Stability of CsPbI₃ with divalent cations incorporated via mechanochemical alloying

Abstract

Cubic CsPbI₃ is a promising perovskite material for optoelectronic applications. This material possesses an energy band gap of 1.7 eV and an optical absorption coefficient of 105 cm⁻¹, and can be synthesized at high temperatures using various methods. However, cubic CsPbI₃ faces a significant challenge with degradation reported under ambient conditions. This degradation results in the formation of a delta phase, which comprises a non-perovskite structure with poor optical and electrical properties at room temperature. This paper proposes the partial substitution of lead in CsPbI₃ with divalent cations, such as Sn²⁺, Mn²⁺, Ni²⁺, and Ca²⁺, to improve the overall stability. These cations were selected because their ionic radii meet the Goldschmidt factor and favor the formation of cubic halide perovskites. For the synthesis of the materials, a solid-state mechanochemical method is used that allows for the incorporation of the above-mentioned cations into the CsPbI₃ matrix in a single step at room temperature. Replacing Pb²⁺ with Sn²⁺ produced the most stable material and resulted in a cubic perovskite structure with similar optical properties to CsPbI₃. In particular, a composition of CsPb_0.6Sn_0.4I₃ resulted in cubic perovskite and did not show degradation when exposed to room temperature, ambient humidity, and atmospheric pressure for up to 25 days. On the other hand, even though Ca²⁺ possesses a smaller ionic radius than Pb²⁺, replacing Pb²⁺ with Ca²⁺ was not effective in stabilizing CsPbI₃ due to the hygroscopic nature of CaI₂. Replacing Pb²⁺ with Mn²⁺ and Ni²⁺ produces stable alloys in a controlled environment (glovebox) at room temperature but quickly decomposes to non-perovskite δ-CsPbI₃, iodine, and metal oxides when exposed to air. The degradation mechanism of these materials was studied in detail using XPS techniques, revealing potential alternatives to produce stable Sn²⁺ containing perovskites with properties similar to those of cubic CsPbI₃ at room temperature without solvents and increased stability under ambient conditions when Pb²⁺ was partially replaced with Sn²⁺.

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Introduction

Cubic cesium lead iodide (CsPbI₃) is a halide perovskite with exceptional optical and electronic properties and has become a promising material for various applications,¹ including solar cells,² LEDs,³ radiation detectors,⁴ and photodetectors.⁵ With an optimal bandgap of 1.73 eV,⁶ CsPbI₃ shows efficient absorption of sunlight across a wide range of wavelengths,⁷ and its high absorption coefficient in the visible spectrum allows for effective light harvesting.⁸ Furthermore, its exceptional charge carrier mobility facilitates efficient charge transport within the material.⁹ Moreover, CsPbI₃ exhibits long carrier diffusion lengths, enabling charge carriers to travel significant distances without recombination.¹⁰ Despite its promising properties, CsPbI₃ is highly susceptible to degradation processes under ambient conditions that can severely impact its performance and stability.¹¹ This is primarily due to the fact that, in ABX₃ halide perovskites, A⁺ cations typically occupy spaces within the [BX₃]⁻ framework to ensure structural stability and charge balance¹² but in the case of CsPbI₃, the Cs⁺ cations are too small to fill the cuboctahedra spaces in the corner-sharing [PbI₆]⁴⁻ network, which is essential for structural stability.¹³

Preserving the cubic phase structure of CsPbI₃ requires a high processing temperature, but as the temperature decreases to room temperature, CsPbI₃ tends to degrade.¹⁴ Despite numerous efforts, maintaining a pure cubic phase of CsPbI₃ at room temperature remains challenging.¹⁵ Hence, there is a growing interest in developing stable materials with properties and structures similar to those of cubic CsPbI₃.¹⁶ Several strategies have been explored to increase the stability and performance of CsPbI₃-based devices such as chemical passivation techniques,¹⁰ interface engineering,¹⁷ and lattice modification approaches.¹⁸ Chemical passivation involves introduction of moisture-resistant molecules or additives to protect CsPbI₃ from moisture-induced degradation.¹⁰ Other alternatives include interface engineering to optimize device architecture and interfaces to prevent moisture penetration and enhance stability.¹⁷ Lattice modification techniques, such as partial substitution of Pb with smaller elements, reduce lattice strain and defects and improve stability.¹⁹ For example, substituting smaller elements on the Pb site can lead to a decrease in lattice strain.²⁰ Additionally, this process can minimize the formation of defects, ultimately resulting in better resistance against external degradation.²¹ This structural stabilization mechanism can effectively suppress phase transitions, lattice distortions, and the formation of non-perovskite phases, thus increasing material stability.

To achieve these effects, the substituting atoms must meet specific criteria. Firstly, a balanced electronic neutrality (or zero charge) must exist between the cations and anions.²² Secondly, the octahedral factor (μ) = R_B/R_X should fall within 0.44 ≤ μ ≤ 0.9.²³ This octahedral factor range enables a favorable geometric configuration, leading to the structurally stable perovskite crystal lattice. Lastly, the Goldschmidt tolerance factor (t), defined as t = (R_A + R_X) √2 (R_B + R_X), has to be within 0.8 ≤ t ≤ 1.²⁴ In this context, R_A, R_B, and R_X represent the ionic radii of the A, B, and X sites, respectively.²⁴ The established acceptable range for the tolerance factor (t) in cubic perovskites is 0.9 ≤ t ≤ 1.²⁵ Therefore, selecting elements with appropriate ionic radii and chemical compatibility with the perovskite lattice is crucial for maintaining structural integrity and avoiding phase segregation.

However, transitioning from the theoretical framework to the practical incorporation of alternative elements into the CsPbI₃ structures presents its challenges. To incorporate alternative elements into CsPbI₃ structures, the standard liquid-based synthesis methods often introduce complex variables like solubility, choice of solvent, stabilizing agents, and temperature control.²⁶ These factors can significantly affect the final structure and composition of the product.²⁷ To avoid these issues, alternative synthesis methods to achieve materials that are structurally close to cubic CsPbI₃ are currently being investigated. In this regard, ball milling, which is a solid-state technique typically conducted at room temperature, has emerged as a promising alternative.²⁸ Ball-milling offers a more straightforward and controlled synthesis process, circumventing the complexities associated with liquid-based methods.²⁹ Although there have been a few studies on synthesizing CsPb_1−xSn_xI₃ using ball milling,³⁰ this method has rarely been explored for stabilizing CsPbI₃ through B-site modification without the use of additives and stabilizing agents under ambient conditions.

In this paper CsPbI₃ perovskite materials with divalent cations replacing Pb²⁺ that have optical and structural characteristics like the cubic phase of CsPbI₃ at room temperature have been studied. The synthesis is based on a mechanochemical method (ball milling) focusing on substituting lead with elements that have a smaller ionic radius that should produce a perovskite structure with enhanced stability. The proposed method differs from other conventional approaches because it excludes ligands, solvents, or stabilizing agents, without using controlled environment enabling a better evaluation of the chemical reactions involved. Our results identified Sn²⁺ as the most effective element to stabilize CsPbI₃ in its solid state and helped understand the degradation mechanisms when Ni²⁺, Mn²⁺, and Ca²⁺ are used. The results described a new approach to improving the stability of CsPbI₃, specifically replacing “B” site elements using simple mechanochemical methods.

Results and discussion

Tolerance and octahedral factors in the CsMI₃ systems (M = Sn²⁺, Ca²⁺, Ni²⁺, and Mn²⁺)

The CsPb_1−xM_xI₃ compounds were characterized optically and chemically to investigate the effect of replacing Pb with smaller divalent elements. The divalent cations include Sn²⁺, Ca²⁺, Ni²⁺, and Mn²⁺, each possessing an ionic radius of 102 pm, 100 pm, 69 pm, and 83 pm, respectively. Fig. 1 shows the relationship between the tolerance and octahedral factors for pure CsMI₃ compounds (M = Cd²⁺, Ca²⁺, Sn²⁺, Ni²⁺, and Mn²⁺). The mapping reveals an interesting trend: substituting Pb²⁺ with Cd²⁺ and Ca²⁺ results in compounds outside the stability range for cubic perovskites, leading to the formation of orthorhombic structures. Conversely, replacing Pb²⁺ with Sn²⁺, Ni²⁺, and Mn²⁺ at specific molar ratios seems to retain the stable cubic perovskite structure range. According to the mapping, Sn²⁺ is the only element that can form a cubic perovskite structure with the formula ABX₃. However, a significant challenge arises from the oxidation of Sn²⁺ to Sn⁴⁺, posing difficulties in achieving this structure under ambient conditions.

Fig. 1 Calculated tolerance and octahedral factors for CsMI₃ perovskites (M = Pb²⁺, Cd²⁺, Ca²⁺, Sn²⁺, Ni²⁺, and Mn²⁺).

X-ray diffraction analysis

The network of cubic CsPb_1−xSn_xI₃ perovskite alloys is composed of Sn(II) and Pb(II) sharing corners, and each surrounded by six halogens forming [PbI₆]⁴⁻ and [SnI₆]⁴⁻ units. The cuboctahedra spaces within the structure are occupied by Cs⁺ cations.

Fig. 2a shows that the XRD pattern of CsPb_1−xSn_xI₃ compounds with a Sn²⁺/Pb²⁺ molar ratio ranges from 0.05 to 0.5 after a 12-hour ball-milling synthesis. The XRD pattern for the 0.05 molar ratio reveals a dominant peak at 13.02°, corresponding to the (012) plane of δ-CsPbI₃. As the molar Sn²⁺/Pb²⁺ ratio increases to 0.1, an additional peak at 14.46° appears, indicating the presence of the (100) planes for cubic CsPbI₃, suggesting the transformation towards the cubic phase. Further increasing the Sn²⁺/Pb²⁺ molar ratio from 0.1 to 0.4 leads to a considerable reduction in the intensity of δ-CsPbI₃ peaks and an increase in the cubic CsPbI₃ phase. Remarkably, for a Sn²⁺/Pb²⁺ molar ratio, the XRD results indicate a pure cubic phase. A slight shift towards higher 2θ degrees is evident in all Sn-containing samples compared to pure CsPbI₃, confirming lattice contraction in CsPb_1−xSn_xI₃. This is attributed to the incorporation of the smaller Sn²⁺ in the lattice structure.

Fig. 2 XRD patterns of 12-h ball-milled CsPb_1−xM_xI₃ for several M²⁺ atoms, including Sn²⁺ (a), Ca²⁺ (b), Ni²⁺ (c), and Mn²⁺ (d), at various M²⁺/Pb²⁺ molar ratios.

For a 0.5 Sn²⁺/Pb²⁺ molar ratio, the orthorhombic CsSnI₃ and cubic CsPbI₃ phases coexist within the structure. XRD peaks at 14.25°, 25°, and 28.7° confirm orthorhombic CsSnI₃, while the peak at 14.46° confirms cubic CsPbI₃. This indicates that a Sn²⁺ molar ratio exceeding 0.4 introduces orthorhombic CsSnI₃ into the initially formed cubic crystalline structure. This is a consequence of the excess of Sn²⁺ in the reaction, enabling the formation of the orthorhombic CsSnI₃ phase, which has a formation energy lower than cubic CsPbI₃ (−1.270 eV per atom for cubic CsPbI₃ and −1.203 eV per atom for orthorhombic CsSnI₃³¹). These results imply that maintaining a Sn²⁺ ratio below 0.5 is ideal to promote a cubic structure.

The XRD patterns of ball-milled CsPb_1−xCa_xI₃ are shown in Fig. 2b. Interestingly, none of the samples showed a cubic phase; instead, all samples exhibited XRD patterns consistent with δ-CsPbI₃. This observation is consistent with Fig. 1 where theoretical aspects of cubic perovskites were discussed. In addition to theoretical considerations, the absence of the cubic phase can be attributed to the highly hygroscopic nature of CaI₂ within the reaction environment. The presence of the δ-CsPbI₃ phase is a consequence of the presence of water in the ambient during the synthesis. The cubic phase cannot be achieved when using Ca²⁺ without using a tightly controlled environment to minimize water during the reaction. These samples were not further analyzed, and the paper focuses on Sn²⁺, Ni²⁺, and Mn²⁺.

Fig. 2c and d show the XRD patterns of CsPb_1−xM_xI₃, ball-milled for 12 hours, with M representing Ni and Mn, respectively. All XRD patterns are consistent with δ-CsPbI₃, with no evidence of the cubic CsPbI₃ phase. Upon opening the reaction container, the initial black powder quickly transitioned to yellow within a few minutes. This indicates degradation upon exposure to air. This was explored through XPS, and a detailed discussion is presented in the following sections.

Stability analysis of CsPb_0.6Sn_0.4I₃ samples

In terms of phase control and stability the CsPb_0.6Sn_0.4I₃ is the most promising composition and is analyzed next in detail. Fig. 3 shows the XPS spectra for C 1s, O 1s, Cs 3d, Pb 4f, Sn 3d, and I 3d regions in CsPb_0.6Sn_0.4I₃ for as prepared samples and those exposed to ambient conditions for 25 days. For clarity, CPSI refers to CsPb_0.6Sn_0.4I₃.

Fig. 3 Comparison of XPS spectra for CsPb_0.6Sn_0.4I₃ samples at 0 days (a) and after 25 days (b) exposed to ambient conditions. CPSI refers to the CsPb_0.6Sn_0.4I₃ composition.

For both as prepared and 25-day exposed CsPb_0.6Sn_0.4I₃ samples, the C 1s band shows two distinct chemical components. The first band at 284.8 eV indicates C–C structures commonly associated with surface-adsorbed carbon. The second band, observed at 286.67 eV, is characteristic of carbon in a C–O chemical environment. This latter band is notably enhanced in the samples that have been exposed to ambient conditions for 25 days, suggesting significant interaction with the environment.

Similarly, the O 1s region shows a single chemical environment in both samples, corresponding to oxygen in an OH environment. This feature is likely a result of the samples and precursors being exposed to moisture in the ambient air. The increased OH intensity in the exposed samples compared to the as-prepared samples further supports this conclusion, indicating an interaction with moisture and oxygen in the air.

The Cs 3d region exhibits a doublet at 723.96 eV and 737.90 eV, corresponding to the Cs 3d_5/2 and Cs 3d_3/2 states, respectively. These bands confirm the presence of Cs atoms in the CsPb_0.6Sn_0.4I₃ structure. These bands remain unchanged in both the as-prepared and 25-day exposed samples, indicating the stability of the Cs atoms in the CsPb_0.6Sn_0.4I₃ perovskites without any significant chemical change.

Similarly, for the Pb 4f region, a doublet is observed at 137.41 eV and 142.30 eV, corresponding to Pb 4f_7/2 and Pb 4f_5/2, respectively. This doublet is consistent with the incorporation of Pb atoms in the CsPb_0.6Sn_0.4I₃ structure and, as in the Cs bands, shows no shift or additional bands in either the as-prepared or the exposed samples, suggesting a stable Pb environment in the CsPb_0.6Sn_0.4I₃ perovskites.

The Sn 3d region shows a doublet at 485.60 eV and 494.1 eV, associated with Sn 3d_5/2 and Sn 3d_3/2, indicative of Sn²⁺ in the CsPb_0.6Sn_0.4I₃ structure. The stable position of these bands in both the as-prepared and the 25-day exposed samples further supports the chemical stability of Sn²⁺ in the CsPb_0.6Sn_0.4I₃ perovskites. This observation effectively eliminates concerns regarding the potential oxidation of Sn²⁺ to Sn⁴⁺ during the synthesis or after ambient exposure.

Lastly, the I 3d region shows a doublet at 618.63 eV and 630.12 eV, corresponding to I 3d_5/2 and I 3d_3/2, confirming the presence of iodine. The absence of significant shifts or new bands indicates a stable iodine environment in the CsPb_0.6Sn_0.4I₃ perovskites.

Fig. 4 illustrates the Kubelka–Munk function derived from UV-vis diffuse reflectance spectra³² and photoluminescence (PL) spectra of as-prepared CsPb_0.6Sn_0.4I₃ and 25-day exposed samples.

Fig. 4 Kubelka–Munk function (a) and photoluminescence (PL) spectra (b) of CsPb_0.6Sn_0.4I₃ samples as prepared and samples exposed to ambient conditions for 25 days. CPSI refers to the CsPb_0.6Sn_0.4I₃ composition.

The Kubelka–Munk function results for both the as-prepared and exposed samples are quite similar, with a bandgap of 1.58 eV. Similarly, the photoluminescence (PL) spectra for both samples show an emission peak at a wavelength (λ_max) of 794 nm with no significant shift in the emission peak for the sample after 25 days of exposure, suggesting that the optical properties remain stable over time. This consistent optical behavior confirms that the CsPb_0.6Sn_0.4I₃ sample retains its inherent characteristics, highlighting its robustness and stability when exposed to ambient conditions for 25 days.

Exploring the degradation mechanisms in CsPb_1−xNi_xI₃ and CsPb_1−xMn_xI₃ perovskites

CsPb_0.9Ni_0.1I₃. The synthesis of CsPb_0.9Mn_0.1I₃ and 3.3% Ni-doped doped CsPbI₃ nanocrystals using solution-based methods at elevated temperatures has been previously reported. These methods also use ligands and stabilizing agents to improve stability and passivation.^33,34 In contrast, our approach explores the stabilization of CsPbI₃ by incorporating smaller elements at the B-site using a simple room-temperature solid-state approach. It is worth noting that the mechanochemical synthesis for alloying CsPbI₃ with Ni²⁺ and Mn²⁺ used in this paper produces a black powder that, upon exposure to ambient conditions, changed to yellow in a few minutes.

XPS was used to thoroughly investigate the synthesized materials and to study the composition of the as-prepared CsPb_0.9Ni_0.1I₃ samples and monitor compositional changes as a function of exposure time. Two batches of samples were prepared: one in a controlled environment (glove box) and another with exposure to ambient conditions. XPS analyses were conducted in an ultra-high vacuum.

Fig. 5 illustrates the XPS spectra for the C 1s, O 1s, Cs 3d, Pb 4f, Ni 2p, and I 3d regions of the CsPb_0.9Ni_0.1I₃ samples for the controlled and ambient-exposed environments. In the controlled environment, the C 1s region showed distinct binding energies for C in various chemical states - C–C, C–O, and C=O at 284.8 eV, 286.2 eV, and 288.4 eV, respectively. This carbon presence is likely due to residual carbon and oxygen trapped in the precursor cavities during sample preparation. In the ambient-exposed samples, these carbon states were similarly observed.

Fig. 5 XPS spectra of CsPb_0.9Ni_0.1I₃ before (a) and after (b) exposure to the ambient conditions, highlighting the C 1s, O 1s, Cs 3d, Pb 4f, Ni 2p, and I 3d regions. CPNI refers to the CsPb_0.9Ni_0.1I₃ composition.

In the O 1s region, samples from the controlled environment showed a single band indicative of C=O or C–OH bonds, consistent with the C 1s findings. However, in contrast, the samples exposed to air showed additional bands at 529.75 eV and 531.35 eV. These new bands correspond to the formation of nickel oxides, specifically NiO and Ni₂O₃, which indicates significant compositional changes following exposure to air.

The Cs 3d region exhibited a distinctive doublet band with binding energies at 724.5 eV and 738.45 eV, indicative of Cs in the original CsPb_0.9Ni_0.1I₃ perovskite. Also, a new doublet band is observed at 726 eV and 739.93 eV for the exposed sample, indicating Cs–I formation and confirming a partial decomposition of the perovskites. In contrast, the Pb 4f region showed minimal changes in its binding energies after exposure, implying a limited impact on the chemical environment for Pb²⁺.

The Ni 2p region showed two initial doublet bands at 855.14 eV and 858.91 eV, associated with Ni²⁺. Post-exposure, an additional doublet band at 858.80 eV was observed, indicating the oxidation of Ni²⁺ to Ni³⁺. This finding is consistent with the O 1s region data.

For iodine, both samples exhibited a doublet band at 620 eV and 631.46 eV, corresponding to the I 3d_5/2 and I 3d_3/2 states, signifying iodine integration into the perovskite structure. In air-exposed samples, a new doublet band at 618.75 eV in the I 3d region suggested the presence of an I–I bond.

As explained in the XRD section, exposure of CsPb_0.9Ni_0.1I₃ samples to ambient conditions predominantly led to the formation of the non-perovskite phase δ-CsPbI₃. XPS analysis further confirmed the presence of Cs–I, NiO, and Ni₂O₃ in the samples. The XPS and XRD data collectively suggest that CsPb_0.9Ni_0.1I₃ perovskite samples undergo a transformation into non-perovskite forms such as δ-CsPbI₃, CsI, NiO, Ni₂O₃, and I₂ when exposed to ambient conditions.

CsPb_0.8Mn_0.2I₃. Fig. 6 shows the XPS spectra for C 1s, O 1s, Cs 3d, Pb 4f, Mn 2p, and I 3d in CsPb_0.8Mn_0.2I₃ for samples in a controlled environment and those exposed to ambient conditions. Bands corresponding to C–C and C–O bonds are observed in both samples, but upon air exposure, an additional band corresponding to C–OOH is observed, suggesting degradation from exposure to the environment.

Fig. 6 XPS spectra of CsPb_0.8Mn_0.2I₃ before (a) and after (b) exposure to the ambient conditions, highlighting the C 1s, O 1s, Cs 3d, Pb 4f, Mn 2p, and I 3d regions. CPMI refers to CsPb_0.8Mn_0.2I₃ composition.

The O 1s region shows a similar behavior to that in Ni-containing samples. Samples in a controlled environment exhibit a C–OH band, while exposure to air results in two new bands at 530.1 eV and 531.22 eV. These new bands are attributed to the formation of Mn–OH and Mn–O bonds, respectively. This change suggests that the sample undergoes decomposition and reacts with H₂O and O₂ molecules in the air, leading to the formation of these new chemical bonds. This evidence of alteration further underscores the sensitivity of CsPb_0.8Mn_0.2I₃ to ambient conditions and the consequent chemical transformations.

The Cs 3d, Pb 4f, and I 3d regions for CsPb_0.8Mn_0.2I₃ samples show the same behavior as in CsPb_0.9Ni_0.1I₃. This suggests a consistent trend in the chemical states of cesium, lead, and iodine across different perovskite compositions when a portion of lead is substituted by either nickel or manganese.

For the Mn 2p region, the samples in a controlled environment exhibit a doublet band at 642.75 eV, which is characteristic of the Mn²⁺ oxidation state. However, after exposure to air, in addition to the Mn²⁺ bands, a broad band appears at 644.25 eV. This additional band is attributed to MnO, indicating the oxidation of Mn²⁺. This finding is consistent with the O 1s region, where the appearance of new bands (such as those for Mn–OH and Mn–O bonds) suggests the oxidation of manganese. The correlation between the Mn 2p and O 1s regions in CPMI samples highlights the oxidation of manganese upon exposure to air, underscoring the material's sensitivity to environmental changes.

Degradation mechanism

Fig. 7 shows the proposed degradation mechanism of CsPb_0.9Ni_0.1I₃ and CsPb_0.8Mn_0.2I₃ when exposed to ambient conditions. The absence of surface passivation in the mechanochemical synthesis used makes both compositions vulnerable to environmental factors such as humidity and oxygen. When exposed to air, the [NiI₆]⁴⁻ and [MnI₆]⁴⁻ units in the perovskite network break down, releasing Ni²⁺ and Mn²⁺ ions. These ions then oxidize, forming species such as Ni₂O₃, NiO, Ni(OH)₂, MnO, and Mn(OH)₂. Furthermore, the decomposition of these perovskites into [MnI₆]⁴⁻ and [NiI₆]⁴⁻ also releases I⁻ ions, which, consequently, form CsI and generate additional I–I bonds.

Fig. 7 Proposed degradation mechanism of CsPb_0.9Ni_0.1I₃ and CsPb_0.8Mn_0.2I₃ samples upon exposure to ambient conditions. The Cs, Pb, M (Ni and Mn), and I are colored orange, gray, green, and red, respectively.

Ultimately, this degradation leads to the formation of the more stable non-perovskite δ-CsPbI₃ phase.

Conclusions

This paper proposes the partial substitution of lead in CsPbI₃ with divalent cations, such as Sn²⁺, Mn²⁺, Ni²⁺, and Ca²⁺, to improve the overall stability. It demonstrates that the stability of CsPbI₃ perovskites can be enhanced through mechanochemical alloying and substitution of Pb²⁺ with Sn²⁺ cations. It is shown that Sn²⁺ is particularly effective in stabilizing the cubic phase of CsPbI₃ while maintaining its structural integrity and optical properties when exposed to ambient conditions for an extended period. A composition of CsPb_0.6Sn_0.4I₃ results in cubic perovskite and did not show degradation when exposed at room temperature, ambient humidity, and atmospheric pressure for up to 25 days. Although Ca²⁺ possesses a smaller ionic radius than Pb²⁺, replacing Pb²⁺ with Ca²⁺ is not effective in stabilizing CsPbI₃ due to the hygroscopic nature of Ca²⁺. Materials in which Pb²⁺ is partially replaced with Mn²⁺ and Ni²⁺ are stable in a controlled environment (glovebox) but quickly decompose to non-perovskite δ-CsPbI₃, iodine, and metal oxides when exposed to air.

Experimental

Materials

Cesium iodide (CsI, 99.9% trace metal basis), lead(II) iodide (PbI₂, 99.999% trace metal basis), tin(II) iodide (SnI₂, 99.99% trace metal basis), and calcium iodide (CaI₂, 99.5% trace metal basis) were acquired from Sigma Aldrich. Nickel(II) iodide (NiI₂, anhydrous, 99.5%) and manganese(II) iodide (MnI₂, ultra-dry, 99.99%) were purchased from Thermo Fisher Scientific. The stainless-steel balls (10 mm), stainless-steel milling jars, and high energy vertical planetary ball mill used for the solid-state reactions were acquired from MSE Supplies LLC.

CsM_xPb_1−xI₃ synthesis

For each composition, 2.5 mmol of cesium iodide (CsI) were mixed with (2.5 times (x)) mmol of the MI₂ (MnI₂, NiI₂, CaI₂, and SnI₂) and (2.5 times (1−x)) mmol of lead(II) iodide (PbI₂). The mixture was placed in stainless steel jars along with two stainless steel balls. The jars were then inserted into the High Energy Vertical Planetary Ball Milling system, and the samples were collected at different time intervals while maintaining a constant rotation speed of 700 rpm.

Materials characterization

The crystalline structures of the samples were analyzed using a Rigaku SmartLab X-ray diffraction system with a CuK_α radiation source having a wavelength (λ) of 1.5406 Å. XRD measurements were executed at a current of 30 mA and a voltage of 40 kV, with a scan speed set at 3 degrees per minute. X-ray photoelectron spectroscopy (XPS) was employed with a PHI 5000 VersaProbe system for chemical analysis, utilizing a monochromatic Al Ka X-ray source at 1486.7 eV. The XPS spectra were acquired in ultra-high vacuum conditions. AAnalyzer software was employed to analyze the XPS results. UV-vis assessments were conducted using an Agilent UV-vis-NIR system with a multi-angle reflection (R) capability. Photoluminescence (PL) measurements utilized an Ocean Optics QE65000 spectrometer, employing a continuous-wave laser with a wavelength of 405 nm for excitation and a 450 nm long-pass filter.

Author contributions

Mahsa Shekarnoush: conceptualization, formal analysis, methodology, and writing – original draft. Francisco S. Aguirre-Tostado: formal analysis and writing – original draft. Manuel Quevedo-Lopez: conceptualization, funding acquisition, supervision, review and editing.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

The authors would like to acknowledge the support provided by the Texas Instruments University Chair in Nanoelectronics at the University of Texas at Dallas.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ma00034j

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