Unveiling Local Electronic Structure of Lanthanide‐Doped Cs₂NaInCl₆ Double Perovskites for Realizing Efficient Near‐Infrared Luminescence

Abstract

Lanthanide ion (Ln³⁺)‐doped halide double perovskites (DPs) have evoked tremendous interest due to their unique optical properties. However, Ln³⁺ ions in these DPs still suffer from weak emissions due to their parity‐forbidden 4f–4f electronic transitions. Herein, the local electronic structure of Ln³⁺‐doped Cs₂NaInCl₆ DPs is unveiled. Benefiting from the localized electrons of [YbCl₆]³⁻ octahedron in Cs₂NaInCl₆ DPs, an efficient strategy of Cl⁻‐Yb³⁺ charge transfer sensitization is proposed to obtain intense near‐infrared (NIR) luminescence of Ln³⁺. NIR photoluminescence (PL) quantum yield (QY) up to 39.4% of Yb³⁺ in Cs₂NaInCl₆ is achieved, which is more than three orders of magnitude higher than that (0.1%) in the well‐established Cs₂AgInCl₆ via conventional self‐trapped excitons sensitization. Density functional theory calculation and Bader charge analysis indicate that the [YbCl₆]³⁻ octahedron is strongly localized in Cs₂NaInCl₆:Yb³⁺, which facilitates the Cl⁻‐Yb³⁺ charge transfer process. The Cl⁻‐Yb³⁺ charge transfer sensitization mechanism in Cs₂NaInCl₆:Yb³⁺ is further verified by temperature‐dependent steady‐state and transient PL spectra. Furthermore, efficient NIR emission of Er³⁺ with the NIR PLQY of 7.9% via the Cl⁻‐Yb³⁺ charge transfer sensitization is realized. These findings provide fundamental insights into the optical manipulation of Ln³⁺‐doped halide DPs, thus laying a foundation for the future design of efficient NIR‐emitting DPs.

The authors achieve efficient near‐infrared (NIR) luminescence in lanthanide‐doped Cs₂NaInCl₆, with a photoluminescence quantum yield of Yb³⁺ up to 39.4% via Cl⁻‐Yb³⁺ charge transfer sensitization. Temperature‐dependent spectroscopic and theory calculations reveal the characteristic local electron structure of t [YbCl₆]³⁻ octahedron in Yb³⁺‐doped Cs₂NaInCl₆, which boosts the NIR emissions of Yb³⁺ and Er³⁺ through Cl⁻‐Yb³⁺ charge transfer sensitization.

1. Introduction

Lead‐free double perovskites (DPs) with A₂B^IB^IIIX₆ stoichiometry have attracted much attention in recent years due to their good stability, low toxicity, and diversity of composition.^{[ 1 ]} These DPs are characterized by a 3D structure composed of alternating [B⁺X₆] and [B³⁺X₆] corner‐sharing octahedron with A⁺ ions occupying the voids. Several combinations for A₂B^IB^IIIX₆ DPs have been reported, wherein B⁺ can be Ag⁺, Na⁺, Li⁺, K⁺ and B³⁺ can be In³⁺, Sb³⁺, Bi³⁺, Tl³⁺, etc.^{[ 2 ]} Despite the attractive photophysical properties of these A₂B^IB^IIIX₆ DPs, their studies were mainly restricted to the visible spectral region. Hitherto, it is quite challenging to realize efficient near‐infrared (NIR) luminescence in these DPs.

To this regard, lanthanide ions (e.g., Yb³⁺, Er³⁺, Tm³⁺) with rich electronic energy levels were proposed for tailoring the optical performances of DPs toward the NIR regions. Among various A₂B^IB^IIIX₆ DPs, Cs₂Na(Ag)InCl₆ DPs have been widely reported as one of the excellent hosts for Ln³⁺ doping owing to the direct bandgap character and high chemical stability. It was reported that NIR emission from Yb³⁺ can be produced in Cs₂AgInCl₆ DPs via the sensitization of self‐trapped exciton (STE).^{[ 3 ]} For example, Kim et al. doped Yb³⁺/Er³⁺ into Cs₂AgInCl₆ nanocrystals, which exhibited characteristic NIR emissions of Yb³⁺ and Er³⁺ peaking at 996 and 1537 nm, respectively.^{[ 4 ]} However, the NIR photoluminescence (PL) quantum yield (QY) of these Ln³⁺‐doped Cs₂AgInCl₆ DPs remains low (<5%). Thus, substantial efforts have been made to overcome such obstacles and to enhance the NIR luminescence of Ln³⁺‐doped DPs. Typically, sensitizer (e.g., Bi³⁺) co‐doping or Na⁺/Ag⁺ alloying strategies have to be adopted. Nag et al. boosted the NIR emission of Ln³⁺ in Cs₂AgInCl₆ through co‐doping with Bi³⁺, which introduced a new optical absorption channel to sensitize the Ln³⁺ dopants of Yb³⁺ and Er³⁺.^{[ 5 ]} Lin et al. synthesized Bi³⁺/Yb³⁺ co‐doped Cs₂Na_0.6Ag_0.4InCl₆ DPs, in which Na⁺/Ag⁺ alloying broke the local site symmetry of Cs₂AgInCl₆ to enhance the NIR emission of Bi³⁺‐sensitized Yb³⁺.^{[ 6 ]} Because the optical transitions of Ln³⁺ are sensitive to the local coordination, the PLQY of Ln³⁺ ions in these lead‐free DPs strongly depends on the crystal structure around Ln³⁺. Unfortunately, the local electronic structure of Ln³⁺‐doped Cs₂NaInCl₆ and Cs₂AgInCl₆ DPs remains essentially untouched yet. To circumvent the complicated energy transfer procedures and difficulty of composition regulation, an unambiguous local structural analysis is a prerequisite to optimizing their optical performance for further applications.

Herein, we propose a facile strategy to boost the NIR luminescence of Ln³⁺ (Yb³⁺ and Er³⁺) in Cs₂NaInCl₆ DPs. Through a theoretical survey of the local electronic structure based on density functional theory (DFT) and Bader charge analysis calculations, we revealed that the characteristic local electronic structure of [YbCl₆]³⁻ octahedron in Cs₂NaInCl₆ DPs can greatly promote the Cl⁻‐Yb³⁺ charge transfer process. Benefiting from the Cl⁻‐Yb³⁺ charge transfer sensitization, intense NIR emission of Yb³⁺ in Cs₂NaInCl₆ DPs was achieved, with an intensity 142.2 times higher than the well‐established Cs₂AgInCl₆:Yb³⁺ counterparts. Temperature‐dependent PL spectroscopic measurements confirmed the efficient energy transfer path from Cl⁻‐Yb³⁺ charge transfer band (CTB) to Yb³⁺.^{[ 7 ]} Furthermore, we also achieved intense NIR emission of Er³⁺ in Cs₂NaInCl₆:Yb³⁺/Er³⁺ through Cl⁻‐Yb³⁺ charge transfer sensitization, the integrated intensity of which was 1510.2 times higher than that of Cs₂NaInCl₆:Er³⁺ counterparts, respectively.

2. Results and Discussion

Cs₂Na _x Ag_{1− x} InCl₆ and Cs₂Na _x Ag_{1− x} InCl₆:Yb³⁺ crystals with different Na/Ag ratios were synthesized via a hydrothermal method (Figure 1a). X‐ray diffraction (XRD) patterns of the crystals can be well indexed into cubic Cs₂AgInCl₆ (ICSD No. 244519) and Cs₂NaInCl₆ (ICSD No. 132718) without any impurities (Figure S1, Supporting Information), which indicates that the as‐prepared Cs₂Na _x Ag_{1− x} InCl₆:Yb³⁺ crystals have the typical double perovskite structure with space group of Fm $\overline{3}$ m (Figure 1b). These crystals were transparent with the size of several millimeters (Figure 1c). The absorption band of Cs₂Na _x Ag_{1− x} InCl₆:Yb³⁺ crystals located in the UV region, and band edges monotonically shifted from 355 to 283 nm as the Na/(Na+Ag) ratio increased from 0 to 1 (Figure S2, Supporting Information).

Figure 1.

a) Schematic diagram of the synthesis of Cs₂Na _x Ag_{1− x} InCl₆ and Cs₂Na _x Ag_{1− x} InCl₆:Yb³⁺ crystals with different Na/(Na+Ag) ratios. b) Crystal structure of Cs₂Na _x Ag_{1− x} InCl₆:Yb³⁺. c) Photographs of Cs₂Na _x Ag_{1− x} InCl₆:Yb³⁺ crystals. d) PL emission spectra of Cs₂Na _x Ag_{1− x} InCl₆:Yb³⁺ crystals excited by 365 nm. e) PL excitation (left) and emission (right) spectra of Cs₂Na _x Ag_{1− x} InCl₆:Yb³⁺ crystals. f) Integrated emission intensity of Yb³⁺ in Cs₂Na _x Ag_{1− x} InCl₆:Yb³⁺ crystals with different Na/(Na+Ag) ratios.

Upon excitation at 365 nm, NIR emission of Yb³⁺ can be produced in these Cs₂Na _x Ag_{1− x} InCl₆:Yb³⁺ DPs. The optimal NIR emission of Yb³⁺ was obtained when Na/(Na+Ag) ratio was 0.6 as reported previously (Figure 1d).^{[ 3} , ^{8 ]} However, it should be noted that the excitation peaks exhibited an obvious blue shift from 350 to 273 nm and the shape of the peaks became sharper with the Na/(Na+Ag) ratio rising from 0 to 1 (Figure 1e). Upon excitation with the best excitation wavelength of these Cs₂Na _x Ag_{1− x} InCl₆:Yb³⁺ DPs, it was observed that the NIR luminescence intensity of Yb³⁺ markedly increased by 135.6 times as the Na/(Na+Ag) ratio increased from 0 to 1 (Figure 1f). According to the PL decays of Yb³⁺, the lifetime of Yb³⁺ increased from 2.72 to 4.52 ms with increasing the Na/(Na+Ag) ratio from 0 to 1 (Figure S2, Supporting Information). Intriguingly, Cs₂NaInCl₆:Yb³⁺ exhibited the highest NIR luminescence intensity and longest PL lifetime of Yb³⁺ among the Cs₂Na _x Ag_{1− x} InCl₆:Yb³⁺ DPs, which had not been reported before.

To explore the NIR luminescence mechanism of Yb³⁺ in Cs₂AgInCl₆ and Cs₂NaInCl₆, we synthesized Cs₂AgInCl₆:Yb³⁺ and Cs₂NaInCl₆:Yb³⁺ DPs with different contents of Yb³⁺. XRD patterns confirmed the pure phase of these samples (Figure S3, Supporting Information). X‐ray photoelectron spectra analysis revealed the existence of Yb³⁺ ions in the as‐prepared DPs (Figure S4, Supporting Information). For Cs₂AgInCl₆, the feeding concentrations of Yb³⁺ were from 50% to 200%, while the actual Yb³⁺ concentrations in the crystal lattice were identified to be only from 1% to 15.5% based on the inductively coupled plasma atomic emission spectra analysis (Table S1, Supporting Information).^{[ 3a ]} By monitoring the Yb³⁺ emission at 994 nm, a broad excitation band (250–400 nm) centered at ≈350 nm was detected (Figure 2a), which was associated with the bandgap absorption of Cs₂AgInCl₆. Upon excitation at 365 nm, Cs₂AgInCl₆:Yb³⁺ with different Yb³⁺ concentrations exhibited weak NIR PL (Figure S5, Supporting Information and Figure 2b). PL decays revealed decreased PL lifetime from 2.76 to 2.54 ms with the concentration of Yb³⁺ from 1.0% to 15.5% (Figure S5, Supporting Information). Diffuse reflectance spectra of Cs₂AgInCl₆:Yb³⁺ exhibited an intense absorption at ≈358 nm (3.47 eV) (Figure 2c), which agrees well with the absorption spectrum of pure Cs₂AgInCl₆.^{[ 2b ]}

Figure 2.

a) PL excitation spectra of Cs₂AgInCl₆:Yb³⁺ and Cs₂NaInCl₆:Yb³⁺ with different Yb³⁺ concentrations. b) Integrated Yb³⁺ emission intensity of Cs₂AgInCl₆:Yb³⁺ and Cs₂NaInCl₆:Yb³⁺ with different Yb³⁺ concentrations. c) Diffuse reflectance spectra of Cs₂AgInCl₆:Yb³⁺ with different Yb³⁺ concentrations. d) Diffuse reflectance spectra of Cs₂NaInCl₆:Yb³⁺ with different Yb³⁺ concentrations.

For Cs₂NaInCl₆, we adopted the same feeding concentrations as those in Cs₂AgInCl₆, resulting in also low concentrations of Yb³⁺ from 0.4% to 8.7% into the Cs₂NaInCl₆ lattice (Tables S2 and S3, Supporting Information). When monitoring the Yb³⁺ emission of 994 nm, a sharp excitation peak at 273 nm was detected, which was ≈70 nm blue‐shift compared with that of Cs₂AgInCl₆:Yb³⁺ (Figure 2a). Meanwhile, the full‐width of half‐maximum (FWHM) of the excitation peak (≈30 nm) was much narrower than that (≈60 nm) of Cs₂AgInCl₆:Yb³⁺. Diffuse reflectance spectrum of pure Cs₂NaInCl₆ exhibited an ultra‐weak absorption band in the visible region and the bandgap was determined to be 4.45 eV (Figure 2d).^{[ 2} , ^{9 ]} However, a new and sharp absorption peak appeared at ≈273 nm when Yb³⁺ was introduced in Cs₂NaInCl₆. With increasing the Yb³⁺ concentration, this absorption peak increased and reached the strongest when the Yb³⁺ concentration was 6.9%. According to the previous report, this sharp excitation peak can be well conformed to the CTB absorption.^{[ 10 ]} Particularly, upon excitation at 273 nm, the NIR luminescence intensity of Yb³⁺ was observed to be 142.2 times higher than that of the Cs₂AgInCl₆:Yb³⁺ counterpart with the optimal doping concentration (Figure 2b). The highest PLQY of Yb³⁺ in Cs₂NaInCl₆:Yb³⁺ reaches 39.4%, which is higher than most of the lead‐free halide DPs (Table S4, Supporting Information). Note that the NIR PLQY of Cs₂AgInCl₆:Yb³⁺ counterpart was less than 0.1% under otherwise identical conditions. Furthermore, the PL lifetime of Yb³⁺ in Cs₂NaInCl₆:Yb³⁺ was determined to decrease from 4.54 to 4.11 ms with the increase of Yb³⁺ concentration from 0.4% to 8.7% (Figure S5, Supporting Information), which was much longer than that in Cs₂AgInCl₆:Yb³⁺.

To shed more light on the NIR luminescent mechanism of Yb³⁺, first‐principles calculations based on hybrid DFT were carried out. We replaced the central In³⁺ ion with Yb³⁺ ion in a 2 × 2 × 2 supercell of Cs₂AgInCl₆:Yb³⁺ and Cs₂NaInCl₆:Yb³⁺ (Figure S8, Supporting Information). The bandgaps of Cs₂AgInCl₆:Yb³⁺ and Cs₂NaInCl₆:Yb³⁺ were determined to be 3.21 and 4.38 eV, respectively, wherein Yb³⁺ made no contributions to the valence band maximum (VBM) or conduction band minimum (CBM) (Figure 3a,b). The partial density of states analysis and orbital distribution profiles of Cs₂AgInCl₆:Yb³⁺ showed that VBM was composed of mixed configuration of Ag 4d and Cl 3p states, and CBM mainly consisted of In 5s states with minor contributions from Ag 4d and Cl 3p states (Figure 3c,d). Such configuration benefited the formation of STE, which resulted from the Jahn–Teller distortion of the connected [AgCl₆]⁵⁻‐[InCl₆]³⁻ octahedron.^{[ 2b ]} For Cs₂NaInCl₆:Yb³⁺, VBM and CBM were essentially composed of Cl 3p states and In 5s states, respectively, which revealed that the orbitals were distributed over the whole supercell with little spatial overlap (Figure 3e,f). Such poor spatial overlap led to the extremely weak edge‐to‐edge transition in this system.^{[ 11 ]} From the above partial density of states analysis, it can be seen that Cl 3p states coupled with Ag 4d states in VBM of Cs₂AgInCl₆:Yb³⁺, which thus weakened the coupling of Cl and Yb and may be adverse to the Cl⁻‐Yb³⁺ charge transfer process in [YbCl₆]³⁻ octahedron. By contrast, VBM of Cs₂NaInCl₆:Yb³⁺ was mainly composed of Cl 3p states without the contributions from Na, benefiting the coupling of Cl⁻ and Yb³⁺ and favoring the Cl⁻‐Yb³⁺ charge transfer process.

Figure 3.

Partial density of states for a) Cs₂AgInCl₆:Yb³⁺ and b) Cs₂NaInCl₆:Yb³⁺. Orbital distribution profiles of c) VBM and d) CBM in Cs₂AgInCl₆:Yb³⁺ (Cs atoms are not displayed). Orbital distribution profiles of e) VBM and f) CBM in Cs₂NaInCl₆:Yb³⁺ (Cs atoms are not displayed).

The different electronic structures of Cs₂NaInCl₆:Yb³⁺ and Cs₂AgInCl₆:Yb³⁺ DPs were further verified by Bader charge analysis. In Cs₂AgInCl₆:Yb³⁺, Ag⁺ and Cl⁻ around Yb³⁺ had charge of +0.642 and −0.655, respectively. Besides, [YbCl₆]³⁻ octahedron had a charge of −2.116, which confirmed that the electron of Cl⁻ ion was delocalized toward Ag⁺ due to the high covalency of the Ag—Cl bond (Figure 4a).^{[ 12 ]} As such, the 3d orbit of Ag⁺ may catch electrons from Cl⁻, which thus impeded the charge transfer from Cl⁻ to Yb³⁺, as revealed by the electron localization function (ELF) analysis (Figure 4b,c).^{[ 13 ]} By contrast, Na⁺ ion in the Cs₂NaInCl₆:Yb³⁺ almost ionized completely with a charge of +0.857 and neighboring Cl⁻ with a charge of −0.753 (Figure 4a). Meanwhile, [YbCl₆]³⁻ octahedron had a charge of −2.623, indicating that the electron may localize in the [YbCl₆]³⁻ octahedron. Moreover, it was determined that the ELF between Na⁺ and Cl⁻ was almost zero due to the ionic bond characteristic (Figure 4e). Such weak interaction between Na and Cl in Cs₂NaInCl₆:Yb³⁺ may greatly promote the Cl⁻‐Yb³⁺ charge transfer process (Figure 4f).

Figure 4.

a) Bader charge analysis and b) ELF of Cs₂AgInCl₆:Yb³⁺. c) Schematic diagram of the structure of Cs₂AgInCl₆:Yb³⁺. d) Bader charge analysis and e) ELF of Cs₂NaInCl₆:Yb³⁺. f) Schematic diagram of the structure of Cs₂NaInCl₆:Yb³⁺.

Furthermore, we carried out temperature‐dependent steady‐state and transient PL spectroscopic measurements to gain deep insights into the excited‐state dynamics of Yb³⁺ in Cs₂NaInCl₆. For pure Cs₂NaInCl₆, blue STE emission located at ≈450 nm with the FWHM of ≈75 nm was observed with temperatures below 200 K (Figure 5a,b). The integrated intensity of STE at 10 K was 26.3 times higher than that at 300 K. Accordingly, the activation energy was determined to be 76 meV (Figure S6, Supporting Information), indicating excellent thermal stability of Cs₂NaInCl₆.^{[ 14 ]} The excitation spectra of STE peaking at ≈290 nm for Cs₂NaInCl₆ were associated with the bandgap absorption. Nevertheless, the excitation spectra of Yb³⁺ exhibited sharp peaks ranging from 265 to 273 nm for Cs₂NaInCl₆:Yb³⁺ (Figure 5c), which was distinct from the excitation spectra of pure Cs₂NaInCl₆, suggesting that they were originated from different processes. Upon excitation at 273 nm, a series of characteristic Yb³⁺ emission peaks were observed (Figure 5d). Besides, several vibronic peaks appeared at temperatures below 200 K, which were attributed to the vibrational modes of [YbCl₆]³⁻ (Figure S7, Supporting Information).^{[ 15 ]} The PL lifetime of ²F_5/2 of Yb³⁺ decreased from 8.17 ms at 10 K to 4.54 ms at 300 K due to the thermal quenching at high temperatures (Figure S6, Supporting Information).

Figure 5.

Temperature‐dependent a) excitation spectra (λ_em = 450 nm) and b) emission spectra (λ_ex = 290 nm) of Cs₂NaInCl₆. Temperature‐dependent c) excitation spectra (λ_em = 994 nm) and d) emission spectra (λ_ex = 273 nm) of Cs₂NaInCl₆:6.9% Yb³⁺. e) PL emission spectra of Cs₂NaInCl₆:6.9% Yb³⁺ at 10 K (λ_ex = 273 nm). f) Schematic illustration of the electronic transitions of Yb³⁺ in Cs₂NaInCl₆.

Particularly, upon excitation at 273 nm at 10 K, two peaks with an energy gap of ≈9766 cm⁻¹ were observed for Cs₂NaInCl₆:Yb³⁺, which agreed well with the energy gap between ²F_5/2 and ²F_7/2 of Yb³⁺ (Figure 5e). These two peaks can be attributed to the transitions from CTB to ²F_7/2 (Yb³⁺) and ²F_5/2 (Yb³⁺), respectively.^{[ 10d ]} Such a result explicitly validated the existence of Cl⁻‐Yb³⁺ CTB.^{[ 16 ]} Thus, the energy transfer process of Yb³⁺ in Cs₂NaInCl₆ was proposed in Figure 5f. Upon UV excitation at 273 nm, the Yb³⁺ ion is excited from the 4f ground state (²F_7/2) to the Cl⁻‐Yb³⁺ CTB, followed by a fast relaxation process to the 4f excited state (²F_5/2) through thermal activation. Then, intense NIR emission of Yb³⁺ at 994 nm can be detected due to the radiative transition from ²F_5/2 to ²F_7/2.

Besides Yb³⁺, another Ln³⁺ dopant, Er³⁺, was employed to produce NIR emissions (Table S5, Supporting Information). Figure 6a shows the PL excitation spectra of Er³⁺ singly doped and Yb³⁺/Er³⁺ co‐doped Cs₂NaInCl₆ DPs. By monitoring the Er³⁺ emission at 1540 nm, the excitation peaks at 380 and 520 nm were detected for Cs₂NaInCl₆:Er³⁺ DPs, which belonged to ⁴I_15/2 → ⁴G_11/2 and ⁴I_15/2 → ²H_11/2 transitions of Er³⁺, respectively. For Cs₂NaInCl₆:Yb³⁺/Er³⁺ DPs, a strong peak at 273 nm corresponding to the Cl⁻‐Yb³⁺ CTB excitation appeared beside the above‐mentioned excitation peaks of Er³⁺. Upon excitation at 273 nm, Cs₂NaInCl₆:Yb³⁺/Er³⁺ DPs showed strong NIR emission peaking at 994 and 1540 nm corresponding to the ²F_5/2 → ²F_7/2 transition of Yb³⁺ and ⁴I_13/2 → ⁴I_15/2 of Er³⁺, respectively (Figure 6b). Note that the optimal integrated NIR intensity of Cs₂NaInCl₆:Yb³⁺/Er³⁺ DPs was 1510.2 times higher than that of Cs₂NaInCl₆:Er³⁺ counterparts (Figure 6b,c). The highest NIR PLQY of Cs₂NaInCl₆:Yb³⁺/Er³⁺ DPs was determined to be 7.9% (Table S4, Supporting Information).

Figure 6.

a) Excitation spectra (λ_em = 1540 nm) and b) emission spectra of Cs₂NaInCl₆ (λ_ex = 273 nm) doped with different contents of Yb³⁺ and Er³⁺. c) Integrated intensity of Yb³⁺ emission (purple) and Er³⁺ emission (pink) in Cs₂NaInCl₆:6.9%Yb³⁺/Er³⁺ with different contents of Er³⁺. d) PL decays of Yb³⁺ in Cs₂NaInCl₆:Yb³⁺/Er³⁺ with different contents Yb³⁺ and Er³⁺ by monitoring the emission at 994 nm. e) PL decays of Er³⁺ in Cs₂NaInCl₆:Yb³⁺/Er³⁺ with different contents Yb³⁺ and Er³⁺ by monitoring the emission at 1540 nm. The initial fast rise portion was enlarged in the inset. f) Schematic diagram of energy transfer process in Cs₂NaInCl₆ with simplified energy levels of Yb³⁺ and Er³⁺.

Moreover, with the increase of Er³⁺ concentration in Cs₂NaInCl₆:6.9%Yb³⁺/xEr³⁺ DPs, it was discovered that the integrated intensity of Er³⁺ emission continuously increased while the Yb³⁺ emission steadily decreased, indicative of the energy transfer from Yb³⁺ to Er³⁺. Meanwhile, the PL lifetime of Yb³⁺ in Cs₂NaInCl₆:6.9%Yb³⁺/xEr³⁺ DPs decreased from 4.29 to 3.06 ms with the content of Er³⁺ increasing from 0.03% to 4.0%, which also verified the enhanced energy transfer from Yb³⁺ to Er³⁺ (Figure 6d). Furthermore, a decreased rising edge from 4.71 to 2.14 ms can be observed from the PL decays of Er³⁺ by monitoring the emission at 1540 nm, revealing the faster electron population process with increasing content of Er³⁺ (Figure 6e). The energy transfer efficiency (η _ET) can be calculated as^{[ 17 ]}

$${\eta }_{\mathrm{ET}}=1-\frac{{\tau }_{\mathrm{s}}}{{\tau }_0}$$ (1)

where τ ₀ and τ _s display the Yb³⁺ lifetime (monitored at 994 nm) in the absence and presence of Er³⁺, respectively. Based on effective lifetime changes of different content Er³⁺‐doped Cs₂NaInCl₆:6.9%Yb³⁺/xEr³⁺ DPs, η _ET were calculated to be 11.3%, 23.4%, 29.5%, and 29.7% with the Er³⁺ content of 0.03%, 2.9%, 3.1%, and 4.0%, respectively. Thus, the energy transfer mechanism in Cs₂NaInCl₆:Yb³⁺/Er³⁺ is illustrated in Figure 6f. Upon excitation to the Cl⁻‐Yb³⁺ CTB, the excitation energy is transferred to the ²F_5/2 (Yb³⁺) level through a fast nonradiative relaxation process, followed by the radiative transition of Yb³⁺ at 994 nm and energy transfer to the well‐matched ⁴I_11/2 level of Er³⁺. Through the nonradiative relaxation from ⁴I_11/2 to ⁴I_13/2, the NIR emission at 1540 nm can be produced due to the ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺.

3. Conclusion

In summary, we have unveiled the different local electronic structures of Ln³⁺ ions‐doped Cs₂NaInCl₆ DPs. Accordingly, a novel strategy for achieving efficient NIR luminescence of Ln³⁺ in Cs₂NaInCl₆ DPs was proposed, resulting in anNIR PLQY up to 39.4% of Yb³⁺ by virtue of the Cl⁻‐Yb³⁺ charge transfer sensitization. Through systematically investigating the PL excitation and emission spectra of Cs₂AgInCl₆:Yb³⁺ and Cs₂NaInCl₆:Yb³⁺, we revealed the superior sensitization paths of NIR emission of Yb³⁺ in Cs₂NaInCl₆ relative to that in Cs₂AgInCl₆. Notably, the Cs₂NaInCl₆:Yb³⁺ exhibited 142.2 times higher NIR PL intensity than the Cs₂AgInCl₆:Yb³⁺ counterparts. Temperature‐dependent PL excitation and emission spectra verified that the proposed Cl⁻‐Yb³⁺ charge transfer sensitization mechanism benefited from the localized electrons of [YbCl₆]³⁻ octahedron in Cs₂NaInCl₆:Yb³⁺, which was also confirmed by the theoretical analysis. Furthermore, efficient NIR luminescence from Er³⁺ with PLQY of 7.9% was also achieved in Yb³⁺/Er³⁺ co‐doped Cs₂NaInCl₆ DPs due to the energy transfer from the Cl⁻‐Yb³⁺ CTB to Er³⁺. These findings provide a universal approach for the development of highly efficient Ln³⁺‐doped NIR luminescent halide DPs, which might pave a new way to manipulate the optical properties of Ln³⁺‐doped DPs toward versatile applications.

Conflict of Interest

The authors declare no conflict of interest.

Supporting information

Supporting Information

Han S., Tu D., Xie Z., Zhang Y., Li J., Pei Y., Xu J., Gong Z., Chen X., Unveiling Local Electronic Structure of Lanthanide‐Doped Cs₂NaInCl₆ Double Perovskites for Realizing Efficient Near‐Infrared Luminescence. Adv. Sci. 2022, 9, 2203735. 10.1002/advs.202203735

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.