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⁺.

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

Computational Simulation

2.1. Preparation of green-emitting Ca_14-xEu_xMg₂[SiO4]₈ (CMS:Eu²⁺)  phosphor

        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⁺ (0.72n 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)

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

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

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.

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.

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