The effectiveness of MgCeAl11O19:Tb phosphor in enhancing the luminous efficacy and color quality of multi-chip white LEDs

Received Nov 13, 2019 Revised Mar 13, 2020 Accepted Mar 27, 2020 In this research paper, we introduced yellow-green MgCeAl11O19:Tb as a new phosphor ingredient to adapt to the quality requirements on the chromatic homogeneity and emitted luminous flux of modern multi-chip white LED lights (MCW-LEDs). The results from experiments and simulation show that employing MgCeAl11O19:Tb phosphor can lead to much better optical properties and therefore is a perfect supporting material to achieve the goals of the research. When the MgCeAl11O19:Tb phosphor is added into the phosphorus composite which already contains YAG: Ce particles, and the silicone glue, it affects the optical properties significantly. In other words, the concentration of this phosphor can determine the efficiency of lumen output and chromatic homogeneity of WLEDs. In specific, as the concentration of MgCeAl11O19:Tb go up, the luminous yield will increase accordingly, though there is an insignificant decrease in CQS. Moreover, if the MgCeAl11O19:Tb concentration reduce a little bit, it is possible to better the correlated color temperature uniformity and lumen efficacy of LED packages. In addition, the Mie scattering theory, Monte Carlo simulation and LightTools 8.3.2 software are employed to analyze and simulate the LED packages’ structure as well as the phosphor compound.


INTRODUCTION
White light-emitting diodes (WLEDs) are considered as a potential light source that can fulfill the fastidious illumination market just because they are advantageous in compact size, energy saving, eco-friendly, and long lifespan. This also the reason why WLEDs havea widespread application over the human life thesedays, from backlighting to street and indoor lightings [1][2][3]. Up to now, the method used to producecommercial phosphor-converted WLEDs is combining the blue LED chips and yellow Y3Al5O12:Ce 3+ (YAG:Ce 3+ ) phosphor embedded with organic resins [4,5]. Nevertheless, when the organic encapsulants is applied to high-power WLEDs, it shows poor thermal and photonic stability, may wane easily and turns yellow after using for a long time. As a result, this degradation probably reduces the luminous efficacy (LE), the chromatic quality, and the long-term reliability [6,7]. Moreover, it is showed that compared to the YAG:Ce 3+ phosphor, which has refractive index at about 1.83, that of the organic encapsulants is lower as its value is approximately 1.5. This difference causes the reflection loss to happen and as a result, the emitted lights from phosphor particles are downgraded [8,9]. Owing to that disadvantage of organic encapsulants, an alternative approach called phosphor-in-glass (PiG) was proposed. To prepare PiG, glass powders and phosphor particles are sintered at a low-temperature (<800°C). Researchers considered PiG as a potential luminescent converter for high-power WLEDs as it has the robustness, high thermal stability, and low thermal expansion coefficient [10][11][12][13][14][15]. Moreover, it is possible for the precursor glass matrix to accomplish a high refractive index for the PiG converter by addinghigh polarizable ions. Last year, we succeeded in developing YAG:Ce 3+ based PiG converter by embedding the yellow phosphor YAG:Ce 3+ with borosilicate glassusing screen-printing and low-temperature sintering methods. The product can yield cool white light with the values of lumen efficacy,correlated color temperature (CCT), and color rendering index (CRI) of 114 lm/W, 5524 K and 69, respectively [16,17]. However, this YAG:Ce 3+ PiG does not generate true white light having high CRI because the emission light is short of red light spectrum. To fulfill the goal of enhancing the CRI, researchers proposed multi-components PiG, which is invented by having red CaAlSiN3:Eu 2+ (CASN:Eu 2+ ) phosphor added into the glass matrix [18][19][20]. Yet, there are noticeable drawbacks of this PiG: the effect of thermal degradation during co-sintering on it, and the interfacial reaction it caused to the glass matrix. These influences probably decrease the quantum efficiency of red phosphor [21,22]. Additionally, when using the PiG converter with flat surface, the total internal reflection (TIR) at the glass-air interface also exits as the refractive indexes of glass and air are different; and as a result, the loss of light trapped in the remote-type LED package occurs.
For the aim of TIR loss reduction and light extraction enhancement, the micro/nano patterned structures were widely applied. Currently, there have been many strategies suggested to produce the patterned structures consisting lithography [23], photoresist reflow, direct laser writing, and inkjet imprinting [24]. But in fact, most of the processes in those strategies consist of at least one photolithographic step which requires much money and time to do as well as leads to heavy pollution. It is possible to produce high precise patterned structure by direct laser writing, but it is high-cost and impracticalfor mass production. Another method proposed to cover the disadvantages is inkjet printingowing to its advantages in production which are mask-less, large-scale, and low-cost. However, the drawback of this method is the limitation caused by inhomogeneous morphology and large feature size. Hence, to successfully produce the YAG:Ce 3+ PiG with high light extraction and color quality, there is a big challenge in choosing the most flexible and effective approach to apply. On the hand, choosing the suitable WLED configuration is also an important decision as it affect the quality evaluation of WLED. Regarding this subject, there are many research have proposed different alterations that achieved certain improvements. For example, the remote phosphor structure is proposed for utilization in many phosphor structures to increase the color quality of White LEDs. There are reports that claimed the two essential quality indicators of WLED, CRI and lumen output, can be enhanced with the application convex-dual-layer remote phosphor geometry containing the red-emitting phosphor Mg8Ge2O11F2:Mn 4+ . The remote phosphor structure with flat dual phosphor layers are also reported to be efficient in enhancing WLED, especially the luminous flux. However, these research concentrate on the light output or CRI while there are other more important optical properties. Moreover, the in-cup phosphor geometry (IGP) and conformal phosphor geometry (CPG) were not mentioned and the phosphor utilized in the experiments is the red-emitting phosphor. Thus, this research come up with an approach which utilize the green phosphor as a new phosphor material and including other aspects of WLED such as scattering coefficient and CQS. The influences of the green phosphor on the optical properties of WLED will be estimated by changing the phosphor concentration.
In this article, the yellow-green MgCeAl11O19:Tb 3+ phosphor is used as a support of increasing chromatic homogeneity and emitted luminous flux to achieve the greatest efficiency in the phosphor compounding of MCW-LEDs during the scattering process. Specifically, the presence of MgCeAl11O19:Tb 3+ particles is capable of determining the illuminating quality and chromatic homogeneity. The research is comprised of 3 stages: (1) Building the MCW-LED model; (2) Adding MgCeAl11O19:Tb 3+ to the phosphor compounding used for the simulated MCW-LEDs; (3) Investigating how the concentration of MgCeAl11O19:Tb 3+ impactthe emission spectra and the scattering of the structure.The research will calculated the emission spectra, scattering coefficient, lumen output, and CQS of WLED with MgCeAl11O19:Tb 3+ in conformal phosphor and in-cup phosphor configuration. The mathematical equations for the optical properties calculation are also presented in this paper. Briefly, combining MgCeAl11O19:Tb 3+ together with YAG:Ce 3+ phosphors is considered as an optimal approach to push the color uniformity and lumen output to the highest value. This research can provide information and support the manufacturers in identify the suitable configuration to fabricate high quality WLED.

PREPARATION AND SIMULATION 2.1. Preparation of yellow-green MgCeAl11O19:Tb 3+ phosphor
To achieve the optimal MgCeAl11O19:Tb 3+ phosphor, it is important to get the preparation carried out in a strict order. The preparation process will go through six steps described as follows. Initially, the materials are 4633 mixed by using an automatic mill. Next, the mixed product is taken into the alumina crucibles and fired with stagnant ait at the temperature of 1650 0 C in 2 hours. After that, powderizing the material by automatic mill and re-firing it in alumina boats at 1600 0 C in the same time as the first firing. However, for the re-firing stage, it is vital to reduce the atmosphere (95% N2/5% H2). Then, the fired mixture is powderized again. Finally, the obtained product will be stored in a well-sealed container.

Construction of MC-WLEDs
To construct a conventional phosphor-conversion white LED, it is essential to dispense the compounding made of transparent silicone binder and phosphor grains into a die-bonded lead frame. However, inside this LED model, the opportunity that the blue lights reach the phosphor particles is diverse considerably depending on different light angle, which leads to the low angular color uniformityof the package [7][8][9]. Thus, the purpose here is to enhance the chromatic homogeneity of the LED packages. Then, researchers came up with the remote phosphor LED package including a phosphor layer or plate to attain this goal [10][11][12]. Hence, this study is carried out with the application of the remote phosphor structure so that we can achievea emitting surface that is uniform with a rectangular shape on an LED package and fulfill the requirement of the ISO 9680 regulation.
According to Figure 1, the remote phosphor LED package was simulated by applying the LightTools 8.3.2 software with the support of Monte Carlo ray tracing. The two LED chips are placed horizontally on the bottom of the reflector cup. Theirwidth, length, and height are 1.33 mm, 1.33 mm, and 0.17 mm, respectively.For the cavity of the LED (width/length/height = 6/5/2.8 mm), its dimensions are width = 44 mm, length = 3.06 mm, and depth = 1.7 mm. In addition, the bottom of the reflector cup was seen as a mirror with the reflectivity of 90%. Meanwhile, the side surfaces get their reflective properties followed Gaussian scattering with 90% reflectivity and 10 0 Gaussian spread. Besides, the transparent glass substrate covered by a phosphor layeris located at the top surface of the reflector cup to protect the LED chips. Besides, the glass substrate and phosphor film whose width and length are the same (5mm x 3.5mm) have the thickness of 0.2 mm and 0.15 mm, in turn.
Inside the LED lamp, just a proportion of blue lights from LED chips can directly get to the interface between the glass substrate and the phosphor layer, while the others have to going through a process of reflection caused by the reflector cup before arriving at the destination. Besides, the structure of the reflector cup and the gap between two LED chips were amended so as to obtain the homogeneous illuminance at the interface. After passing the glass substrate, the blue light will be absorbed by phosphor grains or scattered without any absorption, when they reach the phosphor film. Additionally, the Stokes shift can have some absorbed blue lights down-converted to green-red light. Meanwhile, the unconverted light is released as a non-radiative loss. Finally, the white light is generated at the top emitting surface by the combination of green-red light and blue light which are converted and unabsorbed, respectively. For the phosphor film, it consists of transparent silicone binder, green (LuAG:Ce) and red (CASN:Eu) phosphors to reach the CRI over 90, in which the LuAG:Ce phosphor and CASN:Eu phosphor have the phosphor density of 6g∕cm 3 and 3.1g∕cm 3 , the average particle size of approximate 13 μm and 12 μm, respectively. The excitation, quantum yield, and emission spectra of each phosphor are verified by employing a quantum yield measurement framework. The parameters of phosphors were updated to the optical configuration which then simulated with the Monte Carlo ray tracing and Mie scattering. Figure 1(c) illustrated the conformal phosphor structure (CPG), and Figure 1(d) presented the in-cup phosphor structure (IPG). From Figure 1(c), we can observe that the proposed phosphor compounding covers the chips conformally. In contrast, for the in-cup phosphor structure and for mixing with the silicone lens, the methods of separation are applied. Then, the Mie-theory is used to analyse the scattering processes of these two structures. In addition, the mean phosphor particles diameter adopted in this research is 14.5 μm which is equal to the actual parameter.
The phosphor compounding used in the experiments is comprised of MgCeAl11O19:Tb 3+ particles, YAG:Ce 3+ particles, and the silicone glue that are completely mixed together. Moreover, the sizes of these elements are the similar to the actual ones whose refractive indexes are respectively of 1.85, 1.83, and 1.52. After verifying the refractive index and the size of the phosphor grains, it is possible to determine the emission spectra of phosphor mixture. Figure 2 demonstrates the emission spectra of two phosphor structures along with different concentration values of MgCeAl11O19:Tb 3+ : (a) the conformal with MgCeAl11O19:Tb 3+ concentration from 0% to 4%, and (b) the in-cup with MgCeAl11O19:Tb 3+ varying from 0% to 1.4% sometimes. Based on this chart, the luminous flux of MCW-LEDs can be promoted as soon as the MgCeAl11O19:Tb 3+ phosphor is added into the phosphor compound.

COMPUTATION AND DISCUSSION
In order to confirm the optical properties of phosphor compounding, the scattering coefficient μsca are calculated based on the Mie theory [25]. The following expressions demonstrate the connection among the three elements: the scattering coefficient (SC), the wavelength, and the size of MgCeAl11O19:Tb 3+ particles.
In these expressions, f(D) indicates the size distribution function, cis the phosphor concentration (g/cm 3 ), and Csca,D is the scattering cross-section of the phosphor with the particle diameter D. Besides, () sca C  and m present the scattering cross-section and the particle mass of the phosphor integrated over f(D), respectively. Additionally, () sca P  is known as the scattered power by phosphor particles, while () inc I  are the irradiance intensity. Figure 3 shows the scattering coefficient (SC) of the phosphor layer as the MgCeAl11O19:Tb 3+ phosphor participates in the scattering process.In fact, the concentrations and other factors of MgCeAl11O19:Tb 3+ can cause the SC of phosphorus mixture to change greatly. Additionally, this conclusion also assures the impacts of previous concentrations and stimulation of MgCeAl11O19:Tb 3+ on the color quality of both CPG and IPG structures. As can be seen, the increase in MgCeAl11O19:Tb 3+ concentration leads to the upward tendency of SC no matter what particle size of MgCeAl11O19:Tb 3+ . Moreover, when the MgCeAl11O19:Tb 3+ phosphor particle is set at about 1 μm, SC can reach the highest value and better the chromatic uniformity, without consideration for the larger sizes. Meanwhile, with the size of approximately 7μm, the SC value is more stable, which is independent of the rise in MgCeAl11O19:Tb 3+ concentration. As a result, with this size, the color quality (CQS) of LED lights is advanced. Thus, if the goal is to improve the CQS, the size of MgCeAl11O19:Tb 3+ should be at 1 μm and less than 7 μm. Obviously, the concentration and the size of MgCeAl11O19:Tb 3+ can determine the SC value. That's why MgCeAl11O19:Tb 3+ can be employed to lift up the luminous efficiency and color uniformity of the LEDs. The essential purpose that needs to be accomplished in this article is fulfilling the requirements of the LED product specification. Thus, the ACCT that needs for the effective operation of MCW-LED is in the range of 8500 K. In addition, as the MgCeAl11O19:Tb 3+ phosphor concentration increases, the yellow YAG:Ce 3+ phosphor concentration simultaneously decreases to keep the ACCT at 8500 K.The weight percentage of the LED phosphor layer is presented in the following formula.  Figure 4. Obviously, the presence of MgCeAl11O19:Tb 3+ will probably cause the CCT peak deviation todecline noticeably. In other words, the spatial distribution of MCW-LED with MgCeAl11O19:Tb 3+ is much flatter than the absence of MgCeAl11O19:Tb 3+ . To acquire the high-quality LED products, it is crucial to focus on both performance factors and optimization issues since the optical systems will not be fully optimized if we just concentrate on promoting a single element. In other words, the WLEDs cannot accomplish the highest CQS and efficiency at the same time. Thus, to be able to solve this problem, the spectrum and wide source efficiency must be maximized in monochromatic radiation of 555 nm wavelength to enhance CRI. In this paper, CQS, luminous flux, and CCT P-V deviation values are put into comparison.
From the attained results illustrated in Figures 5 and 6, it is possible to say that the growth of luminous yield is compatible with the increase of MgCeAl11O19:Tb 3+ concentration. Though the higher concentration of MgCeAl11O19:Tb 3+ will better the luminous flux, the CQS will decline. Besides, when the MgCeAl11O19:Tb 3+ concentration decrease but insignificantly, the uniformity of correlated color temperature and lumen efficiency can be enhanced significantly.

CONCLUSION
To further enhance the optical characteristics of modern multi-chip white LED lamps (MCW-LEDs) for application inmore advanced and demanding utilization. Inshort, through results of experiments and simulations of this study, the yellow-green MgCeAl11O19:Tb 3+ has proved its effectiveness in getting the color quality and lumen output of WLEDsimproved. There are two noticeable results that reinforce the efficiency of employing the yellow-green MgCeAl11O19:Tb 3+ phosphor. First, with the application of Mie scattering theory, it demonstrates that the compensation of light scattering for WLEDs leads to the diversity in color uniformity, regardless of ACCT values. Second, the Monte Carlo simulation was applied to compute the lumen output which result showed significant improvement when the amount of MgCeAl11O19:Tb 3+ concentration increased. One small notice is that the high concentration in MgCeAl11O19:Tb 3+ can negatively affect the value of CQS, an important quality indicator for WLED, therefore, keeping MgCeAl11O19:Tb 3+ concentration under a certain percent that does not damage the CQS is most advisable. However, with the superior values achieved in different optical qualities when this type of phosphor is applied compared to previous phosphor configurations, MgCeAl11O19:Tb 3+ phosphor is the still the more advanced choicefor developing phosphorus materials and manufacturing white LED packages. The manufacturers can refer to the results of this research as guideline for the production of high-quality LED or references for further quality development study.