SrBaSiO4:Eu phosphor: a novel application for improving the luminous flux and color quality of multi-chip white LED lamps

Received Sep 11, 2019 Revised Apr 22, 2020 Accepted May 5, 2020 This paper described in detail the chromatic homogeneity and luminous flux influences in producing better quality white LED devices with various phosphor layers (MCW-LEDs). The method is to let Eu-activated strontium–barium silicate (SrBaSiO4:Eu) mixed with their phosphor compounding, which results in notable impact on lighting performance. The increase in concentration of yellow-green-emitting SrBaSiO4:Eu phosphor also promotes the color performance and lumen output of WLED devices at high correlated color temperature around 8500K. This is the first time this approach is applied and it results can be utilized for better understanding of optical properties interaction with phosphor materials. Although SrBaSiO4:Eu receives many positive responses, we still need to limit it concentration for high SrBaSiO4:Eu concentration is detrimental to CQS. The appropriate choice of concentration and size of SrBaSiO4:Eu is the principal factor to decide the performance of MCW-LEDs.

Oh's group proposed the multiple phosphor LED with three primary colors green, yellow, and red, which focus on enabling the luminous flux and color consistency to reach the most satisfactory result [21]. Zhang's team used multi-chromatic materials for better quality of CRI in LED products [22]. The improvement of CRI and optical output is of interest to many researchers, but the increase in color uniformity seems to be ignored. Furthermore, their research subjects is solely white LED with one chip, which gives out low CRI and therefore not really applicable to modern lighting applications that require multiple chips for lighting devices with high CCTs.
Understanding the needs for an advanced study in optical properties enhancement and more relevant results to the current WLED structures, this research present experiments in which the chromatic phosphor is employed in a multi-chip LED configuration. Based on previous studies that targeted at the optical properties such as lumen output, CRI, and chromatic quality, we introduce an innovative research method that exploits the advantageous findings while adding in new aspects for research. For example, the application of red phosphor Mg8Ge2O11F2:Mn 4+ in the dual-layer WLED at 6600 K CCT is confirmed to optimize the lumen output and CRI. On the other hand, the remote phosphor configuration is proven to be the optimal set up for WLED among the three configurations including the in-cup phosphor and conformal phosphor after the achieved lumen output of this configuration is demonstrated as better than others [23][24][25]. However, there are currently a lack of diversity in the applicable types of phosphor for WLED. Therefore, we conducted experiments on a new phosphor, known as the yellow-green phosphor SrBaSiO4:Eu 2+ , on WLEDs with three different phosphor structures respectively. The approach this research suggests allow comparison between performances of SrBaSiO4:Eu 2+ in each phosphor configuration, which assists the identification of the best combination. The yellow-green SrBaSiO4:Eu 2+ phosphor plays a supporting role for chromatic homogeneity and luminous efficacy so that the greatest efficiency could be attained from the phosphor materials in MCW-LEDs during the scattering process. The participation of SrBaSiO4:Eu 2+ particles will decide lighting quality color uniformity. Regarding the remote phosphor structure, there are the remote structure with two flat phosphor layers (FDRP) and the remote structure with two curved phosphor layers (CDRP). However, a preceding study claims that FDRP is the better choice as CDRP yields less lumen while having a more complex fabricating procedure, therefore, the dual-remote WLED in this research is simulated with FDRP arrangement [26][27][28]. This research has 3 phases: (1) Simulating the MCW-LEDs prototype; (2) Applying SrBaSiO4:Eu 2+ as a phosphor material for the MCW-LEDs; (3) Studying the influences of SrBaSiO4:Eu 2+ on the optical properties, including the emission spectra and the scattering coefficient. In sum, mixing SrBaSiO4:Eu 2+ and YAG:Ce 3+ phosphors together is considered as an optimal method to maximize quality and lighting efficiency. The conclusions in this manuscript are useful references for luminous flux and chromatic performance development while also effective in supporting the manufacturers in choosing the suitable configuration for quality WLEDs production.

PREPARATION AND SIMULATION 2.1. Preparation of yellow-green SrBaSiO4:Eu 2+ phosphor
Yellow-green SrBaSiO4:Eu 2+ phosphor particles contain many superb characteristics, for example, great quantum yield and consistency at high temperature with an emission peak up to 2.36 eV, which makes it become more popular recently [29]. The particle sizes and concentration directly affect the luminescence properties of SrBaSiO4:Eu 2+ phosphor. The composition including SrCO3, BaCO3, SiO2, Eu2O3, NH4Cl, and Eu 2+ ion are used as raw materials as shown in Table 1. Some heavy duty and long lifetime fluorescent lights prefer to use SrBaSiO4:Eu 2+ . Therefore, it turns into the most well-known commercialized oxide material in the lighting market. The fabrication process of SrBaSiO4:Eu 2+ is described specifically as follows: Firstly, the materials are soaked in water to be dissolved before being crushed into small particles. Then they will be pulverized as soon as being dried in the air. Secondly, the powder will be heated in a closed quartz tube with CO at 1100ºC for one hour and crushed into powder by dry grinding. Then 5.4 g NH4Cl is added and mixed with this compound by dry grinding. Next, they will be reheated in the capped quartz tubes with CO at 1100ºC for one hour and will be pulverized. Finally, the materials are washed in water several times (pH ranged from 10 to 12) and dried afterward.

Construction of MC-WLEDs
Based on the LightTools 8.5.0 software and Monte Carlo method [30][31][32], the prototype simulation of MCW-LEDs is conducted through two main periods with a flat silicone layer. Setting up and manufacturing the framework and lighting characteristics of MCW-LED at first, then the influences of phosphor compounding with different SrBaSiO4:Eu 2+ concentration is analyzed based on the performance of MCW-LED with different phosphor geometries. The realistic simulated product is present in Figure 1(a) with technical measurements can be seen from Figure 1(b). Among them, the two types of compounding which have average CCT of 8500 K, conformal and in-cup phosphor, are picked to analyze. A distinct description for MCW-LEDs with conformal phosphor structure, in-cup structure, and remote structure with the average CCT of 8500 K was shown in Figure 1(c), (d), and (e) respectively. The components of the MCW-LED is an 8x2.07x9.85 mm at bottom, height, and surface reflector, 0.08 mm phosphor layer in phosphor compound, and 9 LED chips. Each chip is 1.14 mm 2 in acreage and 0.15 mm in height with 1.16 W energy emission for each individual at 453 nm maximum wavelength connected to the base of the structure. As can be seen from Figure 1(c), the chips are covered with phosphor particles diffused by conformal coating. Conversely, separate methods are used for the structure with in-cup phosphor and for mixture with silicone lens. The scattering analysis of these two structures was carried out based on Mie-theory. The measurement for particles in this research is 14.5 μm in size, similar to the real parameter. The phosphor composite is a combination of SrBaSiO4:Eu 2+ , YAG:Ce 3+ , and the silicone gel with the same sizes as the real parameters and own the refractive indexes respectively of 1.85, 1.83, and 1.52. Once the refractive index and diameter of phosphor particles are set, we can study the emission spectra of research subjects. Figure 2 (top) described the emission spectra of the conformal structure WLED at different SrBaSiO4:Eu 2+ concentration fluctuating from 0% toward 24%. Figure 2 (bottom) expresses in details the emission spectra of in-cup structure WLED with different ranges of SrBaSiO4:Eu 2+ concentration which can reach 1.4%. According to this chart, the lighting efficiency of MCW-LEDs will improve after SrBaSiO4:Eu 2+ phosphor are added to the structure.

COMPUTATION AND DISCUSSION
Based on the Mie theory [23-35], the scattering coefficient μsca are computed to verify the optical properties of phosphor compounding. The relationship among the scattering coefficient (SC), the wavelength, and diameter of SrBaSiO4:Eu 2+ particles is expressed in the formula below: where, f(D) is the function of diameter distribution, c is the distribution density of phosphor (g/cm 3 ), Csca,D is the scattering cross-section of the phosphor having particle diameter D. While When SrBaSiO4:Eu 2+ phosphor involves in the scattering, the scattering coefficient (SC) of the phosphor layer is indicated in Figure 3. Concentrations and other agents of SrBaSiO4:Eu 2+ will be the catalyst that makes SC of phosphorus mixture significantly change. This also verifies that previous concentrations and stimulation of SrBaSiO4:Eu 2+ affect the color quality of both CPG and IPG structures. When SrBaSiO4:Eu 2+ concentration increases, SC also tends to develop regardless of SrBaSiO4:Eu 2+ particle size. With a size of about 1 μm, SC can achieve optimum value and making color uniformity better regardless of the larger sizes. Therefore, if the target is CQS, SrBaSiO4:Eu 2+ size can be approximately 1 μm. When it is approximately 7 μm, the SC value of phosphorus is more stable regardless of the increase in SrBaSiO4:Eu 2+ concentration, which brings color quality (CQS) of LED lights a lot of benefits. As a result, if the target is CQS, size less than 7 m can be chosen. Apparently, the SC value depends on the amount of SrBaSiO4:Eu 2+ particles and also their diameter, which is why SrBaSiO4:Eu 2+ can be used to increase luminous efficiency and color uniformity of the LEDs. The appearance of SrBaSiO4:Eu 2+ is one of the factors affecting the color deviation result of MCW-LED as illustrated in Figure 4. The CCT peak deviation decreases significantly when SrBaSiO4:Eu 2+ is involved and keep going as the phosphor concentration increases. That means the spatial distribution of MCW-LED with SrBaSiO4:Eu 2+ is much better than when the SrBaSiO4:Eu 2+ is absence. There must be a balance between performance factors and optimization issues because if we only focus on optimizing one element, optical systems can be weak in other aspects. In addition, the greatest CQS and the luminous efficiency of white LED packages cannot be achieved at the same time. The solution is that the spectrum and wide source efficiency must be maximized in monochromatic radiation of 555 nm wavelength to better CRI. Therefore, the remaining optical properties that need optimization are CQS, luminous flux, and CCT P-V deviation values. Below are the Figure 5 that illustrates the luminous efficacy of WLED with SrBaSiO4:Eu 2+ and Figure 6 that shows concentration continues to grow higher, the luminous flux will still follow, even though CQS will decrease. Therefore, for WLEDs that focus on the CQS values, it is advisable to limit the phosphor concentration under permitted level to prevent damaging CQS. Moreover, SrBaSiO4:Eu 2+ concentration develops backward but not significantly, it will contribute to better CCT stability and greater luminous efficiency.

CONCLUSION
In summary, the effectiveness of the yellow-green SrBaSiO4:Eu 2+ in improving color quality and light output of WLEDs devices was verified through some references in the article. The experimental results from this research are also viable to conclude about the optical properties influences as follows. First, no matter what the average CCT value, color uniformity can be very diverse according to Mie scattering theory. This is the effect of light loss from scattering in white LED packages. Second, through Monte Carlo method, the luminous efficacy is confirmed to increase with SrBaSiO4:Eu 2+ concentration. Particularly, the emitted light will grow as long as SrBaSiO4:Eu 2+ concentration keep increasing at any phosphor geometry. Finally, selecting SrBaSiO4:Eu 2+ phosphor is the appropriate solution for developing phosphorus materials as well as producing white LED packages. Although SrBaSiO4:Eu 2+ phosphor is an effective enhancement material, the concentration of this phosphor must be monitored in application as any amount of SrBaSiO4:Eu 2+ is good for lumen output and CCT deviation, however, CQS can be damaged if the SrBaSiO4:Eu 2+ concentration is too high. This research not only presented a new approach by applying the yellow-green phosphor to WLED but also compared the performance based on the type of phosphor geometry and phosphor concentration, which opens more options for manufacturers to develop their products and leaves valuable references for future studies.