The Light Features and Bredigite Layout for Orthosilicate Phosphor in WLED Devices

Title: The Light Features and Bredigite Layout for Orthosilicate Phosphor in WLED Devices

Authors
Authors and Affiliations

My Hanh Nguyen Thi, Nguyen Le Thai, Thuc Minh Bui, Sang Dang Ho

Corresponding email: hodangsang@tdtu.edu.vn

Published at : 28 Jun 2023
Volume : IJtech Vol 14, No 4 (2023)
DOI : https://doi.org/10.14716/ijtech.v14i4.5785

Cite this article as:
Thi, M.H.N.,Thai, N.L., Bui, T.M., Ho, S.D., 2023. The Light Features and Bredigite Layout for Orthosilicate Phosphor in WLED Devices. International Journal of Technology. Volume 14(4), pp. 911-920

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My Hanh Nguyen Thi	Faculty of Mechanical Engineering, Industrial University of Ho Chi Minh City, Ho Chi Minh City, Vietnam
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
Sang Dang Ho	Faculty of Electrical and Electronics Engineering, Ton Duc Thang University, Ho Chi Minh City, Vietnam

Email to Corresponding Author

Abstract

The Light Features and Bredigite Layout for Orthosilicate Phosphor in WLED Devices

We created a green phosphor Ca_14-xEu_xMg₂[SiO₄]₈ or CMS: Eu²⁺ to be utilized in WLED devices. The phosphor offers a wide spectrum achieving the highest value of 505 nm when excited at 400 nm, as a result of a shift between the excited state of 4f ⁶5d and the ground state of 4f ⁷ in an ion of Eu²⁺. The interactivity of dipole-dipole appeared to be a primary power shift for the electrical multipolar nature of the phosphor. We acquired a critical distance measured at 12.9as well as 14.9 via a critical Eu²⁺ concentration as well as the Dexter hypothesis on power shift. Via an encapsulant, we combined CMS:Eu²⁺ as well as a phosphor in red with a LED device having a value of 395 nm and managed to acquire white illumination having a CRI value measured at 91 at a 20-milliampere forward bias current. In addition, we also examined the layout as well as the light features in CMS:Eu²⁺.

Keywords

Color homogeneity; Double-layer phosphor; Luminous flux; Monte Carlo theory; WLEDs

Introduction

WLED devices have become increasingly common due to their many advantages which include small power drain, high durability, high performance, etc (Alexeev et al., 2021; Chang et al., 2021; Bindai, Annapurna, and Tarafder, 2019). On the other hand, these devices still have certain limitations that prevent them from becoming standard optical devices. It is possible to create white illumination via daubing yellow phosphor on blue chips of LED. Such a procedure may yield a significant lumen exceeding 30 lm/W. On the other hand, it results in low CRI below 65, which is caused by a faint red discharge (Huu and Thi, 2022; Hakim et al., 2021). It is worth noting that the majority of phosphors do not possess an absorption line between 440 nm and 460 nm. This downside is not present in the yellow phosphor

(Chen et al., 2020). Therefore, the industry has managed to a novel WLED form that offers desirable features of hue generation. Many novel phosphors have been created for WLED, such as oxide, oxyfluoride, nitride, oxynitride host latticework, etc (Hsin et al., 2021; Jain et al., 2020) subjected to non-ultraviolet light. While these phosphors have proved to be promising, we would still need further research for the task of eliminating certain problems to achieve strong performance, superior heat consistency, as well as improvement in hue features. When it comes to WLED devices, The best options would be alkaline-earth silicate phosphors as they offer great performance as well as heat and chemical reliability. Some mixtures were acknowledged and proposed, which have a universal structure of Sr₂SiO₄ that crystallizes within the tetragonal space group Pmnb (S. G. No. 62). With this taken into account, our investigation focused on Ca₁₄Mg₂[SiO₄]₈ (CMS, S. G. No. 34, Pnn2) - a bregidite mineral. The earliest examination concerning the powder X-ray data for CMS on the basis of a novel pinwheel setting was done by Araki and his colleagues. According to Sun et al. (2022),

along with Bredigite appeared to be distinctive phases. Li et al. (2020) claimed that

appeared to be based on K₂[SO₄].

The space group pedigree of bredigite results in Pmnn

Pnn2, caused by the tetrahedral’s leaning. This would decrease the symmetrical mechanic to Pnn2 (Ma et al., 2021; Qin, Shi, and Leon, 2020).

Our investigation focuses on the layout as well as light features of the phosphor CMS:Eu²⁺. Specifically, the investigation concerns the power shift of Eu²⁺ with critical concentration as well as Dexter's hypothesis on power shift. We constructed a WLED device by creating a merger between an InGaN chip of LED with

value of 395 nm and CMS:Eu²⁺. Furthermore, we also examined the heat abatement for the luminescence and compared the mechanism in the phosphor Sr₂SiO₄.

Experimental Methods

Computational Simulation

2.1. Preparation of green-emitting Ca_14-_xEu_xMg₂[SiO4]₈(CMS:Eu²⁺) phosphor

Table 1 The constituents and preparation of CMS:Eu²⁺

Table 2 The characteristics of CMS:Eu²⁺

We created the LED device via daubing an InGaN LED with a compound containing CMS:Eu²⁺, a phosphor in red and translucent resin. The electroluminescence was determined by putting a separate LED constructed above m-plane GaN on silver headers linked with golden wires to work under electricity. Afterward, we covered the LED with a compound of phosphor and silicone. The compound was put above the headers and cured. Once the preparation was finished, we set the LED for assessment within a sphere with DC bias forward states. The unit cell for CMS appears to be a structured mixture of Ca:Mg that possesses a desirable polyhedral layout of X^[12]X₂^[9]Y^[10]M^[6][TO₄]₄ with X and Y being the huge polyhedral, M being the octahedron with T being tetrahedron. The desirable upper restriction for Mg presence in bredigite would be Ca₁₄Mg₂[SiO₄]₈. For the layout of bredigite, the MgO₆ octahedra bond through the tetrahedra to create bindings traveling parallel to the axis of

For the layout, Ca would settle in the locations of 2a and 4c, Mg would settle in the location of 2b, while Si and O would settle in the location of 4c. The eight locations of Ca would be related to polyhedral having 9-, 10-, and 12- coordination. These locations would be replaced with the ions of Eu. If the ions merge with the CMS crystal layout, they could replace every cation location which includes Ca²⁺, Mg²⁺, and Si⁴⁺. On the other hand, with the ion radius and the permitted value of oxygen coordination (n) taken into account, Ca²⁺ (1.12

n value of 8), Mg²⁺ (0.72

n value of 6) and Si⁴⁺ (0.26

n value of 4), replacing the ions of Mg²⁺ as well as Si⁴⁺ with those of Eu²⁺ (1.25

n value of 8) would be a challenging task (Shadalou, Cassarly, and Suleski, 2021; Shi et al., 2021).

Figure 1 Photograph of WLEDs

2.2. Spectra optimization of the CCT

Figures 2(b) and 2(c) demonstrate the correlation between the apex location, intensity of discharge, and replacement of Eu²⁺. As the concentration of Eu²⁺ rises, the discharge line with

value of 400 nm will move towards a greater wavelength, which is the result of the repeating absorbing activity among the ions of Eu²⁺ instead of the alterations within the crystal field surrounding Eu²⁺ which can be expressed via the following equation (Shih et al., 2020):

where Dq represents the crystal field of the octahedral symmetrical mechanism. R represents the range separating the center ion from its ligands. Z represents the anion’s charge or valence. e represents the electron’s charge. r represents the d wavefunction’s radius. Therefore, the cleavage of the crystal field would not be the primary cause of the red shift because of the ion radius for Ca²⁺ with 8- coordinate being inferior to Eu²⁺ (1.12

and 1.25

). It is clear in Figure 2(c) that the optimal replacement for Eu²⁺from CMS:Eu²⁺ would be an x value of 0.3. If this value rises, the relative intensity of discharge will occur, caused by the abatement of concentration. It is possible to assess the critical range of power shift or R_c for CMS:Eu²⁺ via the layout data containing the unit cell’s volume (V), the quantity of locations of Eu²⁺ for each cell (N), as well as the critical concentration (X_c).

where R_c represents the mean splitting among the closest ions of Eu²⁺under X_c. With a V value of 1356

³, an N value of 8, and an X_c value of 0.3, the critical shift range for Eu²⁺ would be roughly 12.9

Figure 2 (a) Discharge and emission spectra in CMS:Eu²⁺ when excited at 400 nm under different concentrations of Eu²⁺ (x). (b) The location for the maximal discharge. (c) The relative intensity of discharge along with replacement of Eu²⁺ (x)

The Dexter expression was utilized to determine the shift for the electrical dipole-dipole interactivity as the symmetrical permitted shifts of Eu²⁺ are involved (Singh et al., 2021). The critical range is calculated based on Blasse expression:

where P represents oscillator potency for the ion of Eu²⁺. E represents the energy for the maximal spectrum overlay.

represents the outcome of normalized spectrum forms for discharge as well as excitation. From Figure 2, it is possible to acquire E as well as

via the spectrum information. In the case of P related to the wide absorbing line of

Based on the hypothesis of Dexter, we ought to manage the power shift activity via electric multipole-multipole interactivity (Tian et al., 2020). The following formula will determine the intensity of discharge or I for each trigger ion (Tuyet et al., 2020):

where C represents trigger concentration that abates itself. k and

are constants corresponding to interactivity within an identical excitation state of a host’s latticework. If C >> X_c, the non-radioactive depletions could be caused by a multipolar shift. It is possible to alter the formula (5) (Wang et al., 2020):

where k₁ represents a constant. The

values of 6, 8 and 10 corresponds to dipole-dipole, dipole-quadrupole and quadruple-quadruple. It is possible to assess

via the slop

from the linear dash demonstrated by Figure 3. The Figure puts log(I/C) and log(C) on a logarithmic scale. The result for

would reach -2.16, and as such,

assumes a value of roughly 6. Such an outcome points out the dipole-dipole interactivity would be the concentration abatement activity for the discharge of Eu²⁺. From Figure 4, we can see the diffuse reflectance absorption spectra of CMS:Eu²⁺. It is possible to determine the Kubelka-Munk absorption coefficient (K/S) via a correlation: the absorbance (A) for CMS as well as CMS:Eu²⁺ can be assessed with the reflectances (R) via an expression (Yu et al., 2021):

Using the shown expression, the line gap power or E_g for CMS:Eu²⁺ assumed a value of 2.56 eV. From this result, it might be impossible for the excitation power for n-UV (400 nm, 45000 cm^-1) to shift between the valence line and conduction band. CMS does not display any absorption at below 380 nm and became white as a result. Hence, it is possible to excite the electrons of the 4f ⁷ states in Eu²⁺ via n-UV illumination within the 4f⁶5d state. We determined the quantum performance via a 400-nm excitation cause. The performance reached roughly 23% under room temperature, lower than the 85% result of Sr₂SiO₄:Eu²⁺.

Figure 3 The logarithm for the intensity of discharge for each trigger ion (log I/CEu) along with the logarithm for the concentration of Eu²⁺ (log CEu) from CMS:Eu²⁺ with

value of 400 nm

Figure 4 The diffuse reflectance spectra of (a) CMS and (b) CMS:Eu²⁺ when excited at 400 nm

Between room temperature and 150^oC, the intensities of photoluminescence in CMS:Eu²⁺ and Sr₂SiO₄:Eu²⁺ suffered a penalty of 11% and 18%, which is demonstrated by Figure 5(a). If the task is to examine the heat abatement features, we need to determine the trigger power via Arrhenius expression (Zhang et al., 2020; Yuce et al., 2019):

represents the first intensity. I(T) indicates the intensity under a specific (T) temperature. A represents a constant. E represents the trigger power of heat abatement. k represents the constant of Boltzmann. In Figure 5(b), we can see ln

and 1/(kT). Through utilizing the Arrhenius expression, the values of trigger power (E) for CMS:Eu²⁺ and Sr₂SiO₄:Eu²⁺ appeared to be 0.348 eV and 0.158 eV.

Figure 5 (a) The intensities of discharge reliant on temperature. (b) The heat abatement in Sr₂SiO₄:Eu²⁺ as well as CMS:Eu²⁺ when Arrhenius expression is utilized

Figure 6 Sustaining the median CCT level via the modification of concentration: (a) 3000 K-5000 K; (b) 6000 K-8000 K

With the use of the software LightTools 9.0 and the Monte Carlo approach, the flat phosphor sheet for a multi-chip WLED (MCW-LED) was reproduced (Zhang et al., 2021). The recreation procedure would have two main phases. First, assessing as well as constructing the layout samples along with optical characteristics in MCW-LED lights. Second, managing the phosphor mixture and the light influences through many CMS:Eu²⁺ doping amount percentages. For the assessment of YAG:Ce³⁺ and CMS:Eu²⁺ compound’s effects on WLED lights’ performance, generating variants would be compulsory. Examining two kinds of compounds under the median CCT values of 3000 K, 4000 K, and 5000 K, a remote phosphor layout with two sheets would be necessary. Via Figure 1, the modeling illustration of the MCW-LED with a conformal phosphor structure and a great 8500 K CCT value is presented. It also reveals that the initial MCW-LEDs’ recreation does not involve CMS:Eu²⁺. The WLED components’ dimensions can be listed as follows. A reflector with 2.07×8×9.85 mm for height and bottom- and top-surface length, respectively; a 0.08 mm thick film of conformal phosphor compounding would be daubed on the chips; a set of nine chips with a size of 1.14×0.15 mm (length × height) and radiant-flux value of 1.16 W, and 453 nm peak wavelength for each is attached to LED reflector’s gap.

Results and Discussion

Figure 6 depicts the relation between the utilizing concentration of CMS:Eu²⁺ and YAG:Ce³⁺. In particular, it shows the inverse data trend between these two phosphor amounts: the increase (2-20%) of CMS:Eu²⁺ content leads to the decrease of YAG:Ce³⁺ content, at both 4000 K and 8000 K. This opposite serves as a median to sustain the CCT value and cause changes to either absorptivity or dispersion of light in the WLED model. As a result, we can manage to achieve the desired results for WLED illumination by modifying the CMS:Eu²⁺ concentration.

Figure 7 The emission spectra in WLED devices and the CMS:Eu²⁺ concentrations: (a) 4000 K; (b) 8000 K

Figure 7 illustrates how the concentration of green phosphor CMS:Eu²⁺influence the emission power of the WLED at 4000 K and 8000 K, from which the producer can determine suitable CMS:Eu²⁺ concentration to apply to their LED products. If they want to get high and strong luminescence for backlight application, for example, they can allow a minor deduction in the chromatic features. As depicted in both Figures 7(a) and 7(b), two critical spectral regions for white light generation, 420-480 nm, and 500-640 nm are improved with increasing CMS:Eu²⁺ concentration, suggesting the enhancement in the luminescent output of the WLED model. Additionally, the addition of green phosphor seems to stimulate the emission and dispersion of blue light, which will contribute to increasing the possibility of blue-light conversion and extraction. Thus, the active dispersion and color consistency could be improved. This result is crucial and essential for the application of CMS:Eu²⁺ phosphor to serve the enhancement of WLED-lighting performance, especially at high CCT like 8000 K.

The discussed states can be further demonstrated with the data in Figures 8 and 9. The luminous intensity of the white light from the WLED in connection with CMS:Eu²⁺ concentration is shown in Figure 8. When the concentration of CMS:Eu²⁺ grows in the range of 2-25%, a notable rise is recorded at 5000 K – 8000 K. In Figure 9, the decreases in deviating CCT proportions decline as CMS:Eu²⁺ content becomes higher, suggesting the enhancement in CCT consistency. The green phosphor scattering and absorbing activities could be applied to explain this phenomenon. The green phosphor particles will absorb the blue illumination more significantly than the yellow illumination, as the blue emission wavelength is shorter than the yellow one. This allows greener illuminations to be generated by converting these absorbed illuminations. In other words, the green spectral component is increased as the concentration of CMS:Eu²⁺ is higher, contributing to compensating the green-light energy shortage to result in better light color consistency and luminosity.

Figure 8 The lumen in the WLED devices with respective CMS:Eu²⁺ concentrations: (a) 3000 K-5000 K; (b) 6000 K-8000 K

Figure 9 The hue aberration in the WLED device along with CMS:Eu²⁺ concentrations: (a) 3000 K-5000 K; (b) 6000 K-8000 K

Though color uniformity is a vital and essential factor for high white-light color quality, the color rendering intent and reproduction efficiency should not be ignored. Thus, the CRI (color rendering index) and CQS (color quality scale) are subsequently examined and discussed. Figures 10 and 11 present the data of CRI and CQS, respectively. As shown, CRI values decline gradually with the surge of CMS:Eu²⁺ content percentage. Meanwhile, the CQS displays an increase with the rising concentration of CMS:Eu²⁺ within 2-10%. More than 10% CMS:Eu²⁺ obviously causes the CQS to degrade. Such events can be attributed to green-energy domination, which means the green light is generated excessively, causing the lack of other critical emission colors (blue and yellow). Therefore, the balance and homogeneity of white-light emission colors are damaged, leading to the mentioned degradation in CRI and CQS. However, the rise and stability in CQS with <10% CMS:Eu²⁺ is an important result because the CQS is more complex and difficult to control than the CRI. Specifically, managing CQS means managing to satisfy the three critical factors: color coordinates, CRI, and the observer’s visual inclination. So, we should consider the optical goals to determine the suitable concentration of CMS:Eu²⁺.

Figure 10 CRI in the WLED devices with the respective CMS:Eu²⁺ concentrations: (a) 3000 K-5000 K; (b) 6000 K-8000 K

Figure 11 CQS in the WLED devices with respective CMS:Eu²⁺ concentrations: (a) 3000 K-5000 K; (b) 6000 K-8000 K

Conclusion

Our study investigates how the green phosphor CMS:Eu²⁺ would impact the lighting features in the two-sheet phosphor layout. Judging the Monte Carlo recreations, this phosphor would be appropriate for the task of augmenting hue homogeneity, which can be applied to WLED devices possessing hue temperature measured at 5600 K and greater than 8500 K. Our study has managed to augment the hue output as well as the lumen. Achieving these elements can be fairly difficult in the case of remote phosphor layouts. On the other hand, there is a small downside to CRI as well as CQS. If the CMS:Eu²⁺ concentration rises too high, both of these elements would fall considerably. As such, choosing an appropriate concentration by considering the producer’s requirements is very important. Our study can be useful data for the task of acquiring superior hue homogeneity as well as lumen in WLED devices.

Acknowledgement

This research is supported by Industrial University of Ho Chi Minh City (IUH) under grant number 138/HD-DHCN.

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