Improving Lighting Performance of High Color Temperature White LED Packages Using ( La , Ce , Tb ) PO 4 : Ce : Tb Phosphor

Enhancement of the color uniformity, the lumen output of the multi-chip white LED lamps (MCW-LEDs) at high color correlated temperature is a big challenge for researchers. However, an innovative LED lamp designed with a phosphor compounding, which combines (La,Ce,Tb)PO4:Ce:Tb (LaTb) green phosphor with YAG: CE yellow phosphor, is proposed as an optimal solution to this requirement. Index, using LaTb green phosphor into MCWLEDs could bring a superior optical performance for MCW-LEDs. It is found that the lumen output of this new MCW-LED at a high color temperature of 8500 K signi cantly improves up to 1600 lm compared to MCW-LEDs without LaTb phosphor. The simulation results demonstrated that the CCT deviation sharply decreases from 9000 to 1000 at the LaTb concentration range from 0 to 1.8 %, while the Color Rendering Index ability (CRI) and the Color Quality Scale (CQS) slightly decrease. To obtain the highest lumen output and the best color uniformity, the particle size range within 6 8 μm should be


Introduction
Recently, White Light Emitting Diodes (W-LEDs) with a potential exibility and dominant functions is becoming an important role in our life in illumination applications such as general lighting, medical, lifestyle products [1]. One of the most popular methods of the white light emitting diode technology includes the employment of multiple monochromatic LED chips or the combining blue light emitted from the LED chips with YAG: Ce phosphor to produce white light that is called phosphor converted LED (pc-LEDs). However, the obtained results of lumen output and angular color uniformity are still low due to the intensive reection and the reabsorption of light going back to the package. Therefore, the pursuit of W-LEDs with high luminous eciency, excellent color uniformity, good CRI was widely developed in recent years [2]. LED with remote phosphor structure designed, in which the phosphor layer placed far away from the chip can improve the loss of backscattered light inside LED chip [3]. Although the remote phosphor structure brings the luminous eciency higher than the dispensing and conformal structure, it is dicult in manufacturing technique the concave surface of this structure resulting in a non-uniform phosphor thickness. Moreover, the luminous eciency, uniform color distribution of MCW-LEDs hardly fulll the different requirements of many illumination applications. As a development, the remote phosphor structure is optimized by design with the concentric green, red phosphor rings separated to reduce the backscattering problems. The extraction eciency, the color rendering property of the proposed MCW-LEDs are higher than MCW-LEDs having a mixed phosphor layer [4]. A dierent phosphor conguration, which has a novel double remote micro-patterned phosphor lm, is used to enhance the color uniformity of MCW-LEDs [5]. On the other hand, the conguration of LED's lens is extensively studied to optimize uniform illumination and the illumination eciency. Therefore, there is an exploration of new discoveries about len structure to optimize the light quality of MCW-LEDs. A len with a freeform microlens array or the droplet evaporation structure or a free-form secondary lens structure exhibits better optical eciency than traditional lens [6] and [7]. Also, the luminescent material is one of the important factors that strongly aect the performance of the multi-chip MCW-LEDs, and hence it is studied for purpose overcoming poor light extraction and color uniformity problem. For example, Kaur [8] has presented a new research about LaAlO 3 phosphor that generates the yellow-orange-red emission intensity stronger when combining this phosphor with the blue LEDs. Another study found that the use of the dierent structures of green (Ba, Sr) 2 SiO 4 :Eu 2+ and red CaAlSiN 3 :Eu 2+ phosphor compounding with blue LEDs can help achieve better LED performance [9]. Because of the great potential of this luminescent phosphor material, they were continuously researched and developed.
Although the above approaches are expected to improve the performance of MCW-LEDs, the luminous eciency and the angular color uni-formity of white LEDs are not fully satised many dierent illumination applications, as well as extensive demands in the commercial, LED lighting market. Besides, the papers mainly research on white LED lamps with single-chip at low color correlated temperature. Moreover, in order to satisfy the competitiveness in lighting market and oer better light quality, advanced studies should be extensively conducted to nd the most optimal conguration of LEDs or new phosphor materials that help increase the luminous intensity and color uniformity at such high temperatures.
Hexagonal prism (La,Ce,Tb)PO 4 green phosphor is frequently employed for very high loading and durable uorescent bulbs due to its higher quantum eciency, chemical and thermal stability of this material. (La,Ce,Tb)PO 4 composition obtains from chemical processes of the materials such as citric acid ( Ce 3+ ion can act as an ecient sensitizer to Tb 3+ and with the appropriate increase of Ce 3+ concentration will improve the thermal quenching properties of LaTb phosphor. Therefore, this phosphor compounding is considered to be an ecient host candidate for good optical performance of the MCW-LEDs at high color correlated temperature.
In this paper, we propose adding LaTb green luminescent phosphor to yellow YAG:Ce phosphor compound of the MCW-LEDs to optimize its light extraction, the CCT uniformity and CRI at high color correlated temperature. From the obtained simulation results, we discovered that LaTb green phosphor does not only improve the luminescent properties in traditional uorescent lamps at such high temperature as published in a previous paper [10] but also it could produce the great enhancement for color uniformity and lumen output in in-cup phosphor package of the MCW-LEDs. The weight percentage and size of LaTb phosphor particle are varied from the simulation process to examine closely its eect on the optical performance of the MCW-LEDs. The optical properties of LaTb and YAG:Ce phosphor compounding are studied by analyzing light absorption, scattering as well as light conversion in LEDs at high correlated color temperature. In other words, we car-ried out simulations and calculations to discover the positive eect of concentration and size of LaTb green luminescent phosphor on luminous ux, angular color distribution, CRI and CQS at a high color temperature of 8500 K MCW-LEDs.

Computation
In order to investigate the inuence of LaTb phosphor on the performance of the MCW-LEDs at the high correlated temperature of 8500 K.
The MCW-LED with in-cup phosphor structure is simulated by using the LightTools 8.1.0 program and Mie-theory to analyze the scattering of phosphor particles. To obtain the precise simulation results about the eect of LaTb phosphor on MCW-LED, our study used the parameters of the real -world model of LED for simulation. Figure 1(a) shows an actual MCW-LED with incup phosphor package having average temperature of 8500 K. These parameters about structure of this real MCW-LED are employed for designing a simulated MCW-LED (see Fig. 1(a)).
In the preparation process of the in-cup phosphor structure of MCW-LED, the LaTb and YAG: Ce phosphor compounding are mixed in silicon lens as shown in Fig. 1(b). Therefore, the phosphor layer of the MCW-LED consists of LaTb green phosphor, the yellow phosphor YAG:Ce and the silicone glue. The model structure as in Fig. 1(b) shows the components of simulated the MCW-LED including blue chips, a reector cup, a phosphor layer and a silicone layer. A reector with a 2.07 mm depth, a bottom length of 8 mm and a length of 9.85 mm at its top surface is bonded with these chips. The radiant power of each nice blue chip was designed with 1.16 W, a peak wavelength of 453 nm.
The radius of the green LaTb phosphor particles was changed from 1 µm to 10 µm. The LaTb phosphor particle density varies from 0 1.8 % in the simulation process to optimize color uniformity and lumen output eciency. The refractive index of phosphor particle is set to be 1.85 and 1.83 for LaTb and YAG: Ce, respectively. To maintain the average CCT of white LEDs at (a) A sample LED used in this study.
(b) The simulated the in-cup phosphor package. 8500 K, YAG: Ce phosphor concentration needs to be appropriately changed with the concentration of LaTb green phosphor. To obtain the precise results about the eect of LaTb green phosphor on the optical performance o MCW-LED at high temperature, the optical simulation process is carried out with a change of LaTb particle size and density.
The scattering of LaTb phosphor particles was analyzed by using the Mie-theory. The optical constants of scattering coecient µ sca (λ) and reduced scattering coecient δ sca (λ) can be computed by the below expressions Eq. (1) and Eq. (2): (a) Scattering coecient.
(b) Reduced scattering coecient. where N (r) is the number density of particles, C sca (λ, r) is the scattering cross-section, g is anisotropy factor.
According to the obtained results as shown in Fig. 2(a), the scattering coecient grows with the decrease of the size of LaTb particle, it means that the smaller phosphor particles scatter the incident light stronger and achieve a more color uniform compared with bigger particles. However, if the scattering coecient is so high, the imbalance of the color spectrum of the emitting light will appear. This leads to the disadvantage in the light quality of MCW-LED. The highest color uniform can be achieved at a size of 1 µm. Generally, the scattering coecient is the same with the wavelengths varied from 380 nm to 780 nm. The reduced scattering coecient increases with the change of the wavelength at a small size of 1 µm as shown in Fig. 2(b). At the small size of 1 µm, the reduced scattering is slightly sensitive to the incident wavelength. Therefore, the optical performance of the MCW-LED in the wavelength range from 380 to 780 nm will not be stable. Thus, the use of LaTb particle size of 1 µm doesn't oer better color quality of the MCW-LED. Meanwhile, the reduced scattering doesn't change with the wavelength of the range within 6 10 µm.

Results and Discussion
The simulation results presented that LaTb particles oer a positive impact on the luminous ux of MCW-LEDs. It is found that the luminescence characteristics of LaTb green phosphors are strongly inuenced by particle concentration and size. LaTb green luminescent phosphor is known to be a good candidate for enhancing the green light and thermal quenching phenomenon. Therefore, adding this particle in YAG:Ce phosphor layer will help LED achieve the higher brightness at the high temperature of 8500 K. In order to obtain precise results about optical features of LaTb, we carried out the simulations with the various concentrations from 0 to 1.8 % corresponding to each size with range within 1 10 µm. The lines of the graph representing the luminous ux sharply increase and can reach maximum value in the range of the concentration from 0 % up to 1.3 % and the size within 1 5 µm as depicted in Fig. 3. Luminous ux improves on all particle sizes when adding LaTb particles into phosphor compounding with increasing the concentration. As is known in many previous papers, the small particles usually provide less luminous ux than bigger ones due to the unwanted backward scattering inside MCW-LEDs [12]. It means that there is more probability of light trapped inside a package and less the light escaping from LEDs when the small particles are applied. Thereby, the light emission intensity decreases and hardly achieve the maximum value for small particles at a high correlated color temperature of 8500 K. However, LaTb green phosphor with dominant luminescent characteristics such as chemical and thermal stability can help overcome shortcomings about small particle sizes at high temperature to produce a higher lumen output for MCW-LEDs. With large particle size of 6 10 µm, the trend of light propagates stronger in the forward direction and weaker in the backward direction of LED chips; thereby the large particles show the advantages for lumen output. In another word, adding LaTb particles with dierent particle sizes to YAG:Ce phosphor oers a remarkable increase in the lumen output of the in-cup phosphor package at a high temperature of 8500 K.
Additionally, the eect of LaTb phosphor on angular color uniformity that representing by mean color deviation parameter (CCT) is simulated and analyzed. The concept of mean color deviation is determined by minus between CCT(Max) and CCT(Min) as following D-CCT = CCT (Max) -CCT (Min). Where CCT(Max), CCT(Min) is the maximum value, minimum value of CCT respectively. D-CCT parameter can be used to evaluate the inuence of this phosphor on the color uniformity level of LEDs [13]. The high CCT of the package is attributed to the diculty in obtaining a high color uniformity due to some issues of phosphor at this temperature. As a result, larger color deviation is usually generated by MCW-LEDs at this temperature. The color correlated temperature deviation obtained from the simulation process with various concentrations and sizes of LaTb particles in MCW-LEDs at 8500 K shows in Fig. 4. The CCT deviation signicantly decreases when the concentration of LaTb particle increases in all dierent particle sizes. It means that LaTb particle shows a great eect on the color uniformity of the MCW-LEDs when it is added into phosphor compounding with the concentration range within 0 1.8 % and the size range from 1 µm to 10 µm. Therefore, the excellent color uniformity capacity can be achieved in MCW-LEDs with this new method. The CCT deviation of white LEDs with smaller particles from 1 µm to 5 µm remarkably decreases from 4000 K to 500 K as shown in Fig. 4.
The CCT deviation value can reach the minimum value less than 500 K at 1 µm and concentration at 0.4 %. In the case of the particles with the bigger sizes from 5 µm 10 µm, the tendency  of the color temperature deviation exhibits the features similar as the small particles. However, it is easy to realize that the lines of the graph for smaller particles drop faster than bigger ones, and thus, smaller particles can generate better scattering than bigger ones. This could be deduced that smaller particles of LaTb phosphor have more chance to scatter the incident lights in many dierent directions inside LEDs. As a result, the smaller particles make the color uniformity of MCW-LEDs atter and better. The use LaTb phosphor into yellow YAG: Ce phosphor compounding makes the color uniformity better for all particles compared to that of non-LaTb case. The larger color temperature deviation is usually generated as MCW-LEDs illuminate at high CCT, but the LaTb phosphor compounding will help achieve a higher color uniformity at high CCT for all particle sizes with range within 1 10 µm compared to the phosphor compounding without LaTb particles.  Moreover, the eect of LaTb particles in the phosphor compounding of the MCW-LEDs on CRI, CQS is investigated in this paper. As shown in Fig. 5, the CRI value slightly decreases in size range from 6 µm to 8 µm. The reason is that adding LaTb green luminescent phosphor resulting in green light supplementation and deciency of red components in the spectrum of the MCW-LEDs, at this moment decreasing the CRI. It is essential to have a broader spectrum of light sources to increase CRI, and thus, the maximum eciency of the white LED wouldn't be able to achieve. The CQS index, which evaluates the overall color quality of the MCW-LEDs about color delity, chromatic discrimination, and observer preferences, shows slight decrement similarly as CRI at a concentration around 1.5 % and the particle 6 8 µm as Fig. 6.
Although LaTb particles from 1 µm to 5 µm help MCW-LEDs signicantly enhance lumen output and color uniformity for the LaTb particles with the concentration of 1.5 % and size range within 6 8 µm should be the best choice.
In this size and concentration range, the most optimal color uniformity and lumen output can achieve accompanying with the insignicant decrease of CRI, CQS values as well. 4. Conclusion In this article, the eect of LaTb green luminescent phosphor on the MCW-LED performance at 8500 K is analyzed and demonstrated in detail. The optical simulation reveals that the LaTb particles can signicantly improve both luminous ux and color uniformity at high color correlated temperature. We analyzed the eect of size and concentration of LaTb particle on CCT, luminous ux, CRI and CQS. For the particle sizes smaller than 5 µm, the lumen output and the color uniformity are remarkably enhanced with the adopting phosphor concentration smaller 1.5 %. Meanwhile, the concentration range within 0 1.8 % should be added for particle sizes from 6 µm to 10 µm. The optimal size and concentration of LaTb particles should be chosen from 6 µm to 8 µm and 1.5 % respectively. The lowest CCT value, the highest luminous ux accompanying with an insignicant decrease in CRI, CQS could be achieved in this range. It could be concluded that, by adding LaTb green phosphor with the concentration range within 0 1.8 % and the particle size of 6