Triple-doped Dy³⁺/Tb³⁺/Eu³⁺ activated Na₂Ca₄(PO₄)₃F halo-phosphors for next-generation WLEDs and solar cell efficiency enhancement

Abstract

This study reports the synthesis and characterization of Na₂Ca₄(PO₄)₃F phosphor doped with Dy³⁺, Tb³⁺, and Eu³⁺ ions were synthesized using a conventional solid-state method. The phase formation was confirmed by X-ray diffraction (XRD). Functional group analysis through Fourier transforms infrared spectroscopy (FTIR) and morphological evaluation via scanning electron microscope (SEM) and EDS revealed good crystallinity and elemental distribution. The triple-doped phosphor exhibited color-tunable emissions under near-UV excitation, covering the blue, green, yellow, and red regions, resulting in efficient white light output. Photoluminescence (PL) studies indicated effective energy transfer among the dopant ions. Additionally, the phosphor was applied as a down-conversion layer on silicon solar cells using the doctor blade technique. The coated cells showed a 16.33% improvement in efficiency compared to uncoated cells. The combined optical and photovoltaic performance highlights the potential of Na₂Ca₄(PO₄)₃F: Dy³⁺/Tb³⁺/Eu³⁺ as a multifunctional material for white light-emitting diodes and solar energy devices.

Introduction

The growing global energy demand, driven by population growth and technological advancement, necessitates the development of sustainable, efficient, and eco-friendly energy solutions. Among the various innovations in lighting technologies, white light-emitting diodes (WLEDs) have garnered considerable attention due to their high luminous efficacy, long operational lifespan, low power consumption, and environmental safety. These attributes make WLEDs a promising alternative to conventional fluorescent and incandescent lighting sources^1,2,3,4,5. The most common approach to produce white light in WLEDs involves combining a blue-emitting InGaN chip with a yellow-emitting YAG: Ce³⁺ phosphor. Despite its success, this combination suffers from limitations such as a high correlated color temperature (CCT), low color rendering index (CRI), and undesirable blue light leakage, potentially causing photobiological risks^6,7,8. An alternative strategy involves ultraviolet (UV) excitation of RGB (red-green-blue) phosphors to achieve better color balance. However, this method often leads to light reabsorption, spectral mismatch, and complex fabrication processes due to multilayered coatings^8,9. To overcome these drawbacks, researchers have focused on single-component phosphors capable of generating white light under near-UV or blue excitation. This approach offers advantages such as simpler device design, better thermal stability, and reduced fabrication costs^10,11,12. In this context, rare-earth (RE³⁺)-doped inorganic phosphors are extensively studied for their sharp line emissions, tunable photoluminescence, and excellent thermal and chemical stability.

Among them, Dy³⁺, Tb³⁺, and Eu³⁺ ions are ideal candidates due to their complementary emissions in the blue, green, yellow, and red regions, enabling full-spectrum white light emission when appropriately combined^13,14. Phosphate-based host materials such as Na₂Ca₄(PO₄)₃F have attracted increasing attention due to their excellent thermal stability, strong absorption in the UV region, high chemical resistance, and ability to accommodate multiple rare-earth dopants^13,15,16,17. Furthermore, fluoride incorporation into the phosphate matrix reduces the phonon energy, which suppresses non-radiative losses and enhances luminescence efficiency¹⁶.

Considering the limited studies on Na₂Ca₄(PO₄)₃F as a host material, the present investigation focuses on the synthesis and characterization of Dy³⁺, Tb³⁺, and Eu³⁺ doped, co-doped, and triple-doped phosphor using a conventional solid-state diffusion method. The primary objective is to explore the luminescence behavior and energy transfer mechanisms among these rare earth ions to achieve tunable and efficient white light emission. Structural, morphological, vibrational, and thermal analyses are carried out to understand the material suitability for lighting applications. Furthermore, to assess its potential in photovoltaic applications, the phosphor was coated on silicon solar cells, and the resulting device performance was evaluated. This dual-functional approach aims to demonstrate the viability of Na₂Ca₄(PO₄)₃F: Dy³⁺/Tb³⁺/Eu³⁺ as a promising single-phase material for both next-generation WLEDs and enhanced solar energy conversion.

Even though there has been a lot of progress over the years, but still required highly effective and reasonably priced solar converters¹⁸. The world population on the rise, the energy demand has also increased, leading to growing concern about the impact of greenhouse gas emission on climate change¹⁹. There is strong motivation among researchers across the globe to explore alternative and sustainable methods of energy production. So here, numerous scientists and researchers published research papers on enhancement of solar efficiency of down-converting inorganic phosphor by dye-sensitized solar cells (DSSCs). Nannan Yao and co-authors et al. presented on the ZnO: Eu³⁺, Dy³⁺ down-conversion synthesized by precipitation method and used to prepare the photo anode of dye-sensitized solar cells²⁰. Xiuting Luo et al. explored on Aerosol synthesis and luminescent behavior of CaAl₂O₄:Eu²⁺, Nd³⁺ down-conversion material for enhanced light harvesting of dye sensitized solar cells¹⁹. Toshi S. Dhapodkar and other et al. reported on Dy³⁺, Tb³⁺, Eu³⁺ activated/co-activated/triple-activated Mg₂₁Ca₄Na₄(PO₄)₁₈ prepared by melt quenching technique²¹. Abhijeet R. Kadam et al. studied on Energy transfer mechanism of KAlF₄:Dy³⁺, Eu³⁺ co-activated down-conversion phosphor sol gel method²². Solar energy is one of the many sources of renewable energy providing an abundant, Eco-friendly and noiseless source of power that has vast potential in meeting the growing global energy consumption demand^23,24. The utilization of dye-sensitized solar cells (DSSCs) is an effective method of generating electricity for indoor electronic devices like wireless sensors. DSSCs are known for their affordability and accessibility as well as their capacity to produced lightweight and flexible solar module^25,26.

In this work, the series of newly synthesized triple-doped Dy3+/Tb3+/Eu3 + of Na2Ca4(PO4)3 F phosphor with different concentration of dopants was successfully prepared by using traditional solid-state synthesis route. The analysis of properties and possible application of prepared phosphor in WLEDs and photovoltaics is presented below.

Experimental methods

Material preparation

To synthesize powder sample of Na₂Ca₄(PO₄)₃F phosphor dopped with Dy^3+, Tb^3+, Eu³⁺, a conventional high temperature solid state diffusion method was employed. The scheme of samples preparation is shown on Fig. 1. Table 1. Gives an account of all the precursors used for the preparation of phosphor along with their purity and their sources.

Table 1 Details of precursors used for Preparation of phosphor.

These materials were weighed according to stochiometric proportion and ground together in an agate mortal pestle until a fine mixture was obtained. the resulting homogenous mixture transferred to an alumina crucible and sintered at 800 °C for 24 h in muffle furnace under an air atmosphere. Once the product was obtained, it was furnace cooled to room temperature and ground again to obtain fine powders, which were then subjected to further characterization. Tables 2, 3 and 4 provide the stochiometric amount of the compounds employed in the synthesis of every series presented in this study. The balanced chemical equation for the solid-state synthesis of prepared material;

Na₂​CO₃​ + 4CaCO₃ ​+ 3NH₄​H₂​PO₄ ​+ NH₄​F → Na₂​Ca₄​(PO₄​)₃​F + 4NH₃ ​+ 5CO₂ ​+ 5H₂​O

Table 2 Shows the stochiometric amount of materials utilized for Na₂Ca₄(PO₄)₃F: Dy^3+, Tb^3+, Eu³⁺ activated phosphors.

Table 3 Shows the stochiometric amount of materials utilized for Na₂Ca₄(PO₄)₃F: Dy³⁺/Eu^3+, Dy³⁺/Tb^3+, Tb³⁺/Eu³⁺ activated phosphors.

Table 4 Shows the stochiometric amount of materials utilized for Na₂Ca₄(PO₄)₃F: Dy³⁺/Tb³⁺/Eu³⁺ activated phosphors.

Instrumentation used

In order to access the phase purity of prepared phosphor Na₂Ca₄(PO₄)₃F: Dy^3+, Tb^3+, Eu³⁺, powder X-ray diffraction (XRD) pattern was obtain using Cu Kα radiation (λ = 1.5405 Å) and analyzed on a RIGAKU mini flex 600. Fourier transform infrared spectroscopy (FTIR) instrument BRUKER ALPHA II was employed to determine the vibrational characteristics of the sample. Photoluminescence (PL) measurement including excitation and emission spectra as well as lifetime, were conducted using SHIMADZU spectroflurophotometer RF 5301. The morphological properties of the Na₂Ca₄(PO₄)₃F: Dy^3+, Tb^3+, Eu³⁺ phosphor was analyzed using scanning electron microscope (SEM) EVO18 ZEISS. Additionally, the light and color coordinate parameter were determined by using Commission de I Eclairage (CIE) chromaticity coordinate diagram. The efficiency of the solar cell was evaluated by comparing the I-V characteristics of the coated cell to that of a blank cell with the help of JETSPIN LED SOLAR SIMULATOR.

Fig. 1

Flowchart for the synthesis of Dy³⁺, Tb³⁺, Eu³⁺-activated Na₂Ca₄(PO₄)₃F phosphor.

Results and discussion

Phase and structural identification

The phase purity and structural identification of newly synthesized triple doped Na₂Ca₄(PO₄)₃F: xDy³⁺/Tb³⁺/Eu³⁺ (x = 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0 mol %) phosphor by high temperature solid state method at 800⁰C were assessed by using XRD. The XRD spectra of obtained sample recorded in the range of 20^o – 80^o with a step size of 0.02^o. The XRD pattern of undoped and doped Na₂Ca₄(PO₄)₃F: xDy³⁺/Tb³⁺/Eu³⁺ phosphor with different concentration displayed in Fig. 2. The XRD pattern of the synthesized phosphor shows prominent diffraction peaks at 2θ = 25.8°, 31.7°, 32.9°, 34.7°, 39.6°, 46.5° and 49.3°, which can be assigned to the (0 0 12), (1 1 9), (0 2 7), (2 0 8), (2 1 4), (0 0 21), and (0 3 9) planes respectively. Here, all the positions and diffraction peaks of proposed sample are well treated with standard Inorganic Crystal Structure Database (ICSD) data with reference no. is 00-033-1228. The obtained material is completely crystalline nature in a Rhombohedral system with the space group R-3 m, with no other phases or allotropic phases will observe which indicating a pure crystalline substance was achieved. The intense diffraction peaks in synthesized material as made have a high crystalline character which is good for producing outstanding WLEDs.

Fig. 2

XRD patterns of prepared phosphors.

Functional group examination

Figure 3. depicted the FTIR spectrum of the phosphor sintered at 800^oC by using solid state diffusion method with measurement within the range of (3500 − 500 Cm⁻¹) at room temperature. The spectrum provided valuable insights into the actual formation and identification of functional groups present in the prepared phosphor^27,28,29. The FTIR spectrum shows that there are five bands that will be observed at 545 cm⁻¹, 611 cm⁻¹, 1011 cm⁻¹, 1119 cm⁻¹ and 1428 cm⁻¹. The band in the range between 545 cm⁻¹ to 615 cm⁻¹ is due to the ν₃ asymmetric stretching mode of PO₄³⁻ groups. The shoulder between 1000 cm⁻¹ to 1200 cm⁻¹ is assigned to the symmetric stretching mode v1 (P-O-P) of (PO₄)₃ groups³⁰. The absorption peak at 1428 cm⁻¹ is assigned to the asymmetric stretching mode (ν₃) of residual carbonate (CO₃²⁻) groups, which likely originated from incomplete decomposition of carbonate-based precursors such as CaCO₃ or Na₂CO₃ during the high-temperature solid-state synthesis process. Here there is no OH molecule peak in the proposed material was not observed, likely due to the preparation method involving high-temperature solid-state reaction^16,17.

Fig. 3

FTIR spectrum of undoped sample and Na₂Ca₄(PO₄)₃F: xDy³⁺/Tb³⁺/Eu³⁺ (x = 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0 mol %) phosphor.

Photoluminescence study (PL)

Photoluminescence of single doping in phosphor

Na ₂ Ca ₄ (PO ₄ ) ₃ F: Dy ³⁺ phosphor

Figure 4(a). Displays the PL excitation spectrum of the Na₂Ca₄(PO₄)₃F: Dy³⁺ phosphor with emission wavelength 482 nm. The dominant excitation peak of the sample comes at 351 nm followed by a secondary peak at 365 nm. These peaks correspond to electron transition within Dy³⁺, specifically ⁶H_15/2-⁶P_7/2 and ⁶H_15/2-⁶P_5/2 respectively^31,32,33. Additionally, Dy³⁺ exhibit characteristic excitation peaks at 325 nm, 388 nm and 428 nm corresponding to the electron transitions ⁶H_15/2 - ⁴K_15/2, ⁶H_15/2− ⁴M_21/2 and ⁶H_15/2−⁴G_11/2. Figure 4(b-d) illustrates the PL emission spectra of Na₂Ca₄(PO₄)₃F: xDy³⁺ phosphor (x = 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0 mol %) at 351 (Fig. 4.b), 365 (Fig. 4.c) and 388 nm (Fig. 4.d) between the 450–600 nm range. Here, there are two prominent emission peaks observed. One at 482 nm in the blue band and another at 574 nm in the yellow band. These peaks correspond to the electron transition ⁴F_9/2 - ⁶H_15/2 and ⁴F_9/2 - ⁶H_13/2 respectively^34,35,36.

Fig. 4

(a) Shows PL excitation spectra of synthesized Na₂Ca₄(PO₄)₃F: Dy³⁺ phosphor with different varying concentration located at emission 482 nm. Figure 4. (b-d) The PL emission spectra of synthesized Na₂Ca₄(PO₄)₃F: Dy³⁺ phosphor with different varying concentration located at excitation b) 351 nm, c) 365 nm, and d) 388 nm.

Na ₂ Ca ₄ (PO ₄ ) ₃ F: Tb ³⁺ phosphor

The Tb³⁺ ion doping in Na₂Ca₄(PO₄)₃F fluorophosphate phosphor material has been used in a similar PL study. Figure 5. Displays the excitation and emission spectra at 545 nm (Fig. 5.a) and 378 nm (Fig. 5.b) of the Na₂Ca₄(PO₄)₃F: Tb³⁺ phosphors. The excitation peaks observed in the spectra are attributed to f-d and f-f transition of the Tb³⁺ ions from the ground state to higher excited state. Specifically, the excitation peaks are observed at wavelength 340, 369 and 378 nm. The pecks at 378 nm originate from lower ⁷F₆ transition to higher state ⁵G₆. Among these excitation peaks, the two most intense peaks occur at 369 nm and 378 nm as shown in Fig. 5.a. When the excitation wavelength is set at 378 nm (Fig. 5.b), the emission spectra of Na₂Ca₄(PO₄)₃F: Tb³⁺ in the range of 400–600 nm are recorded. In the emission spectra, all peaks corresponding to the transition from the 5D state to the 7 F state have the same position except for differences in intensity. The emission pecks at 380, 415, 438 nm correspond to the transition ⁵D₃ - ⁷F_6,5,4, while the peaks at 490, 545, 590 nm correspond to the transitions ⁵D₄ - ⁷F_6,5,4 respectively^37,38,39,40. The peak at 545 nm exhibits relatively high intensity, indicating strong emissions in the green region.

Fig. 5

(a) The PL excitation spectra of synthesized Na₂Ca₄(PO₄)₃F: Tb³⁺ phosphor with different varying concentration located at emission 545 nm. Figure 5. (b) The PL emission spectra of synthesized Na₂Ca₄(PO₄)₃F: Dy³⁺ phosphor with different varying concentration located at excitation b) 378 nm.

Na ₂ Ca ₄ (PO ₄ ) ₃ F: Eu ³⁺ phosphor

Figure 6. (a) Represents the PL excitation spectrum of Na₂Ca₄(PO₄)₃F: Eu³⁺ with monitoring at 616 nm. The excitation spectrum can be divided into two sections. Firstly, there is a wide range of wavelengths from 200 to 300 nm, forming a broad band. This band is attributed to the charge transfer band (CTB) from O²⁻ to Eu^{3+41,42,43,44}. Secondly, there are distinct excitation peaks ranging from 300 to 600 nm. These sharp peaks are associated with the forbidden intra − 4f transitions of Eu³⁺. Specifically, the peaks occur at 363 nm (⁷F₀→⁵D₄), 383 nm (⁷F₀→⁵G₃), 395 nm (⁷F₀→⁵L₆), 416 nm (⁷F₀→⁵D₃), and 465 nm (⁷F₀→⁵D₂)^45,46. Among these transitions, the ⁷F₀→⁵L₆ transition at 395 nm is the most pronounced that spectra indicating that the Na₂Ca₄(PO₄)₃F: Eu³⁺ compound holds promise as a phosphor for near-ultraviolet (n-UV) based LEDs. The PL emission spectra Na₂Ca₄(PO₄)₃F: Eu³⁺ phosphor located at different excitation wavelengths 246 nm, 395 nm and 466 nm presented in Fig. 6(b-d). The emission spectra exhibit distinct peaks ranging from 570 to 650 nm, which can be attributed to the intra - configurational f-f transition of Eu³⁺. The PL spectrum primarily comprises two intense and sharp pecks at 595 and 616 nm, corresponding to the characteristics transition of Eu³⁺ ions due to ⁵D₀ - ⁷F_J (J = 1,2) respectively^{47,48,49,50,51,52}. The most prominent emission peak at 616 nm corresponds to the ⁵D₀ - ⁷F₂ transition.

Fig. 6

(a) The PL excitation spectra of synthesized Na₂Ca₄(PO₄)₃F: Tb³⁺ phosphor with different varying concentration located at emission 616 nm. Figure 6(b-d). The PL emission spectra of synthesized Na₂Ca₄(PO₄)₃F: Eu³⁺ phosphor with different varying concentration located at excitation b) 246 nm, c) 395 nm, and d) 466 nm.

Basically, the concentration quenching mechanism is related with non-radiative energy transfer and this phenomenon completely depends on the critical transfer distance (R_C) among Dy³⁺, Tb³⁺ and Eu³⁺ ions. So here, the photoluminescence emission intensity of rare earth ions goes on increasing with respect to the enhancing the concentration up to 2.5 mol % Dy³⁺, Tb³⁺ and Eu³⁺ after that emission intensity is decreasing because of concentration quenching mechanism displayed in Fig. 7(a-c). Blasse formula is used to calculate the Critical transfer distance (R_C) as below^53,54,55:

$$\:{R}_{C} = 2 {\left(\frac{3V}{4\prod\:{X}_{C}N}\right)}^{1/3}$$

(1)

Where, V denotes unit cell of volume, Xc represents the highest concentration of Dy³⁺, Tb³⁺ and Eu³⁺ ions and N is the number of available sites for the activator in the unit cell. According to standard ICSD file, the unit cell volume is equal to 1787.64, N = 6 and the value is Xc = 2.5 mol % Dy³⁺, Tb³⁺ and Eu³⁺ ions. putting all these parameter and constant values in above equation then we got the Rc value is 28.344 Å which value is greater than 5. Therefore, we conclude that multipole interactions are responsible for the concentration quenching in non-radiative transition between Na₂Ca₄(PO₄)₃F and rare-earth ions.

To identify the specific type of multipolar interaction, Förster–Dexter theory was employed and expressed as;

$$\:{{\frac{I}{x}=k[1+\:.x}^{\frac{}{3}}\:]}^{-1}$$

(2)

taking the logarithm of both sides:

$$\:{log}\frac{1}{x}=c-\frac{\theta\:}{3}{log}x$$

(3)

Where, I is the emission intensity, X is the activator ion concentration, K and β are constants, and θ is an empirical parameter that characterizes the nature of the interaction including exchange interaction, dipole-dipole (d-d), dipole-quadrupole (d-q) and quadrupole-quadrupole (q-q) interactions for θ = 3, 6, 8 and 10 respectively. This linear fit graph is plot between log (X) Vs log (I/X) yields a straight line with a slope of -θ/3 as shown in Fig. 8. (a-c).

The slope of this fit is −0.7887, −0.9708 and − 0.1.1218. The calculated slope from the experimental data corresponds to θ = 2.366, 2.912 and 3.365, indicating that the exchange interaction is the dominant mechanism responsible for concentration quenching in the Dy³⁺/Tb³⁺/Eu³⁺ doped Na₂Ca₄(PO₄)₃F phosphors.

Fig. 7

(a-c) The concentration quenching plot of singly doped Dy³⁺, Tb³⁺ and Eu³⁺ synthesized Na₂Ca₄(PO₄)₃F: Eu³⁺ phosphor with different varying concentration.

Fig. 8

(a-c) The linear fit plot of singly doped Dy³⁺, Tb³⁺ and Eu³⁺ synthesized Na₂Ca₄(PO₄)₃F: Eu³⁺ phosphor.

Optical behavior based on co-doping and triple doping

Na ₂ Ca ₄ (PO ₄ ) ₃ F: Dy ³⁺ /Tb ³⁺ co-activated phosphor

Examining the criteria of spectrum overlapping is crucial to determining the requirements for energy transfer for rare earth ions⁵⁶ as shown in Fig. 9. There are three types of overlapping for energy transfer mechanism namely as excitation-excitation overlap, emission-emission overlap and excitation-emission overlap¹¹. The PL excitation spectrum of Na₂Ca₄(PO₄)₃F: 2.5 mol % of Dy³⁺, xTb³⁺ (x = 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0 mol %) co-doped synthesized phosphor monitored at emission wavelength 545 nm displayed in Fig. 10. (a). From this excitation spectra show, there are three intense peaks that will be observed at 351 nm, 365 nm and 378 nm, respectively. Figure 10. (b-d) Shows the PL emission spectra of Dy³⁺/Tb³⁺ co-activated Na₂Ca₄(PO₄)₃F with different varying concentration excited by 351 nm, 365 nm and 378 nm in the range from 400 to 600 nm. From this emission spectrum shows several prominent peaks of Dy³⁺ centered at 490 nm and 576 nm corresponding to the ⁴F_9/2 → ⁶H_15/2, ⁴F_9/2 → ⁶H_13/2 respectively and 545 nm spectra due to ⁵D₄→⁷F₅ transition of Tb³⁺ ions. It is evident that when the concentration of Dy³⁺ ions increases, the PL intensity of Tb³⁺ ions continues to decrease. These results indicate that the emission in these spectra changes from blue/yellow to green. As a result, energy is transferred from Dy³⁺ to Tb³⁺.

Fig. 9

Spectral overlaps between Dy³⁺, Tb³⁺, and Eu³⁺ for energy transfer criterion.

Fig. 10

(a) The PL excitation spectra of synthesized Na₂Ca₄(PO₄)₃F: Dy³⁺/Tb³⁺ phosphor with different varying concentration located at emission 545 nm. Figure 10. (b-d) The PL emission spectra of synthesized Na₂Ca₄(PO₄)₃F: Dy³⁺/Tb³⁺ phosphor with different varying concentration located at excitation b) 351 nm, c) 365 nm, d) 378 nm.

Na ₂ Ca ₄ (PO ₄ ) ₃ F: Dy ³⁺ /Eu ³⁺ co-activated phosphor

The synthesized co-activated phosphor material has been investigated by keeping one lanthanide ion constant with highest intensity and other lanthanide ions is different concentration. So, in this scenario excitation overlapping is possible by Dy³⁺ excitation and Eu³⁺ excitation in proposed synthesized material. The PL excitation band of 2.5 mol % of Dy³⁺, xEu³⁺ (x = 0.5, 1.0, 1.5,2.0,2.5 and 3.0 mol %) co-doped Na₂Ca₄(PO₄)₃F phosphor monitoring at 616 nm depicted in Fig. 11. (a). In this scenario, 395 nm excitation wavelength is most prominent intense peaks as compared to other spectra like 465 nm, 535 nm and 590 nm respectively. So here we plot the emission spectrum at excitation wavelength 395 nm. Figure 11. (b) illustrate the PL emission spectra of Na₂Ca₄(PO₄)₃F: 2.5 mol % Dy³⁺, x mol % Eu³⁺ (x = 0.5, 1.0, 1.5,2.0,2.5 and 3.0 mol %) material located at excitation wavelength 395 nm. From this emission spectra, there are four prominent peaks of Dy³⁺ and Eu³⁺ will be observed here at 482 nm and 576 nm respectively which corresponding to ⁴F_9/2 → ⁶H_15/2 and ⁴F_9/2 → ⁶H_13/2 transitions of Dy³⁺ while the peak located at 590 and 616 nm attributed to ⁵D₀ – ⁷F₁ and ⁵D₀ –⁷F₂ transition of Eu³⁺ ions. It can be clearly seen that the photoluminescence intensity of Dy³⁺ ions goes on diminishing with respect to the enhancing concentration of Eu³⁺ ions. These findings show that these spectra shift from blue/yellow to orange/red emission. This results in the transfer of energy from Dy³⁺ to Eu³⁺.

Fig. 11

(a) PL excitation spectra of synthesized Na₂Ca₄(PO₄)₃F: Dy³⁺/Eu³⁺ phosphor with different varying concentration located at emission 616 nm. Figure 11. (b) PL emission spectra of synthesized Na₂Ca₄(PO₄)₃F: Dy³⁺/Eu³⁺ phosphor with different varying concentration located at excitation b) 395 nm.

Na ₂ Ca ₄ (PO ₄ ) ₃ F: Tb ³⁺ /Eu ³⁺ co-activated phosphor

Figure 12. Displayed the PL emission spectra of synthesized Na₂Ca₄(PO₄)₃F: 2.5 mol % Tb³⁺, x mol % Eu³⁺ (x = 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0 mol %) phosphor excited by 378 nm. So therefore, under this situation, excitation overlapping between Tb³⁺ and Eu³⁺ excitation in the suggested proposed material is achievable. From this spectrum we observe that two intense peaks will observe at 490 and 545 nm corresponding to the ⁵D₄-⁷F_6,5 of Tb³⁺ rare earth ions and another two peaks will identify in the same emission spectra located at 590 and 612 nm due to ⁵D₀ – ⁷F₁ and ⁵D₀ –⁷F₂ of Eu³⁺ ions. From Fig. 12, The concentration of Eu³⁺ ions rises, the emission of Eu³⁺ increases while the PL emission intensity of Tb³⁺ decreases. This mechanism shows the complete energy transfer from Tb³⁺ to Eu³⁺.

Fig. 12

PL emission spectra of synthesized Na₂Ca₄(PO₄)₃F: Tb³⁺/Eu³⁺ phosphor with different varying concentration located at excitation 378 nm.

Na ₂ Ca ₄ (PO ₄ ) ₃ F: Dy ³⁺ /Tb ³⁺ /Eu ³⁺ triple-doped phosphor

The energy transfer in lanthanide doped co-activated has been analyzed for Dy³⁺/Tb³⁺, Dy³⁺/Eu³⁺, and Tb³⁺/Eu³⁺ co-doped synthesized phosphor as previously discussed in detailed above. The study found that energy was transferred from Dy³⁺ to Tb³⁺, Eu³⁺, and Tb³⁺ to Eu³⁺. To achieve perfect white light emission, triple-doping energy transfer was done from Dy³⁺ to Tb³⁺ to Eu³⁺. In this present scenario, the highest concentration of Dy³⁺ and Tb³⁺ were held constant, whereas the concentration of Eu³⁺ changed. Figure 13. (a) displayed the PL excitation spectra of doped with Dy³⁺/Tb³⁺/Eu³⁺ doped Na₂Ca₄(PO₄)₃F synthesized material with different concentration under emission wavelength 616 nm. From this PL excitation spectra, there are number intense and prominent excitation peaks will be observed around at 350 nm, 365 nm, 370 nm, 382 nm, 395 nm, 420 nm, 465 nm, 527 nm and 535 nm respectively of Dy³⁺/Tb³⁺/Eu³⁺ rare earth ions. Figure 13. (b-d) shows the PL emission spectra synthesized Na₂Ca₄(PO₄)₃F: Dy³⁺/Tb³⁺/Eu³⁺ phosphor monitored at different excitation wavelength under 365 nm, 382 nm and 395 nm. From emission plot we observe that there is multiple emission peaks will be showing around at 482 nm (blue spectra) due to ⁴F_9/2→ ⁶H_15/2 transition of Dy³⁺ ions, 545 nm (green spectra) due to ⁵D₄ →⁷F₅ transition of Tb³⁺ ions, 575 nm (yellow spectra) due to ⁴F_9/2→⁶H_13/2 transition of Dy³⁺ ions, Small peaks will be observe at 595 nm (orange spectra) and 616 nm (red spectra) are caused by the ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ transitions of Eu³⁺ ions, respectively. Figure 14. PL emission spectra of synthesized Na₂Ca₄(PO₄)₃F: Dy³⁺/Tb³⁺/Eu³⁺ phosphor of optimum concentration. So, the suggested triple doped synthesized Na₂Ca₄(PO₄)₃F: Tb³⁺/Eu³⁺ phosphor emits full white light when all these photoluminescence emissions spectra are combined into a single spectrum. Based on the above presented research, the energy transfer mechanism was proposed and it is displayed on Fig. 15. it is explained with the help of works energy level diagram.

Fig. 13

(a) PL excitation spectra of synthesized Na₂Ca₄(PO₄)₃F: Dy³⁺/Tb³⁺/Eu³⁺ phosphor with different varying concentration located at emission 616 nm. Figure 13. (b-d) PL emission spectra of synthesized Na₂Ca₄(PO₄)₃F: Dy³⁺/Tb³⁺/Eu³⁺ phosphor with different varying concentration located at excitation (a) 365 nm, (b) 382 nm, (c) 395 nm.

Fig. 14

PL emission spectra of synthesized Na₂Ca₄(PO₄)₃F: Dy³⁺/Tb³⁺/Eu³⁺ phosphor of optimum concentration.

Fig. 15

Energy level diagram for Dy³⁺, Tb³⁺, and Eu³⁺ triple activated Na₂Ca₄(PO₄)₃F phosphor energy transfer phenomenon.

Morphological features and energy dispersive x-ray spectroscopy

SEM is a widely employed method for characterizing phosphor material. In this study, both the SEM image and energy dispersive x-ray spectroscopy (EDS) pattern were utilized to examine the morphological properties and compositions of the synthesized sample¹⁰. Figures 15 and 18(a-d). Represents the SEM image of Na₂Ca₄(PO₄)₃F and Na₂Ca₄(PO₄)₃F: Dy³⁺/Tb³⁺/Eu³⁺ sample at room temperature. Clearly, the samples are made up of anomalous microparticles with size varies from 2 to 50 μm. The microparticles are composed of aggregates of small and irregular shaped particles which look like stacking platelet-like particles. Further, the irregular and granular layered structure indicates the crystallographic nature of the powder^57,58,59. Figure 17. Displays the energy dispersive X-ray spectroscopy (EDS) spectrum and elemental mapping Na₂Ca₄(PO₄)₃F phosphor, which indicates the purpose of confirming the composition of the prepared phosphor. As we can see that, the presence of distinct and intense peaks corresponding to Na, Ca, P, O and F in the spectra validated the successful synthesis of Na₂Ca₄(PO₄)₃F prepared phosphor. Figure 19. Show the EDS spectrum and elemental mapping of Na₂Ca₄(PO₄)₃F: Dy³⁺/Tb³⁺/Eu³⁺ phosphor. From this figure, The EDAX and elemental mapping image of the triple-doped sample reveals a uniform and homogeneous distribution of all constituent elements across the analyzed region. The clear and consistent signals from the dopant elements confirm their successful incorporation into the host matrix. This uniform dispersion supports the formation of a single-phase material and indicates effective synthesis, which is essential for achieving consistent optical and structural properties in the doped phosphor system.

Fig. 16

SEM image spectrum of Na₂Ca₄(PO₄)₃F phosphor.

Fig. 17

EDS spectrum and elemental mapping of Na₂Ca₄(PO₄)₃F phosphor.

Fig. 18

SEM image spectrum of Na₂Ca₄(PO₄)₃F: Dy³⁺/Tb³⁺/Eu³⁺ phosphor.

Fig. 19

EDS spectrum and elemental mapping of Na₂Ca₄(PO₄)₃F: Dy³⁺/Tb³⁺/Eu³⁺ phosphor.

TGA-DTA analysis

Thermogravimetric and Differential thermal analysis (TGA-DTA) instrument used to evaluate the thermal stability⁶⁰ of obtained Na₂Ca₄(PO₄)₃F sample. Figure 20. depict the TGA-DTA plot of proposed Na₂Ca₄(PO₄)₃F phosphor measured from room ambient temperature up to 500 °C. The TGA curve exhibited minimum weight loss up to approximately 250 °C, indicating the sample stability within this range. A slight weight loss observed beyond this temperature is likely due to the decomposition of residual impurities or structural changes within the material. The DTA curve showed an endothermic/exothermic peak at 200 °C, suggesting a phase transition or thermal decomposition event. Overall, the data confirm that Na₂Ca₄(PO₄)₃F possesses good thermal stability making it suitable for high-temperature applications.

Fig. 20

The TGA-DTA plot of Na₂Ca₄(PO₄)₃F phosphor.

Photo chromaticity analysis

The CIE chromaticity coordinates play a crucial role in evaluating the luminous properties of synthesized material^61,62. These color coordinates are determined using the chromaticity coordinate calculation method based on the CIE 1931 system^63,64. Specifically, through color calculator software. Figure 21, depict CIE chromaticity diagram from Na₂Ca₄(PO₄)₃F: 2.5 mol % Dy³⁺- 2.5 mol % Tb³⁺- x mol % Eu³⁺ (x = 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0 mol %) phosphors. The measured CIE chromaticity coordinates of Na₂Ca₄(PO₄)₃F: Dy³⁺/Tb³⁺/Eu³⁺ phosphor obtain with an excitation wavelength of 395 nm are presented in Table 5. Interestingly the CIE values of Na₂Ca₄(PO₄)₃F: Dy³⁺/Tb³⁺/Eu³⁺ phosphor are very close to the commercially available phosphor Y₂O₃:Eu³⁺ (0.61,0.39). Consequently, when excited by UV light, the Na₂Ca₄(PO₄)₃F: Dy³⁺/Tb³⁺/Eu³⁺ phosphor emits a white color making suitable for color tuning in display devices and solid-state lighting applications. The correlated color temperature (CCT) is used to analyze the quality of emitted light of material which can be calculated by using McCamy empirical equation as below^65,66,

$${\rm CCT= 437n^3 + 3601n^2 + 6861n+5517}$$

(4)

where, the value of n denoted as a slope given by (x-x_e)/(y_e-y), where the epicenter of point x_e and y_e are 0.332 and 0.186 respectively. The color purity of suggested triple doped phosphor material has been assessed by using given formula^67,68,

$$\:Colorpurity=\frac{\sqrt{{\left(X-{X}_{i}\right)}^{2}+{\left(Y-{Y}_{i}\right)}^{2}}}{\sqrt{{\left({X}_{d}-{X}_{i}\right)}^{2}+{\left({Y}_{d}-{Y}_{i}\right)}^{2}}}*100\:\%$$

(5)

Where, the X and Y denote CIE co-ordinates, X_d and Y_d denotes dominant wavelength of X and Y co-ordinates and X_i, Y_i denotes the coordinates of the illuminate point. The calculation of X, Y, X_d, Y_d, color purity and CCT for varying concentration of Dy³⁺/Tb³⁺/Eu³⁺ in the synthesized phosphor are shown in Table 5. The obtained CCT values indicate that prepared phosphors could be beneficial for warm light applications since the standard CCT value for such application is below 5000 K. The prepared phosphor Na₂Ca₄(PO₄)₃F: 2.5 mol % Dy³⁺/2.5 mol % Tb³⁺/0.5 mol % Eu³⁺ phosphor exhibited the highest CCT value is 4926. The corresponding chromaticity coordinates were found to be x = 0.3433 and y = 0.3097. So therefore, this proposed synthesized material of Na₂Ca₄(PO₄)₃F: Dy³⁺/Tb³⁺/Eu³⁺ phosphor which is potential application in the various domain like solid - state lighting and white light emitting diodes applications.

Fig. 21

Represent CIE chromaticity diagram from Na₂Ca₄(PO₄)₃F: 2.5 mol % Dy³⁺- 2.5 mol % Tb³⁺- x mol % Eu³⁺ (x = 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0 mol %) phosphors.

Table 5 The complete overview of photochromic analysis of triple doped Na₂Ca₄(PO₄)₃F: 2.5 mol % Dy³⁺- 2.5 mol % Tb³⁺- x mol % Eu³⁺ (x = 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0 mol %) prepared phosphors.

Solar cell efficiency enhancement of synthesized phosphor

The Doctor Blade technique is very important to improve the efficiency of solar cells by allowing for the creation of homogenous and regulated thin coatings of inorganic samples. This technique ensures accurate thickness control, resulting in improved light absorption and lower recombination losses. This method is especially useful for inorganic phosphor material, perovskite solar cells, organic photovoltaics and quantum dot solar cells. Furthermore, this approach is inexpensive, scalable and works with a variety of materials. Initially, the current (I) and voltage (V) properties of blank solar cell were calculated on LED solar simulator maintain at temperature 40^OC. The synthesized triple doped Na₂Ca₄(PO₄)₃F: Dy³⁺/Tb³⁺/Eu³⁺ phosphors were prepared by using solid state method at 800^OC. This synthesized sample crushed finely at mortar pestle for minutes and then crushed fine powder coated on a silicon solar cell by using doctor blade technique. After that the coated silicon cell is dried for 10 h. at an ambient temperature and again, we can calculate I-V characteristics of coated solar cell with the help of JETSPIN solar simulator. Using basic laboratory instruments, like an ammeter and voltmeter, the current and voltage of the solar cell were measured by connecting wires in accordance with the block diagram shown in Fig. 22. The blank and coated solar cells is placed in the LED solar simulator at 40 °C. Variable resistance was used to adjust the output voltage of the solar cell, and current was measured accordingly. The efficiency of solar cell was calculated by using given formula as follows²¹,

$$Efficiency \% = \frac{Slope\; of\: coated\; cell}{slope\: of\: without\: coated\: cell }*100\%$$

(6)

So therefore, we can calculate the solar cell efficiency of the cell and observe that the efficiency of solar cell has increased by 16.33% than the blank solar cell. The current (I) and voltage (V) behavior of the blank and coated solar cell displayed on Fig. 23. The relationship between voltage (v) Vs current density (mA/cm²) of Na₂Ca₄(PO₄)₃F: Dy³⁺/Tb³⁺/Eu³⁺ prepared phosphors as shown on Fig. 24.

Fig. 22

Schematic representation for block diagram of solar cell efficiency setup.

Fig. 23

The plot between voltage (v) Current (mA) of Na₂Ca₄(PO₄)₃F: Dy³⁺/Tb³⁺/Eu³⁺ prepared phosphors.

Fig. 24

The relationship between voltage (v) Vs current density (mA/cm²) of Na₂Ca₄(PO₄)₃F: Dy³⁺/Tb³⁺/Eu³⁺ prepared phosphors.

Solar spectrum

The down converting phosphor doped with Dy³⁺/Tb³⁺/Eu³⁺ of Na₂Ca₄(PO₄)₃F triple activated prepared sample function as downshifting materials that can absorb high-energy photons and transmit these lower-energy photons depicted on Fig. 25. Maximum energy is lost due to thermalization when solar energy is transformed into electrical energy. Coating a downshifting layer of material on the silicon solar cell can stop these losses by preventing thermalization losses and acting as a down-conversion material that smoothly transforms high-energy photons into lower-energy for the creation of electron-hole pairs. Therefore, proposed study concluded that Na₂Ca₄(PO₄)₃F: Dy³⁺/Tb³⁺/Eu³⁺ triple activated phosphors are the potential candidate for the upcoming generation solar cells.

Fig. 25

PL excitation and emission spectra Na₂Ca₄(PO₄)₃F: Dy³⁺/Tb³⁺/Eu³⁺ triple activated phosphors used in down conversion compared with the solar spectrum.

Conclusion

In summary, Dy³⁺, Tb³⁺, and Eu³⁺ activated Na₂Ca₄(PO₄)₃F phosphors were successfully synthesized via a conventional solid-state method. Structural analysis confirmed the formation of a pure rhombohedral phase with good crystallinity. The prepared phosphors exhibited multi-color emissions covering blue, green, yellow, and red regions resulting in tunable white light output suitable for WLEDs applications. Additionally, the application of the phosphor layer on silicon solar cells enhanced the photovoltaic efficiency by 16.33%, demonstrating its potential as a down-conversion material for solar energy conversion.

For future work, optimization of dopant concentrations, particle morphology, and coating techniques is recommended to further enhance the optical performance and solar conversion efficiency. Exploring alternative synthesis routes and integrating these phosphors with advanced solar cell could provide promising pathways for next-generation lighting and photovoltaic applications. Table 6 gives coparative study of recent rare-earth-doped phosphors for WLEDs and solar cell applications (2019–2024).

Table 6 Comparison of recent rare-earth-doped phosphors for WLEDs and solar cell applications (2019–2024).