Ca₃La₂Te₂O₁₂:Mn⁴⁺,Nd³⁺,Yb³⁺: an efficient thermally-stable UV/visible–far red/NIR broadband spectral converter for c-Si solar cells and plant-growth LEDs

Abstract

A series of novel Mn⁴⁺,Nd³⁺,Yb³⁺-doped Ca₃La₂Te₂O₁₂ (CLTO) phosphors were prepared by substituting Te⁶⁺ for W⁶⁺ in the Ca₃La₂W₂O₁₂ compound using a high-temperature solid-state reaction method. A Mn⁴⁺ singly-doped CLTO phosphor (CLTO:0.004Mn⁴⁺) showed a bright deep red-light emission corresponding to a narrow band around 707 nm (14 144 cm⁻¹) due to a Mn^{4+ 2}E_g → ⁴A_2g spin-forbidden transition upon 365 nm UV excitation, which largely overlapped the absorption spectrum of plant phytochrome P_fr. The excitation spectrum (λ_em = 707 nm) presented a broad band ranging from 250 nm (40 000 cm⁻¹) to 600 nm (16 667 cm⁻¹), which can be decomposed into four Gaussian bands peaking at 318 nm (31 431 cm⁻¹), 355 nm (28 148 cm⁻¹), 410 nm (24 385 cm⁻¹), and 476 nm (20 992 cm⁻¹), corresponding to a Mn⁴⁺–O²⁻ charge transfer (CT) transition, and Mn⁴⁺ transitions ⁴T_1g ← ⁴A_2g, ²T_2g ← ⁴A_2g and ⁴T_2g ← ⁴A_2g, respectively. Moreover, three kinds of Mn⁴⁺ emission sites were analyzed with the assistance of time-resolved spectra. With the single incorporation of Nd³⁺/Yb³⁺ into CLTO:Mn⁴⁺, energy transfer phenomena from Mn⁴⁺ to Nd³⁺/Yb³⁺ ions can be observed, which proceeded via non-radiative resonant and phonon-assisted mechanisms, respectively, resulting in the broadband spectral conversion of the UV/blue to NIR light. When Nd³⁺ and Yb³⁺ were co-doped into CLTO:Mn⁴⁺ to form the tri-doped CLTO phosphors, a successive energy transfer process, Mn⁴⁺ → Nd³⁺ → Yb³⁺, was determined based on the simultaneous energy transfer processes Mn⁴⁺ → Nd³⁺ and Nd³⁺ → Yb³⁺ in the co-doped samples, further enhancing the broadband spectral conversion process of the UV/blue to NIR region, which can be absorbed by photosynthetic bacteria and show high response when applied to c-Si solar cells. More attractively, the luminescence thermal stabilities of both CLTO:0.004Mn⁴⁺ and CLTO:0.004Mn⁴⁺,0.04Nd³⁺,0.20Yb³⁺ showed excellent performance, and the temperature-dependent luminescence properties of the Mn⁴⁺,Nd³⁺,Yb³⁺ tri-doped materials have been investigated for the first time. These results indicate that this kind of phosphor can be potentially applied to improving spectral conversion efficiency for c-Si solar cells and plant-growth far-red/NIR LEDs. In addition, this report provides a strategy wherein hosts for Mn⁴⁺ doping can be well enriched by substituting Te⁶⁺ for W⁶⁺ in certain tungstate compounds, which is highly desired in searching for novel red-emitting phosphors.

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1. Introduction

Solving the energy crisis is a global issue mankind is currently facing. As one of the most clean and sustainable energies, solar energy is always harnessed by utilizing solar cell devices to convert it into electricity. At present, there are several kinds of solar cells and technologies that are well developed such as crystalline silicon (c-Si) (mono/polycrystalline silicon), non-crystalline silicon, GaAs, CdTe, perovskite, organic, and dye-sensitized solar cells.^1–8 So far, c-Si type solar cells, first-generation solar energy-conversion devices, have occupied the major market on account of their relatively mature fabrication technology and low price compared to others. However, their low energy conversion efficiencies of around 22% and 25% for industrial application and lab research, respectively, which are under the Shockley–Queisser limit of about 33.7%,⁹ still need to be improved. The main issue that limits their solar energy conversion efficiency is the spectral mismatch between the phonon energy of the incident sunlight and the band gap of c-Si semiconductor materials. This indicates that only a small portion of the solar spectrum lying at 900–1100 nm can be utilized and converted into electricity; in addition, low-energy photons cannot be absorbed and the excess energy of high-energy photons will be released in the form of heat via charge carrier thermalization or electron–hole recombination effects.^10–12 Therefore, spectral converters showing giant potential for integration with solar cells to reduce the non-absorption and thermal losses are developed. They are often used as photo-luminescent layers to absorb the photons that cannot be effectively captured by solar cells and convert them to suitable wavelengths for specific type solar cells.¹³ This strategy offers the advantages of unnecessarily modifying the solar cells’ standard structure and flexible optimization toward a specific type solar cell.

Rare earth ion doped luminescent materials converting UV/n-UV/visible light into near infrared emission around 1000 nm have attracted lots of attention because of their potential application in improving the photovoltaic efficiency in c-Si solar cells, among which Yb³⁺ is considered to be the most promising candidate for this purpose since it possesses only one excited state level, ²F_5/2, and approaches the excellent spectral response of c-Si solar cells.^14,15 As reported before, diverse ions such as Tm³⁺, Pr³⁺, Tb³⁺, Er³⁺, and Ho³⁺ were first selected as the sensitizers for Yb³⁺ ions to realize spectral conversion.^16–22 It is unsatisfactory that only a small portion of the sunlight can be converted as a result of their natural f–f transition. In order to overcome this disadvantage and obtain better conversion results, several other kinds of ions such as Ce³⁺, Eu²⁺, Yb²⁺, Bi³⁺, and Cr^3+23–28 with broadband absorptions were adopted as the effective sensitizers to realize broadband spectral conversion for Yb³⁺, mainly through the quantum cutting method. Apart from Yb³⁺, Nd³⁺ can also be considered as an alternative activated luminescent ion owing to its emission bands around 900 (⁴F_3/2 → ⁴I_9/2) and 1064 nm (⁴F_3/2 → ⁴I_11/2), which are also within the high spectral response range of 900–1100 nm for c-Si semiconductor materials.²⁹

Mn⁴⁺, a transition metal ion, is popular as an activator in a large number of compounds,^30–35 typically in oxides and fluorides, to produce red or far-red emissions around wavelengths ranging from 620 nm to 750 nm when it is located in octahedra, and often acts as a red-component supplement for white light-emitting diodes (w-LEDs) to improve their device performance. It often shows broadband absorption originating from Mn⁴⁺–O²⁻ charge transfer transition and Mn⁴⁺ d–d inner transitions ⁴T₁ ← ⁴A₂, ⁴T₁ ← ⁴A₂ and ⁴T₂ ← ⁴A₂ in the UV and visible regions, which indicates its potential for broadband spectral conversion of UV/visible to red light. It was reported before that an effective energy transfer from Mn⁴⁺ to Yb³⁺ ions took place in certain phosphors, indicating their potential applications in improving the conversion efficiency of c-Si solar cells.^36–39 Moreover, the energy transfer process from Mn⁴⁺ to Yb³⁺ ions would be a probable phonon-assisted non-resonant energy transfer process, which often needs nearly five phonons since the energy gap between the Mn^{4+ 2}E_g and Yb^{3+ 2}F_5/2 levels is quite large. In spite of the visible spectral overlap between common singly doped Nd³⁺ excitation and Mn⁴⁺ emission bands in certain systems, few reports were focused on Mn⁴⁺ → Nd³⁺ resonant energy transfer properties. In addition, rarely has attention been paid to the energy transfer from Nd³⁺ to Yb³⁺ ions via a down-shifting process, which is different from previously reported down-conversion processes⁴⁰ in some phosphor systems. Recently, the luminescence properties of Ca₃La₂W₂O₁₂:Mn⁴⁺ as a red phosphor have been investigated.⁴¹ On the basis of the similar ionic radii of Te⁶⁺ and W⁶⁺, we search for a suitable host (Ca₃La₂Te₂O₁₂) through the reasonable substitution of Te⁶⁺ for W⁶⁺ in Ca₃La₂W₂O₁₂ to be doped with Mn⁴⁺, Nd³⁺ and Yb³⁺ ions, which show improved luminescence thermal stability in Mn⁴⁺ doped systems and clear Mn⁴⁺ → Yb³⁺, Mn⁴⁺ → Nd³⁺ and Nd³⁺ → Yb³⁺ energy transfer processes in co-doped systems, in addition to a successive energy transfer process, Mn⁴⁺ → Nd³⁺ → Yb³⁺, which has rarely been investigated in Mn⁴⁺,Nd³⁺,Yb³⁺ tri-doped systems. All these energy transfer processes would effectively realize the spectral conversion from UV/visible to NIR emission in phosphors. Furthermore, blue (400–480 nm), red (600–680 nm) and far-red (FR, 680–780 nm) lights play important roles in photosynthetic, phototropic and photomorphogenetic processes,^42,43 respectively, for plant cultivation, and the near infrared (NIR, 715–1050 nm) light absorbed by photosynthetic bacteria can accelerate the synthesis of nutrients for assisting biological nitrogen fixation,⁴⁴ which can indirectly prompt plant growth. Currently, light-emitting diodes (LEDs) with the merits of high-efficiency, energy saving, non-polluting nature, and long lifetimes are considered to replace traditional incandescent and fluorescent lamps as next generation light source candidates. In particular, the spectral composition of LEDs can be flexibly controlled via the phosphor-converted technology to match well with the target emission wavelengths that different plants and photosynthetic bacteria require. Therefore, it is attractive to achieve a single-phase red to NIR emission to meet the requirements of plant growth and photosynthetic bacteria based on the Mn⁴⁺ → Yb³⁺, Mn⁴⁺ → Nd³⁺ and Mn⁴⁺ → Nd³⁺ → Yb³⁺ energy transfer processes in this kind of Ca₃La₂Te₂O₁₂:Mn⁴⁺,Nd³⁺,Yb³⁺ phosphor, which can avert the complicated combination of single color-emitting phosphors, spectral re-absorption and color instability arising from the different aging speeds of phosphors. In fact, spectral conversion materials should exhibit good spectral conversion, and physical, chemical and thermal stability properties for their real applications. Herein, the luminescence thermal stability of the as-prepared Mn⁴⁺,Nd³⁺,Yb³⁺ tri-doped CLTO phosphors, which exhibit excellent properties, has been investigated for the first time. Accordingly, this kind of phosphor would be a possible component candidate for c-Si solar cells and plant-growth LEDs. In addition, this report provides a good example to enrich hosts for Mn⁴⁺ doping with red emission by substituting Te⁶⁺ for W⁶⁺ in certain compounds.

2. Experimental

2.1 Material synthesis

Polycrystalline powders with the chemical composition formulae Ca₃La₂Te_2−xO₁₂:xMn⁴⁺ (CLTO:xMn⁴⁺, x = 0–0.02), Ca₃La_2−a/bTe_1.996O₁₂:0.004Mn⁴⁺,aNd³⁺/bYb³⁺ (CLTO:0.004Mn⁴⁺,aNd³⁺/bYb³⁺, a = 0–0.06, b = 0–0.40), Ca₃La_1.96−yTe₂O₁₂:0.04Nd³⁺,yYb³⁺ (CLTO:0.04Nd³⁺,yYb³⁺, y = 0–0.40), Ca₃La_1.8−cTe_1.996O₁₂:0.004Mn⁴⁺,0.20Yb³⁺,cNd³⁺ (CLTO:0.004Mn⁴⁺,0.20Yb³⁺,cYb³⁺, c = 0–0.0.06), Ca₃La_1.96−dTe_1.996O₁₂:0.004Mn⁴⁺,0.04Nd³⁺,dYb³⁺ (CLTO:0.004Mn⁴⁺,0.04Nd³⁺,dYb³⁺, d = 0–0.40), Ca₃La_1.8T₂O₁₂:0.20Yb³⁺ (CLTO:0.20Yb³⁺), and Ca₃La_1.96Te₂O₁₂:0.04Nd³⁺ (CLTO:0.04Nd³⁺) were prepared via a high-temperature solid-state reaction route. Raw materials CaCO₃ (A.R.), TeO₂ (99.9%), MnCO₃ (99.9%), and high purity La₂O₃ (99.99%), Yb₂O₃ (99.99%), and Nd₂O₃ (99.99%) were first weighed according to the chemical stoichiometric ratios and then thoroughly mixed and ground in an agate mortar for about 15 min with the addition of proper amount of ethanol. After that, the homogenous mixtures were dried in an oven followed by grinding for another 1 min and were transferred to ceramic crucibles one by one. They were sintered in an oven at 1100 °C for 10 h with the increasing rate of 5 °C min⁻¹. Eventually, the as-prepared naturally cooled products were ground again for the following measurements.

2.2 Characterization

The crystal structure and phase purity were investigated using powder X-ray diffraction (PXRD) performed using a Thermo Scientific ARLX’TRA diffractometer equipped with a Cu Kα (λ = 1.5405 Å) source, maintaining the scanning rate of 5° min⁻¹ with the scattering angle range (2θ) from 15° to 65°. The Fourier transform infrared (FT-IR) and Raman spectra were recorded on a FT-IR-RAMAN-DRIFT NICOLET 6700 setup. Photoluminescence (PL), quantum yield (QY) and luminescence lifetimes in the visible and infrared regions were recorded on an Edinburgh Instruments FLSP 920 UV-vis-NIR spectrofluorimeter, equipped with a 450 W continuous xenon lamp and a 60 W pulsed μF 920 xenon lamp and an integrated sphere. The setup has a Hamamatsu R928P red-sensitive photomultiplier tube (PMT) to detect the luminescence located in the 200–870 nm wavelength range, together with a liquid-nitrogen cooled (−80 °C) Hamamatsu R5509-72 PMT to record near-infrared luminescence up to 1700 nm. All the measurements above were performed at room temperature. In addition, the measurements of temperature-dependent PL spectra of the selected samples were performed on an FLS 920 spectrofluorimeter with the assistance of a temperature controller Model 336 from Lakeshore Company.

3. Results and discussion

3.1 Phase, structure, FT-IR and Raman spectra

Fig. 1a shows the PXRD patterns of the as-prepared CLTO host and Mn⁴⁺, Nd³⁺, Yb³⁺ doped CLTO samples. It is observed that all the patterns are assigned to the reported Ca₃La₂W₂O₁₂ compound (JCPDS no. 49-0965),⁴⁵ indicating that a pure phase has been obtained with the rational substitution of Te⁶⁺ for W⁶⁺ atoms in phosphors due to their similar ionic radii and valence states; moreover, the introduction of Mn⁴⁺, Nd³⁺ or Yb³⁺ dopants into the CLTO host does not induce any significant structural change. Although the crystal structures of neither the CLTO nor Ca₃La₂W₂O₁₂ compounds have been obtained up to now, it is reported that they have similar structures to the Ca₅Re₂O₁₂ compound.⁴⁶ As approximately depicted in Fig. 1b, it crystallizes in a rhombohedral system with the space group R3̄m. There are three kinds of Te⁶⁺ sites [coordination number (CN) = 6] for Mn⁴⁺ occupancy and four kinds of La³⁺ sites for Nd³⁺/Yb³⁺ (CN = 6, 8, 9) substitution based on their similar ionic radii [Mn⁴⁺ ions (CN = 6, radius (r) = 0.53 Å), Te⁶⁺ (CN = 6, r = 0.56 Å); Yb³⁺/Nd³⁺ (CN = 6, r = 0.87/0.98 Å; CN = 8, r = 0.98/1.12 Å; CN = 9, r = 1.05/1.19 Å), La³⁺ (CN = 6, r = 1.05 Å; CN = 8, r = 1.18 Å; CN = 9, r = 1.20 Å)]. As proposed, a difference in charge between Mn⁴⁺ and Te⁶⁺ can be found, which would result in oxygen vacancies according to the balanced formula, and simultaneously induce a change in the TeO₆ octahedra; this may prompt the luminescence of Mn⁴⁺. The FT-IR spectra of the representative CLTO host and Mn⁴⁺, Nd³⁺, Yb³⁺ doped CLTO samples in the range of 500–3000 cm⁻¹, depicted in Fig. 1c, show two main peaks at 673 and 704 cm⁻¹, which can be assigned to the stretching Te–O vibrations in [TeO₆]⁶⁻ groups.⁴⁷ Another observed peak at 1463 cm⁻¹ commonly originates from the vibration of the OH⁻ bonds in absorbed water from air on the surfaces of the samples. It can be found that the profiles of the FT-IR spectra for the three representatives are similar to each other, indicating little change in the structure after the incorporation of dopants Mn⁴⁺, Nd³⁺, and Yb³⁺ into the CLTO host, which is consistent with the XRD analysis result above. The Raman spectrum for the CLTO host in Fig. 1d presents several main peaks at 314, 438, 566 and 715 cm⁻¹, due to the valence vibrations of O–Te–O and Te–O bonds,⁴⁸ in which the maximum phonon energy in the crystal lattice is around 715 cm⁻¹. This may be beneficial for the luminescence thermal stability of the as-prepared Mn⁴⁺,Yb³⁺,Nd³⁺ doped materials.

Fig. 1 (a) PXRD patterns for the as-prepared representative samples of the Ca₃La₂Te₂O₁₂ host and Mn⁴⁺,Nd³⁺,Yb³⁺ doped CLTO samples, in addition to the standard reference of the Ca₃La₂W₂O₁₂ compound (JCPDS card no. 49-0965). (b) Approximate crystal structure of the Ca₃La₂Te₂O₁₂ compound and the coordination environments of the cations. (c) FT-IR spectra of the representative CLTO host and Mn⁴⁺,Nd³⁺,Yb³⁺ doped CLTO samples. (d) The Raman spectrum for the CLTO host.

3.2 PL properties of Mn⁴⁺ doped CLTO phosphors

Fig. 2a exhibits the PL excitation and emission spectra of the CLTO:0.004Mn⁴⁺ sample. It can be observed that the emission spectrum consists of a narrow band ranging from 640 to 800 nm around 707 nm (14 144 cm⁻¹) upon 365 nm excitation, corresponding to the Mn^{4+ 2}E_g → ⁴A_2g transition. The bright deep-red color under 365 nm UV lamp excitation (the inset in Fig. 2a) shows its intensive emission. The quantum yield of this representative sample is determined to be 53.6%, a relatively high one, as shown in Fig. S1 (ESI†). The excitation spectrum detected at 707 nm comprises a broad band from 250 nm (40 000 cm⁻¹) to 600 nm (16 667 cm⁻¹), which can be decomposed into four Gaussian bands peaking at 318 nm (31 431 cm⁻¹), 355 nm (28 148 cm⁻¹), 410 nm (24 385 cm⁻¹), and 476 nm (20 992 cm⁻¹), corresponding to the Mn⁴⁺–O²⁻ charge transfer transition, and Mn⁴⁺ transitions ⁴T_1g ← ⁴A_2g, ²T_2g ← ⁴A_2g and ⁴T_2g ← ⁴A_2g, respectively, as presented in Fig. 2a. With increasing doped Mn⁴⁺ content into CLTO, the intensity of the emission spectrum first increases due to more luminescent centers and then decreases owing to the concentration quenching effect in spite of their similar profiles. The optimal Mn⁴⁺ concentration is determined to be x = 0.004, as shown in the inset of Fig. 2b. The critical distance (R_c) between Mn⁴⁺ in this kind of phosphor can be approximately determined via the following formula:^49,50where X_c is the critical content of the dopant Mn⁴⁺, N is the number of available Te⁶⁺ sites in a unit cell, and V is the unit cell volume. As the ionic radii of W⁶⁺ and Te⁶⁺ are the same (0.56 Å), we propose that the substitution of Te⁶⁺ for W⁶⁺ seldom causes a change in crystal lattice parameters. As clarified above, herein, X_c = 0.004, N = 2 × 18 = 36, and V = 4569.7 ų. Therefore, the value of R_c is calculated to be 38.28 Å, indicating a little possibility of exchange interaction (R_c ≤ 5 Å) for the non-radiative energy transfer process here. Consequently, the electric multi-pole interactions would take place in this kind of phosphor, which can be determined using the following formula derived by Blasse and Van Uitert:^51,52where x, I and β refer to the activator (Mn⁴⁺) concentration beyond the critical content, integrated emission intensity and a constant for a given matrix, respectively. θ = 6, 8 and 10 refer to the electric dipole–dipole, dipole–quadrupole and quadrupole–quadrupole interaction mechanisms, respectively. In order to determine the specific θ value, the relationship between log(x) and log(I/x) is plotted and displayed in Fig. 2c, which can be fitted with a straight line, showing a slope of −1.521 = −θ/3; thus θ = 4.563 is close to 6, implying that an electric dipole–dipole interaction predominates the energy transfer process between Mn⁴⁺ ions in the CLTO:Mn⁴⁺ phosphors. The decay curves (λ_ex = 365 nm, λ_em = 707nm) of the selected samples CLTO:xMn⁴⁺ (x = 0.002, 0.004, 0.012 and 0.020) are used to further illustrate the concentration quenching in this kind of phosphor in detail, as presented in Fig. 2d, which can be fitted with a double-exponential function:⁵³

I(t) = I₀ + A₁ exp(−t/τ₁) + A₂ exp(−t/τ₂) (3)

where I(t) and I₀ are the emission intensities at times t and 0, respectively, both A₁ and A₂ are the constants, and τ₁ and τ₂ refer to the luminescence lifetimes for the quick and slow decays, respectively. As a result, the average lifetimes (τ) are determined to be 1.271, 0.961, 0.817 and 0.730 ms corresponding to x = 0.002, 0.004, 0.012 and 0.02, respectively, by calculation using the following equation:

τ₁ = (A₁τ₁² + A₂τ₂²)/(A₁τ₁ + A₂τ₂) (4)

A decreasing trend (the inset in Fig. 2d) is clearly seen for the lifetime upon increasing the Mn⁴⁺ concentration from x = 0.002 to 0.02, which is also related to the total relaxation rate expressed using the following equation:⁵⁴where τ₀ is the radiative lifetime, A_nr is the non-radiative rate attributed to multiphonon relaxation, and P_t is the energy transfer rate between Mn⁴⁺ ions. As the Mn⁴⁺ concentration is increased, the distance between Mn⁴⁺ ions becomes shorter and shorter, which implies that both the probability of energy transfer to luminescent killer sites and the energy transfer rate between Mn⁴⁺–Mn⁴⁺ ion groups increase. As a consequence, the lifetime decreases monotonically with the increase of Mn⁴⁺ content in CLTO.

Fig. 2 (a) PL excitation and emission spectra of CLTO:0.004Mn⁴⁺ and Gaussian deconvolution for the emission spectrum; the inset shows the luminescence image under 365 nm UV lamp excitation for this sample. (b) Variation of emission spectra for CLTO:xMn⁴⁺ (x = 0–0.02) samples as a function of Mn⁴⁺ concentration x; the inset shows the variation of the corresponding emission intensity. (c) Linear fitting for log(I/x) versus log x beyond the critical concentration (x ≥ 0.004). (d) Decay curves for CLTO:xMn⁴⁺ (x = 0.002–0.02) samples; the inset shows the dependence of the decay lifetime on Mn⁴⁺ concentration.

In order to distinguish three possible kinds of Mn⁴⁺ luminescent centers, the decay curves for CLTO:0.004Mn⁴⁺ (λ_ex = 365 nm) monitored at different wavelengths of 680, 705, 715 and 750 nm are shown in Fig. 3a, all of which can be fitted with a bi-exponential function mentioned above; therefore, the decay lifetimes are determined to be 1.123, 0.955, 0.974 and 0.720 ms, respectively. It can be found that the lifetime decreases first followed by a slight increase and it subsequently decreases again upon increasing the wavelength from 680 to 750 nm. This phenomenon illustrates that three kinds of emission bands tend to be located between 680 and 705 nm, 705 and 715 nm and 715 and 750 nm, respectively. Moreover, the time-resolved spectra, shown in Fig. 3b, were recorded with a delayed time from 0.2 to 1.8 ms to further show the different kinds of sites for Mn⁴⁺ in the CLTO:0.004Mn⁴⁺ phosphor. It can be seen that the profile of the emission spectrum changes a little; that is to say, it varies and the emission peak gradually shifts to a shorter wavelength with increasing delayed time. These phenomena can be reflected by the homogeneous increments of the emission intensity ratios for 680/705 nm, 680/715 nm and 680/750 nm in the inset of Fig. 3b.

Fig. 3 (a) Decay curves monitored at different emission wavelengths (λ_ex = 365 nm, λ_em = 680, 705, 715 and 750 nm) for CLTO:0.004Mn⁴⁺; the inset shows the variation of the corresponding decay lifetime. (b) Time-resolved emission spectra for CLTO:0.004Mn⁴⁺ excited at 365 nm; the inset shows the variations of the intensity ratios for 680/705 nm, 680/715 nm and 680/750 nm as a function of delayed time.

3.3 Energy transfer properties in Mn⁴⁺,Nd³⁺,Yb³⁺ doped CLTO phosphors

PL excitation and emission spectra for CLTO:0.004Mn⁴⁺ are shown in Fig. 4a and d, and have been described above in Section 3.2. Fig. 4b shows the PL excitation and emission spectra for CLTO:0.04Nd³⁺, in which two primary narrow bands around 907 nm (⁴F_3/2 → ⁴I_9/2) and 1067 nm (⁴F_3/2 → ⁴I_11/2) due to Nd³⁺ upon maximum excitation at 584 nm are observed. Monitored at 1067 nm, several sharp excitation lines peaking at 359 nm (⁴D_1/2 ← ⁴I_9/2), 432 nm (²P_1/2 ← ⁴I_9/2), 468 nm (⁴G_9/2 ← ⁴I_9/2), 526 nm (⁴G_7/2 ← ⁴I_9/2), 584 nm (⁴G_5/2 ← ⁴I_9/2), 684 nm (⁴F_9/2 ← ⁴I_9/2), 741 nm (⁴S_3/2 ← ⁴I_9/2), 821 nm (⁴F_5/2 ← ⁴I_9/2) and 881 nm (⁴F_3/2 ← ⁴I_9/2) can be observed.^55,56 By comparing the emission spectrum of Mn⁴⁺ in Fig. 4a and the Nd³⁺ excitation spectrum in Fig. 4b, a significant spectral overlap can be found (marked with a green dashed line frame), indicating that a possible resonant energy transfer from Mn⁴⁺ to Nd³⁺ ions would take place in Mn⁴⁺,Nd³⁺ co-doped CLTO samples. In order to elucidate this energy transfer possibility, the excitation and emission spectra for Mn⁴⁺,Nd³⁺ co-doped CLTO:0.004Mn⁴⁺,0.04Nd³⁺ are recorded and plotted in Fig. 4c, in which the emission spectrum shows both Mn⁴⁺ and Nd³⁺ emission bands upon 365 nm excitation, and the excitation spectra monitored at 707 nm (Mn⁴⁺ emission) and 1067 nm (Nd³⁺ emission) exhibit similar broadband profiles to Mn⁴⁺ singly doped samples, besides the characteristic excitation band around 584 nm (marked with a green dashed line oval) and other bands (marked with a green dashed line frame) due to Nd³⁺. These phenomena imply that the excitation energy of Nd³⁺ mainly originates from Mn⁴⁺; that is, the energy transfer from Mn⁴⁺ to Nd³⁺ ions occurs in this kind of co-doped sample. As to CLTO:0.20Yb³⁺, its weak emission shows a narrow band with a peak at 978 nm upon 324 nm excitation originating from the Yb^{3+ 2}F_5/2 → ²F_7/2 transition, see Fig. 4e. In addition, an excitation spectral band centered at 324 nm appears when monitored at an emission wavelength of 978 nm, which may have originated from the Yb³⁺–O²⁻ charge transfer transition.⁵⁷ In Fig. 4f, the emission spectrum for CLTO:0.004Mn⁴⁺,0.20Yb³⁺ presents both Mn⁴⁺ and Yb³⁺ bands upon 365 nm excitation, in which the emission intensity of Yb³⁺ is enhanced by over 50 times compared to that of CLTO:0.20Yb³⁺ upon 324 nm excitation. Moreover, two excitation spectra monitored at 709 and 978 nm exhibit analogous profiles to Mn⁴⁺ singly doped samples. These phenomena confirm that efficient energy transfer from Mn⁴⁺ to Yb³⁺ ions in CLTO:Mn⁴⁺,Yb³⁺ can also take place despite the scarcely observed spectral overlap between the Mn⁴⁺ emission and Yb³⁺ excitation spectra in the green dashed line frame in Fig. 4d and e. It is proposed that the energy transfer process can proceed via a phonon-assisted process similar to a Cr³⁺–Yb³⁺ group,²⁷ which generally requires five phonons. As mentioned in Fig. 1d, the phonon energy is about 715 cm⁻¹; upon multiplying by five, the value is about 3575 cm⁻¹, which is close to the energy gap of 3919 cm⁻¹ between the Yb^{3+ 2}F_5/2 and ²E_g levels. This suggests its possible phonon-assistant energy transfer process from Mn⁴⁺ to Yb³⁺ ions in this system. From the emission spectrum of CLTO:0.04Nd³⁺ in Fig. S2a (ESI†) and the excitation spectrum of CLTO:0.20Yb³⁺ in Fig. S2b (ESI†), a rough overlap between them (marked with a green dashed frame) can be found, which indicates energy transfer from Nd³⁺ to Yb³⁺ ions in Nd³⁺,Yb³⁺ co-doped systems. This can be validated by the similar excitation spectra monitored at around 1067 nm (Nd³⁺ emission band) and around 978 nm (Yb³⁺ emission band) for CLTO:0.04Nd³⁺,0.20Yb³⁺, as illustrated in Fig. S2c (ESI†). In addition, the emission band upon 584 nm excitation including both Nd³⁺ and Yb³⁺ emissions further confirms the energy transfer process from Nd³⁺ to Yb³⁺ ions here. In order to show the influence of luminescence on the concentration of dopants, the content-dependent emission spectra (λ_ex = 365 nm) for CLTO:0.004Mn⁴⁺,aNd³⁺ (a = 0–0.06) and CLTO:0.004Mn⁴⁺,bYb³⁺ (b = 0–0.40) are depicted in Fig. 5a and b, respectively. It can be found that the emission intensity of Mn⁴⁺ monotonically decreases with increasing Nd³⁺/Yb³⁺ concentration, while that of Nd³⁺/Yb³⁺ increases to a maximum a = 0.04/b = 0.20 first and then decreases with further dopants owing to their self-concentration quenching effect. Analogous results are obtained for CLTO:0.04Nd⁴⁺,yYb³⁺ (y = 0–0.40), as illustrated in Fig. S3 (ESI†); that is, the Nd³⁺ emission intensity (λ_ex = 584 nm) monotonically decreases with increasing Yb³⁺ content y and the maximum intensity of Yb³⁺ occurs when y = 0.20. These phenomena elucidate that the energy transfers from Mn⁴⁺ to Nd³⁺, Mn⁴⁺ to Yb³⁺, and Nd³⁺ to Yb³⁺ ions can proceed in the respective co-doped CLTO systems. Decay lifetimes, an efficient supplement for the confirmation of the energy transfer phenomenon, of Mn⁴⁺ for CLTO:0.004Mn⁴⁺,aNd³⁺ (a = 0–0.06) and CLTO:0.004Mn⁴⁺,bYb³⁺ (b = 0–0.40) (λ_ex = 365 nm, λ_em = 707 nm) are determined to be 0.961, 0.855, 0.816, 0.613 and 0.533 ms corresponding to a = 0, 0.01, 0.02, 0.04 and 0.06, and 0.961, 0.941, 0.849 and 0.767 ms corresponding to b = 0, 0.10, 0.20 and 0.40, respectively, by fitting the decay curves for these samples in Fig. 5c and d with a bi-exponential function mentioned above in eqn (3). Similarly, the Nd³⁺ decay lifetimes (λ_ex = 584 nm, λ_em = 907 nm) are determined to be 117, 79.9, 47.2 and 37.5 μs corresponding to y = 0, 0.10, 0.20 and 0.40, respectively, for CLTO:0.04Nd³⁺,yYb³⁺ samples (y = 0–0.40) in Fig. S4 (ESI†). We can observe that the decay lifetimes of Mn⁴⁺ and Nd³⁺ monotonically decline with the corresponding increasing Nd³⁺/Yb³⁺ and Yb³⁺ concentrations in the respective co-doped samples, further indicating the energy transfer processes of Mn⁴⁺ → Yb³⁺, Mn⁴⁺ → Nd³⁺ and Nd³⁺ → Yb³⁺.

Fig. 4 PL excitation and emission spectra for CLTO:0.004Mn⁴⁺ (a and d), CLTO:0.04Nd³⁺ (b), CLTO:0.004Mn⁴⁺,0.04Nd³⁺ (c), CLTO:0.20Yb³⁺ (e) and CLTO:0.004Yb³⁺,0.20Yb³⁺ (f).

Fig. 5 PL emission spectra of CLTO:0.004Mn⁴⁺,aNd³⁺ (a = 0–0.06) (a) and CLTO:0.004Mn⁴⁺,bNd³⁺ (b = 0–0.40) (b) excited at 365 nm. Decay curves of CLTO:0.004Mn⁴⁺,aNd³⁺ (a = 0–0.06) (c) and CLTO:0.004Mn⁴⁺,bNd³⁺ (b = 0–0.40) (d) (λ_ex = 365 nm, λ_em = 707 nm).

The energy transfer efficiencies (η_T) from Mn⁴⁺ to Nd³⁺/Yb³⁺ ions can be approximately calculated using the formula below:⁵⁸

η_T = 1 − τ/τ₀ (6)

where τ₀ and τ correspond to the decay lifetimes of Mn⁴⁺ in the absence and presence of Nd³⁺/Yb³⁺ in the CLTO:0.004Mn⁴⁺,aNd³⁺/bYb³⁺ samples. Accordingly, the values of η_T (listed in Table 1) increase from 11.03%/2.8% to 44.54%/20.89%, as illustrated in Fig. 6 left and right, respectively, for a/b = 0/0 to 0.06/0.40. The increasing trends of η_T indicate more and more efficient energy transfer processes from Mn⁴⁺ to Nd³⁺/Yb³⁺ ions in the CLTO:0.004Mn⁴⁺,Nd³⁺/Yb³⁺ samples with increasing dopant Nd³⁺/Yb³⁺ concentration.

Table 1 Decay lifetimes and energy transfer efficiencies (η_T) (λ_ex = 365 nm, λ_em = 707 nm) for CLTO:0.004Mn⁴⁺,aNd³⁺ (a = 0–0.06) and CLTO:0.004Mn⁴⁺,bYb³⁺ (b = 0–0.40) samples

Concentration (a/b)	Decay time (ms)	Energy transfer efficiency (ηT)
a = 0	0.961	—
a = 0.01	0.855	0.1103
a = 0.02	0.816	0.1509
a = 0.04	0.613	0.3621
a = 0.06	0.533	0.4454
b = 0	0.961	—
b = 0.10	0.941	0.028
b = 0.20	0.849	0.1165
b = 0.40	0.767	0.2019

---

Fig. 6 Energy transfer efficiencies (η_T) for CLTO:0.004Mn⁴⁺,aNd³⁺ (a = 0.01–0.06) (left) and CLTO:0.004Mn⁴⁺,bYb³⁺ (b = 0.10–0.40) (right).

In the subsequent section, we will investigate the PL properties of Mn⁴⁺,Nd³⁺,Yb³⁺ tri-doped CLTO phosphors based on their validated Mn⁴⁺ → Yb³⁺, Mn⁴⁺ → Nd³⁺ and Nd³⁺ → Yb³⁺ energy transfer processes in this system. As shown in Fig. 7a, upon 365 nm excitation, with the increase of Nd³⁺ concentration, the emission intensities of Mn⁴⁺ in the spectra for CLTO:0.004Mn⁴⁺,0.20Yb³⁺,cNd³⁺ (c = 0–0.06) decrease monotonically, and the emission intensities of Nd³⁺ and Yb³⁺ gradually increase, indicating that the addition of Nd³⁺ in CLTO:0.004Mn⁴⁺,0.20Yb³⁺ will result in energy transfer from Mn⁴⁺ to Nd³⁺ ions, and simultaneously, some energy from Nd³⁺ will be transferred to Yb³⁺ to enhance the Yb³⁺ emission. The excitation spectra monitored at 707 (Mn⁴⁺), 978 (Yb³⁺) and 1067 nm (Nd³⁺), as shown in the inset in Fig. 7a, exhibit similar profiles, among which the excitation spectra for 978 and 1067 nm display an exceptional excitation band around 584 nm(the green dashed oval) and several peaks over 700 nm (the green dashed frame) attributed to Nd³⁺. These phenomena imply the existence of the successive energy transfer process from Mn⁴⁺ to Nd³⁺ and then to Yb³⁺ ions. The emission intensities of the Mn⁴⁺ and Nd³⁺ bands in CLTO:0.004Mn⁴⁺,0.04Nd³⁺,dYb³⁺ (d = 0–0.40) in Fig. 7b show a monotonic decline, with increasing Yb³⁺ concentration, while the Yb³⁺ emission intensity increases to the maximum when d = 0.10, beyond which it decreases as a result of its self-concentration quenching. The decay times (λ_ex = 584 nm, λ_em = 907 nm) for CLTO:0.004Mn⁴⁺,0.04Nd³⁺,dYb³⁺ (d = 0–0.40) samples also decrease upon increasing the Yb³⁺ concentration, as shown in Fig. S5 (ESI†), from 187.8 μs to 44.7 μs. These results likewise indicate the Mn⁴⁺ → Nd³⁺ → Yb³⁺ successive energy transfer process in the Mn⁴⁺,Nd³⁺,Yb³⁺ tri-doped CLTO phosphor.

Fig. 7 PL emission spectra (λ_ex = 365 nm) of CLTO:0.004Mn⁴⁺,0.20Yb³⁺,cNd³⁺ (c = 0–0.06) (a) and CLTO:0.004Mn⁴⁺,0.04Nd³⁺,dYb³⁺ (d = 0–0.40) (b). The inset in (a) shows the comparison of excitation spectra monitored at 707, 978 and 1067 nm for CLTO:0.004Mn⁴⁺,0.20Yb³⁺,0.04Nd³⁺.

The emission and excitation spectra of the representative sample CLTO:0.004Mn⁴⁺,0.20Yb³⁺,0.04Nd³⁺ and the general solar spectrum are exhibited in Fig. 8a, showing that the broadband UV/visible spectrum can be converted to the NIR region to better match the high response region of c-Si semiconductor materials, which would be possible to promote the utilization of rare solar spectral response parts (300–600 nm) in the solar spectrum. In addition, the absorption spectrum of plant far-red absorption type phytochrome P_fr,³⁷ a color protein promoting plant cultivation, which is compared to the emission spectra of the CLTO:0.04Mn⁴⁺ and CLTO:0.004Mn⁴⁺,0.04Nd³⁺,0.10Yb³⁺ samples shown in Fig. 8b. It is manifested that there is a significant overlap between the absorption spectrum of P_fr and the Mn⁴⁺ emission band, which indicates the possibility of using CLTO:0.04Mn⁴⁺ in plant cultivation far-red LEDs. An extra NIR emission appears for CLTO:0.004Mn⁴⁺,0.04Nd³⁺,0.10Yb³⁺, besides the Mn⁴⁺ far-red emission, which can be absorbed by photosynthetic bacteria (extensively distributed in oil, water and floral tissue) to accelerate the synthesis of nutrients for assisting biological nitrogen fixation to indirectly boost plant cultivation. This indicates that far-red light and NIR emissions from the CLTO:0.004Mn⁴⁺,0.04Nd³⁺,0.10Yb³⁺ phosphor can cooperatively promote plant growth.

Fig. 8 (a) The general solar spectrum, spectral response of c-Si solar cells, and PL emission (λ_ex = 365 nm) and excitation (λ_em = 978 nm) spectra for the CLTO:0.004Mn⁴⁺,0.20Yb³⁺,0.04Nd³⁺ sample. (b) The comparison of the absorption spectrum of plant phytochrome P_fr and the emission spectra (λ_ex = 365 nm) of the CLTO:0.04Mn⁴⁺ and CLTO:0.004Mn⁴⁺,0.04Nd³⁺,0.10Yb³⁺ samples.

A simple schematic with respect to the electronic energy level transitions and energy transfer processes in Mn⁴⁺,Nd³⁺,Yb³⁺ doped CLTO systems is depicted in Fig. 9. Mn⁴⁺ singly doped CLTO would occupy three kinds of Te⁶⁺ sites to generate an approximate overall far-red emission around 707 nm (14 144 cm⁻¹) attributed to the Mn^{4+ 2}E_g → ⁴A_2g transition under UV/visible light excitation with wavelengths from about 250 nm (40 000 cm⁻¹) to 600 nm (16 667 cm⁻¹). Four excitation bands peaking at 318 nm (31 431 cm⁻¹), 355 nm (28 148 cm⁻¹), 410 nm (24 385 cm⁻¹), and 476 nm (20 992 cm⁻¹) correspond to the Mn⁴⁺–O²⁻ charge transfer transition and Mn⁴⁺ transitions ⁴T_1g ← ⁴A_2g, ²T_2g ← ⁴A_2g and ⁴T_2g ← ⁴A_2g, respectively. For Nd³⁺ and Yb³⁺ ions, substituting four kinds of La³⁺ sites in the CLTO host produces their characteristic excitations and emissions. In the Mn⁴⁺/Nd³⁺/Yb³⁺ double-doped CLTO systems, the energy at the Mn^{4+ 2}E_g level can be transferred to Nd^{3+ 4}F_7/2,⁴S_3/2 or ⁴F_5/2,²H_9/2 levels (process ) to enhance the emission bands around 907 nm (11 025 cm⁻¹) (⁴F_3/2 → ⁴I_9/2) and 1067 nm (9372 cm⁻¹) (⁴F_3/2 → ⁴I_11/2) followed by relaxation to the ⁴F_3/2 level via a resonant process due to the spectral overlap between the Mn⁴⁺ emission and Nd³⁺ excitation bands, and the Yb^{3+ 2}F_5/2 level to reinforce the emission band around 978 nm (10 225 cm⁻¹) (process ) with the assistance of approximately five phonons mentioned above due to the large energy gap (3919 cm⁻¹) between the Mn^{4+ 2}E_g and Yb^{3+ 2}F_5/2 levels. Besides, the energy at the Nd^{3+ 4}F_3/2 level can also be transferred to the ²F_5/2 level to enhance the Yb³⁺ 978 nm emission (process ) ascribed to their close energy levels. When Mn⁴⁺,Nd³⁺ and Yb³⁺ are tri-doped into CLTO, the Mn⁴⁺ → Nd³⁺ → Yb³⁺ successive energy transfer process can be implied to further enhance the Yb³⁺ emission based on the energy transfer processes mentioned above to better match the spectral response of c-Si solar cells and the absorption spectra of photosynthetic bacteria.

Fig. 9 A schematic energy level diagram illustrating the energy level transitions and energy transfer in CLTO:Mn⁴⁺,Nd³⁺,Yb³⁺ phosphors.

3.4 Luminescence thermal stability

The temperature-dependent luminescent properties are some of the most important parameters for phosphors used for LEDs or c-Si solar cells because they often need to work at a relatively high temperature. Herein, the temperature-dependent luminescence emission spectra for CLTO:0.004Mn⁴⁺ and CLTO:0.004Mn⁴⁺,0.04Nd³⁺,0.20Yb³⁺, upon 365 nm excitation, are depicted in Fig. 10a and b, respectively. Generally, with increasing operating temperature, in both of them, the higher the temperature, the lower the emission intensities of the phosphors, which is attributed to the increasing non-radiative transition process. The insets on the left show that the integrated emission intensities of CLTO:0.004Mn⁴⁺ in Fig. 10a and CLTO:0.004Mn⁴⁺,0.04Nd³⁺,0.20Yb³⁺ in Fig. 10b are maintained at 79.83% (much higher than that of Ca₃La₂W₂O₁₂:Mn^4+ 41) and 91.44% (150 °C) compared to those at room temperature, respectively, which indicate their excellent luminescence thermal stabilities. Moreover, the Mn⁴⁺ emission peaks present a slight red-shift and become wider and wider with increasing temperature in Fig. 10a and b, which can be attributed to the expansion of the cell unit and the reinforcement of the vibration mode under higher and higher temperature treatments.⁵⁹

Fig. 10 Temperature-dependent PL emission spectra (λ_ex = 365 nm) for CLTO:0.004Mn⁴⁺ (a) and CLTO:0.004Mn⁴⁺,0.04Nd³⁺,0.20Yb³⁺ (b) samples. The insets in (a) and (b) show variations of emission integral intensity on temperature and decay curves for CLTO:0.004Mn⁴⁺ (λ_ex = 365 nm, λ_em = 707 nm) and CLTO:0.004Mn⁴⁺,0.04Nd³⁺,0.20Yb³⁺ (λ_ex = 365 nm, λ_em = 978 nm) samples, respectively.

To further characterize the thermal stabilities of the as-prepared CLTO:0.004Mn⁴⁺ and CLTO:0.004Mn⁴⁺,0.04Nd³⁺,0.20Yb³⁺ phosphors, the temperature-dependent decay curves of Mn⁴⁺ (λ_ex = 365 nm, λ_em = 707 nm) in the CLTO:0.004Mn⁴⁺ phosphor and Yb³⁺ (λ_ex = 365 nm, λ_em = 978 nm) in the CLTO:0.004Mn⁴⁺,0.04Nd³⁺,0.20Yb³⁺ phosphor are shown in the right insets of Fig. 10a and b, respectively. It is found that the decay lifetime of Mn⁴⁺ decreases monotonically upon increasing the temperature from 298 K to 473 K. The lifetime of Mn⁴⁺ can be calculated by fitting the curves with a bi-exponential function well mentioned in eqn (4), which is determined to be 0.961 ms at room temperature (298 K) and decreases to 0.566 ms at 473 K. In order to clarify the dynamic process of Mn⁴⁺ in Mn⁴⁺ doped CLTO with increasing temperature, a simple model based on the temperature-dependent Stokes and anti-Stokes transition possibilities of vibronic emission can be expressed as:⁶⁰

where D is the offset coefficient, ħω refers to the energy of the coupled vibronic mode, T is the temperature using the K mode, and k is the Boltzmann constant. By considering eqn (8) and (9), we can deduce that both Stokes and anti-Stokes transition possibilities will increase with increasing temperature. Moreover, it is reported that the decay lifetime is inversely proportional to the sum of the radiative transition possibilities [W_a(T) + W_S(T)] and non-radiative transition probabilities, which therefore decrease monotonically with increasing temperature. As for Yb³⁺ in the CLTO:0.004Mn⁴⁺,0.04Nd³⁺,0.20Yb³⁺ sample, the decay curves can be fitted well with a single exponential equation:⁶¹

I = I₀ exp(−t/τ) (10)

where I and I₀ are the luminescence intensities at times t and 0, and τ is the decay lifetime for the fitting result. Accordingly, the τ value is determined to be 0.367 ms at room temperature (298 K) and slightly decreases to 0.341 ms at 473 K, which is ascribed to the declined lifetimes of the excited states of Yb³⁺ ions arising from the increase of the non-radiative relaxation rate.⁶² The results for decay lifetimes are consistent with the variation of emission intensities above, further indicating that the Mn⁴⁺,Nd³⁺,Yb³⁺ doped CLTO phosphors have excellent thermal stability due to the luminescence quenching effect, suggesting that this kind of phosphor system can be a possible component candidate for plant growth far-red and NIR LEDs and c-Si solar cells.

4. Conclusions

In conclusion, a series of novel Mn⁴⁺,Nd³⁺,Yb³⁺ doped CLTO phosphors were synthesized by substituting Te⁶⁺ for W⁶⁺ in the Ca₃La₂W₂O₁₂ compound via a high-temperature solid-state reaction route. A Mn⁴⁺ singly-doped CLTO phosphor showed broadband excitation (λ_em = 707 nm, 14 144 cm⁻¹) ranging from 250 nm (40 000 cm⁻¹) to 600 nm (16 667 cm⁻¹), which could be decomposed into four Gaussian bands peaking at 318 nm (31 431 cm⁻¹), 355 nm (28 148 cm⁻¹), 410 nm (24 385 cm⁻¹), and 476 nm (20 992 cm⁻¹), corresponding to Mn⁴⁺–O²⁻ charge transfer transition and Mn⁴⁺ transitions ⁴T_1g ← ⁴A_2g, ²T_2g ← ⁴A_2g and ⁴T_2g ← ⁴A_2g, respectively. A narrow far-red emission arising from Mn^{4+ 2}E_g → ⁴A_2g spin-forbidden transition appeared upon 365 nm UV excitation. Moreover, three kinds of Mn⁴⁺ sites were analyzed using the time-resolved spectral technology. After co-doping Nd³⁺/Yb³⁺ into CLTO:Mn⁴⁺, energy transfer from Mn⁴⁺ to Nd³⁺/Yb³⁺ was observed, which realized the effective spectral conversion of UV/visible to NIR light for possible utilization in c-Si solar cells. In addition, the Mn⁴⁺,Nd³⁺,Yb³⁺ tri-doped CLTO further enhanced this kind of spectral conversion based on the confirmed successive energy transfer process Mn⁴⁺ → Nd³⁺ → Yb³⁺ in phosphors. More importantly, the Mn⁴⁺,Nd³⁺,Yb³⁺ phosphors show excellent luminescence thermal stabilities. Results indicate that the as-prepared materials can be possible component candidates for use in c-Si solar cells and plant-growth far-red and NIR LEDs. In addition, this work provides a strategy wherein hosts for Mn⁴⁺ doping can be well enriched by substituting Te⁶⁺ for W⁶⁺ in certain tungstate compounds, which is highly desired in the search for novel red-emitting phosphors.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This project was financially supported by Ghent University's Special Research Fund (BOF; postdoctoral fellowship BOF16/PDO/159).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8qm00603b

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