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Unity photoluminescence quantum yield and superior stability achieved in lanthanide-doped lead-free Cs₂NaInCl₆ double perovskites for highly efficient light emitting diodes†

Hamza Shaili^a, Tae Kyong John Kim^b and Clemens Burda*^a
aDepartment of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106, USA. E-mail: burda@case.edu
^bSwagelok Center for Surface Analysis of Materials, School of Engineering, Case Western Reserve University, Cleveland, Ohio 44106, USA

First published on 18th June 2025

Introduction

To address these issues, lead-free double perovskites have gained significant attention as ideal alternatives for light-emitting applications. They offer tunable properties to generate superior luminescence through doping engineering, exceptional stability in diverse environments, and environmental compatibility.^13–15 Double perovskites (DPs), characterized by the general formula Cs₂A⁺B³⁺X₆ (A⁺ = Ag⁺, Na⁺, Cu⁺, and K⁺; B³⁺ = Bi³⁺, Sb³⁺, and In³⁺; X = Cl⁻, Br⁻, and I⁻), exhibit versatile properties depending on their composition, making them suitable for potentially many applications.^16–18 Despite their potential, however, most pristine DPs suffer from low photoluminescence quantum yields (PLQYs) owing to their indirect bandgap characteristics.^19,20

Lanthanide doping has proven to be a powerful strategy to tailor the optical properties of solid-state materials, particularly perovskites. Multiple reports have shown that doping can significantly enhance the luminescence of double perovskite, especially using rare earth elements. The sensitization of self-trapped excitons (STEs) can result in strong visible-light emission alongside near-infrared (NIR) emissions.^21–23 Notable examples include an 82.5% PLQY reported by Tran et al. for Yb³⁺-doped Cs₂AgBiBr₆ with excellent stability,²⁴ Zhou et al. reported strong white photoluminescence generated from STEs using Sb³⁺/Bi³⁺-codoped Cs₂NaInCl₆ with a 77% PLQY,²⁵ and Zhen et al. were able to achieve a white emission with 75.89% PLQY using Sb³⁺-doped Cs₂NaInCl₆ double perovskite.²⁶ Furthermore, Saikia et al. achieved a blue and short-wave infrared (SWIR) emission from Sb³⁺–Er³⁺-codoped Cs₂NaInCl₆ showing an impressive 93% PLQY,²⁷ marking the highest internal emission quantum yield achieved for doped double perovskites to date.

In this study, we present a breakthrough in the synthesis of highly fluorescent doped-Cs₂NaInCl₆ microcrystals with warm white emission. Using a simple precipitation method, we successfully synthesized Ag⁺, Bi³⁺, and lanthanide (Ln³⁺ = Yb³⁺, Er³⁺) tri-doped Cs₂NaInCl₆ microcrystals, achieving unity photoluminescence quantum yield (PLQY) for the first time. Specifically, the Yb³⁺-doped sample exhibited a 100% PLQY, while the Er³⁺-doped sample reached 99.8% PLQY. These samples displayed strong broadband emissions driven by self-trapped excitons (STEs), with warm white-light and NIR emissions at ∼990 nm for Yb³⁺ and ∼1540 nm for Er³⁺. Most remarkably, the samples retained 98% of their PLQY over 60 days, confirming their exceptional stability. To demonstrate the practical application of these materials, we assembled a white light-emitting diode (WLED) by coupling the powder samples with a UV LED chip, generating warm white emission with high efficiency. Our findings propose tri-doped Cs₂NaInCl₆ microcrystals as transformative materials for next-generation light-emitting applications, addressing the challenges of sustainability and efficiency for future LED lighting technologies.

Results and discussion

The double-perovskite (DPs) samples were synthesized using a multi-step precipitation method.^28–31 Structural characterization was performed to confirm the phase quality to detect any possible secondary phases present in the samples. Fig. 1a illustrates the powder X-ray diffraction (XRD) pattern for the various samples. The XRD data clearly show that the prepared double perovskite samples are of high quality and correspond to the face-centered cubic structure of Cs₂NaInCl₆ (ICSD no. 132718) with a space group of Fmm (Fig. 1b). No secondary phases were detected in any of the presented samples. With increasing Yb³⁺ and Er³⁺ content, the XRD peaks gradually shifted towards smaller angles. This shift is attributed to lattice deformation caused by the difference among the radii of Na⁺ (1.02 Å), In³⁺ (0.81 Å), Ag⁺ (1.15 Å), Yb³⁺ (0.93 Å), Er³⁺ (0.89 Å), and Bi³⁺ (1.22 Å). Thus, these observations are an indication of the successful insertion of dopants into the host lattice, inducing strain and resulting in the formation of [BiCl₆]³⁻, [AgCl₆]⁵⁻ and lanthanide [LnCl₆]³⁻ octahedrons which contribute to strong self-trapped exciton (STE) emission. Crucially, the XRD data confirms that the doping process does not compromise the phase purity or quality of the samples.

	Fig. 1 (a) XRD pattern for the undoped and (Yb3+ and Er3+) doped Cs2Na0.9Ag0.1In0.95Bi0.05Cl6 microcrystals. (b) Structure of the Cs2NaInCl6 host lattice. (c) and (d) SEM micrographs for the 5%Yb3+- and 5%Er3+-doped Cs2Na0.9Ag0.1In0.95Bi0.05Cl6 crystals, respectively. (e) Photograph of Yb3+-doped Cs2Na0.9Ag0.1In0.95Bi0.05Cl6 dispersed in toluene and drop-cast onto a glass substrate and then placed on top of a 365 nm UV lamp under ambient conditions. (f) Raman analysis of the pure and the various doped samples.

To investigate the size, morphology of the crystals, and elemental distribution, we performed scanning electron microscopy (SEM) alongside X-ray energy dispersive spectroscopy (EDS) analysis. Fig. 1c presents SEM micrographs of the Yb³⁺- and Er³⁺-doped Cs₂Na_0.9Ag_0.1In_0.95Bi_0.05Cl₆ microcrystals. For the Yb³⁺-doped Cs₂Na_0.9Ag_0.1In_0.95Bi_0.05Cl₆ sample, highly crystalline microcubes with varying sizes were observed suggesting high crystallinity and quality of the prepared microcrystals. In contrast, the Er³⁺-doped Cs₂Na_0.9Ag_0.1In_0.95Bi_0.05Cl₆, exhibited dissimilar shapes and sizes, indicating a different crystallization behavior. EDS analysis was conducted to quantify the stoichiometry of the samples. The data in Table S1 and S2 (ESI†) illustrate the stochiometric nature of the prepared sample by having a near-perfect balance of the main elements and dopant content in the samples.

X-ray photoelectron spectroscopy (XPS) analysis of the samples is shown in Fig. S2 and S3 (ESI†). The XPS data reveal the presence of the main elements (Na, Cs, In, and Cl) alongside the dopants (Ag, Er, and Yb). The dopant peaks were assigned by their specific binding energies of 366, 168, and 183 eV corresponding to Ag 3d, Er 4d, and Yb 3d, suggesting the successful insertion of dopant ions into the Cs₂NaInCl₆ host lattice. Although the bismuth peak was detected, it overlapped with a cesium peak. Quantitative elemental analysis using Multipack software aligns well with our EDS results, reaffirming the near-stoichiometric nature of the prepared samples. To assess luminescence properties, the 5%Yb³⁺-doped Cs₂Na_0.9Ag_0.1In_0.95Bi_0.05Cl₆ microcrystals were dispersed in DMSO and drop-cast onto a glass substrate as illustrated in Fig. 1e. When exposed to a 365 nm UV lamp, the coated sample exhibited intense warm white light emission, highlighting its strong photoluminescence.

Further vibrational analysis of lanthanide-doped Cs₂Na_0.9Ag_0.1In_0.95Bi_0.05Cl₆ microcrystals was conducted to inspect the purity of the sample and the effect of dopant insertion on the crystal structure and vibrational properties. The Raman spectra of the samples are displayed in Fig. 1f, where two distinct peaks at 145 and 295 cm⁻¹ are attributed to the A_1g and 2A_1g vibrational modes of the octahedral structure. The differences in peak intensities and broadening observed are indicative of lattice strain generated by dopant incorporation. This strain primarily impacts the stretching vibrations of the octahedra, leading to the observed broadening of the Raman peaks.³²

To explore the doping effect on the optical properties, UV-vis diffuse reflectance and steady-state photoluminescence (PL) measurements were performed. The diffuse reflectance spectra in Fig. 2a illustrate an increasing reflectance of the microcrystals as doping increases, starting from 400 nm and reaching high reflectance percentages in the 550–900 nm range in good agreement with the emission and PLQY data. The 5%Yb³⁺-doped Cs₂Na_0.9Ag_0.1In_0.95Bi_0.05Cl₆ exhibited the highest reflectance around the absorption edge (∼340 nm) and decreased to around 30% at 400 nm and spiked further to nearly 100% in the visible range. The same trend was observed for the other samples with relatively different intensities ranging from 75 to 95%.

	Fig. 2 (a) Diffuse reflectance spectra of (Yb3+- and Er3+-) doped Cs2Na0.9Ag0.1In0.95Bi0.05Cl6 microcrystals with different dopant concentrations. (b) and (d) PL spectra of (Yb3+- and Er3+-) doped Cs2Na0.9Ag0.1In0.95Bi0.05Cl6, both excited at 332 nm. (c) NIR emission from the 5%Yb3+ doped Cs2Na0.9Ag0.1In0.95Bi0.05Cl6 microcrystals excited at 332 nm at high power. (e) Schematic diagram of a luminescence mechanism for (Yb3+ and Er3+) doped Cs2Na0.9Ag0.1In0.95Bi0.05Cl6 under 332 nm. (f) Photoemission contour plot vs. time of 5%Yb3+ doped Cs2Na0.9Ag0.1In0.95Bi0.05Cl6.

Photoluminescence measurements were carried out across a range of excitation wavelengths (300–350 nm) and the most intense emission was recorded at 332 nm. The pristine material showed a weak emission in the visible range (450–670 nm) as shown in Fig. S4 (ESI†). On the other hand, Fig. 2b and d reveal the PL emission for the various doped samples. Upon lanthanide doping, the Cs₂Na_0.9Ag_0.1In_0.95Bi_0.05Cl₆ sample exhibited broad emission spanning 425–750 nm, originating from STE recombination driven by Jahn–Teller distortion of [AgCl₆]⁵⁻ octahedra.³³ The introduction of silver and bismuth dopants significantly enhances the STE PL intensity producing an outstanding emission yield as the dopant concentration increased. With Bi³⁺ (5%) and Ag⁺ (10%) concentrations fixed, varying amounts of Yb³⁺ and Er³⁺ doping were introduced leading to a significant increase in the emission and STE activity. These self-trapped excitons (STEs) are excitons (bound electron–hole pairs) that localize in the material due to strong interactions with the lattice itself. In materials with soft lattices and strong electron–phonon coupling, such as halide double perovskites (in our case: doped Cs₂NaInCl₆), the presence of an exciton can locally distort the crystal structure. This distortion creates a potential well that effectively “traps” the exciton in place, even in the absence of any defects or impurities. Because the trapping is self-induced and does not rely on additional factors like lattice imperfections, it is often referred to as “self-trapping”. In addition, the samples maintained the same broadband emission spectra spanning the entire 425–750 nm visible-light range with stronger emission attributed to enhanced STEs as a result of the localized states below the conduction band minimum created by dopant, particularly by lanthanide incorporation. No distinguished peak corresponding to any specific transition was detected individually as the emission was strong and broad enough to cover the entire range. When the lanthanide doping exceeded 5%, a decrease in the intensity of the emission was noticed as evidenced by a weaker emission at 10% of Yb³⁺ or Er³⁺ content.

The lanthanide doping not only improves the visible-range emission properties but also triggers NIR emission into the host materials owing to the newly introduced energy levels within the material. NIR emission was detected when the samples were excited at higher intensity, as shown in Fig. 2c. The NIR emission is strong evidence that the lanthanides were successfully incorporated within the Cs₂NaInCl₆ crystal structure, as supported by XPS data. Unlike the broadband visible emission, the NIR emission intensity increased with higher lanthanide doping. The observed NIR emission for Yb³⁺ primarily originates from the ²F_5/2 → ²F_7/2 transition, while the Er³⁺-induced emission is assigned to ⁴I_9/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 transitions in the visible range. The IR transition ⁴I_13/2 → ⁴I_15/2 from Er³⁺ was beyond the detection limit of our instrument. This energy transfer process is illustrated in Fig. 2e. To further elucidate energy transfer mechanisms, the streak image of 5%Yb³⁺-doped Cs₂Na_0.9Ag_0.1In_0.95Bi_0.05Cl₆ was recorded and is shown in Fig. 2f. The profile reveals excellent agreement between the steady-state emission spectrum and time-resolved emission behavior, characterized by intense, broad emission exhibiting a gradual decay. This confirms efficient energy transfer within the doped material, highlighting its potential for high-performance optical applications.

Time-resolved photoluminescence (TRPL) measurements were carried out on the different DPs prepared with varying dopant concentrations to investigate their fluorescence decay behavior. The PL decay curves of Yb³⁺- and Er³⁺-doped Cs₂Na_0.9Ag_0.1In_0.95Bi_0.05Cl₆ were measured upon excitation at 332 nm and are shown in Fig. 3a and b, respectively. The TRPL decay curves were fitted using tri-exponential functions, reflecting multiple relaxation dynamics within the samples. The pristine Cs₂NaInCl₆ sample showed very short lifetime carriers which are consistent with our previous findings and indicated by the very low PLQY.³⁶ The Cs₂Na_0.9Ag_0.1In_0.95Bi_0.05Cl₆ sample PL decay curves showed two components having a short-lived component (τ₁ = 274.81 ns) and a longer-lived component was also found (τ₂ = 1490.21 ns) as shown in Fig. S3 (ESI†). For 5%Yb³⁺-doped Cs₂Na_0.9Ag_0.1In_0.95Bi_0.05Cl₆, the PL decay curves exhibited a shorter-lived component (τ₁ = 85.59 ns), which might be generated as the result of energy transfer or shallow trap states. Furthermore, two longer-lived components were observed (τ₂ = 581.85 ns, τ₃ = 2076.42 ns) corresponding to the spin-triplet excited-state transitions introduced by the dopant ions.³⁴ However, due to the broad emission, the individual transitions could not be distinctly resolved. Similarly, for the 5%Er³⁺-doped Cs₂Na_0.9Ag_0.1In_0.95Bi_0.05Cl₆ sample, the PL decay curve included a shorter-lived component (τ₁ = 78.89 ns) and two longer-lived components (τ₂ = 603.71 ns, τ₃ = 1909.46 ns). The average lifetime of the Cs₂Na_0.9Ag_0.1In_0.95Bi_0.05Cl₆ sample was 1003.79 ns, and for 5%Yb³⁺-doped sample was 1290.851 ns, while the average lifetime for the 5%Er³⁺ sample was 1210.898 ns (Table S5, ESI†). Note that Yb³⁺ doping generated longer-lived excitons compared to Er³⁺ ions. The short component is generally related to the nonradiative filling of surface and/or trap states, while the longer lifetime components arise from the slower radiative relaxation process.³⁵ The tri-exponential fluorescence decay observed in these double perovskite samples likely originates from the various relaxation processes within the octahedral structures ([YbCl₆]³⁻ or [ErCl₆]³⁻) accompanied by Jahn–Teller-type distortions.²³

	Fig. 3 PL decay curves for (a) 5%Yb3+-doped Cs2Na0.9Ag0.1In0.95Bi0.05Cl6 and (b) 5%Er3+-doped Cs2Na0.9Ag0.1In0.95Bi0.05Cl6. (c) and (d) PLQY values as a function of lanthanide doping concentrations in % for (c) Yb3+ and (d) Er3+ ions. (e) PL intensity and (f) PLQY values recorded over the course of 60 days on a 20-day period on the samples exposed to ambient conditions.

Photoluminescence quantum yield (PLQY) was measured at room temperature immediately after the synthesis and every 20 days for the course of 60 days. The undoped Cs₂NaInCl₆ exhibited a low PLQY of 2.3% which is consistent with the literature value.³⁶ Upon co-doping with Bi³⁺ and Ag⁺, the PLQY increased drastically to around 68%, as shown in Fig. 3c and d. After extensive optimization, the ideal dopant concentrations were determined to be 5% for Bi³⁺ and 10% for Ag⁺. Upon inducing Yb³⁺ and Er³⁺ ions, the PLQY was significantly enhanced and reached a remarkable 99.8% for the 5%Er³⁺-doped Cs₂Na_0.9Ag_0.1In_0.95Bi_0.5Cl₆, and unity PLQY of 100% for 5%Yb³⁺-doped Cs₂Na_0.9Ag_0.1In_0.95Bi_0.5Cl₆ (Fig. S4, ESI†). We scanned the PL excitation spectrum over a range of wavelengths and the peak emission was found upon 332 nm excitation, which was then used equally for the steady-state PL and TR-PL measurements. The PLQY values were consistent in all the synthesized batches having values ranging between 100% (highest) and 99.7% (lowest) for Yb³⁺-doped Cs₂Na_0.9Ag_0.1In_0.95Bi_0.05Cl₆ samples and 99.8% (highest) to 99.5% (lowest) for the Er³⁺-doped Cs₂Na_0.9Ag_0.1In_0.95Bi_0.05Cl₆ samples as can be shown in Fig. S8 (ESI†) in detail. It is noteworthy that a further increase in doping concentration of lanthanide elements decreased the PLQY. These results demonstrate the critical role of dopant optimization in unlocking the peak emission properties in double perovskites.

To evaluate the phase stability and retention of the PL emission and PLQY of the samples after air exposure, we conducted measurement immediately after the samples were prepared and periodically over the course of 60 days, measuring PL data every 20-days. For the 5%Yb³⁺-Cs₂Na_0.9Ag_0.1In_0.95Bi_0.05Cl₆ sample, the PL measurements revealed exceptional stability, retaining 99% of its emission after 20 days and 98% after 60 days. Structural analysis via XRD confirmed high crystal phase stability with no signs of phase deterioration, as shown in Fig. S6 (ESI†). Notably, these results demonstrate superior phase stability under ambient conditions compared to other perovskite materials (Fig. S5, ESI†). The PLQY of the 5%Er³⁺-Cs₂Na_0.9Ag_0.1In_0.95Bi_0.05Cl₆ sample exhibited minimal degradation over the same period, decreasing slightly to 99% after 20 days and stabilizing at 98.5% after 60 days (Fig. 3f). These findings underscore the excellent environmental robustness of the doped Cs₂Na_0.9Ag_0.1In_0.95Bi_0.05Cl₆ material, making it a candidate for long-term optoelectronic applications. PLQY data is summarized in Table S6 (ESI†).

To test the suitability of our samples for light-emitting diode (LED) applications, we assembled a white-light emitting diode (WLED) by combining Cs₂Na_0.9Ag_0.1In_0.95Bi_0.05Cl₆, Yb³⁺-doped and Er³⁺-doped Cs₂Na_0.9Ag_0.1In_0.95Bi_0.05Cl₆ with a 365 nm UV-LED chip. The luminescence spectrum of the WLED based on Yb³⁺ and Er³⁺-doped Cs₂Na_0.9Ag_0.1In_0.95Bi_0.05Cl₆ is shown in Fig. 4b and d, covering nearly the entire visible region of the solar spectrum with a peak emissions at 560 nm and 590 nm, respectively. The Commission Internationale del’Éclairage (CIE) color coordinates of the WLED based on the Yb³⁺-doped sample were measured as (0.4892, 0.4823). The device achieved a color rendering index (CRI) of 91.3 with a luminous efficiency of 41 lm W⁻¹, and a correlated color temperature (CCT) of 2826 K, aligning well with the standard for warm white lighting (2000–3000 K).³⁷ Similarly, the Er³⁺-doped Cs₂Na_0.9Ag_0.1In_0.95Bi_0.05Cl₆-based WLED exhibited comparable performance, with CIE coordinates of (0.4931, 0.4743), a CRI of 89.5, a luminous efficiency of 39 lm W⁻¹, and a CCT of 2733 K as shown in Fig. 4e. For stability evaluation, measurements were conducted on two WLED devices: one freshly prepared and another continuously illuminated for 12 hours. The CRI was measured every two hours of operation, showing minimal decrease, as can be seen in Fig. S7 (ESI†). At the same time, the EL emission profiles of both devices were nearly identical, with only a minor variation observed, highlighting the stability and reliability of these materials for practical WLED applications. Furthermore, the samples were placed in 90% humidity and 95 °C temperature for further stability testing. Structural and optical characterization were performed on the sample to analyze its stability. Fig. S10 (ESI†) shows the XRD analysis of the fresh sample compared to the sample placed in high temperature and humidity. The XRD of these samples showed minimal decrease in signal intensity without signs of a phase change. Fig. S9 (ESI†) shows the emission spectrum of the fresh sample compared to the high-humidity/high-temperature sample and the sample immersed in water. The same trend was observed with a minimal decrease in emissions, and no extra emission peaks arose.

	Fig. 4 (a) Photograph of the WLED assembled by coating the DPs sample with a commercially 365 nm LED chip and luminescence emission of the WLED measured at ambient conditions for (a) and (b) 5%Yb3+ Doped Cs2Na0.9Ag0.1In0.95Bi0.05Cl6 and (c) and (d) 5%Er3+ Doped Cs2Na0.9Ag0.1In0.95Bi0.05Cl6, respectively. (e) Color coordinate and the color gamut of WLEDs with bottom one related to Cs2Na0.9Ag0.1In0.95Bi0.05Cl6, the middle one to 5%Er3+ Doped Cs2Na0.9Ag0.1In0.95Bi0.05Cl6 and the top one related to 5%Yb3+ Doped Cs2Na0.9Ag0.1In0.95Bi0.05Cl6.

Conclusion

Data availability

Conflicts of interest

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