Enhancement of luminous flux and color quality of white light-emitting diodes by using green (Y,Gd)BO 3 :Tb 3+ phosphor

In the study, we analyzed and clarified the effect of green (Y,Gd)BO 3 :Tb 3+ phosphor on chromatic homogeneity and optical performance of multi-chip white light emitting diodes (MCW-LEDs). Thereby there is a solution to get the best luminous efficiency. In addition, (Y,Gd)BO 3 :Tb 3+ is known as one of the factors that has a significant impact on lighting performance, so it needs to add the (Y,Gd)BO 3 :Tb 3+ phosphor in the structure of LEDs to combine with the yellow phosphor YAG:Ce 3+ to receive the best results. Therefore, the concentration and size of (Y,Gd)BO 3 :Tb 3+ should be choose carefully so that the presentation of MCW-LEDs would be more incredible. The results show that when the concentration of green-emitting (Y,Gd)BO 3 :Tb 3+ phosphor tends to increase, it also helps the color homogeneity and the lumen efficiency of MCW-LEDs with the average correlated color temperature (CCT) of 5600 K-8500 K become better.


INTRODUCTION
In recent years, researchers have been always aiming to obtain the improvement in the light efficiency of white LEDs. In addition, they also have put a lot of effort into achieving better optical output and color uniformity for white light emitting diodes (LEDs) [1][2][3]. Finally, Anh and his colleague recognized a truth that the performance of multi-chip white LED (MCW-LED) lights can be promoted through the mixture of YAG:Ce 3+ phosphor with SiO2 particles [4]. Moreover, the yellow YAG:Ce 3+ phosphor should be blended with the silicone glue so that it could create a mixture that emits yellow light and absorbs blue light emitted from blue chips, and then generates white light. Before white lights are generated, the lights need to go through two processes: first, disintegrating in phosphorus compounds, and second, transmitting through phosphorus particles [5][6][7][8]. During this process, the yellow YAG:Ce 3+ phosphor particles will absorb the blue-light energy and weaken it. However, after every scattering the converted yellow light bcomes more vehement. In addition, the light intensity distribution also affects the color of the LED light. Specifically, the emitted light will become more bluish as it reaches the surface of LED, while it tends to be more yellowish when it is almost parallel to the surface of the LED. Thus, if the distribution of the light intensity changes, it can result in a yellow circle surrounding the white light, which is also known as yellow ring phenomenon [9][10][11][12]. When the white light quality and lumen output receive the optimized methods for phosphorus or optical structure of white LEDs, there will be a positive difference. Won, together with his partner, proved the strong influence of phosphor geometry on the luminous intensity of white light emitting diode (WLED) devices yielding better color quality scale (CQS). In this project, they combined the LED chips with green (Ba,Sr)2SiO4:Eu 2+ and red CaAlSiN3:Eu 2+ phosphors with various phosphor designs to achieve optimal efficiency [13][14][15]. However, when the Green-Amber-Red multi-package LED is proposed, its priority is to obtain the best lumen output and CRI [16][17][18][19][20]. The multi-chromatic phosphor is used in order to serve a purpose in attaining the better quality for the CRI of the LED lamp. The problem that scientists have always been concerned about is how to improve CRI and optical output, but they ignore the enhancement in the uniformity of correlated color temperature (CCT). Moreover, the WLED packages these articles concentrated on is the single-chip one. In other words, their results did not really work with high CCT multi-chip white LEDs [21][22][23].
In this paper, the participation of green (Y,Gd)BO3:Tb 3+ phosphor helps the chromatic homogeneity and the luminous performance positively transform in the phosphorus compound utilized in MCW-LEDs during the light scattering process. Depending on the density of (Y,Gd)BO3:Tb 3+ particles, it will propose the development trend of the lighting quality as well as the color homogeneity. The research contains 3 stages: 1) building the model of MCW-LEDs; 2) blending (Y,Gd)BO3:Tb 3+ with the phosphor compound in the simulated MCW-LEDs; 3) carrying out the investigation on how (Y,Gd)BO3:Tb 3+ concentration affect the optical performance, such as the emission spectra and the scattering. To sum up, if (Y,Gd)BO3:Tb 3+ and YAG:Ce 3+ phosphors are used in LED structures, it will contribute to improving the uniformity of light color and the luminous flux.

PREPARATION AND SIMULATION 2.1. Preparation of green (Y,Gd)BO3:Tb 3+ phosphor
(Y,Gd)BO3:Tb 3+ is a potential phosphor material for LED packages due to their high luminous efficiency. Before starting the preparation process, it is essential to understand the nature of (Y,Gd)BO3:Tb 3+ . It comprises four grassroots: 51.9 g of Y2O3 (accounting for 23% of the compound), 83.4 gram of Gd2O3 (having the same percentage with Y2O3), 15 grams of Tb4O7 (very small amount) and 85.9 gram of H3BO3 (major mole). The process of fabricating (Y,Gd)BO3:Tb 3+ is demonstrated as follows. Initially, dry blending above ingredients. Next, firing the mixture in capped quartz tubes with air flows and at 400-500 0 C for 2 hours. After that, putting slurry powder in boiling 10% HCI for 30 minutes. Then, decanting it with boiling water until neutral. Finally, drying it at 110 0 C. The process of creating (Y,Gd)BO3:Tb 3+ is seen as successful when the mixture can requires the following optical properties. During the emission, the compound has green color. In addition, its emission peak is 544 mm, and -3.40 eV, +4.88 eV and +8.40 eV are the index of excitation efficiency by UV.

MC-WLED simulation
LightTools 8.1.0 program and the Monte Carlo method are applied to construct the MC-WLEDs simulation with the addition of green phosphor (Y,Gd)BO3:Tb 3+ . Moreover, it is crucial to observe and evaluate the influences of (Y,Gd)BO3:Tb 3+ concentrations on the optical performance of W-LED packages, see Figure 1 (a) and Figure 1 (b). Additionally, the WLED used in the study has average CCT of 5600 K and 8500 K. The reflector, one of the elements comprising WLED lamp, has parameters of 8 mm in bottom length, 2.07 mm in height, and 9.85 mm in top surface length. Each LED chip embedded to the reflector's cavity owns 1.14 mm 2 square base area, a height of 0.15 mm, the radiant power of 1.16 W, and a wavelength that peaks at 453 nm. The 0.08 mm thick phosphor layers are placed over the nine LED chips. Figure 1 (c) shows a clear description of a MCW-LED package with remote phosphor structure of two phosphor layers: yellow YAG:Ce 3+ and green (Y,Gd)BO3:Tb 3+ layers. Moreover, it is important to note that the green phosphor (Y,Gd)BO3:Tb 3+ is above the yellow YAG:Ce 3+ layer. By applying the Mie-theory, the scattering of phosphor particles can be analyzed easily. In this study, the actual parameter average diameter for all phosphorus particles was 14.5 mm. Figure 2 describes the emission spectra of WLEDs with a dual-remote phosphor compound doping 10% (Y,Gd)BO3:Tb 3+ . It can be seen obviously from this chart that the yielded luminescence of MCW-LEDs becomes better after (Y,Gd)BO3:Tb 3+ particles are added into phosphor compound.

COMPUTATION AND DISCUSSION
The Mie-scattering theory [24][25][26] plays an inprotant role in carrying out the computation of scattering coefficient μsca to verify optical properties of phosphor compounding. The connection among the scattering coefficient (SC), the wavelength, and the size of (Y,Gd)BO3:Tb 3+ particles is described by the following equations: Here, f(D) is the size distribution function, c is the phosphor concentration (g/cm 3 ), Csca,D is the scattering cross-section of the phosphor with a particle diameter of D. ̄( ) indicates the scattering cross-section and ̄ shows the particle mass of the phosphor integrated over f(D).
( ) and ( ) are the scattered power by phosphor particles and the irradiance intensity, respectively.
According to Figure 3, we calculate the scattering coefficient through the scattering process of (Y,Gd)BO3:Tb 3+ phosphor. The concentrations and different triggers of (Y,Gd)BO3:Tb 3+ will be a catalyst that causes SC of the phosphor compound to fluctuate significantly. When the concentration of (Y,Gd)BO3:Tb 3+ increases, SC also become higher at any particle size of (Y,Gd)BO3:Tb 3+ phosphor. With a size of approximately 1 μm, SC reaches the highest value as well as makes the color uniformity develop better. Hence, the (Y,Gd)BO3:Tb 3+ size of approximately 1 μm should be chosen if the target is CQS. Otherwise, it is around 7 μm, the SC of the phosphor layer tends to be much more steady without concerning the increase in (Y,Gd)BO3:Tb 3+ concentration, which gives the color quality (CQS) of the LEDs so many benefits. As a result, if the target is CQS, the lighter size of 7 μm can be selected. Obviously, the SC value is dependent on not only the concentration but also the size of (Y,Gd)BO3:Tb 3+ , which is the reason why (Y,Gd)BO3:Tb 3+ is recommended for the enhancement of the lumen efficacy and color quality of WLEDs.
In this article, the LED product specification must be strictly adhered. Hence, the enhancement of MCW-LED should be bound with the average CCTs. Accordingly, when the concentration of the (Y,Gd)BO3:Tb 3+ phosphor increases, a strong reduction in yellow phosphor YAG:Ce 3+ concentration is required for stabilizing the average CCTs. The CCT peak-valley deviation tends to decrease sharply when the concentration of (Y,Gd)BO3:Tb 3+ increases, as can be seen in Figure 4. In other words, the spatial color distribution of MCW-LEDs with (Y,Gd)BO3:Tb 3+ is much flatter compared to the absence of (Y,Gd)BO3:Tb 3+ . To accomplish the best improvement for WLED packages, it is important to ensure that performance properties and optimization issues are in balance. In other words, we need to focus on advancing both of them, not just focus on one element as this will make the optical system become weak in the other aspect, or in another manner, CQS and the luminescence efficacy of a white LED lamp cannot contemporaneously achieve the best results. Therefore, we need to obtain good CRI, and at the same time, a broad source spectrum and efficiency must be maximized at monochromatic radiation with a wavelength of 555 nm. Considering this issue, the paper makes the indexes of CQS, lumen output, and the P-V deviation of CCT three competitors. In Figure 5 and Figure 6, the lumen output and the color quality scale are described following the concentration of (Y,Gd)BO3:Tb 3+ . As can be seen, the increase in (Y,Gd)BO3:Tb 3+ concentration will support the enhancement of the luminous flux, but will degrade the CQS. However, if there is a small reduction in the concentration of (Y,Gd)BO3:Tb 3+ , it will help to increase both the CCT homogeneity and the luminescence efficacy.

CONCLUSION
In conclusion, the potency of using green (Y,Gd)BO3:Tb 3+ phosphor in the process of acquiring better performance for CCT uniformity and luminous intensity of white LED devices is demonstrated through findings in the article. Firstly, according to Mie scattering theory, the color uniformity is very diverse, regardless of the average CCT value, because of the light-scattering compensation to the WLEDs. Besides that, with the support from Monte Carlo simulation, the difference in lumen values is presented and TELKOMNIKA Telecommun Comput El Control  Enhancement of luminous flux and color quality of … (My Hanh Nguyen Thi) 949 discussed by concentration-dependent (Y,Gd)BO3:Tb 3+ . Indeed, the lumen output increases together with the change of (Y,Gd)BO3:Tb 3+ concentration. Meanwhile, that increase leads to a decrease in CQS. However, if we reduce a small amount of (Y,Gd)BO3:Tb 3+ concentration, the LED structure can attain better CQS and luminous flux. In short, (Y,Gd)BO3:Tb 3+ phosphor can be a right answer for the inquiry into the development of optical properties of phosphorus layers and MCW-LED fabrication.