Evidence and engineering of x-ray luminescence in lead free perovskite Cs₂AgInCl₆ with Bi substitutions

Because of the vast variety of applications in non-destructive real-time medical diagnosis, product investigation, crystal structure determination, scientific study, and astronomy, exploration for x-ray detectors dated back more than 100 years soon after the discovery of x rays.^1–4 The functioning concept of modern x-ray detectors is based on two mechanisms: photon to current and photon to UV–visible photon downconversion. In the first process, x-ray energy was transformed directly into electrical signals, but in the second method, x rays were transformed into photons by scintillator phosphors and then detected by a photodetector operating at lower photon energies.³ Both mechanisms for developing low-cost large-area and high-conversion-coefficient materials must be investigated. As a result, both are fundamentally and equally appealing for practical purposes.

Organic and inorganic hybrid lead perovskites spurred as an exciting class for their many exceptional x-ray detector properties. Besides their good detection efficiency solution processing, low cost fabrication MAPbI₃ thin films, MAPbBr₃ single crystals, revolutionized the research trends for x-ray detectors offering a compelling combination of fast photo-response and a high absorption cross section for x rays.⁵ Photodiodes and photoconductors synthesized by the solution process demonstrate exceptional for x-ray sensitivity and responsibility of 25 μCmGyair⁻¹ cm⁻³ and 1.9 × 10⁴ carriers/photon, respectively.⁵ A comparative analysis of the x-ray scintillation properties of three-dimensional (3D) MAPbI₃ and MAPbBr₃, along with two-dimensional (2D) (EDBE)PbCl₄ hybrid perovskite crystals, reveals several advantageous characteristics. These include low fabrication costs, a low intrinsic trap density (n₀ ∼ 10⁵–10⁷ cm⁻³), and a rapid nanosecond response time of 5.95 × 10⁻⁴.⁶ Thermo-luminescence measurements have revealed that perovskite crystals possess a much lower trap density compared to conventional oxide scintillator materials.

Current x-ray imagers use about 3336 g of lead per square meter, which far exceeds the EU RoHS regulation limit of 1000 ppm. The highest reported detection limit for lead halide perovskite materials is 0.036 μGyair s⁻¹.^7,8 This suggests that there is still room for improvement, as reducing the radiation dose during medical examinations could significantly decrease the cancer risk associated with x-ray exposure. Moreover, x-ray security screening systems also require detectors with low detection limits, given that each inspection uses an x-ray dose of 0.25 μGy. Therefore, it is essential to develop lead-free, solution-processed scintillators (SCs) for x-ray detection, aiming to achieve better sensitivity and lower detection limits.⁹ 

Compounds of the form A₂B^IB^IIIX₆ have emerged as promising lead-free perovskite alternatives. In these compounds, A represents small cations, B^I includes monovalent metal ions such as Na, Ag, and K, and B^III comprises trivalent metal ions such as Bi and In.^9,10 The halides X can be Br, Cl, or F. Among these, Cs₂AgBiBr₆ and Cs₂AgInCl₆ (CAIC) have shown promising photovoltaic properties, including long carrier recombination lifetimes and good stability against air and moisture. However, Cs₂AgBiBr₆ has an indirect bandgap, whereas CAIC possesses a direct bandgap.^9,11,12 Recent studies have extensively explored these materials for advanced applications. For instance, Cs₂AgBiBr₆ single crystals have been used to develop sensitive x-ray detectors. By largely eliminating Ag⁺/Bi³⁺ disorder and improving crystal resistivity, these detectors achieved a minimum detectable dose rate as low as 59.7 nGyair s⁻¹.^13,14 Cs₂AgBiBr₆-based x-ray detectors utilize the photon-to-current method. However, the indirect bandgap of Cs₂AgBiBr₆ hinders its optical properties for photon-to-light emission in x-ray detectors. In contrast, the direct bandgap of CAIC enhances its suitability for such applications.^15–17 

CAIC has a direct bandgap of 3.2 eV and can be synthesized both as powder and as single crystals. Although CAIC has parity forbidden transitions, which can be controlled, CAIC has applications as a visible-blind UV detector.¹⁸ Given the new interesting physical and chemical phenomena of CAIC, the optical properties deserve further investigation especially for x-ray scintillation applications, which are still void.

Here, we present a new characteristic, x-ray luminescence for CAIC for scintillation applications. Furthermore, we modulated the x-ray luminescence by Bi alloying and varying power of x-ray source. Our work highlights x-ray luminescence for Cs₂AgIn_(1−x)Bi_xCl₆ (0 ≤ x ≤ 50) (CAIBC) for x-ray scintillations. Thermo-luminescence investigations were carried out to explore the trap density and states of CAIBC. CAIC shows greater stability for x-ray exposure as no bleaching effect or reduction in luminescence observed as happened in laser excitation.

Figure 1(a) represents the XRD pattern of CAIBC powder, grinded from crystals. When Bi alloyed from 15% to 50% into the host, the XRD pattern reveals that the CAIBC retains the same structure with the same cubic phase of $Fm3̄m$ space group and no secondary peak appears in the XRD pattern. The characteristic diffraction peak (220) of the XRD pattern shifts to the lower angle from 23.92° for CAIC to 23.63° for Cs₂AgIn_0.5Bi_0.5Cl₆. The shifting of XRD peaks to smaller angles provides evidence of change in chemical equilibrium in the structure. Figure 1(b) shows the Rietveld refined results of CAIC powder. The Rietveld refined results reveal that CAIC has a cubic structure with a lattice parameter of a = b = c = 10.48 Å, consistent with the literature report.^10,19 The Rietveld refinement of the XRD patterns for varying Bi concentrations demonstrates a linear increase in lattice parameters, correlating with shifts in the XRD peaks. As the Bi concentration increases, the Ag–Cl bond length decreases, while the In–Cl bond length increases, attributed to a larger size of Bi compared to In, exerting pressure within the unit cell. Notably, the crystal phase and space group remain unchanged up to a 50% Bi concentration, as detailed in Table I.²⁰ 

Figure 1(c) shows well shaped and large size crystals, which represents the smooth growth of crystals during our synthesis process.

X-ray photoelectron spectroscopy (XPS) was used to characterize the chemical composition of CAIC and Cs₂AgIn_0.70Bi_0.30Cl₆ as shown in Fig. 2. The observed peak positions are consistent with the presence of Cs⁺, In³⁺, Ag⁺, Cl⁻, and Bi³⁺ in the sample. The change in energies of In³⁺ also infers the change in chemical composition and presence of Bi with alloying.

X-ray scintillation yield is inversely proportional to the optical bandgap. Low-bandgap perovskites of CAIBC are expected to have more light yields.^19–23 This is because the bandgap of CAIC decreases with Bi alloying. The absorption coefficient, α, of x rays is determined by α ∝Z⁴/E³, where Z is the atomic number and E is the radiation energy. Lead free double perovskites contain the heaviest stable element and have an average Z value of 53.1, higher than MAPbI₃ (Z = 48.9), MAPbBr₃ (Z = 45.1), and α-Se (Z = 34), thus allowing for more efficient x-ray attenuation.¹³ Cs₂AgInCl₆ SCs reported for low trap-state density with a high ON–OFF ratio.¹⁸ The bandgap of Cs₂AgInCl₆ decreased from 3.1 to 2.68 eV with an increase in Bi substitution.¹⁹ Finally, long emission wavelengths ranging from 350 to 1200 nm enable optimal detection of scintillation.²⁴ 

The x-ray excited luminescence and photoluminescence spectra of CAIBC crystals under ambient conditions are shown in Fig. 3. Both x-ray excited luminescence and photoluminescence spectra of CAIBC have broadband peaks. For CAIC, both spectra exhibit peaks centered at 607 and 609 nm, respectively. With 50% Bi alloying, both spectra exhibit peaks centered at 660 and 658 nm, respectively. The x-ray excited luminescence peak is fitted with two Gaussian curves. The integrated areas of both curves are given in Table of the supplementary material. Peak 1 and peak 2 are related to defect states and self-trapped excitons (STEs), respectively. With Bi alloying STE become stable more up to 30% and x-ray luminescence become high stand with more alloying up to 50% defect state contributions become more strong and eventually x-ray luminescence decreased. The Gaussian fittings are shown in Fig. S2 of the supplementary material. The photoluminescence (PL) spectra of CAIBC under 325 nm laser excitation show that all alloyed samples exhibit broad PL emissions similar to pure CAIC compounds. The original CAIC, being a direct gap semiconductor, has weak emissions due to parity-forbidden transitions. Bi alloying introduces lattice distortion, which breaks inversion symmetry, modifies parity, and results in enhanced light emission.^25,26 This mechanism is attributed to self-trapped excitons (STEs), which dominate the photoluminescence in Bi-alloyed CAIC. The lattice’s soft nature, structural distortion, and the Huang–Rhys factor play crucial roles in this process.^27,28 As Bi is introduced, the Bi–Cl bond length decreases, while the In–Cl bond length increases as given in Table I, affecting the Huang–Rhys factor and, consequently, the luminescence intensity.^25,29 In all perovskite crystals, the x-ray excited luminescence and photoluminescence spectra are remarkably similar. This similarity indicates a straightforward dominant scintillation mechanism. Upon x-ray absorption, high-energy excitations undergo thermalization through ionizations and excitations of atoms until excitons are generated. The x-ray excited luminescence partly arises from the excitonic emission of the perovskites, with self-trapped excitons playing a role in the scintillation process.

The parameters were derived from the fitting of first-order glow curves in Fig. 4, where T_max(K), E (eV), ln σ (s − 1), τ, and n₀V are the temperature for glow curve peaks, the trap depth, the logarithmic frequency factor, lifetime, and the total initial number of traps, respectively.³² 

However, the glow curves of CAIBC in Figs. 4(a)–4(d) have been fitted using one and two components. The corresponding fitting parameters are shown in Table II. All crystals have relatively low trap densities, with the depth energy (E) varying from ∼0.073 to 0.0.116 eV for CAIC. For CAIBC, the depth energy (E) varies from 0.062 to 0.374 eV. The fastest room temperature lifetimes (τ) of CAIBC are of the order of nanoseconds. The decay curves of CAIBC are presented in the supplementary material. Detailed information about lifetime is presented in Table S2. Correspondingly, logarithmic frequency factors (ln σ) are all below 5 for Cs₂AgInCl₆, which is much smaller than ln σ ∼ 30 typically found in pristine or activated oxide. For CAIBC, the logarithmic frequency factor (ln σ) is below 10, which is much smaller than ln σ ∼ 30 except Cs₂AgIn_0.85Bi_0.15Cl₆, which has 31.4. For Cs₂AgIn_0.85Bi_0.15Cl₆, higher In σ ∼ 30 could be due to abrupt changes in bandgap with Bi alloying.³³ 

The x-ray excited luminescence emission intensities of the Cs₂AgIn_0.70Bi_0.30Cl₆ perovskite crystals as a function of x-ray intensity are reported in Fig. 5. As Cs₂AgIn_0.70Bi_0.30Cl₆ exhibits maximum x-ray excited luminescence emission, so intensity dependent measurements are carried out for this material. The intensity of the x-ray source was varied by the source voltage and current. The source voltage varied 45, 40, 35, 30 and 25 kV with current 200, 150, 100, 50 and 30 mA, respectively. All the peaks are centered at 631 nm for all the intensities. As shown in the inset of Fig. 6, the integrated intensities increase linearly with excitation source power. This linear relation also suggests that emission does not arise from permanent defect states. So, it concludes that emission center remains the same for all the intensities. Emission is contributed by STE and FE in the structure, and no permanent defects are present in Cs₂AgIn_0.70Bi_0.30Cl₆. X-ray intensity dependent studies broaden the scope for the application of CAIBC as an x-ray detector for low intensity x rays.

CAIC is not stable when exposed to a laser source. A permanent prominent bleaching effect is observed with laser excitation. Due to laser bleaching, quenching in PL as well as new peaks emerge in the XRD pattern as shown in Fig. 6. The color of Cs₂AgIn_0.7Bi_0.3Cl₆ changes from yellow to gray with exposure time as shown in the inset of Fig. 6(b). However, with x-ray exposure at a high energy with 45 KeV and 200 mA excitation power for different time intervals, Cs₂AgIn_0.7Bi_0.3Cl₆ remains stable. No bleaching effect was observed as shown in the inset of Fig. 6(a). The inset of Fig. 6(a) shows normalized intensity as a function of time. The x-ray excited luminescence also remains stable with time as shown in the inset of Fig. 6(a).

Our findings confirm CAIC as a new lead free halide double perovskite. Lead-free halide double perovskite single crystals show great potential as scintillator materials because of their low production costs, minimal intrinsic trap density, and fast response time in the nanosecond range. Thermo-luminescence measurements have demonstrated that these perovskite crystals exhibit a much lower trap density compared to both lead halide and conventional oxide scintillator materials. As light yield is directly related to bandgap, so our bandgap engineering CAIBC made it more suitable for high light yields. The broad versatility of CAIBC achieved through bismuth (Bi) substitution allows for easy tuning of its scintillation properties. For example, the emission characteristics are dependent on the Bi concentration, demonstrating high sensitivity. The emissive properties of CAIC were further enhanced through structural and dimensional engineering of the perovskite. This new lead-free halide perovskite exhibits high sensitivity to x-ray power. Overall, our research indicates that CAIBC double perovskite single crystals are highly promising for x-ray scintillation due to their nontoxicity, exceptional sensitivity, fast response time, and excellent stability.

See the supplementary material for the synthesis and systematic diagram of the setup built for x-ray excitation luminescence and luminescence spectra.

This work was supported by the Experimental Center of University of Science and Technology of China and the Supercomputing Center of University of Science and Technology of China.

The author has no conflicts to disclose.

Hassan Siddique: Conceptualization (equal).

The data that support the findings of this study are available within the article and its supplementary material.