Using Trivalent Eu/Tb Codoped Orthophosphate Compound Mixing SiO2 Particles to Obtain Better Color Uniformity and Luminosity for White LED

A light-conversion phosphor composition consisting of K₃Lu(PO₄)₂: Tb³⁺, Eu³⁺ (KLP:TE) phosphor, and SiO₂ particles was developed and utilized for white light-emitting diodes (WLEDs). KLP:TE phosphor was developed using a high-heat solid-state reaction. KLP:TE phosphor offers modifiable luminescence as well as an effective powershift. Fluorescent-related computations were used to evaluate KLP:TE luminescence performance. Tb³⁺/Eu³⁺ doping ratio plays an important role in controlling the powershift between ions of Tb³⁺ and Eu³⁺, primarily through electric dipole-dipole (d-d) achieving a high PS effectiveness of approximately 98.36%. Subsequently, KLP:TE@SiO₂ was integrated into an ultraviolet (UV) LED (light-emitting diode) package. While phosphor concentration remained constant, SiO₂ concentration varied. As a result, the performance of WLEDs was effectively regulated with SiO₂ concentration modification. The device generates white illumination with higher luminosity and color-distribution uniformity with higher SiO₂ doping concentration. It is possible to use this emerging phosphor for high-power WLEDs implementations.  

Band space; Color-distribution uniformity; Phosphor; Power transition; Warm WLED

Phosphor-converted white light-emitting diodes (WLEDs) exhibit several outstanding characteristics, including high power converting effectiveness, hue adjustability, long lifespan, small size, environmental friendliness, and dependability (Anh and Lee, 2024; Le et al., 2024). These qualities enable WLEDs as a viable replacement for traditional incandescent and fluorescent lamps, and a typical WLEDs consists of two main components. The first is a blue or near-ultraviolet (NUV) LED (light-emitting diode) chip, while the second includes at least one phosphor material derived from rare-earth-doped compounds (Tung et al., 2024; My et al., 2022; Le et al., 2022). Phosphor plays an important role in determining the total illuminating effectiveness, hue reproduction, and heat steadiness of the resulting white illumination, making it a crucial element (Tran et al., 2020a; 2020b; Loan et al., 2020). Until recently, the most common and straightforward approach to fabricating commercial WLEDs involved the addition of a yellow phosphor Y₃Al₅O₁₂:Ce³⁺ (YAG: Ce) to one blue LED chip (Jia et al., 2016). However, white illumination generated from the device developed has a color deficiency in the green as well as red areas, which severely restricts its wide-scale applicability and results in deficient chromatic rendering indices (CRI<80) and cold-white light due to highly correlated chromatic temperature (CCT>4500 K) (Dang et al., 2021).

In order to improve the effectiveness of WLEDs, there is a need to enhance the green or red radiation proportion (Thai et al., 2023). According to an investigation, red phosphors are known to execute better red emission for the white light spectrum (Hu et al., 2023). Another report showed red-light improvement, the luminescence was occasionally degraded, and CRI did not reach the expected value (Huu et al., 2022). One of the problems with such results is that the available red phosphors on the market give an inadequate red emission band for high-color rendering tasks.

The traditional InGaN blue chip has several challenges, including the reduction in efficiency and excessively emitted blue light (Verzellesi et al., 2013). These problems can be mitigated by replacing the blue-emitting InGaN chip with an ultraviolet (UV)-violet chip (Loan et al., 2021). In the last decades, the industry has been transitioning toward the adoption of high-power UV LED. This LED type has been applied in many sectors, such as medical diagnostics, imaging, spectroscopy, biosensing, and material curing (Kneissl et al., 2019; Widiyati and Poernomo, 2018). However, phosphor performances are often unstable and degraded under high-power operation (Kim et al., 2017). Therefore, research topics focus on generating red phosphors for the development of LED devices with a great CRI as well as acceptable CCT. In these conditions, it is important to investigate tri-hue light-emitting phosphors that are effectively stimulated through NUV radiation while meeting the requirements for WLEDs devices (Kazakovsky et al., 2020; Desnijder et al., 2019).

Due to their exceptional luminous qualities and distinct emitting ranges, rare-earth (RE) ions play an important in current display illumination, photodetection, optic amplification, and other relevant applications. It is well known that the central RE ions' illumination radiation is primarily caused by the efficient power shift (PS) between the triplet condition for the ligand and the crystal field conditions (Salerno, 2021). Consequently, PS plays a crucial role in both theoretical research and practical applications related to the color tuning of phosphors. Tb³⁺ ions are the most prevalent activators in phosphors. According to the number of dopants, their radiation is the consequence of either the  transformation inside a blue zone or the activity (J = 6-2) inside one green area (Thi et al., 2023). The interaction between Tb³⁺ ions becomes stronger with increasing Tb³⁺ dosage, resulting in a cross-relaxation among the ⁵D₃ as well as ⁵D₄ states along with the induction for the  transition with mostly green radiation (Royer et al., 2019; Tian et al., 2019). However, the red element from different phosphor samples incorporated with Tb³⁺ as well as Eu³⁺ can be compensated for by the magnetic dipole transition  along with electric dipole transformation of Eu³⁺ ions (Thi et al., 2023).

This research applied K₃Lu(PO₄)₂:Tb³⁺,Eu³⁺ phosphor (KLP:TE) to create a UV-LED package. K₃Lu(PO₄)₂ was selected as the host material, while (PO₄)₃- orthophosphates are known to be a good phosphor group for doping Eu³⁺ and Tb³⁺. These incorporations can offer extremely efficient high-power transformation and UV-energy absorption (Wu et al., 2020; Choi et al., 2019). A novel high-heat solid-state KLP:TE phosphor yielding controllable emission in the orange-red region was developed. Then, the phosphor was mixed with the original phosphor composition containing YAG:Ce³⁺ and silicone gel in order to improve the red-light spectrum. SiO₂ particles were added to obtain significant scattering improvement in the WLEDs package. The luminescence computation of the created KLP:TE phosphor was described. Subsequently, LED light performance was demonstrated in the presence of KLP:TE@SiO₂ composition. The concentration of KLP:TE was fixed while varying SiO₂ doping amounts. This regulation of the scattering effect increases the likelihood of achieving enhanced color uniformity, rendering efficiency, and luminous output in WLEDs. The reported data demonstrate promoted luminous flux and notable color-deviation decrease when increasing the SiO₂ doping amount. However, the chromatic rendering results do not meet the expectations due to the lack of green and deeper red regions to achieve the full-chromatic spectrum for the generation of white light. The results demonstrated the application possibility for KLP:TE@SiO₂ in UV WLEDs in terms of accomplishing improved color-distribution uniformity and luminosity.

2.1. Substances and combination

The typical high-temperature solid-state reaction was used to create KLP:TE phosphor. Concentrations of Tb³⁺ and Eu³⁺ ions were determined at 0.1 and 0.06 mol, respectively. The materials required for the development process with steps carried out during the process are shown in Table 1 (Sheu et al., 2019).
 

Table 1 Constituents and creation process of K₃Lu(PO₄)₂: Tb³⁺, Eu³⁺

Materials	Purity	Process
K2CO3	99%	- All materials are weighed with predetermined amounts.
Lu2O3	99.99%	- These powders are homogeneously mixed and ground in one agate mortar.
NH4H2PO4	99%	- The attained composition is pre-heated at 800oC in 4 hours.
Tb4O7	99.99%	- The composition is then sintered at 1150oC for 4 hours in a muffle furnace in the air surrounding.
Eu2O3	99.99%	- After that, the obtained sample was removed and allowed to cool at normal temperature.
K2CO3	99.99%	- The sample was pulverized to acquire powders ready for WLEDs creation.

2.2. LED manufacture

        KLP:TE concentration was fixed and blended with YAG:Ce³⁺ phosphor, and SiO₂ particles in silicone. The powder combination was combined with one UV chip (1 W, ex = 365 nm) and OE6550 silica gel in the form of a fixing agent to develop WLEDs (Tanaka et al., 2021). Figure 1 below shows the WLEDs simulation carried out during the tests. The concentration of SiO₂ is modified in the range of 0-25 wt.%.

Figure 1 WLEDs formation depictions: (a) WLEDs device, (b) Binding schema, (c) Illustrated device, (d) Recreated device in program LightTools

2.3. Characterization

Following the development process of KLP:TE samples, their characteristics were examined. For this task, various tools were used, and the characteristics as well as their matching tools are shown in Table 2 (Li et al., 2021, Li and Zhen, 2020).

 

Table 2 Characteristics and Determining Tools

Characteristics	Tools
XRD patterns	D8 Advance diffractometer with  tube and a voltage reaching 40 kilovolts as well as current reaching 30 milliamperes
Rietveld refinement	GSAS program
Chemical blends along with element valence	XPS using an ESCALAB Xi+ electron spectrometer
Diffuse reflectance spectra	UV-Vis-NIR spectrophotometer
Photoluminescent excitation/emission spectra	Fluorescence spectrophotometer accompanied by Xe light (400 V, 150 W)
Quantum performance	Spectrofluorometer with a 150 W xenon light source
Fluorescent degradation arches	FS5 spectrofluorometer accompanied by 150-W CW Xe arc light with no ozone

3.1. Luminescent computation of phosphor

With the exception of Ce³⁺, most trivalent rare earth ions produce a series of relatively narrow emission lines due to intrinsic 4fⁿ -4fⁿ shifts barely influenced by the base's molecule. However, the host plays a part in the discharge characteristics in the 4f-4f dischargers because it affects the relative potency of the discharge lines (through local symmetry-related picking principles). The splitting of the emission lines in dependence on the crystal field, and the quantum efficiency (by virtue of the existence of non-radiative pathways and temperature abatement). Certain rare earth ions generate rays that may be seen. Notable rare earth ions include Tb³⁺ (green discharge, main apex under 545 nm) as well as Eu³⁺ (orange to red discharge, key maximum around 600 or 620 nm), which have both been extensively used in cathode ray tubes and fluorescent light phosphors. Since 5d states, along with charge transfer conditions (CTC), typically lie below 350 nm, the primary challenge in adapting these materials for LED applications is the lack of efficient and broad emission pathways in the near-UV to blue spectral region.

By sensitizing with the proper addition of co-dopants, the recreation spectrum may extend toward greater wavelengths. For instance, the addition of Ce³⁺ may sensitize Tb³⁺ discharge. There were reports of extra (wide band) routes for Eu³⁺ being created by the addition of Bi³⁺. As previously mentioned, narrow-line emission under 460, 540, as well as 610 nm may be combined to provide effective white light emission, WLEDs can be designed with low color rendering quality but high illuminating efficiency. The principal emission peaks of Tb3+ and Eu3+ align with the required red and green components. Using red phosphors doped with Eu3+ helps prevent green phosphor dominance, enhancing color balance radioactivity from being absorbed again, a drawback for red phosphors made using Eu²⁺ dopant.

The host's absorbing band is primarily responsible for KLP:TE phosphors' close 200 nm absorption characteristics. The power gap can be estimated by utilizing the Kubelka-Munk function to further distinguish if K₃Lu(PO₄)₂ would be a direct or indirect band gap substance, demonstrated using Equation 1 (Mednikov et al., 2020):

where hv is the photo energy, represents reflectivity, and E_g stands for optic band space energy. The combination is regarded as a direct band space substance when n = 2. The indirect band space is indicated by n = 12.     

Weak blue illumination emissions are produced at 417 and 438 nm by a few stimulated states of Tb³⁺ that move straight from ⁵D₃ to ⁷F₆. Other stimulated states undertake the non-radiative shift, relax towards the bottom excitation state ⁵D₄, but afterward suffer the radioactive degradation towards ⁷F₆, which emits a powerful green illumination. PS procedure among Tb³⁺/Eu³⁺ 4f stimulated states is facilitated by the energy mismatch. The addition of Eu³⁺ to KLP:TE results in partial power transmission from ⁵D₄⁷F_J shift between Tb³⁺ and Eu³⁺ accompanied by later alleviation towards ⁵D₀. The last orange-red emission results from the power radiation’s decline from ⁵D₀ to ⁷F₀.

It is discovered that a double-exponential degradation function may suit the degradation curve nicely (Sezer et al., 2019), as shown in Equation 2:

I(t) denotes Tb³⁺ emitting strengths under the delay time t; ₁ and ₂ denote the exponential element's fast and slow fluorescent durations; and A₁ and A₂ denote the specific constants Calculations for the median duration  in Equation 3 (Rabaza et al., 2020):

In the Tb/Eu PS procedure, the transferring probability  may be represented as shown by Equation 4:

where  represent the lives of Tb³⁺ with and without Eu³⁺ for the equal sensitizer doses, in turn. Subsequently, the formula below calculates the power-shift effectiveness  from ions of Tb³⁺ to Eu³⁺ can be computed with Equation 5:

where signifies the duration for Tb³⁺ in the co-doped Tb/Eu phosphor samples;  signifies the radiative degradation lifetime of Tb³⁺; and T signifies the power-shift effectiveness. The transmission effectiveness increases from 26.93% to almost 100% with a rise in Eu concentration (Liao et al., 2019), as calculated using Equation 6:

 signifies the combined concentration for Tb³⁺ as well as Eu³⁺. N signifies the formula unit amount in the unit cell. V signifies the unit cell’s volume. N reaches 3, while V reaches 612.973128  under  (sum percentage for sensitizer Tb³⁺ as well as trigger Eu³⁺) for K₃Lu(PO₄)₂ host. Therefore, the PS process should be assigned to the multipolar interaction since the associated R_c is equivalent to 10.274  which is substantially bigger than 5 

The multipole interaction governs the resonance PS between Tb³⁺ and Eu³⁺ from KLP:TE phosphors. Reisfeld's theory, along with Dexter's expression, presented in Equation 7, helps us assess the PS process (García et al., 2020):

where I_s and I_so signify the luminescent strengths of the sensitizer Tb³⁺ in the case of having and lacking Eu³⁺. C signifies the Eu³⁺ presence used for incorporation. n values of 6, 8, 10 correspond to dipole-dipole (d-d), dipole-quadrupole (d-q), quadrupole-quadrupole (q-q) interactivities. signify the illumination quantum performance (QP).

The emitting strengths for Tb³⁺ and Eu³⁺ are reduced by thermal quenching to 68.1% and 84.0% under room temperature. Arrhenius equation, included in Equation 8, is used to calculate the trigger power  and further evaluate the heat steadiness (Bullough et al., 2019):

where A signifies one constant. k signifies the Boltzmann constant (8.617 × 10^-5 eV K^-1).  are the luminous strengths of KLP:TE under ambient heat and a specified heat level, accordingly.

3.2. Phosphor Influence on WLEDs performances

The concentration of SiO₂ is adjusted to impact the scattering property of the whole phosphor compound containing YAG:Ce³⁺ and KLP:TE phosphors. Scattering coefficients (SCs), regarding the increased concentration of SiO₂ from 0 wt.% to 25 wt.% in the composition, were shown in Figure 2. According to observation, high SiO₂ concentrations stimulate SCs, thereby enabling greater transmission and conversion of scattered blue-chip light into longer-wavelength light. When the forward emission exhibits an increase in dispersed blue light, coupled with a decrease in blue-light reabsorption and backscattering, such a phenomenon has the potential to enhance luminosity.

Figure 2 Scattering coefficients when varying SiO₂ contents in KLP:TE@SiO₂ composition

 

To achieve this objective, it is essential to maintain a lower concentration of YAG:Ce³⁺ yellow phosphor as SiO₂ concentration rises. The adjustment also contributes to minimizing fluctuations in the angular CCT range. Figure 3 shows the reduction in YAG:Ce³⁺ concentration with increasing SiO₂ concentration, and Figures 4 and 5 show the stability of CCT at higher concentrations. Furthermore, Figure 4 demonstrates how, at higher doping concentrations, the phosphor may reduce its CCT fluctuation (D-CCT). With 25wt.% SiO₂, D-CCT eventually reaches its lowest point at about 225 K, which is lower by 45 K than the value when no SiO₂ was used.

Figure 3 YGA:Ce phosphor dosage values when varying SiO₂ contents in KLP:TE@SiO₂ composition

Figure 4 CCT values when varying SiO₂ contents in KLP:TE@SiO₂ composition

 

Figure 5 Color difference values when varying SiO₂ contents in KLP:TE@SiO₂ composition

 

Figure 6 shows how the increase in SiO₂ proportion did not always lead to an increase in the luminous brightness of white light emission. According to the results, using 0 – 5wt.% SiO₂ leads to a considerable brightness reduction while using 25wt.% SiO₂ provided the greatest luminosity. As shown in Figure 5, the D-CCT value at 25wt.% SiO₂ is the highest. This exhibits an uneven color distribution and lower blue emission intensity due to greater back-scattering and reabsorption. Higher SiO₂ doses in particular would promote the light conversion from blue to yellow or orange-red as phosphor absorbs more backscattered blue light. 

Figure 6 Luminescence strength when varying SiO₂ contents in KLP:TE@SiO₂ composition

 

The quantity of SiO₂ increases and leads to the thickening of the phosphor layer. This would cause the converted light to reflect off various surfaces several times, decreasing the power of the overall emission spectrum. In other words, if the dosage of SiO₂ is too high, a greater amount of converted light has the potential to be reflected, resulting in a reduction in luminous intensity and an increase in CCT values. As a result, it was determined that 25wt.% of SiO₂ was the appropriate amount for the simulated WLEDs in order to improve luminous intensity and color uniformity (Aydin et al., 2021).

The concentration of SiO₂ not only impacts the brightness but also significantly influences the color rendering performance of WLEDs. The evaluation of color rendition values through hue rendering indicators (CRI) and hue quality scale (CQS) measurements showed a consistent decline as SiO₂ concentration increased up to 25wt.% (Figures 7 and 8). The observable decreases in CRI and CQS can be attributed to imbalanced blue, green, and yellow-orange patterns. As previously mentioned, the increased scattering resulting from the higher SiO₂ dose creates an imbalance, causing the illumination emission hue to shift further toward the yellow-orange region. Consequently, excessive dispersion leads to a decrease in both CRI and CQS. Further investigations into additional characteristics of this phosphor, such as particle size, will be conducted in the ongoing research to effectively manage CRI and CQS (Oliveira et al., 2019).

Figure 7 CRI values of WLEDs when varying SiO₂ contents in KLP:TE@SiO₂ composition

Figure 8 CQS values when varying SiO₂ contents in KLP:TE@SiO₂ composition

 

The emission spectra of KLP:TE@SiO₂ phosphor-based WLEDs are shown in the supplementary materials section. The entire white emission band demonstrated how phosphor may increase the power of blue and orange-red radiation. The highest emission points may be observed in the blue (450 nm) and yellow-orange (600 nm) areas. This can be attributed to the effect of scattering improvement provided by SiO₂ presence in the phosphor compound. Particularly, lighting efficiency may be changed by modifying SiO₂ amount settings to alter the scattering and absorption patterns of WLEDs light output. When higher SiO₂ is introduced, scattering productivity is more active, leading to higher light absorption of the phosphor compound. Therefore, UV-light utilization by KLP:TE phosphor is probably simulated, resulting in better conversion for blue light and orange-red light related to the trivalent Tb and Eu ions.

In conclusion, this research successfully developed and utilized a light-conversion phosphor compound containing K₃Lu(PO₄)₂: Tb³⁺, Eu³⁺ orange-red phosphor, SiO₂ particles, and YAG:Ce³⁺ yellow phosphor for WLEDs using UV chip. KLP:TE phosphor was synthesized through a high-heat solid-state reaction, offering adjustable luminescence properties and effective powershift capabilities. The electric d-d interaction was found to contribute to an elevated powershift effectiveness of approximately 98.36%. Subsequently, the illumination properties of WLEDs were controlled with SiO₂ concentration modification. It is possible to induce higher luminosity and improved color-distribution uniformity by increasing SiO₂ doping concentrations. Either CRI or CQS reduces with a highly doped SiO₂ amount in the phosphor compound. In general, this emerging phosphor composition showed great potential for high-power WLEDs applications, offering the ability to generate white illumination with enhanced brightness and uniformity. Further exploration and utilization of this phosphor composition could lead to significant advancements in high-power WLEDs implementations.

Acknowledgements

The authors would like to thank Professor Hsiao-Yi Lee, National Kaohsiung University of Science and Technology, for providing software and experiment support.

Author Contributions

Phan Xuan Le: Conceptualization, Methodology, Software, Validation, Investigation, Resources, Data Curation, Writing, Review, Editing, Visualization, Supervision, Project administration

Nguyen Thi Phuong Loan: Conceptualization, Methodology, Software, Validation, Investigation, Resources, Data Curation, Writing, Review, Editing, Visualization

Conflict of Interest

The authors declare no conflicts of interest.

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