Enhancing the CRI and lumen output for the 6600 K WLED with convex-dual-layer remote phosphor geometry by applying red-emitting MGSR3SI2O8:EUMN phosphor

Thuc Minh Bui, Nguyen Thi Phuong Loan, Phan Xuan Le, Nguyen Doan Quoc Anh, Anh Tuan Le, Le Van Tho Faculty of Electrical and Electronics Engineering, Nha Trang University, Vietnam Faculty of Fundamental 2, Posts and Telecommunications Institute of Technology, Vietnam Faculty of Electrical and Electronics Engineering, HCMC University of Food Industry, Vietnam Faculty of Electrical and Electronics Engineering, Ton Duc Thang University, Vietnam Institute of Tropical Biology, Vietnam Academy of Science and Technology, Vietnam


INTRODUCTION
With outstanding properties of long lifespan, high reliability and endurance, eco-friendly and power-saving characteristics, and small size, white light-emitting diodes (LEDs) have a worldwide usage in illuminating applications, particularly in aspects of automatic, billboard, and low-temperature lightings [1][2][3]. Remarkably, their recent advances in fast switching properties have got them widely used in smart lighting which is another auto control technology. In fact, there have been a variety of methods that combine the LED chips and the phosphor to attain white light. The very first and cheapest package applying that combination consists of a single blue chip and a single yellow phosphor, but the resulted CRI is very low. Therefore, focusing on improving the CRI, the package was built with the participation of a red and a blue chip as well as a single layer of yellow phosphor. The color rendering index CRI is a quantitative measure of light quality compared to the natural light or black-body radiation. Thus, when CRI has a high value this means the applied phosphor has a broad emission spectrum which helps to better the light quality for LEDs light, similar to the continuous spectrum of the black-body radiation [4][5][6]. With the goal of customizing the chromaticity and CRI for LED lamps, many reports with the topics of combining phosphors with LED chips to accomplish the optimization and prediction of the spectrum have been given. Specifically, these reports focus on the application of the empirical and mathematical models for the analysis and decomposition of the spectrum of LED chip [7][8][9][10]. Nevertheless, constructing a white LED package with tunable correlated color temperatures (CCTs), high efficiency, and superior CRI is a real challenging task to accomplish [11,12]. Previously, the structure of a blue chip combined with two phosphor materials was applied to reach a high value of CRI, but the efficacy is poor because of the Stokes shift [13][14][15][16]. Then, the implement of the two red and blue chips together with one phosphor resulted in an increase in CRI and efficacy but was unable to tune the color, in comparison with the structure consisting of two phosphors. For those drawbacks, another white LED package comprised of the two different LED chips and two phosphors has been proposed to achieve high CRI and luminous output as well as be able to keep a tunable color. In this article, we suggest a color structure that is designed based on Beer's law and linear conversion for the demand of producing white LEDs with subtle color difference [17][18][19][20][21]. In addition, this W-LED module was manufactured by applying yttrium aluminum garnet (YAG) and nitride-based phosphors in blue and red LEDs with high CRI and luminous efficiency. For the WLED production, the phosphors that are mixed in silicone glues with different proportions and densities are combined with the red and blue LED chips. From the results of the article, it is possible to construct and apply this proposed color design model easily.

PREPARATION AND SIMULATION 2.1. Preparation
Mg 8 Ge 2 O 11 F 2 :Mn 4+ is a composition of the four other chemical materials, including MgO, MgF 2 , MgCO 3 , GeO 2 with the mole and weight expressed in Table 1. The preparation process of MgSr 3 Si 2 O 8 :Eu 2+ ,Mn 2+ includes two main stages of firing that need to be followed in a strict order to get the best final composition for the research. Before the first firing, four aforementioned materials must be mixed by ball-milling. Then, the attain mixture is fired in capped quartz tubes in with air at 1200 0 C within two hours. Next, the fried material will be powderized by using dry ball-milling method. Finally, the powder goes through the second firing in open quartz boats overnight for about 16 hours at 1200 0 C. The attain product has deep red emission color with the emission peak of 1.88 eV and the excitation efficiency of over 3.40 eV, which is beneficial enough to be applied in the simulation process.

Simulation
The applications of the Light Tools program and Mie-theory play a crucial role in carrying out this work, as it is easier to simulate the dual-layer phosphor structure of WLED, based on the analysis of phosphor scattering phenomenon and the investigation in the impacts of MgSr 3 Si 2 O 8 :Eu 2+ ,Mn 2+ phosphor on the WLEDs' performance at the 6600 K correlated temperature. Before structuring the in-cup phosphor configuration of WLEDs, it is necessary to prepare the required phosphor layer by mixing the chemical compounding of the MgSr 3 Si 2 O 8 :Eu 2+ ,Mn 2+ phosphor and the yellow YAG:Ce 3+ with the silicon glue, as demonstrated in Figure 1. The simulated model of WLEDs includes the following constituents: nine blue chips, a reflector cup, a phosphor layer, and a silicone layer. Additionally, the parameters of each part are expressed as follows.

RESULTS AND DISCUSSION
As can be seen from Figure 2, the change in the concentration of red phosphor MgSr 3 Si 2 O 8 :Eu 2+ ,Mn 2+ is opposite to that of the YAG:Ce 3+ yellow phosphor. This opposite change demonstrates two aspects, the first one is to maintain the ACCTs and the second one is its influence in the scattering and absorption processes of phosphor layers inside the WLEDs, which does have a great effect on the color quality and lumen output of WLEDs. Hence, selecting the appropriate MgSr 3 Si 2 O 8 :Eu 2+ ,Mn 2+ concentration is very important as it is the factor determining the WLEDs' color quality. Vividly from the chart, when there is an increase in MgSr 3 Si 2 O 8 :Eu 2+ ,Mn 2+ concentration from 2% to 24% wt., the concentration of yellow phosphor YAG:Ce 3+ declines to retain the ACCTs, which is similar to the WLEDs structure with the ACCT of 6600 K. Figure 3 presents the emission spectra of the WLED package with 6600 K ACCT where the impacts of different concentrations of MgSr 3 Si 2 O 8 :Eu 2+ , Mn 2+ are demonstrated obviously. Besides, the synthesis of the spectral regions as shown in Figure 3 actually forms the white light. As can be seen, when the MgSr 3 Si 2 O 8 :Eu 2+ ,Mn 2+ concentration rises from 2% to 22%, the emission spectrum increases significantly in the wavelength range of 680 nm -738 nm. However, this change will be insignificant if there is no spectral increase in the two range of 420 nm -480 nm and 500 nm -640 nm. Moreover, the growth of the spectrum in the wavelength range of 420 nm -480 nm raises the luminous flux of blue light (blue-light scattering). Thus, it can be addressed that a higher color temperature will lead to a higher spectral emission. This result is very important to the application of MgSr 3 Si 2 O 8 :Eu 2+ ,Mn 2+ for the color quality management of the WLEDs with high color temperature. Furthermore, with these findings, the research paper could assure the ability of MgSr 3 Si 2 O 8 :Eu 2+ ,Mn 2+ in enhancing the chromatic quality of WLED packages with both low color temperature (6600 K) and high color temperature (7700 K). Therefore, the suitable MgSr 3 Si 2 O 8 :Eu 2+ ,Mn 2+ concentration is decided according to the requirements from manufacturers. If the goal is to achieve a high color quality for WLED products, a small reduction in luminous flux is acceptable.   Figure 4 is the upward trend in the color rendering index following the increase of MgSr 3 Si 2 O 8 :Eu 2+ , Mn 2+ phosphor concentration. This trend can be explained by the absorption process of the red MgSr 3 Si 2 O 8 :Eu 2+ ,Mn 2+ phosphor layer. Specifically, as soon as the red phosphors MgSr 3 Si 2 O 8 :Eu 2+ ,Mn 2+ absorb the blue light emitted from the blue chips, they will turn these blue lights into red lights. Moreover, besides the blue light, this red phosphor MgSr 3 Si 2 O 8 :Eu 2+ ,Mn 2+ also absorbs the yellow light. However, when drawing a comparison between these two absorption processes, it has turned out that the blue light from LED chips is absorbed more strongly than the yellow light because of the absorption properties of the material, which certainly leads to the larger amount of red light components inside the WLEDs when MgSr 3 Si 2 O 8 :Eu 2+ ,Mn 2+ is added. Consequently, the color rendering index (CRI) reaches a higher value. CRI is an important factor that needs to be focused on when choosing a modern model of WLEDs, so the higher the CRI is, the more expensive the LED products become. Nonetheless, CRI cannot fully evaluate the WLEDs' color quality since it is just one of the measurement factors. Therefore, a new index called color quality scale (CQS) is introduced to be an alternative because it covers three different factors, including the CRI, the viewers' preference and the color coordinates. The CQS shows a considerable improvement with the concentration of the red phosphor MgSr 3 Si 2 O 8 :Eu 2+ ,Mn 2+ , as illustrated in Figure 5. It is obvious to admit that the red phosphor MgSr 3 Si 2 O 8 :Eu 2+ ,Mn 2+ can enhance the color quality of white light LEDs with dual-layer phosphor structure. Thus, this result plays an important role in accomplishing the goal of color quality enhancement. In addition, another advantage of this type of phosphor is its low cost which is beneficial to mass production, and thus, that MgSr 3 Si 2 O 8 :Eu 2+ ,Mn 2+ is widely used in this industry can be easily understood. However, the downside when utilizing this phosphor material is that it can cause a decrease in the lumen output of WLEDs. In this part, we will show and explain the mathematical model of the transmitted blue light and converted yellow light in the double-layer phosphor structure to obtain a significant enhancement of the LED efficacy. The asymmetrical SPD of monochrome LED is typically modeled with Gaussian function [22,23]: The SPD of a white LED utilizing yellow YAG phosphor and blue LED chip can hypothetically be viewed as the aggregate of the blue and yellow spectra. However, in fact, the supposed yellow phosphor radiates light in both of the yellow and green spectra as demonstrated from the deliberate spectra in Figure 3. In the event that a blue and a yellow range are picked, the contrast between the essentially estimated SPD and twofold shading (blue and yellow shading) range model can be spoken for a green range. In this way, considering the practical circumstance, a green range can be added to the twofold range model to form the accompanying investigative tri-spectrum (B-G-Y) model represented by (3) and subsequently altered as (4). η Ratio of specific spectra to white spectrum, dimensionless.
λ 1 , λ 2 Wavelengths at half of the peak intensity. Therefore, the SPD modeling for the phosphor-coated white LED can be expressed as a tricolor spectrum, which can be considered as an extended Gaussian model. The scattering of MgSr 3 Si 2 O 8 :Eu 2+ ,Mn 2+ phosphor particle was analyzed by using the Mie-theory. In addition, the scattering cross section C sca for spherical particles can be computed by the following expression by applying the Mie theory [24]. The transmitted light power can be calculated by the Lambert-Beer law [25]: where I 0 , L, and µ ext represent the incident light power the thickness of phosphor layer (mm), and the extinction coefficient, respectively. Moreover, the extinction coefficient can be demonstrated as the following formula: µ ext = N r .C ext , in which N r indicates the number density distribution of particles (mm -3 ), while C ext (mm 2 ) is known as the extinction cross-section of phosphor particles. It can be implied from (5) that the dual-layer remote phosphor results in the larger luminous efficiency for the LED packages than the single-layer phosphor. Hence, the benefit of using dual-layer remote phosphor layer in yielding better lumen output is successfully demonstrated in this study. On the other hand, the concentration of red phosphor MgSr 3 Si 2 O 8 :Eu 2+ ,Mn 2+ greatly affects the optical path of this dual-layer remote phosphor structure. Vividly, the reduction factor µ ext has a direct proportion to the MgSr 3 Si 2 O 8 :Eu 2+ ,Mn 2+ concentration, but an inverse ratio to the light transmission power. Thus, as the thickness of the two phosphor layers are fixed, a decrease in luminous flux can occur with the growth of MgSr 3 Si 2 O 8 :Eu 2+ ,Mn 2+ concentration, leading to the decline in all five CCTs, as shown in Figure 6. Obviously, when the concentration of MgSr 3 Si 2 O 8 :Eu 2+ ,Mn 2+ reaches 24% wt, the luminous flux decreases dramatically. However, this drawback in lumen output can be accepted due to the great benefits that the red phosphor MgSr 3 Si 2 O 8 :Eu 2+ ,Mn 2+ brings to the WLEDs, including the better CRI and CQS. In addition, dual-layer remote phosphor can yield higher luminous flux than the single-layer phosphor without the presence of red phosphors. Therefore, the only issue here is the purpose of manufactures for determining the most appropriate phosphor concentrations of MgSr 3 Si 2 O 8 :Eu 2+ ,Mn 2+ when mass producing WLEDs products.

CONCLUSION
From this article, a simple method for better performance of adjusting the color with white-light LED modules while retaining the high CRI and luminous efficacy is proposed. Depending on the manufacturers' requirements, a color design model is built based on the applications of Beer's law and linear conversion which can support the different white-light LEDs. The simulated and experimental spectra show positive overlapped results. Moreover, the biggest difference between the measured and simulated CIE 1931 color coordinates is identified by approximately 0.0063 with the 6600 K CCT. Hence, it is possible to structure the proposed color design model and get it applied easily.