Luminescence properties and energy transfer of the near-infrared phosphor Ca₃In₂Ge₃O₁₂:Cr³⁺,Nd³⁺

Abstract

The double doping strategy based on energy transfer is an effective way to regulate the NIR spectral distribution. In this work, Ca₃In₂Ge₃O₁₂:xNd³⁺ (CIG:xNd³⁺) and Ca_3−xIn_1.93Ge₃O₁₂:0.07Cr³⁺,yNd³⁺ (CIG:0.07Cr³⁺,yNd³⁺) phosphors are successfully prepared via a high-temperature solid-state method. CIG:0.07Cr³⁺ shows broadband emission centered at 804 nm, which covers most of the excitation peaks of Nd³⁺ ions. Under excitation at 480 nm, Cr³⁺ can provide effective energy transfer to Nd³⁺. In addition, CIG:0.07Cr³⁺,0.15Nd³⁺ has good temperature stability, and maintains 68.98% of the room-temperature intensity at 150 °C. The phosphors can convert short-wave photons to long-wave photons and enhance solar cell utilization, demonstrating the potential application of this material in solar spectral conversion technology.

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1 Introduction

Generally, the absorption spectrum of an instrument in the near-infrared (NIR) region is mainly due to the multiplication and combination of hydrogen-containing groups such as –CH, –NH and –OH, and these absorption bands are mainly distributed in the range of 700–1500 nm.^1–4 Therefore, NIR light can be used in security and surveillance, modern agriculture, the security and safety of international products and medical diagnosis. In order to achieve comprehensive and accurate detection, a broadband emission with uniform intensity is required. NIR phosphor-converted LEDs (NIR pc-LEDs) are one of the most promising NIR light sources with significant advantages such as long life, high efficiency, low cost, compactness and spectral tunability, and the choice of NIR phosphor plays a key role in the device performance. Currently, the lowest cost and highest efficiency commercial LED chips are blue chips.^5–7 If a NIR phosphor is combined with blue chips, a phosphor-converted NIR LED can be developed, whose emission can be adjusted on demand by the NIR phosphor, which also has the advantages of high efficiency, long service life and compact structure. It is one of the most promising NIR light sources. Therefore, it is crucial to develop efficient NIR broadband emitting phosphors that can be excited by blue LED cores.^8,9

To meet the broadband emission and high efficiency requirements of pc-LEDs, high-performance NIR emitting materials doped with Cr³⁺ ions are often required in the NIR region.^10–15 They have been of interest to researchers for applications in material analysis, fluorescent labeling, solar cells and biomedical monitoring.¹⁶ Taking solar cells as an example, silicon solar cells have a forbidden bandwidth of 1.1 eV (∼1100 nm), which has a better utilization rate for photons with energy slightly higher than the forbidden bandwidth and low lattice heat loss. The selection of suitable NIR light-emitting materials can improve the photovoltaic efficiency of solar cells by converting UV light, which is more distributed in sunlight but less utilized by solar cells, into NIR light, which can be used efficiently. Among the many NIR luminescent materials, fluorescent materials doped with rare earth ions account for a large proportion. Trivalent rare earth ions are selected in a suitable combination to meet the fluorescence requirements from the UV band all the way to the IR band, which have abundant energy levels.^17,18 Zhu et al. reported Cr³⁺-doped AlP₃O₉ displaying broadband NIR emission from 650–1000 nm with ∼76% luminescence internal quantum efficiency (IQE).¹⁹ KGaP₂O₇ exhibits a broadband NIR emission peak at 815 nm when Cr³⁺ is added in a weak crystal field environment.²⁰ In addition, the garnet-structured Gd₃Y_0.5In_0.5Ga₄O₁₂ phosphor displayed a broadband NIR emission peak at 760 nm, and broadband emission at 650–1050 nm was achieved by adjusting the six-coordinated crystal field environment occupied by Cr³⁺ with an IQE of 81.8%.²¹ Although these materials are suitable for the NIR region, they are not well suited to the application requirements. After a series of exploratory studies, it was found that transition metal ions or rare earth ions in the NIR range could be doped into a suitable substrate to obtain phosphors that could convert light from short-wave to long-wave photons. When applied to solar cells, these can improve the utilization of short-wave photons to a certain extent, which in turn improves the photovoltaic conversion efficiency of solar cells.²² Dong et al. synthesized a series of LaGaO₃:Cr³⁺,Ln³⁺ (Ln = Yb, Nd, Er) phosphors to achieve efficient NIR emission from c-Si solar cells through energy transfer.²³

In this work, Ca₃In₂Ge₃O₁₂:xNd³⁺ (CIG:xNd³⁺) and Ca_3−xIn_1.93Ge₃O₁₂:0.07Cr³⁺,yNd³⁺ (CIG:0.07Cr³⁺,yNd³⁺) phosphors were successfully prepared. Because there is a large overlap between the solar spectral energy distribution and the excitation spectrum of Cr³⁺, CIG:Cr³⁺ might convert the most intensive energy distribution in the solar spectral energy distribution to a long emission at around 800 nm. Nd³⁺ has high luminescence intensity and its emission wavelength at around 1100 nm matches the strongest point of the response curve of c-Si solar cells.^24,25 Spectral conversion materials with excitation at 480 nm and emission at 1100 nm were synthesized using the energy transfer of Cr³⁺ → Nd³⁺. This may be of great significance for the development of rare earth ions and their applications in new energy fields.

2 Experimental

2.1 Synthesis

A series of CIG:Cr³⁺/Nd³⁺ phosphors were successfully synthesized by a high-temperature solid-state method. CaCO₃, In₂O₃, GeO₂, Cr₂O₃ and Nd₂O₃ were weighed as raw materials and then mixed and ground in an agate mortar. The mixture was sintered at 1250 °C for 6 h in air. After cooling, the mixture was ground to a powder using an agate mortar and then tested.

2.2 Characterization

XRD patterns were obtained on a Bruker D8 Advance X-ray diffractometer at 40 kV and 40 mA with Cu-Kα (λ = 1.54056 Å) irradiation. The specific test parameters are as follows: scan range = 10°–80°, time per steps = 0.15, steps = 3000. Rietveld refinement of the measured XRD data was performed using the GENERAL STRUCTURE ANALYSIS SYSTEM (GSAS) software.²⁶ The photoluminescence excitation (PLE) and photoluminescence emission (PL) spectra were recorded on a HORIBA Fluorolog-3 fluorescence spectrometer with a 450 W Xe lamp as the excitation source, and the temperature spectra were tested with an external device. The diffuse reflectance spectra were measured on a Hitachi U-4100, using BaSO₄ as a standard reference.

3 Results and discussion

3.1 Crystal structure and phase identification

As shown in Fig. 1, CIG exhibits a garnet structure, the space group is Ia3̄d, and it is a symmetric cubic structure with the lattice parameters a = b = c = 12.58 Å, α = β = γ = 90°, and Z = 8. Ca²⁺ (R = 1.12 Å) combines with O²⁻ to form CaO₈ dodecahedra, while In³⁺ (R = 0.80 Å) and Ge⁴⁺ (R = 0.39 Å) ions occupy the six-coordinated octahedral site and the four-coordinated tetrahedral site, respectively. In addition, as shown in the oblique tangent diagram of the cell, each dodecahedron is connected to six octahedra by a common edge and each tetrahedron is connected to four octahedra by a common vertex. The flexible structure provides a rich coordination environment for the activator.

Fig. 1 Crystal structure of CIG.

The XRD patterns of CIG:xNd³⁺ (x = 0.01, 0.03, 0.07, 0.1, 0.3, 0.5) and CIG:0.07Cr³⁺,yNd³⁺ (y = 0.01, 0.05, 0.15, 0.3) are presented in Fig. 2. The main diffraction peaks of all the samples are well consistent with the standard diffraction cards of PDF #04-004-0594, suggesting the samples are all pure phases. XRD refinement was conducted for the samples using the GSAS software, and the results of CIG:0.1Nd³⁺ and CIG:0.07Cr³⁺,0.15Nd³⁺ are displayed in Fig. 2c and d. It is clearly seen that the measured values match well with the calculated results, and the obtained refinement parameters χ² < 10, R_p < 10%, and R_wp < 15% are within the error range, which justify the refinement results. More detailed refinement results of CIG:xNd³⁺ (x = 0.01, 0.03, 0.07, 0.1, 0.3, 0.5) are given in Table 1. The space group, crystal phase, α/β/γ and other crystal parameters of the six samples correspond to each other, and the doping of Nd³⁺ does not change the lattice parameters of CIG. As the ionic radius of Ca²⁺ (1.12 Å, CN = 8) is close to that of Nd³⁺ (1.11 Å, CN = 8), the cell parameters a/b/c and the cell volume V do not change significantly. This subtle change is difficult to monitor only by the macroscopic characterization of XRD, which is also confirmed by the lack of significant change in the XRD diffraction peaks with increasing Nd³⁺ concentration (Fig. 2a). Nevertheless, a slight downward trend in cell volume was observed through more careful refinement of the data (the cell parameters are retained to six decimal places). As shown in Fig. 3, the fitting results demonstrate a slight shrinkage of the lattice and eventually a decrease in cell volume with the incorporation of the smaller Nd³⁺.

Fig. 2 (a) XRD diffraction patterns of CIG:xNd³⁺ (x = 0.01, 0.03, 0.07, 0.1, 0.3, 0.5). (b) XRD diffraction patterns of CIG:0.07Cr³⁺,yNd³⁺ (y = 0.01, 0.05, 0.15, 0.3). (c) XRD refinement mapping of CIG:0.1Nd³⁺. (d) XRD refinement mapping of CIG:0.07Cr³⁺,0.15Nd³⁺.

Table 1 Results of the XRD refinement of CIG:xNd³⁺ (x = 0.01, 0.03, 0.07, 0.1, 0.3, 0.5)

	CIG:0.01Nd^3+	CIG:0.03Nd^3+	CIG:0.07Nd^3+	CIG:0.1Nd^3+	CIG:0.3Nd^3+	CIG:0.5Nd^3+
Space group	Ia3̄d	Ia3̄d	Ia3̄d	Ia3̄d	Ia3̄d	Ia3̄d
Symmetry	Cubic	Cubic	Cubic	Cubic	Cubic	Cubic
V (Å^3)	1994.639	1994.649	1994.625	1994.388	1994.428	1993.955
a/b/c (Å)	12.587933	12.587964	12.587914	12.587415	12.587500	12.586505
α/β/γ (deg)	90	90	90	90	90	90
χ^2	2.002
Rp (%)	8.13	9.57	9.96	9.29	9.61	9.98
Rwp (%)	12.26	13.06	13.59	13.12	14.87	14.97

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Fig. 3 Cell volume of CIG:xNd³⁺ (x = 0.01, 0.03, 0.07, 0.1, 0.3, 0.5).

3.2 Luminescence properties of CIG:Nd³⁺

Fig. 4a shows the PL spectrum excited by an 808 nm laser. CIG:0.1Nd³⁺ emits NIR light in the range of 850–1450 nm with a peak at 1057 nm. The three peak positions from left to right correspond to the ⁴F_3/2 → ⁴I_9/2, ⁴F_3/2 → ⁴I_11/2, and ⁴F_3/2 → ⁴I_13/2 transitions of Nd³⁺.²⁷ The emission band covers the biology first window and biology second window of the NIR light range. Fig. 4b presents the excitation spectra of 1057 nm; there are many excitation bands that correspond to the ⁴I_9/2 → ²G_9/2, ⁴I_9/2 → ⁴G_9/2, ⁴I_9/2 → ⁴G_7/2, ⁴I_9/2 → ⁴G_5/2 + ²G_7/2, ⁴I_9/2 → ²H_11/2, ⁴I_9/2 → ⁴F_9/2, ⁴I_9/2 → ⁴F_7/2 + ⁴S_3/2 and ⁴I_9/2 → ⁴F_5/2 transitions at 200–800 nm.²⁷ The optimum excitation is located at 800 nm, which is similar to the position of the emission spectrum of CIG:0.07Cr³⁺ and provides the necessary conditions for the energy transfer from Cr³⁺ to Nd³⁺.²⁸ Fig. 4c presents the PL spectra of CIG:Nd³⁺ under 808 nm excitation, and all the samples exhibit broad emission in the range of 850–1450 nm. As shown in Fig. 4d, the emission intensity first increases and then decreases with increasing doping concentration and reaches a maximum at x = 0.1, while the peak position does not change with the increase in the doping concentration.

Fig. 4 (a) Emission spectra of CIG:0.1Nd³⁺ under the excitation of an 808 nm laser. (b) Monitoring the excitation spectrum of CIG:0.1Nd³⁺ at 1057 nm. (c) Emission spectra of CIG:Nd³⁺ under the excitation of an 808 nm laser. (d) Plot of the main peak intensity and peak position of CIG:xNd³⁺ with the concentration of Nd³⁺.

3.3 Luminescence properties and energy transfer of CIG:Cr³⁺,Nd³⁺

As shown in Fig. 5a, the PL spectrum of CIG:0.07Cr³⁺ overlaps with the two strongest excitation peaks of CIG:0.1Nd³⁺, indicating that Nd³⁺ will effectively absorb the light of Cr³⁺, which is necessary to generate energy transfer from Cr³⁺ to Nd³⁺. Meanwhile, the PLE spectrum of CIG:0.07Cr³⁺ shows the most intense absorption band at 480 nm, while the excitation of CIG:0.1Nd³⁺ is very weak at 480 nm, indicating that the excitation of Nd³⁺ luminescence in CIG:0.07Cr³⁺,yNd³⁺ will mainly originate from the emission of Cr³⁺ under 480 nm excitation. As displayed in Fig. 5b, the intensity of the 1057 nm peak increases and then decreases with an increase in the Nd³⁺ doping concentration, and the intensity of the Cr³⁺ peak decreases continuously. The emission intensity reaches a maximum at 0.15 Nd³⁺. The decay curves of Cr³⁺ are shown in Fig. 6, and the lifetime of Cr³⁺ can be calculated using eqn (1) and (2). The results show that the lifetime of Cr³⁺ decreases with the increase in the Nd³⁺ doping concentration, indicating that Cr³⁺ → Nd³⁺ energy transfer occurs.²⁸

I = I₀ + A₁exp(−t/τ₁) + A₂exp(−t/τ₂) (1)

τ* = (A₁τ²₁ + A₂τ²₂)/(A₁τ₁ + A₂τ₂) (2)

Fig. 5 (a) PLE and PL spectra of CIG:0.07Cr³⁺ and CIG:0.1Nd³⁺. (b) The emission spectra of CIG:0.07Cr³⁺,yNd³⁺ (y = 0.01, 0.05, 0.15, 0.3) upon 480 nm excitation.

Fig. 6 Decay curves and lifetimes of CIG:0.07Cr³⁺,yNd³⁺ under the 480 nm excitation.

With the increase in the doping concentration, the distance between Nd³⁺ gets smaller and the non-radiative jump becomes larger. The luminescence intensity of Nd³⁺ reaches its highest when the concentration and the energy transfer reach a balance. Meanwhile, there is a slight blue shift of the Cr³⁺ emission peak, which is due to the substitution of Nd³⁺ for Ca²⁺ suppressing the substitution of Cr³⁺ for Ca²⁺, and the dodecahedral lattice long-wave luminescence of Cr³⁺ is reduced.

The energy transfer process of Cr³⁺/Nd³⁺ in CIG can be explained as follows. When the sample is excited by 480 nm light, the photoelectrons located on the ⁴A₂(⁴F) energy level of Cr³⁺ transition upward to multiple excitation bands. Then, these photoelectrons fall to the lowest excited state ⁴T₂(⁴F) through non-radiative transition and the photoelectrons transition downward from the lowest excited state to the ground state, which results in NIR emission at 804 nm. This transition is matched with the excitation of Nd³⁺. The photoelectrons in the ground state of Nd³⁺ ions (⁴I_9/2) are excited to transition upward to multiple excitation bands, and these photoelectrons then fall to the lowest excited state ⁴F_3/2 through non-radiative transition. The photoelectrons transition downward from the lowest excited state to the ground state, producing three emissions ⁴F_3/2 → ⁴I_9/2, ⁴F_3/2 → ⁴I_11/2 and ⁴F_3/2 → ⁴I_13/2. The three emission bands correspond to the peak positions of 889 nm, 1057 nm, and 1339 nm, which are the energy transfer processes of Cr³⁺ → Nd³⁺ in CIG:0.07Cr³⁺,Nd³⁺. In order to investigate the energy transfer, Fig. 7 shows the emission intensity and energy transfer efficiency of Cr³⁺ and Nd³⁺. The efficiency of energy transfer can be obtained by the following equation:²⁹

Fig. 7 (a) Emission intensities of Cr³⁺ and Nd³⁺ in CIG:0.07Cr³⁺,yNd³⁺ (y = 0.01, 0.05, 0.15, 0.3). (b) Energy transfer efficiency with different Nd³⁺ concentrations.

I_S represents the emission intensity of Cr³⁺ for different Nd³⁺ doping concentrations, and I_S0 represents the emission intensity of Cr³⁺ when y is 0. With the increase in the doping concentration, the energy transfer efficiency of Cr³⁺ → Nd³⁺ gradually increases to 71.28%, indicating that it is feasible to transfer the energy at 480 nm to the long-wavelength region via the energy transfer of Cr³⁺ → Nd³⁺. In order to investigate the mechanism of energy transfer, Blasse proposed the strategy to distinguish the exchange interaction and electrical multipolar interaction in energy transfer using the critical distance R_c of the activator and sensitizer. If the critical distance R_c < 5 Å, exchange interaction plays a major role, and if R_c > 5 Å, multipolar interaction will dominate, and the critical distance can be calculated by the following equation:³⁰

where V represents the unit cell volume, X_c is the sum of the concentrations of both ions when the emission intensity of Cr³⁺ is reduced to half that of undoped Cr³⁺ after doping Nd³⁺, and N represents the number of cations that can be occupied in the matrix. The calculated critical distance is R_c = 18 Å, which is much larger than 5 Å. Therefore, the type of energy transfer between Cr³⁺ → Nd³⁺ is a multipolar interaction. Multipolar interactions are further divided into dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions. This can be determined by Dexter energy transfer theory and the Reisfeld approximation:^31,32where I_S0 and I_S represent the emission intensities of Cr³⁺ in CIG:0.07Cr³⁺ and CIG:0.07Cr³⁺,yNd³⁺ (y = 0.01, 0.05, 0.15, 0.3), respectively. C is the sum of the concentrations of both compounds. When n is 6, 8, and 10, it corresponds to the three types of multipolar interactions, namely dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions. The relationship between I_S0/I_S and is shown in Fig. 8, the curves of which are fitted linearly. When n = 8, the fit R² is the highest, indicating that the fit is the best and most appropriate. In summary, the type of energy transfer for Cr³⁺ → Nd³⁺ is a dipole–quadrupole interaction (multipolar interaction).

Fig. 8 (a)–(c) Relationship between I_S0/I_S and , respectively.

Slightly different from a pc-LED, a solar cell usually operates at temperatures below 80 °C. The temperature stability of commercial phosphors is critical to their application performance. Fig. 9a presents the thermal spectra of CIG:0.07Cr³⁺,0.15Nd³⁺ at 25–150 °C. The integrated intensity ratio at 75 °C can reach 91.36% of that at room temperature. In addition, the integrated intensity of CIG:0.07Cr³⁺,0.15Nd³⁺ at 150 °C can still reach 68.98% of that at room temperature, as calculated by the integrated area. As displayed in Fig. 9b, the thermal quenching activation energy of CIG:0.07Cr³⁺,0.15Nd³⁺ was further calculated using the Arrhenius equation (eqn (6)).^33,34

Fig. 9 (a) Thermal spectra of CIG:0.07Cr³⁺,0.15Nd³⁺ in the range of 25–150 °C and the inset shows a comparison of the intensities. (b) Calculated activation energy fitting curves for CIG:0.07Cr³⁺,0.15Nd³⁺.

The ln[(I₀/I) − 1] and 1/KT relations were fitted to a straight line, and the absolute value of the slope of the line is the thermal quenching activation energy E_a. The value of E_a was calculated to be 0.374 eV, which indicates that the phosphor has good temperature stability and commercial value.

3.4 Ca_2.85In_1.93Ge₃O₁₂:0.07Cr³⁺,0.15Nd³⁺ application prospects in solar spectral conversion technology

Fig. 10a shows the potential application of CIG:0.07Cr³⁺,0.15Nd³⁺ in solar spectral conversion technology. Solar spectrum AM 1.5 does not correspond to the peak position of the spectral response curve of c-Si solar cells.³⁵ In order to avoid such energy loss and improve the efficiency of solar cells, the CIG:0.07Cr³⁺,0.15Nd³⁺ phosphor can be introduced. It is noted that the best excitation of Cr³⁺ in the visible region is consistent with the peak position of the solar spectrum AM 1.5, and the phosphor emission is consistent with the strongest part of the spectral response curve of the c-Si solar cell by the energy transfer of Cr³⁺ → Nd³⁺; thus, it has the potential to convert short-wave photons into long-wave photons and enhance the efficiency of solar cells. As displayed in Fig. 10b, by putting a thin layer of high-transmittance CIG:0.07Cr³⁺,0.15Nd³⁺ on the EVA film of a crystalline silicon cell, some short-wave photons are converted into long-wave photons, and more photons are absorbed in the best spectral response of the c-Si solar cell, thus obtaining better photoelectric conversion efficiency.

Fig. 10 (a) Spectral response curves of CIG:0.07Cr³⁺,0.15Nd³⁺ with solar spectrum AM 1.5 and c-Si solar cells. (b) Application of CIG:0.07Cr³⁺,0.15Nd³⁺ in silicon-based solar cells.

4 Conclusions

In summary, CIG:0.1Nd³⁺ and CIG:0.07Cr³⁺,0.15Nd³⁺ phosphors are successfully synthesized via a high-temperature solid-state method. The physical phase structure, luminescence properties, energy transfer mechanisms, thermal stability and the future applications are investigated in detail. The XRD and refinement results show that the obtained phosphors were all pure phases. The energy transfer from Cr³⁺ to Nd³⁺ was demonstrated by comparing the excitation and emission spectra of CIG:0.07Cr³⁺,yNd³⁺ (y = 0.01, 0.05, 0.15, 0.3) and the decay curve of Cr³⁺ ions. The energy transfer efficiency is calculated to be 71.28%. CIG:0.07Cr³⁺,0.15Nd³⁺ possesses excellent temperature stability, reaching 91.36% and 68.98% of the intensity at room temperature at 75 °C and 150 °C, respectively. The activation energy is calculated to be 0.374 eV. Finally, the application of the CIG:0.07Cr³⁺,0.15Nd³⁺ phosphor in solar cells is demonstrated. This phosphor has the potential to convert short-wave photons into long-wave photons and improve the efficiency of solar cells, and thus has broad application prospects.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

The work is supported by the National Natural Science Foundation of China (No. 61805285).

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