Near-infrared optical properties of Cs₂KScF₆:Cr³⁺ phosphor for high-power light-emitting diodes

1 Introduction

Over the past decades, NIR spectroscopy technique has been extensively studied and emerged in burgeoning fields such as food composition analysis, biological imaging and plant cultivation due to its strong penetration and harmless to the organism (Rajendran et al., 2021; Liu et al., 2023; Huang et al., 2023). In these practical applications, NIR light sources are entrusted the crucial role (Zhang et al., 2022). In spite of the fact that traditional NIR light sources have a broad spectral region, their large volume and short serving lifetime are the leading cause that enable them to lose competitiveness as NIR light sources for portable spectrometers (Liu et al., 2021; Zhou et al., 2020; Chan et al., 2022). Nowadays, pc-LEDs consist of NIR phosphors and blue LEDs exhibit the peculiar superiorities with respect to compactness, high-efficiency and long-lasting, which are also highly anticipated (Wang et al., 2023; Suo et al., 2022). Nevertheless, the photoelectric properties of NIR pc-LED devices mainly lie on the coated NIR luminescent material. Hence, it is of great significance to develop NIR phosphors with excellent luminous properties for the construction of miniature NIR light sources for pc-LED devices.

Currently, the path to obtain NIR-emitting materials is by introducing rare-earth (i.e., Sm³⁺, Eu²⁺, Pr³⁺) or transition metal (i.e., Ni²⁺, Mn⁴⁺, Cr³⁺) ions into some oxide or fluoride matrices (Yang et al., 2023; Zhou et al., 2018; Jin et al., 2023; Chen et al., 2023). In particular, the transition metal Cr³⁺ ion with d³ electron configuration is considered as a promising activator on account of its effective absorption in blue-light regions and tunable spectral position based on crystal field engineering (Song et al., 2023). The emission characteristics of Cr³⁺ ions basically depend on the crystal field strength of the host material. Generally, broadband NIR emission originated from spin-allowed ⁴T₂ → ⁴A₂ transition could be realized when Cr³⁺ ion is affected by a weak crystal field, whereas narrowband emission derived from spin-forbidden ²E → ⁴A₂ transition will occur under the influence of a stronger coordination field (Liu et al., 2022; Lin et al., 2017). Based on this observation; a series of thermally stable Cr³⁺-doped fluoride compounds with wide NIR emission bands were developed. For instance, LiMgAlF₆:Cr³⁺ phosphor shows desirable thermal stability (93.9 % at 423 K), but the output power of the LED devices is only 24.3 mW at 300 mA (Hu et al., 2023). Two types of K₃Al/GaF₆:Cr³⁺ NIR phosphors present moderate thermal stability, but the conversion efficiency of the LEDs is less than 1 % (Lee et al., 2020). A typical garnet-structured Na₃Al₂Li₃F₁₂:Cr³⁺ phosphor has almost zero thermal-quenching (99 % at 423 K), but the NIR pc-LED device received a dissatisfactory output power of 14.3 mW (Huang et al., 2021). These NIR phosphors show relatively high thermal stability, but the photoelectric performances of the LED devices are always unsatisfactory. However, the photoelectric properties of LED devices are also worthy of attention since they are crucial parameters to evaluate the optical performance of LED devices. In order to improve the photoelectric properties, researchers have paid great efforts to explore different Cr³⁺-activated fluoride NIR phosphors, e.g., K₂LiGaF₆:Cr³⁺ (106.1 mW at 300 mA) (Li et al., 2024), Cs₂KIn_1−yAl_yF₆:Cr³⁺ (215.88 mW at 320 mA) (Wang et al., 2023). Even though a large number of Cr³⁺-activated NIR luminescent materials have been reported, the development of Cr³⁺-doped NIR phosphor used for high-power LEDs that simultaneously meet good thermal stability and favorable photoelectric properties is still desirable.

In this work, Cs₂KScF₆ was selected to incorporate Cr³⁺ ions for the synthesis of NIR phosphor with high thermal stability through a co-precipitation method. The crystal structure, bandgap, micro-morphology, as well as luminescent properties of Cs₂KScF₆:Cr³⁺ were thoroughly investigated and characterized. The as-prepared phosphor emits NIR light peaking at 778 nm owing to the weak crystal field. Thanks to the excellent thermal stability of Cs₂KScF₆:Cr³⁺ (70 % at 423 K), the NIR pc-LED prototype was fabricated by integrating the obtained NIR phosphor into the InGaN chip. Under the drive current of 320 mA, the high output power of 72.28 mW and the photoelectric conversion efficiency of 6.83 % are achieved. Employing this high-power NIR pc-LED device as light source, the potential of Cs₂KScF₆:Cr³⁺ phosphor used for night vision imaging was evaluated.

2 Experimental section

Sample Synthesis and LED fabrication. Sc₂O₃ (Adamas, 99.9 %), CsF (Aladdin, 99.9 %), KF (Aladdin, 99.9 %), HF (Adamas, 40 wt%), Cr(NO₃)₃·9H₂O (Adamas, 99 %) and absolute ethanol (Adamas, 99.7 %) were selected as raw materials and used without additional purification. The precursor of (NH₄)₃CrF₆ was prepared based on the method provided in the literature (Yu et al., 2022). The XRD pattern shown in Fig. S1 of Supplementary Material (SM) demonstrates the pure phase of the precursor. A series of Cs₂KScF₆:xCr³⁺ (x = 0.01–0.09) samples were synthesized at RT through a facile co-precipitation method. The specific synthesis procedure is described as follows: firstly, 2.5 mmol of Sc₂O₃ was weighed and added to a 50 mL plastic tube containing 5 mL of HF solution, and magnetically stirred for 1 h until Sc₂O₃ was completely dissolved. Then, 12 mmol of KF, 15 mmol of CsF and proportional (NH₄)₃CrF₆ were poured into the above solution in turn and continuously stirred for 8 h. After the reaction, the soluble impurities were removed by washing with absolute ethanol for three times. Finally, the precipitates were dried at 80 °C for 6 h in an oven to obtain the Cs₂KScF₆:xCr³⁺ (x = 0.01, 0.03, 0.05, 0.07, 0.09) products.

The LED device was prepared by coating the as-prepared Cs₂KScF₆:Cr³⁺ phosphor on the surface of a commercially available blue InGaN chip (San'an Optoelectronics Co., Ltd, ∼ 450 nm, 3 W). To be specific, the phosphor was firstly mixed with a UV structural adhesive (Shenzhen Tegu New Materials Co., Ltd.) at a mass ratio of 1:1, and then coated on the surface of a blue LED chip, finally dried at 120 °C for 1 h for the fabrication of NIR pc-LED device.

Characterization and theoretical computation. The phase purity of Cs₂KScF₆:xCr³⁺ samples was substantiated by X-ray diffraction (XRD) patterns obtained from a Bruker D8 Advance powder diffractometer with Cu Kα radiation (λ = 1.5406 Å). The operating voltage, current, scanning rate, angular range, and step size were 40 kV, 40 mA, 10°/min, 10°-70°, and 0.02°, respectively. The powder diffraction data used for Rietveld refinement were tested on the same diffractometer with a scanning rate of 1°/min and an angular range of 5°-120°. The morphology and chemical composition were analyzed from a FEI Nova NanoSEM 450 scanning electron microscope (SEM) equipped with an energy-dispersive spectrometer (EDS) attachment. Electron paramagnetic resonance (EPR) spectra were examined on a Bruker EMXplus-6/1 spectrometer. The X-ray photoelectron spectroscopy (XPS) was collected from a Thermo Scientific K-Alpha XPS system. The internal quantum efficiency (IQE), external quantum efficiency (EQE), as well as decay curves were recorded from a steady-state fluorescence spectrophotometer (FLS1000, Edinburgh Instrument) with an additional integrating sphere. The photoluminescence excitation (PLE) and photoluminescence (PL) spectra were obtained using an AVANTES Avaspec Mini 2048CL-SHB3 fiber spectrophotometer. Thermal quenching behavior was conducted on the same spectrophotometer with a temperature-control instrument and sample cooling/heating stage. The moisture resistance test was carried out in a BPS-50CL constant temperature and humidity chamber with a temperature of 85 °C and a humidity of 85 %. The electroluminescent (EL) spectra of the packed NIR pc-LEDs were executed by an OHSP-350 M LED fast-scan spectrophotometer (Hangzhou Hopoo Light & Color Technology Co., Ltd.). The thermographs and working temperatures of the LED devices recorded under drive current of 200 mA were acquired through a thermal imager (FLIR E8). An industrial camera (Canon XA10E) loaded with natural and NIR modes was used to shoot the demonstration images.

The General Structure Analysis System (GSAS) software was used to perform the Rietveld refinement for XRD data. Visualization for Electronic and Structural Analysis (VESTA Ver. 3.4.4) software was applied to construct crystal structure of the Cs₂KScF₆ host. Based on the density functional theory (DFT), the electronic structure of Cs₂KScF₆ were assessed by a CASTEP module of the Materials Studio package. The PBE functional was employed to approximate the exchange–correlation potential. The cutoff energy and k-point mesh were set as 300 eV and 3 × 3 × 3 Monkhorst-Pack grid, respectively. The electronic convergence criterion was10^-5 eV, and the atomic positions and lattice parameters were relaxed to 0.02 eV/Å.

3 Results and discussion

The XRD patterns of a series of Cs₂KScF₆:xCr³⁺ (x = 0.01–0.09) samples with various doping concentrations and standard diffraction card (JCPDS No. 21–0851, cubic Fm $\overline{3}$ m space group) are shown in Fig. 1a, in which all the XRD diffraction peaks are in good agreement with the standard JCPDS card, indicating that the five samples have the same pure phase as the host material and the incorporation of Cr³⁺ ions does not change the cubic structure of Cs₂KScF₆ host (Wu et al., 2023). The role of Cr³⁺ ion has changed with the increasing doping concentration from x  = 0.01 to x  = 0.09, specifically from the dopant to a key composition of Cs₂KScF₆ matrix, manifesting that the target Cs₂KScF₆:xCr³⁺ (x = 0.01–0.09) phosphors have been synthesized.

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Fig. 1

(a) XRD patterns of Cs₂KScF₆:xCr³⁺ (x = 0.01–0.09) samples. (b) Refined XRD results of Cs₂KScF₆:0.05Cr³⁺ product. (c) Crystal structure diagram of Cs₂KScF₆ host. (d) EPR spectra of Cs₂KScF₆:0.05Cr³⁺ sample and Cs₂KScF₆ host.

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The XRD diffraction data of Cs₂KScF₆:0.05Cr³⁺ sample in the angular range of 5-120° are employed to carry out Rietveld structure refinement by the GASA software for verifying the structural changes after incorporating Cr³⁺ into the host. Fig. 1b shows the structural refinement pattern obtained by the Rietveld method, the reliable residual factors are R_WP=9.60 %, R_P=8.91 % and χ² = 2.321 for Cs₂KScF₆:0.05Cr³⁺ product, which further indicate that the crystal structure remains unchanged even though Cr³⁺ ions were introduced (Li et al., 2021). The detailed crystallographic parameters of Cs₂KScF₆ host and refined parameters of Cs₂KScF₆:0.05Cr³⁺ phosphor are listed in Table S1; and the built crystal structure diagram is shown in Fig. 1c. In the cubic structure, Sc³⁺ and K⁺ are coordinated with six F^- ions to form [ScF₆] and [KF₆] octahedron cages, while Cs⁺ is filled in the cavity surrounded by four [ScF₆] and four [KF₆] octahedrons, they are alternately connected by sharing a common vertex to establish a double-perovskite structure. According to the substitution rule of the same valence state and close ion radius, Cr³⁺ (r_6-coor. = 0.615 Å) tends to occupy the lattice site of Sc³⁺ (r_6-coor. = 0.745 Å) (Qing et al., 2022). Furthermore, the partial replacment of small Cr³⁺ for large Sc³⁺ site induces the lattice shrinkage and the decreasing of interplanar spacing, resulting in the unit cell volume of Cs₂KScF₆:Cr³⁺ sample being smaller than that of Cs₂KScF₆ matrix (Zhang et al., 2022). Specifically, the lattice parameter and unit cell volume of the undoped matrix are a = 9.33 Å, and V=812.21 Å³ respectively, while the doped sample has reduced to 9.19 Å and 776.96 ų (Table S1).

As known, Cr³⁺ ion has three d-electrons in the unfilled 3d shell with a total spin of S=3/2. When the Cr³⁺ ion enters the octahedral site and connects with six neighbouring ions to form an [CrF₆] octahedron, the ground state ⁴F term divides into ⁴A_2g, ⁴T₁, and ⁴T₂ sublevels, and then the EPR signals can be observed. As shown in Fig. ld, the sharp EPR signal with g-factor of 1.95 verifies the presence of Cr³⁺ ions that locate in the octahedral sites (Miao et al., 2021). On the contrary, no EPR peak appears in the blank reference of the Cs₂KScF₆ matrix. Moreover, the XPS spectrum also confirms the signal that comes from chromium ion in the obtained Cs₂KScF₆:0.05Cr³⁺ sample (Fig. 2a). It is well-known that the chromium ions have mutiple valence states, thus the high-resolution spectrum of Cr 2p was used to verify its valance state (Fig. 2b). Apparently, the double peaks at 578.7 eV and 588.8 eV belong to Cr 2p_3/2 and Cr 2p_1/2 orbits in Cr³⁺-doped Cs₂KScF₆ compound, which strongly support the conclusion that chromium ion exists in trivalent state and only Cr³⁺ ion devotes to the NIR luminescence in this case (Xiahou et al., 2021).

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Fig. 2

(a) XPS spectrum of Cs₂KScF₆:0.05Cr³⁺ phosphor. (b) High-resolution Cr-2p XPS spectrum. (c, d) Calculated band structure, partial and total densities of states of Cs₂KScF₆ host.

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The SEM image exhibits that Cs₂KScF₆:Cr³⁺ phosphor crystallizes in regular particulate shape, and the smooth surfaces and an average size of 2 μm imply the high crystallinity of Cs₂KScF₆:Cr³⁺ (Fig. S2a). The EDS results and element mapping diagram (Fig. S2b and S2c) demonstrate that Cr³⁺ ions are uniformly dispersed throughout the sample particles. Based on the first-principles calculations and electron configurations of Cs (5p⁶6s¹), K (3p⁶4s¹), Sc (3d¹4s²), F (2s²2p⁵), the electronic structure of Cs₂KScF₆ host was calculated (Fig. 2c and 2d). The flat valence band and dispersive conduction band indicate that the host material has a direct bandgap, which is beneficial to realizing band-to-band luminescence because of its higher PL transition probability toward the indirect one (Ming et al., 2019). The wide bandgap value (6.835 eV) is not only preferable to incorporate Cr³⁺ energy levels, but also is effective to hinder thermal ionization and maintain good thermal stability (Chu et al., 2023). The valence band top is mainly dominated by F-2p orbitals; while the conduction band bottom is donated from Sc-3d orbitals, indicating that Sc-3d and F-2p orbitals partially hybridize to form Sc-F bond, and thus affecting luminescent properties of Cs₂KScF₆:Cr³⁺ (Fig. 2d) (Nie et al., 2021).

Fig. 3a expounds the PLE and PL spectra of Cs₂KScF₆:0.05Cr³⁺ phosphor monitored at RT. Two absorption bands located at ∼ 435 and ∼ 657 nm are observed in the PLE spectrum, which correspond to Cr³⁺:⁴A₂ → ⁴T₁(⁴F) and ⁴T₂(⁴F) spin-allowed transitions, respectively (Trueba et al., 2011). The stronger blue excitation band around 435 nm (FWHM=78 nm) perfectly covers the emission band of commercial InGaN chip (∼20 nm) (Qing et al., 2024), indicating the resultant Cs₂KScF₆:Cr³⁺ phosphor can be effectively pumped by blue InGaN chip. Moreover, two obvious dips were inspected from the ⁴A₂ → ⁴T₂ transition, which are mainly due to the destructive interference of strongly coupled ⁴T₂ levels on weakly coupled ²E/²T₁ levels, resulting in the reduction of excitation peak and the generation of other peaks (Nie et al., 2022). The PL spectrum presents a wide NIR emission band peaking at 778 nm (FWHM=95 nm), which is assignable to the Cr³⁺:⁴T₂ → ⁴A₂ transition in a weak octahedral crystal field (Adachi, 2020). According to the Tanabe-Sugano diagram (Fig. S3), Cr³⁺ emits broad NIR emission from ⁴T₂ to ⁴A₂ in a weak coordination field with Dq/B<2.3, whereas produces sharp line or narrowband emission from ²E to ⁴A₂ in a strong coordination environment (Zhou et al., 2022). Referring to the two excitation peak locations of 435 and 657 nm from PLE spectrum, the Dq, B and Dq/B values were calculated to be 1522 cm⁻¹, 875 cm⁻¹ and 1.74 (eqn (S1) and (S2) in the SM), respectively. The above results reveal that Cr³⁺ ions experience a weak crystal field in Cs₂KScF₆ to present NIR emission, which is similar to the luminescence behavior of Cr³⁺-activated fluoride phosphors reported previously (Wang et al., 2023).

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Fig. 3

(a) PLE and PL spectra of Cs₂KScF₆:0.05Cr³⁺ phosphors recorded at RT. (b) Concentration-dependent PL spectra of Cs₂KScF₆:xCr³⁺ (x = 0.01–0.09) samples and the inset presents the relationship between PL intensity and Cr³⁺ concentration. (c) PL decay curves of Cs₂KScF₆:xCr³⁺ phosphors. (d) Relationship between log(I/x) and log(x).

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For optimizing the luminescent quality of the as-obtained phosphors, Cs₂KScF₆:xCr³⁺ (x = 0.01–0.09) samples with various Cr³⁺ contents were prepared, and the corresponding PL spectra were measured (Fig. 3b). As the rising of Cr³⁺ doping content, the integral NIR emission intensity augments continuously until the critical doping level x  = 0.05. Affected by concentration quenching effect, the intensity starts to decrease when x value was further increased (Zhang et al., 2022). Thus, the optimal Cr³⁺ content is fixed as x  = 0.05 (the inset in Fig. 3b). The PL decay curves reveal the concentration quenching mode of Cs₂KScF₆:Cr³⁺ phosphor (Fig. 3c). All the decay curves live up to a single exponential equation, indicating that Cr³⁺ ions replaced the single site of Sc³⁺ in Cs₂KScF₆ host (Yuan et al., 2023). The average lifetime fitted by eqn (S3) is gradually dropped from 0.243 ms to 0.096 ms with the increase of x value, attributing to the intensified energy transfer among Cr³⁺-Cr³⁺ pairs. Based on the Blass formula given in eqn (S4), the critical distance (R_c) between adjacent Cr³⁺ ions was estimated as 19.51 Å in this case, which indicates multipolar interaction may be the reason for the non-radiative energy transfer in Cs₂KScF₆:Cr³⁺ phosphor (Wang et al., 2022). According to the Dexter theory, the fitting results obtained from eqn (S5) of log(I/x) against log(x) are provided (Fig. 3d), the corresponding θ value is equal to 2.403 (close to 3). One conclusion can be drawn that the energy transfer among the nearest or next nearest Cr³⁺ ions leads to the concentration quenching of Cr³⁺ in Cs₂KScF₆ host (Dai et al., 2019). Under such circumstance, the IQE and EQE values of the optimal Cs₂KScF₆:0.05Cr³⁺ sample is measured as 47.7 % and 8.25 %, manifesting that its prospect in NIR pc-LED devices. The integrated emission intensity of Cs₂KScF₆:0.05Cr³⁺ phosphor can preserve 84.1 % of the initial value after aging for 36 h under a high temperature of 85 ℃ and a high humidity of 85 % conditions, and still remain 74.9 % after being aged for 132 h, indicating that the as-prepared material has strong moisture resistance (Fig. S4).

The temperature-dependent PL spectra of Cs₂KScF₆:0.05Cr³⁺ measured from 298 K to 473 K are depicted in Fig. 4a, in which the PL intensity continuously diminishes along with the temperature goes up. It could be observed that 70 % of initial intensity is preserved at 423 K, strongly suggesting that the Cs₂KScF₆:Cr³⁺ phosphor possesses good luminous thermal stability (the red dot in Fig. 4b). Besides, the red-shift tendency of spectral position appears as elevated temperature, which could be traced to lattice expansion and decreasing crystal field strength at higher temperatures (Fig. S5) (Liu et al., 2022; Zhang et al., 2023). The FWHM enlarges significantly from 95 nm to 111 nm (the blue dot in Fig. 4b), which is imputed to intensified electron–phonon coupling (EPC) effect that takes the Huang-Rhys factor (S) as an evaluation parameter. Fig. 4c shows the fitting relationship of FWHM² versus 2kT referring to eqn (S6), a trustworthy correlation coefficient of R² = 0.987 and S value of 1.68 indicate that Cs₂KScF₆:Cr³⁺ phosphor is subjected to a relatively low EPC effect to resist non-radiative relaxation process, as a result, achieving well-pleasing PL thermal stability.

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Fig. 4

(a) Temperature-dependent PL spectra of Cs₂KScF₆:0.05Cr³⁺ phosphor from 298 K to 473 K. (b) The relationship between temperature and relative PL intensity, FWHM value. (c, d) The fitting results of FWHM² versus 2kT and ln[(I₀/I_T)-1] versus 1/kT. (e) Configurational coordinate diagram for transition mechanism of Cr³⁺ ions.

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A configuration coordinate diagram (Fig. 4e) could be used to illustrate the thermal-quenching mechanism of Cs₂KScF₆:Cr³⁺ phosphor. The energy difference from the bottom of ⁴T₂ state to the intersection point of ⁴A₂ and ⁴T₂ levels (C point) is determined as the activation energy (ΔE). The electrons of ⁴A₂ level are excited to the high-energy ⁴T₂ level under blue light illumination, and then most of them will return to the low-energy state in the form of NIR emission. If the temperature continues to rise, the electrons in the ⁴T₂ excited state could be further thermally excited to C point and then directly fall back to the ground state through non-radiative transition. During this transition process, energy consumption happens because of thermal activation and multi-phonon effects. To further explain the thermal quenching behavior of the Cs₂KScF₆:Cr³⁺ phosphor, the ΔE value was calculated by eqn (S7) (Li and Zhong, 2022). As shown in Fig. 4d, a linear relationship with a slope of −0.340 is plotted by employing ln[(I₀/I_T)-1] as a function of 1/kT, and the ΔE value is estimated to be 0.340 eV. Large activation energy demonstrates that the energy barrier is difficult to be crossed to induce non-radiative relaxation, resulting in the desirable PL thermal stability of Cs₂KScF₆:Cr³⁺ phosphor. Therefore, the large ΔE value contributes to reducing the non-radiative transition probability and inducing the low thermal quenching, and finally achiving a favorable PL thermal stability (Yan et al., 2022).

Given the excellent PL properties as well as low thermal-quenching behavior of Cs₂KScF₆:Cr³⁺ phosphor, an NIR pc-LED prototype was fabricated to evaluate the device performance of the synthesized material. Fig. 5a shows the EL spectra of the NIR pc-LED device measured at various drive currents ranging from 20 mA to 320 mA. In the whole spectra range, a primary NIR emission that comes from Cs₂KScF₆:Cr³⁺ phosphor and a weak blue emission that originates from InGaN chip can be clearly observed. Moreover, the growing drive current boosts a stable rise in NIR emission intensity without saturation, demonstrating that the Cs₂KScF₆:Cr³⁺ phosphor has great potential for high-power NIR pc-LED application. Fig. 5b presents the NIR output power and photoelectric conversion efficiency of NIR pc-LED devices against drive currents. Increasing the drive current from 20 mA to 320 mA, the output power monotonously enhances from 4.70 mW to 72.28 mW, superior to photoelectric performance of many reported NIR pc-LED devices (Table S2). Meanwhile, owing to the efficiency degradation of the blue LED chip, the conversion efficiency changes from 8.86 % to 6.83 %, which is also higher than most NIR phosphors reported so far (Table S2). For confirming the stability of the as-obtained phosphor in high-power LEDs, its dynamic process was monitored in real-time using an NIR thermal imager. The corresponding thermographs and working temperatures of the NIR pc-LED device operated at 200 mA for different irradiation times are presented in Fig. S6 and Fig. 5c, respectively. The operating temperature fluctuates around 42 °C at a constant current of 200 mA, which shows that the phosphors have good thermal conductivity and stability, the potential application for Cs₂KScF₆:Cr³⁺ phosphor could be found in high-power pc-LEDs.

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Fig. 5

(a) EL spectra of the fabricated NIR pc-LED measured at various drive currents in the range of 20–320 mA. (b) NIR output power and photoelectric conversion efficiency of NIR pc-LED device under different drive currents. (c) Working temperatures of NIR pc-LED device operating at 200 mA for different irradiation times. (d) Photographs of the fruits under natural and NIR light irradiation.

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Benefiting from the strong penetration of NIR light and the excellent luminescent properties of the developed NIR phosphor, the application demonstrations were implemented in night vision by using the LED device manufactured with Cs₂KScF₆:Cr³⁺ phosphor as light source. Fig. 5d displays images of the fruits taken by an industrial camera under natural and NIR modes. Compared with the colorful fruit photos taken in natural light, when lighting the NIR pc-LED lamp, the recognizable black-and-white photos of fruit that cannot be observed at night were also captured by the NIR camera. The above demonstration results confirm application potential of the Cs₂KScF₆:Cr³⁺ phosphor for high-power LED devices in the field of night vision.

4 Conclusion

In summary, a fluoride Cs₂KScF₆:Cr³⁺ NIR-emitting phosphor with double-perovskite structure was presented in this work. The Cs₂KScF₆:Cr³⁺ sample launches NIR light peaking at 778 nm (FWHM=95 nm) from ⁴T₂ → ⁴A₂ spin-allowed transition of Cr³⁺ activators. In view of the weak EPC effect, Cs₂KScF₆:Cr³⁺ phosphor shows good anti-thermal quenching behavior (70 % at 423 K). Coating the obtained phosphor on the surface of the InGaN chip to fabricate NIR pc-LED device, high output power of 72.28 mW and photoelectric conversion efficiency of 6.83 % at 320 mA drive current were achieved. This device was used as a light source to evaluate its prospect in night vision imaging, indicating the Cs₂KScF₆:Cr³⁺ phosphor have great potential in high-power LED devices.

CRediT authorship contribution statement

Lingxiang Chu: Writing – original draft, Methodology, Investigation, Data curation. Enbei Yang: Writing – review & editing. Han Ma: Writing – review & editing. Jing Wan: Writing – review & editing, Supervision, Conceptualization. Tao Yang: Writing – review & editing. Yi Qin: Writing – review & editing. Yanqing Ye: Writing – review & editing. Qiang Zhou: Writing – review & editing, Supervision, Resources, Project administration. Zhengliang Wang: Writing – review & editing.