Improved Color Uniformity In White Light-Emitting Diodes Using LiLu(MoO4)2:Sm3+ Combined SiO2 Composite

The research herein concerns the composite of red phosphor of LiLu(MoO₄)₂:Sm³⁺ (LMOS), yellow phosphor YAG:Ce³⁺, SiO₂ particles, and silicone gel. The LMOS phosphor is created via the sol-gel procedure and supposed to yields significant heat consistency. The concentration of this LMOS phosphor is fixed at around 10 wt.%  and the concentration of SiO₂ particles is modifed. This is to influence the scattering performance of the composite to achieve the better color distribution. After sample creation, we analyzed the luminescence of the LMOS in the composite and the effects of the composite with different SiO₂ dosages on the commercial light-emitting diode (LED). When excited via 405-nm ultraviolet, the samples generate red ray under 648 nm matching the shift between ⁴G_5/2 and ⁶H_9/2 for the ion of Sm³⁺. With high SiO₂ amounts, the color difference is reduced, and the luminosity is enhanced. The correlated color temperature is also lower, resulting in a warmer white light for the packed LED. However, the color rendering index declines, which could be attributed to the green and blue color deficiency while the red color is dominant. From the tested outcomes, LiLu(MoO₄)₂:Sm³⁺@SiO_2­ composite is validated to be effective at improving chromatic uniformity for white-ray diodes. 

Blue InGaN chips; Color rendering index; Correlated color temperature; White-light-emitting diodes

Phosphors incorporated with rare-earth ions demonstrate significant utility in the domains of solid-state illumination, solar panels, and heat detectors, owing to their capability to induce diverse transitions involving 4f-4f or 4f-5d energy levels. (Tung, An and Anh, 2023; Yu et al., 2021; Yan et al., 2020). As of today, the fluorescent as well as incandescent outdated lights can be altered in favour of white-light-emitting diodes (WLEDs) because this light source generation offers various qualities including significant performance, low energy consumption, greater durability, etc (Henry and Guruviah, 2020; Kumar et al., 2020; Hansen et al., 2022). These devices are often created by incorporating yellow YAG:Ce³⁺ phosphors and blue InGaN chips. However, this method is deficient in red element within the total spectrum, yielding small color rendering index (CRI) as well as high-level correlated color temperature (CCT) (Bouchakour, Borni, and Brahami, 2021; Finch, Moreno, and Shapiro, 2021; Choi et al., 2019).  Red phosphors such as Y₂O₂S:Eu³⁺ and CaS:Eu²⁺ lack the efficiency in assimilating within the near-ultraviolet zone. Furthermore, sulfide phosphors degrade after an extensive use, subsequently harming the durability in LED devices. Hence, the exploration and enhancement of new red phosphors capable of producing consistent heat output when stimulated by near-ultraviolet radiation is of paramount importance (Royer et al., 2019; Tian et al., 2019).

Following this, studies placed their aim at creating various phosphors with null heat abatement, including those incorporated with Eu²⁺, Eu³⁺, Mn²⁺, Tb³⁺, Mn⁴⁺, as well as Sm³⁺. The sample Ca₂InSbO₆:Sm³⁺ yielded null heat abatement when heat level reaches 480 K, an outcome expected for LED devices with significant performance (Guennoun et al., 2021). As such, the task of augmenting phosphors’ heat abatement attributes remains particularly difficult when it comes to creating WLED devices based on conversion phosphor. Back to previous eras, various studies were carried out to assess dial molybdates incorporated with rare-earth ions having composition shown as MLn(MoO₄)₂ (with M being Li⁺, Na⁺ and Ln being La³⁺, Gd³⁺, Lu³⁺, Y³⁺) in the form of inorganic substances, garnering significant notice as they can be diversely utilized for  solid-state lasers, scintillators as well as WLED devices (Ranjith et al., 2020; Soltic et al., 2019).

The phosphor bases possessing the CaMoO₄ scheelite formation would be assessed through a unique disarrangement for the surroundings for the ions of RE³⁺ with the Ca²⁺ locations from CaMoO₄ contingently inhabited by half of the alkali metal ion M⁺ as well as half of the rare earth ion Ln³⁺ (Attanayake, Okuya, and Murakami, 2020). Dual molybdate blends typically possess wide, potent charge shift line within the near-ultraviolet zone as well as desirable heat consistency, proving to be a fitting base substance that can be diversely utilized for WLED devices incorporated with rare-earth phosphors (Praveen et al., 2021; Königs, Mayr, and Buchner, 2019). The ions of Sm³⁺ would be a typical trigger for orange-red discharge as it has shifts between ⁴G_5/2 and ⁶H_J with J as 5/2, 7/2, 9/2, 11/2, being the best source when it comes illumination and exhibition (Astuti et al., 2020). In addition, certain phosphors incorporated with Sm³⁺ were created and utilized for WLED at some points in the past. The LiLu(MoO₄)₂:Sm³⁺ (LMOS) phosphor created with sol-gel method could provide red-light component for the WLED’s spectrum, which is supposed to effectively improve the chromatic rendition of the package (Irawan et al., 2020). However, this phosphor performance is barely investigated for conventional white LED with blue InGaN chips. Therefore, in this work, the LMOS phosphor is utilized for the goal of improving color uniformity and light distribution of the conventional LED device. This work creates a phosphor composite comprising the LMOS red phosphor, YAG:Ce³⁺ yellow phosphor, SiO₂ scattering improvement particles, and silicone gel, called LMOS@SiO₂ hereafter. This composite is applied for the white LED with blue InGaN chips. In this composite, we adjust the doping dosage of SiO₂ while keeping the dosage of LMOS phosphor constant. This is to regulate the lighting performance of the WLED with LMOS@SiO₂ via the light-scattering modification. Judging the outcomes, LMOS@SiO₂ composite proves to be useful for reducing color deviation level while improving luminosity for the WLED device.

2.1. Creating LMOS phosphors

      The LMOS samples were created using the sol-gel procedure. The ingredients used for the procedure were obtained in a pure, uncontaminated form. Table 1 lists said ingredients along with the procedure’s steps, which also apply to LiLu_{(1- x)}(MoO₄)₂:x% Sm³⁺ with x values of 0.5, 1, 3, 5, 7, 10 (Winberg-Wang, 2019).

Table 1 Ingredients and steps to create LMOS

2.2. Assessing attributes

Following the creation of the samples, assessments were carried out to determine their attributes. Appropriate tools were utilized for this task. Table 2 below details attributes along the tools used (Altenberg-Vaz and Inanici., 2021).

Table 2 Attributes and associated tools used for the samples

3.1.  Luminescence calculation for LMOS

       The power gap will be determined via ultraviolet-observable absorbing spectra by applying the expression below (equation 1) (Nahon, Beckers, and Blanpain, 2019):

? signifies the absorptivity coefficient. h signifies Planck constant. v signifies optical periodicity. K signifies constant. n signifies constant corresponding to various forms of electron shifts with values of 1/2, 2, 3/2, 3 respectively applying to directly allowed, indirectly allowed, directly forbidden, indirectly forbidden shifts.

        For the task of generating non-radioactive power shift among the surrounding Sm³⁺ triggers, two distinct forms of non-radioactive power shift exist: reciprocating interactivity as well as multi-state interactivity. Assessing the critical range would be necessary to identify the best one. Power shift via multi-polar interactivity manifests if the critical range exceeds 5 Å. On the other hand, reciprocating interactivities manifest if critical range goes below 5 Å. For the task of clarifying the concentration abatement, the critical range will be determined by the expression below (equation 2) (Ali et al., 2019):

V signifies cell’s volume as 289.854 Å. ?_c signifies the critical concentration. N signifies the cation amount within the latticework, equal to 2. The yielded critical range reaches 20.974 Å, noticeably exceeding 5 Å. As such, the non-radioactive power shift would be deemed multi-polar interactivity. Based on Dexter’s work, it is possible to utilize an expression for the task of approximating the potential forms of said interactivity (equation 3) (Pierson et al., 2021):

       

The CIE coordinates for LMOS when excited at 405 nm appeared to be (0.5806, 0.4145), resulting in the display of red discharge within the hue scale. For the task of illustrating the capabilities for LMOS, we assessed the sample’s correlated color temperature (CCT) as well as hue clarity. The McCamy expression below will determine the CCT (equation 4) (Chen and Wu, 2019):

x_e (0.03320) and y_e (0.1858) denote hue focal points with x, y being the CIE coordinates. The yielded correlated color temperature output reached 1692.08. The hue clarity would be essential for the task of assessing the luminescent output for LMOS and will be determined via the expression below (equation 5) (Xue and Xie, 2020):

The yielded hue clarity reached 98.94% with hue coordinates (x, y) for LMOS as (0.5806, 0.4145), the white illuminating site (x_i, y_i) as (0.3333, 0.3333), as well as prevalent wavelength site (x_d, y_d) as (0.5834, 0.4158).

        It is possible to fit the luminescent degradation arch by utilizing dual-exponential expression (equation 6) (Mukunda, Joshi, and Mahato, 2022):

I(t) signifies the discharge intenseness under time t. I₀ signifies the surrounding’s intenseness. A₁, A₂ signify fitting constants.  signify the rapid and slow elements for the degradation arch. The expression below will determine the median durations (equation 7) (Eisazadeh, Allacker, and Troyer, 2021):

3.2.  LMOS@SiO₂ effects on WLED outputs as a function of varying SiO₂ amounts

        In the prepared composite, the concentration of YAG:Ce³⁺ phosphor must be changed corresponding to different doping amount of the SiO₂ particles. This situation is demonstrated in Figure 1. In the graph, the amount of SiO₂ varies from 0 wt.% to 50 wt.%, followed by the decline in YAG:Ce³⁺ concentration. Particularly, these changes appear to be an inverse mechanism, with the YAG:Ce amount almost linearly decreasing from its peak at 0 wt.% SiO₂ to the bottom at 50 wt.% SiO_2. As the amount of SiO₂ surges, the scattering coefficient is heightened, leading to better dispersion for the illumination. Besides, the generated light by blue chip can be effectively propagated then transmitted through the phosphor composite. As a result, the amount of re-absorbed light by phosphors is reduced, the luminescence will be heightened when the blue-ray dispersion in the direct path surges while the blue-ray rear-dispersion is diminished (Rubeis et al., 2021). So, the decreasing concentration of YAG:Ce³⁺ phosphor contributes to improving the scattering performance in the presence of increasing SiO₂ dosage.

        With the enhancing scattering performance, higher SiO₂ dosages also affect correlated color temperature (CCT) levels, as shown in Figure 2. In the figure, the CCT levels recorded in the viewing angles of ±90^o with SiO₂ amounts range of 0-50 wt.%. In general, in ±20^o, the CCT level declines and becomes relatively flattened as we increase the SiO₂ concentration. This indicates that the white light is warmer when exposing to human eyes directly. On the other hand, in ±90^o, the CCT levels exhibit a significant fluctuation, but still lower than the value obtained with 0 wt.% SiO₂ in the package. The deviation of CCT range is particularly calculated and displayed in Figure 3. Apparently, with different SiO_­2 dosages, the CCT show notable fluctuations. However, as the SiO₂ amount increases continuously to 50 wt.%, significant CCT-deviation reduction is achieved. The CCT is at its lowest with the SiO₂ amount of 50 wt.%. The other notable CCT-deviation reduction levels are also obtained with SiO₂ concentration of 30-35 wt.%. Though with the amount of 40-45 wt.% SiO₂, the CCT-level aberration sharply goes up, it is lower than the CCT level of the composite sample without SiO_2. This indicates that the light dispersion is influenced and enhanced with the presence of SiO₂, revealing the enhancement in converted red-light proportion by LMOS phosphor.

        Conversely, for the lumen of the WLED shown in Figure 5, the considerable and continuous rise is noticed regarding the higher SiO₂ doping amounts. This event could be a result from the better blue-light discharge induced by greater forward light dispersion and lower light re-absorbance. It is noteworthy that increasing the particle concentration of SiO₂ can enhance the transformation of illumination from blue to yellow or red-orange. This effect is primarily attributed to the broader scattering range, allowing more blue light to be mixed, to interact with the phosphor particles in the composite, and be converted. However, if the scattering is too much, the transformed light may undergo rear-reflection, thereby possibly reducing the intensity of luminescence and resulting in a higher correlated color temperature (CCT) level (Costa et al., 2019). Thus, we predict that if the SiO₂ is more than 50 wt.%, the luminous flux of the WLED with LMOS@SiO₂ sheet will decline. Yet, this will be analyzed in future works.

Figure 1 YAG:Ce concentration under influence of increasing SiO₂ amount

Figure 2 Angular CCT range monitored with different SiO₂ amounts

Figure 3 CCT-range deviation responding to different SiO₂ amounts

Figure 4 LED lumen responding to different SiO₂ amounts

        The output of hue generation of the WLED device is also affected by the dosage of SiO₂. As the particle size increases from 0 wt.% to 50 wt.%, color rendering index (CRI) and color quality scale (CQS) shows a noticeable deterioration in a relatively linear manner, see Figures 5 and 6, respectively. This observed decline is possibly the result of the lack of green and blue spectra in the white light emission. Under great dosages of SiO₂, the heightened dispersion generates more orange-yellow elements, resulting in the mentioned emission-color deficiency since the ray’s discharge hue generally favors the orange-yellow zone. As such, unnecessary dispersion can cause inferior CRI as well as CQS (Guerra et al., 2020).

Figure 5 CRI under influence of SiO₂ particle size

Figure 6 CQS under influence of SiO₂ particle size

        In the previous eras where the scientific fields were unfamiliar with hue assessment, color rendering index (CRI) was created and was deemed the only way to evaluate the quality of hue generation in a ray when compared to natural lights. Eventually, the flaws of CRI became a concern (Danny et al., 2020). Notably, its mechanism to measure hue desirability in lighting devices was not reliable for utilizing outdated hue samples and inaccurate hue scale. In the case of incandescent lights with perfect CRI output of 100, it was reported that the human sight did not witness the correct hue recreation based on the standard of sun ray (Thi et al., 2023). For fluorescent light which yielded a low CRI output, its ability to illuminate the environment was found to be superior. When it comes to LED devices, they generate desirable light while not generating significant temperature, a factor that is heavily assessed by CRI considering that luminescent light was unheard of back in the old times. As a result, high-quality WLED devices end up yielding an inferior CRI when compared to the more outdated lights mentioned above. Color quality scale (CQS), on the other hand, overcome the drawbacks of its predecessor by assessing more facets, such as human preference as well as additional hue samples, thus yielding more accurate results that fit our perception and proven to be a better hue-output indicator for WLED devices (Thai et al., 2023).

The study herein concerns phosphor LiLu(MoO₄)₂:Sm³⁺@SiO_2­ composite for the conventional WLED model with blue chips. The LMOS phosphor possesses the tetragonal setting and generates red ray under 648 nm when excited via near-ultraviolet ray under 405 nm. The primary concentration abatement nature in Sm³⁺ appeared to be the result of dipole-dipole interactivities. By fixing the concentration of LMOS phosphor and varying the SiO₂ doping amount, the illumination properties including color distribution and luminosity of the WLED can be regulated. Specifically, increasing the SiO₂ content leads to a decrease in color discrepancy and an improvement in brightness. Additionally, it results in a lower correlated color temperature, creating a warmer white light for the compact LED. However, this increase in SiO₂ content also causes a decline in the color rendering index, which can be attributed to a deficiency in green and blue colors while the red color remains dominant. The outcomes prove that LiLu(MoO₄)₂:Sm³⁺@SiO_2­ can be used to generate red rays and induce color uniformity for conventional WLED devices.

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