Utilization of BaAl1.4Si0.6O3.4N0.6:Eu2+ Green-emitting Phosphor to Improve Luminous Intensity and Color Adequacy of White Light-emitting Diodes

Title: Utilization of BaAl1.4Si0.6O3.4N0.6:Eu2+ Green-emitting Phosphor to Improve Luminous Intensity and Color Adequacy of White Light-emitting Diodes

Authors
Authors and Affiliations

Nguyen Le Thai, Thuc Minh Bui, Anh Tuan Le, Dieu An Nguyen Thi

Corresponding email: leanhtuan1@tdtu.edu.vn

Published at : 18 Jan 2023
Volume : IJtech Vol 14, No 1 (2023)
DOI : https://doi.org/10.14716/ijtech.v14i1.5666

Cite this article as:
Thai, N.L., Bui, T.M., Le, A.T., Thi, D.A.N., 2023. Utilization of BaAl1.4Si0.6O3.4N0.6:Eu2+ Green-emitting Phosphor to Improve Luminous Intensity and Color Adequacy of White Light-emitting Diodes. International Journal of Technology. Volume 14(1), pp. 119-128

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Nguyen Le Thai	Faculty of Engineering and Technology, Nguyen Tat Thanh University, Ho Chi Minh City, Vietnam
Thuc Minh Bui	Faculty of Electrical and Electronics Engineering, Nha Trang University, Nha Trang City, Vietnam
Anh Tuan Le	Faculty of Electrical and Electronics Engineering, Ton Duc Thang University, Ho Chi Minh City, Vietnam
Dieu An Nguyen Thi	Faculty of Electrical Engineering Technology, Industrial University of Ho Chi Minh City, Ho Chi Minh City, Vietnam

Email to Corresponding Author

Abstract

Utilization of BaAl1.4Si0.6O3.4N0.6:Eu2+ Green-emitting Phosphor to Improve Luminous Intensity and Color Adequacy of White Light-emitting Diodes

BaAl_1.4Si_0.6O_3.4N_0.6:Eu²⁺ exhibiting broad excitation and emission bands with intense green emission centred at 510 nm is applied to produce high-performance white-light-emitting diodes (WLEDs). The preparation of the green phosphor utilizes NaNO₃ molten salt to attain the purity phase and enhanced luminescence strength boosting the crystalline growth. The influences of BaAl_1.4Si_0.6O_3.4N_0.6:Eu²⁺ on the lighting intensity and color adequacy are investigated at three correlated color temperatures (CCTs) of 3000 K, 4000 K and 5000 K. The lighting output of the WLEDs with high CCTs (4000 – 5000 K) is deemed as enhanced with increasing green-phosphor concentration. The lower CCT shows greater lumen output when using a lower concentration of BaAl_1.4Si_0.6O_3.4N_0.6:Eu²⁺. This tendency also takes place in the case of color uniformity. Conversely, the high concentration of the phosphor is not favourable to the color rendition property of the WLED because of the excessive green-light proportion. It is recommended to keep the concentration of BaAl_1.4Si_0.6O_3.4N_0.6:Eu²⁺ staying below 10 wt% for better color fidelity.

Keywords

BaAl1.4Si0.6O3.4N0.6:Eu2+; Color adequacy; Color quality scale; Color rendering index; Luminous intensity; White light-emitting diodes

Introduction

WLEDs have been recognized as a crucial and potential source for modern-day illumination applications, such as household lighting, display devices, microprocessors, and multiplexers (Karadza, et al., 2022; Zhong et al., 2019). Hence, the development of WLED structure yielding higher effectiveness to meet certain optical requirements for certain application field is always critical. One of the most common types of WLEDs is the phosphor-converting WLED, in which the phosphor acts as an agent to convert the emitted light from the near-UV or blue LED chips. The YAG:Ce³⁺ phosphor with great yellow emission is widely utilized for traditional LED (Zhou et al., 2020; Chen et al., 2019). The luminescence of the WLED using Ce³⁺-doped YAG is excellent, with a significant quantum efficiency (about 90%), thus, it is suitable for high-intensity LED. Unfortunately, the color rendering of this phosphor is not impressive, which limits the growth of WLED in fields that require high color reproduction performance (Su and Gao, 2020; Nahavandi et al., 2020). Therefore, introducing good phosphor materials for high luminous intensity with sufficient chromatic rendition has been the objectives for researchers. It is reported that these mentioned issues could be addressed by getting the blue, green and red phosphor excited under the near UV LED to compensate for the lack of green and red spectra which exists in the case of using only YAG:Ce³⁺ (Wang et al., 2021; Anh, Le, and Lee, 2019). The green phosphor might be a good choice for attaining good CCT consistency and high luminous intensity of WLEDs (Keshri et al., 2020; Hao et al., 2019).

The rare earth ions with broadband emission (Eu²⁺ and Ce³⁺) are good for obtaining excellent quantum efficacy with great thermal stability. This can be ascribed to their broad emission spectrum that could cover the near-UV and blue wavelength critical for achieving good chromatic rendering ability and short decay time that could prevent saturation effects. Choosing suitable host compositions for them to magnify their advantages. The oxynitride/nitride phosphor is extensively investigated as the host compound for this type of rare-earth dopant since it provides high chemical and thermal stability. The luminescence of europium-doped (oxy)nitride phosphor is reported to have significant strength. For instance, the intense emission color of green can be recorded in the cases of ?-sialon:Eu²⁺ and MSi₂O₂N₂:Eu²⁺ (M = Ca, Sr, Ba) (Xu et al., 2021; David, Sahlhoff, and Wisser, 2019). M₂Si₅N₈ (M = Ca, Sr, Ba) and MAlSiN₃ (M = Ca or Sr) exhibited peak emission of about 600 nm or event at broader band of wavelengths (Ji et al., 2020; Wang et al., 2019).

This paper therefore utilizes the europium-doped (oxy)nitride phosphor to develop the WLED configuration. The WLED model uses the green phosphor of BaAl_1.4Si_0.6O_3.4N_0.6:Eu²⁺ as the additional phosphor material, besides the original YAG: Ce³⁺, to stimulate the scattering effects, reducing the energy loss by light trapped and backward scattering, which is essential for high chromaticity and luminescence of a WLED. The WLEDs are simulated with three preset CCTs: 3000 K, 4000 K, and 5000 K. The concentration of phosphor is modified between 5 and 10 wt%. The phosphor is prepared via molten salt synthesis to accomplish a desirable purity phase and crystal growth. The finding shows that the phosphor is a good choice for high-luminescence WLEDs that need adequate color rendering properties.

Experimental Methods

2.1. Preparation of BaAl_1.4Si_0.6O_3.4N_0.6:Eu²⁺

The molten salt NaNO₃ is used for the preparation of the BaAl_1.4Si_0.6O_3.4N_0.6:Eu²⁺ with 5% mol of Eu-doping concentration. The purpose of using NaNO₃ is to attain the purity of the phosphor sample as this salt is easily melted at relatively low temperatures and plays a role as a high-temperature solvent to enhance the efficiency of reactants’ diffusion (Ma et al., 2021). The phosphor composition includes BaCO₃, Al₂O₃, Eu₂O₃, ?-Si₃N₄, and NaNO₃. The phosphor synthesis is started with grinding BaCO₃, Al₂O₃, and Eu₂O₃ and followingly going through a 3-hour calcination process at 1450^oC. Then, the product is cooled down to room temperature. NaNO₃ and ?-Si₃N₄ are then mixed with the attained powder, and the compound is re-ground for an hour. Subsequently, it is burned for 6h at 1300^oC in the reducing atmospheric condition of N₂/H₂ (8:1). Note that the mass ratio of the raw ingredients and the molten salt NaNO₃ is 1:2. After the heating stage, the powder stays in the furnace and is cooled down to room temperature before being was with deionized water to get rid of the residues. After that, it is put into a drying oven and heated at 80^oC for 5h (Xi et al., 2021; Elkarim et al., 2021).

The prepared phosphor performs uniform-dispersed morphology with a standard spherical shape while demonstrating high crystallization and dispersibility. This is because the NaNO₃ liquid solvent creates an absorbing layer on the formed crystalline, preventing the aggregation phenomenon by penetrating ions Na⁺. Besides, the liquid environment provided by the NaNO₃ molten salt is beneficial to the crystal growth, contributing to forming the spherical shape for the synthesized phosphor BaAl_1.4Si_0.6O_3.4N_0.6:Eu²⁺. Furthermore, this enhancement in crystalline formation is more beneficial to the luminous efficacy of the phosphor material (Li et al., 2021; Zhang et al., 2020).

2.2. BaAl_1.4Si_0.6O_3.4N_0.6:Eu²⁺ photoluminescence analysis

The synthesized phosphor BaAl_1.4Si_0.6O_3.4N_0.6:Eu²⁺ exhibits the broad excitation region of UV-to-blue wavelength. Its strong green emission spectrum is recognized in the wide range of 460 nm – 600 nm, peaking at 510 nm. This could be attributed to the transitions of Eu²⁺ ion from 4f⁶5d¹ to 4f⁷. Besides, the intensity of phosphor luminescence is affected by the molten salt addition. The luminous intensity could be enhanced with the increasing amount of NaNO₃, yet the following decrease can be recognized since the excessive liquid phase medium causes the incomplete reaction leading to the probability of impurity and surface defects for the phosphor particles. With the applied mass ratio of 1:2 for raw materials – NaNO₃, the intensities under 365 nm and 450 nm excitation bands are heightened by 17% and 13%, respectively.
Besides, the emission intensity’s increase and subsequent decrease is also caused by the concentration quenching when doping Eu²⁺. The concentration quenching influences could be examined with the distance between the doped ions in the host lattice, also known as the energy-transfer critical distance (R_C) (Quesnel et al., 2021; Yang et al., 2021). This R_C can be expressed as the following computation:

x_cis the critical doping concentration of Eu²⁺ in the phosphor composition, Z and V are the amount of Eu²⁺-substituted cations within the unit cell and the unit-cell volume, respectively. In this work, V and Z are determined to be 831.19 Å and 8, respectively. while the x_c of the ion Eu²⁺ is equal to 0.05. As a result, the computed R_C is 15.84 Å. Generally, the R_C is smaller than 4 Å, but in the case of green phosphor, a larger R_C is determined, indicating that the energy transfer among the ions Eu²⁺ does not mainly take place by the exchange interaction. Consequently, the multipolar interaction might be responsible for the mechanism of Eu²⁺ energy transfer, which could be calculated based on the theory of Dexter (Le et al., 2021; Zhu et al., 2019):

x, k and is the doping concentration of Eu²⁺ are the doping concentration of Eu²⁺, and constants under the same source of excitation for the given host, respectively. I is the emission intensity, and indicates the emission strength per Eu²⁺ concentration. Q shows the multipolar interactions’ constraints. Note that Q = 6, 8, 10 corresponding to the interactions of dipole-dipole, dipole-quadrupole, or quadrupole-quadrupole. Accordingly, computed Q is about 5.79, nearly 6, meaning that the multipolar interaction that controls the electron transfer mechanism and the effect of Eu²⁺-doping concentration quenching is the dipole-dipole.

The performance of the used phosphor is also influenced by thermal quenching. The thermal quenching of BaAl_1.4Si_0.6O_3.4N_0.6:Eu²⁺ can be examined with the following formula:

I₀ indicates initial intensity, T is the given temperature at which the intensity I(T) can be determined. A is a pre-exponential factor and constant, k_B is a Boltzmann constant (El-Ghoroury et al., 2020; Abeysekera et al., 2020), and

shows thermal-quenching activation energy which is determined as 0.2509 eV. The stability in thermal factor could be well performed by this BaAl_1.4Si_0.6O_3.4N_0.6:Eu²⁺ with large activation energy for thermal quenching. In other words, the thermal quenching of the prepared green-emitting phosphor is small.

Results and Discussion

One of the critical factors to get the improvement in the color and luminescence performances of WLEDs is the reduction of the backscattering effect. The common approach is to enhance scattering efficiency to perform better light extraction and color blend. The concentration of the yellow phosphor YAG:Ce³⁺ greatly affects this. Too high concentration or thicker film of YAG:Ce³⁺ can induce significant thermal quenching that is disadvantageous to the light conversion but initiates the internal reflection loss. Thus, it may decrease color balance and light intensity. In other words, the emitted light tends to lean to the yellow region or becomes yellowish.

Figure 1 The emission spectra of WLEDs as a function of BaAl_1.4Si_0.6O_3.4N_0.6:Eu²⁺ concentration at different CCTs: (a) 3000 K; (b) 4000 K; (c) 5000 K
The addition of green-emitting phosphor (GEP) BaAl_1.4Si_0.6O_3.4N_0.6:Eu²⁺ performs significant changes in the concentration levels of YAG:Ce3+, which is presented in Figures 1. Regardless of the particle size and WLED’s average CCTs, the reduction in the yellow phosphor concentration with the higher BaAl_1.4Si_0.6O_3.4N_0.6:Eu²⁺ concentration is observed, regardless of the average CCTs. This phenomenon contributes greatly to the color-temperature stability, indicating noticeable benefits to the scattering and conversion effectiveness of the phosphor structure of the simulated WLED.

Figure 2 luminous flux of WLEDs as a function of BaAl_1.4Si_0.6O_3.4N_0.6:Eu²⁺ concentration: (a) 3000 K; (b) 4000 K; (c) 5000 K
The emission power is also influenced by this change in phosphor concentrations, as depicted in Figures 2. Overall, GEP’s presence helps to promote the intensity of green emission spectra. However, unlike the exhibited concentration of YAG:Ce³⁺ in Figures 1, the performance of the WLED spectra depends on the monitored average CCTs. At a low CCT of 3000 K, a higher concentration of GEP brings lower performance, see Figures 2 (a). Meanwhile, 4000 K and 5000 K average CCTs, the difference between the two concentration levels of GEP is negligible. Nonetheless, the improvement in blue-light emission bands is recognized for the lower concentration (5 wt%) of BaAl_1.4Si_0.6O_3.4N_0.6:Eu²⁺. Both blue (420 – 480 nm) and green (500 – 600 nm) bands are involved in forming white light. Consequently, their enhanced performance could induce the white-light generation efficacy. The higher emission lines in the blue wavelengths also expressed that the blue-light scattering and conversion are stimulated, which might be remarkably beneficial to the color uniformity of WLED.

The luminous flux (LF), which is expected to get benefits from the addition of the GEP BaAl_1.4Si_0.6O_3.4N_0.6:Eu²⁺, is expressed in Figures 3. Since the sum power is CCT-dependent, the luminous flux is influenced not only by the concentration but also the CCT values. 3000-K WLED package presents higher LF with a smaller weight percentage of the GEP, which could be described by the emission spectra of the package shown in Figures 2 (a). Conversely, the stronger LF with the higher GEP concentration (10 wt%) is recorded in the 5000-K and 4000-K WLEDs. The increase of the LF in these two cases could be ascribed to the significant decrease of YAG:Ce³⁺ with a greater amount of BaAl_1.4Si_0.6O_3.4N_0.6:Eu²⁺ in the package. The lower the concentration of the yellow phosphor, the higher the probability of the light extracted from the LED and the lower the energy loss by reabsorption. This also marks an important use of BaAl_1.4Si_0.6O_3.4N_0.6:Eu²⁺ for controlling the luminous flux of the WLED at high CCTs.

Figure 3 The CCT of WLEDs as a function of BaAl_1.4Si_0.6O_3.4N_0.6:Eu²⁺ concentration: (a) 3000 K; (b) 4000 K; (c) 5000 K

Figure 4 The color rendering index of WLEDs as a function of BaAl_1.4Si_0.6O_3.4N_0.6:Eu²⁺ concentration: (a) 3000 K; (b) 4000 K; (c) 5000 K

Figure 5 The color quality scale of WLEDs as a function of BaAl_1.4Si_0.6O_3.4N_0.6:Eu²⁺ concentration: (a) 3000 K; (b) 4000 K; (c) 5000 K

The addition of GEP BaAl_1.4Si_0.6O_3.4N_0.6:Eu²⁺ can help attain sufficient uniformity of white-light chromaticity since it stimulates the blue emission and yellow-green emission spectra. Besides, the scattering losses are incredibly reduced as the low concentration of yellow-emitting YAG:Ce³⁺ is obtained. To investigate the the chromatic feature of white-light generated from pumping blue LED, it is essential to assess the CCT differences for color consistency and color rendering index (CRI) and color quality scale (CQS) for the rendering ability. The attained results for these parameters are demonstrated in Figures 4, 5, and 6, respectively. Overall, the increase in BaAl_1.4Si_0.6O_3.4N_0.6:Eu²⁺ concentration is not favourable to the rendering parameters but the color consistency. In particular, the consistency in chromatic is recognized with 10 wt% at 4000 K and 5000 K, see Figures 4 (b) and (c), while 5 wt% could be a better choice for the WLED at 3000 K, see Figures 4 (a). On the other hand, at all CCTs, 10 wt% BaAl_1.4Si_0.6O_3.4N_0.6:Eu²⁺ exhibits lower CRI and CQS. With 5 wt% BaAl_1.4Si_0.6O_3.4N_0.6:Eu²⁺, it is possible to attain the highest CRI and CQS around 60 at 3000 K and 4000 K CCTs, while at 5000 K CCT, the highest values of these factors can be 65, refer to Figures 5 and 6. This reduction in color rendering factors might be caused by the high proportion of generated green light, which is outstanding over the other light color elements of blue and yellow (Seminara et al., 2021). Besides, the enhanced scattering when increasing phosphor concentration promotes the conversion efficiency yet initiates the imbalance in the color distribution (Sun, You, and Teo, 2022; Khalil et al., 2019). Though the CRI is not as high as expected, the CQS on the other may be sufficient for general lighting. CQS is harder to manage than the CRI, especially at the high CCTs such as 5000 K. This is because the CQS is more complex in assessing elements which include the CRI, the visuality of eyes, and the color coordinate, combining with a larger number of color samples than the original CRI factor (Huu and Thi, 2022). Hence, the color adequacy for in terms of CCT consistency and rendition could be achieved with BaAl_1.4Si_0.6O_3.4N_0.6:Eu²⁺. The adjustment of this GEP concentration is recommended depending on the CCT values after taking the luminous requirement into consideration.

Conclusion

This work used BaAl_1.4Si_0.6O_3.4N_0.6:Eu²⁺ phosphors to enhance the optical factors of the WLED. The phosphor has broad excitation and emission spectra that is suitable for being combined with near-UV or blue-pumping LEDs. The strong emission intensity is recognized in the green spectral wavelength and with a peak observed at 510 nm. The strong intensity of the phosphor might be owing to the accomplished purity and better crystallization when using NaNO₃ molten salt as a liquid solvent. The influences of BaAl_1.4Si_0.6O_3.4N_0.6:Eu²⁺ on the lighting intensity present that at low CCT of 3000 K, the power strength is improved with low (5wt%) concentration of BaAl_1.4Si_0.6O_3.4N_0.6:Eu²⁺. Meanwhile the higher CCTs of 4000 K and 5000 K prefer the higher concentration (10wt%). The color uniformity has the same tendency as the luminous intensity. On the other hand, CRI and CQS are reduced with increasing concentration of BaAl_1.4Si_0.6O_3.4N_0.6:Eu²⁺. This may be because of the excessive green light color leading to the light tend to lean over the green region. This is not favourable to color fidelity, which requires balancing color distribution. Therefore, the solution is to use the green phosphor BaAl_1.4Si_0.6O_3.4N_0.6:Eu²⁺ with a concentration lower than 10 wt% for better color fidelity.

Acknowledgement

This research is supported by Industrial University of Ho Chi Minh City (IUH) under grant number 121/HD-DHCN.

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