitted blue light and converted yellow light for double layer remote phosphor package with the phosphor layer thickness of h are expressed as follow: The h is the thickness of each phosphor layer while the subscripts "1" and "2" indicate the number of layers, single layer or double-layer remote phosphor package. The conversion coecient for blue light converting to yellow light is illustrated as β, and γ is the reection coecient of the yellow light. The intensities of blue light (PB) and yellow light (PY) are the light intensities from the blue LED, indicated by PB 0 . αB; αY are parameters which indicate the proportions of blue and yellow lights' energy loss during the scattering process in the phosphor layer.
The lighting eect of pc-LEDs with the double-layer phosphor structure improved significantly in comparison with a single layer structure: (5) By using the Mie-theory [20], the scattering of phosphor particles was studied, and the scattering cross section C sca for spherical particles is also computed. The Lambert-Beer law [21] can calculate the transmitted light power: I 0 is the incident light power, L is the phosphor layer thickness (mm), and µ ext is the extinction coecient which can be expressed as µ ext = N r C ext , where N r is the number density distribution of particles (mm −3 ). Cext (mm 2 ) is the extinction cross-section of phosphor particles.
Equation (5) certies that the use of additional phosphor layers enhances the luminous emission of WLEDs. The increase in luminous emission aects red phosphor and green phosphor concentrations, causing them to rise. To preserve the ACCTs when the concentrations of red phosphor and green phosphor increase, the yellow phosphor concentration decreases. The vital point in reducing the yellow phosphor concentration is to prevent light loss due to the backscattering characteristic. Furthermore, a reduced yellow phosphor concentration makes light transmission energy become stronger, according to Lambert-Beer's Law in Equation (6). Therefore, the higher the concentrations of the phosphor layer Ba 2 Li 2 Si 2 O 7 :Sn 2+ ,Mn 2+ or CaMgSi 2 O 6 :Eu 2+ ,Mn 2+ are, the more powerful the luminous ux emitted. However, this is unfavorable for CQS as that red or green light components exceed a certain limit as this will cause color imbalance, which reduces the obtained CQS.
According to results available in Fig. 4 "This is an Open Access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium provided the original work is properly cited (CC BY 4.0)."