Deciphering the degradation mechanism of the lead-free all inorganic perovskite Cs2SnI6

Organic-inorganic perovskite materials are revolutionizing photovoltaics with high power conversion efficiencies, but experience significant environmental degradation and instability. In this work, the phase stability and decomposition mechanisms of lead-free all inorganic Cs2SnI6 perovskite upon water and moisture exposure were systematically investigated via in situ synchrotron X-ray diffraction, environmental SEM, and micro-Raman spectroscopy. A critical relative humidity (80%) is identified below which Cs2SnI6 perovskite is stable without decomposition. Under higher humidity or aqueous environment, Cs2SnI6 perovskite decomposes into SnI4 and CsI through etch pits formation and stepwave propagation, leading to rapid crystal dissolution. A partial reversibility of the Cs2SnI6 perovskite upon dissolution and re-precipitation with subsequent dehydration was identified, suggesting a self-healing capability of Cs2SnI6 and thus enhanced air stability. Mechanistic understanding of the Cs2SnI6 degradation behavior can be a vital step towards developing new perovskites with enhanced environmental stability and materials performance. The perovskite Cs2SnI6 has been investigated using numerous in-situ techniques to understand its environmental stability. While organic–inorganic perovskites show great promise for photovoltaic applications, their poor stability remains a problem. Perovskite Cs2SnI6 shows enhanced stability properties, however, how it degrades is not fully understood. Now a team lead by Jie Lian at the Rensselaer Polytechnic Institute, USA, has investigated the phase stability and decomposition mechanisms of Cs2SnI6 when exposed to moisture. This systematic study used in-situ synchrotron X-ray diffraction, environmental scanning electron microscopy and micro-Raman spectroscopy and discovered that there was a critical relative humidity of 80% above which Cs2SnI6 decomposes into SnI4 and CsI. They attribute the degradation process to etch pit formation on the surface of the crystals, which results in the dissolution of the entire crystal through a stepwave model.


INTRODUCTION
Organic-inorganic perovskites have attracted significant attention from researchers in recent years due to their impressive properties for photovoltaic applications, including high absorption coefficients, long carrier diffusion length, and solution processability, [1][2][3][4] boosting the photovoltaic power conversion efficiency (PCE) over 20%. [5][6][7] Despite endeavors to optimize cell design and achieve excellent device efficiencies, technical barriers to the implementation of perovskite solar cells remain such as the toxicity of lead, and the long-term environmental instability of perovskite compositions. 8 Particularly, the organic-inorganic hybrid perovskite (CH 3 NH 3 PbI 3 ) undergoes a rapid decomposition to PbI 2 in the presence of moisture, resulting in a substantial decrease of the device performance. [9][10][11][12][13] It suffers an 80% drop in PCE over a 24 h period and retains less than 5% after 6 days when the cells were stored in ambient conditions. 9 In order to address the critical issues of environmental toxicity of Pb and the associated instability, Cs 2 SnI 6 has been explored as an alternative lead-free perovskite, which displays high degree of air stability at room temperature due to the oxidation state of Sn 4 + . [14][15][16] In addition to the air stability, Cs 2 SnI 6 has a direct band gap of~1.3 eV and high absorption coefficient (over 10 5 cm −1 ). 14,15 It has been successfully adopted as a lead-free light absorber material for solar cells and can retain over 90% in PCE for over 30 days in air. 17 However, the achieved PCE of Cs 2 SnI 6 is relatively low (~1%) in air, as compared with CsSnI 3 solar cells fabricated under tightly controlled environments, 15,17 and therefore device engineering may be required to further improve the PCE. Due to the conditions expected in the deployment of solar cells, it is vital to understand the moisture stability and the interaction behaviors of Cs 2 SnI 6 with water or moisture. Previous work indicates that Cs 2 SnI 6 is stable in ambient conditions with low relative humidity (RH) (30-50% RH), and can be stored in ambient conditions for over 2 months without obvious decomposition. 16 However, this material is still susceptible to rapid decomposition upon exposure to high humidity or aqueous conditions. A mechanistic understanding of the degradation process of the lead-free all inorganic perovskite has yet to be achieved.
In this work, we report a systematic investigation of the perovskite's degradation processes via in situ techniques with a goal of elucidating the underlying mechanisms governing their environmental stability. Through a careful control of RH, the morphological changes were monitored using in situ environmental scanning electron microscope (SEM). The dissolution process occurred through the formation of etch pits and subsequent generation of etching steps due to the propagation of the etch pits through the bulk crystal. Interestingly, the wellshaped octahedra crystals recrystallized with the evaporation of water or dehydration, indicating a strong self-healing capability of Cs 2 SnI 6 . Variations in crystallinity and phase transformations were characterized by in situ micro-Raman spectroscopy and synchrotron X-ray powder diffraction (XRD), indicating a phase decomposition of Cs 2 SnI 6 into CsI and SnI 4 . The improved understanding of perovskite environmental stability and degradation maybe useful for optimization of the materials preparation and further development of new types of perovskite with enhanced materials performance and environmental stability.

RESULTS AND DISCUSSION
To investigate the underlying mechanisms governing the environmental stability of Cs 2 SnI 6 , in situ environmental SEM was performed to observe the microstructural evolution at the micro-and/or nanoscale. Figure 1 shows the in situ environmental SEM images of the dissolution and recrystallization processes of a Cs 2 SnI 6 crystal upon controlling the water vapor pressure (also see Supplementary Information; Supplementary Video S1). The asprepared Cs 2 SnI 6 had a crystallite size~10 µm with an octahedral shape, and was placed on an Si substrate held at 15°C (Fig. 1a). The RH in the chamber was controlled by a leak valve. As the water vapor pressure increased, the well-facet Cs 2 SnI 6 crystal was maintained-even after increasing an RH to 80% in 55 min, as shown in Fig. 1b. To further confirm the phase stability in moist air, the interaction of Cs 2 SnI 6 with water vapor was monitored by in situ synchrotron X-ray powder diffraction under high humidity (~80%) at room temperature. For an exposure period in excess of 7 h, in situ XRD of Cs 2 SnI 6 powders suggests the retention of the perovskite structure, with no decomposition peaks visible, as shown in Fig. 2a. This demonstrates that Cs 2 SnI 6 is stable in the moist air, up to 80% RH, which is consistent with the results obtained during in situ environmental SEM and results reported previously. 14,18,19 It should be noted that small dots emerged on the crystal surface, appearing at 45% and disappearing at~60%. These dots are believed to be Cs 2 SnI 6 quantum dots formed during the synthesis process. As pointed out from our previous report, 16 Cs 2 SnI 6 has significant solubility in many common organic solvents, such as ethanol, methanol, and dimethylformamide. The synthesis process in this work utilized methanol and n-butyl acetate organic solvents. Due to the solubility of Cs 2 SnI 6 in these organic solvents, it is highly possible that small particles reprecipitated on the crystal surface during the drying process, as observed from the original powder (Fig. 1a). In addition, Cs 2 SnI 6 powders prepared using ethanol in the place on n-butyl acetate displayed numerous small particles precipitates on powders (Fig. S1). Despite the presence of these small particles across preparation techniques, all of the Cs 2 SnI 6 powders in this study have not displayed any impurities in the XRD characterization. Similarly, Kapil et al. 20 previously reported the presences of quantum dots formed on the bulk Cs 2 SnI 6 material. Therefore, the small dots observed in our in situ experiment are likely quantum dots of Cs 2 SnI 6 . These small particles would easily absorb water vapor due to their high surface energy, showing high brightness. As the RH increased to~60%, these dots disappeared, indicating their preferential dissolution when the particle size is reduced to the nano-metered scale.
Further increments of the water vapor pressure and relatively humidity to 95% resulted in increasingly rough surfaces of the Cs 2 SnI 6 crystals with small hillocks formed as a result of the water vapor condensation. The dissolution process began at preferential sites instead of a uniform dissolution on the 3D crystals. The hillocks grew to a shape of small water drops and evolved into larger droplets on the surface as the RH increased to saturated water vapor condition. Under the saturated environment, the The dissolution process of a Cs 2 SnI 6 crystal under increasing relative humidity. g-i The recrystallization process upon reducing water vapor pressure. The scale bar is 5 µm Fig. 2 In situ synchrotron X-ray diffraction patterns of Cs 2 SnI 6 powders a exposed to~80% RH environment at room temperature and b with water drops added on the sample W. Zhu et al. droplets can no longer be held on the surface and instead flow on the substrate, forming a solution around the crystal (Fig. 1c-e). Figure 1f shows the triangle-shaped residue in the dissolved solution after the crystal was held in saturated water vapor (RH > 95%) for over 30 min, suggesting a nearly complete dissolution of the original Cs 2 SnI 6 perovskite. A reversible reaction occurs between Cs 2 SnI 6 and water ( Fig. 1f-i) with the evaporation of water molecules to~85% RH. The residue could act as nucleation sites from which well-shaped octahedral crystals precipitate and grow.
The degradation behavior and reversible reaction of Cs 2 SnI 6 with water were also confirmed using in situ synchrotron X-ray diffraction (XRD), as shown in Fig. 2b. Once Cs 2 SnI 6 was exposed to water, the diffraction peaks of Cs 2 SnI 6 perovskite decreased rapidly. Within 10 min of water exposure, the powder was fully dissolved, and no obvious crystallinity was observed (Fig. S2). Interestingly, as the water evaporated, the diffraction patterns which can be assigned to Cs 2 SnI 6 reappeared, suggesting Cs 2 SnI 6 re-precipitated from the solution (Figs 2b and S2). It is worth noting the recovered diffraction intensity of Cs 2 SnI 6 is lower than the original one, indicating a partial recovery process. These results indicate a critical humidity (~80% RH) exists below which the decomposition of Cs 2 SnI 6 will not occur and the material remains stable. The reversible reaction also suggests a strong selfhealing capability of the Cs 2 SnI 6 perovskite upon dehydration, providing strong evidence of the enhanced environmental stability as compared with organic-inorganic hybrid perovskite. 11,21,22 Detailed mechanisms of the dissolution process were further analyzed using ex situ SEM observation, in which the formation of etch pits and dissolution surface steps were visible. In the dissolution stepwave model, the surface defects, e.g., surface steps or edge dislocations, act as the energetically favorite sites for crystal dissolution. Figure 3a shows the fine octahedral shape of Cs 2 SnI 6 crystals displaying a layered structure with clearly visible growth steps, indicating the layer-by-layer growth into the bulk crystal. These surface steps and dislocations introduce strain energy into the crystal, which is energetically favorable to the formation of etch pits. 23 The dislocation strain energy can be calculated by the following equation 24 : (1) where µ is the isotropic bulk shear modulus (egs/cm 3 ), b is the size of the Burgers vector (Å), r h (Å) and r (Å) are the size of the hollow core and the distance from the dislocation center, respectively. Based on this equation, the center of the initial dislocation defects has a higher strain energy, and therefore the dissolution process moves away from the center and travels across the crystal surface, generating etch pits. Further etch steps can be generated from the step far from the center of the dislocation, and the continuous movement of these steps into the crystal will form a series of steps, based on the dissolution stepwave model. 23,25 The stepwaves from each etch pit combine to lower the crystal surface area (Fig. 3b), controlling the bulk crystal dissolution rate.
With the continuous dissolution, the etch pits grow in both the vertical and horizontal directions, resulting in a drop in the surface and hollow holes in the crystals as shown in Fig. 3b, c. Figure 3d displays a schematic view of the dissolution model highlighting the formation mechanism of etch pits and the combination of stepwaves resulting in the overall crystal dissolution. The etch pits and stepwave dissolution mechanism are more clearly visible when Cs 2 SnI 6 was treated with methanol. Cs 2 SnI 6 has a lower solubility in methanol as compared to water. It is more evident that the dissolution process of Cs 2 SnI 6 involved the formation of etch pits and the combination of stepwaves from each etch pit eventually leads to the hollow structure of Cs 2 SnI 6 crystals. Figure 4a shows that the triangle etch pits are visible and surrounded by relatively-flat surfaces while exposed to methanol. The stepwaves from these etch pits combined and formed larger pits during the dissolution process (Fig. 4b). The larger pits eventually formed a large hollow pit in the crystal with the etch pits propagating perpendicularly through the bulk crystal, as shown in Fig. 4c. A close look at the etch pit suggests that the dissolution process proceeds along the <111> direction, forming a hexagonal dissolution pit. Figure 5 shows SEM images of various locations on the substrate after the recrystallization process, indicating the partial reversibility of the process. In Fig. 5a, multiple octahedral crystals can be observed with a hollow structure, which are not fully dissolved under the saturated condition. These residual crystals may act as new nucleation sites during the recrystallization process, as evidenced by the presence of small crystals on the surface (Fig. 5a). During the dissolution process, water condensed on the substrate and formed droplets containing decomposition Fig. 3 a As-prepared Cs 2 SnI 6 crystals, exhibiting the layered structure and lots of surface defects. b The combing etch pits, lowering the entire crystal surface. c The pit formation in the Cs 2 SnI 6 crystal. Scale bars are 5 µm. d The schematic graph of the formation of etch pits and the stepwaves from both etch pits combine, leading to a drop of the crystal surface Fig. 4 a Etch pits obtained with methanol, which presents wellshaped triangles. b SEM image of the combing etch pits. c The etched Cs 2 SnI 6 crystal with a hollow structure. d A high magnification SEM image of the pitting, indicating dissolution along <111> directions products. As the water evaporated, perovskite crystals reprecipitated out, forming a ring-like pattern that consisted of small crystals, as shown in Fig. 5b. It was observed that crystals on the edge are larger than the central crystals. This may be explained by a phenomenon known as a coffee ring effect, 26 which is caused by capillary flow from the interior to the edge due to the differential evaporation rate. Dendrites and needle-like precipitates were also observed throughout the sample, reinforcing the partial reversibility of Cs 2 SnI 6 and the dissolution-precipitation mechanism (Fig. 5c, d). A similar partially reversible process resulting from water interaction has also been reported in the organic-inorganic perovskite. 13,22,27 Synchrotron X-ray powder diffraction was used to analyze the phase compositions of the re-precipitated materials after the water was evaporated. As shown in Fig. 6a, the Cs 2 SnI 6 perovskite phase is clearly visible at different locations, confirming the selfhealing capability and reversible reaction with water. In addition, SnI 4 was also identified at P1, suggesting the process is not entirely reversible. Rietveld refinement of the precipitated products indicates that the phase fraction of Cs 2 SnI 6 and SnI 4 are 55% and 45%, respectively (Fig. S3). Besides, CsI could also be identified at P2 and P3. Therefore, synchrotron XRD confirms coexistence of recrystallized Cs 2 SnI 6 , and residual decomposition products CsI and SnI 4 aggregated at different regions. The CsI precipitated as large sized crystals, characterized by the diffraction spots shown in Fig. 6b. Post experiment SEM images indicate that small particles deposited on and around the recrystallized Cs 2 SnI 6 could be SnI 4 . Large dendrites were also observed similar with the in situ environmental SEM results, which may be CsI (Fig. S4). The broad peak at 2θ~5°is attributed to the formation of Sn(OH) 4 , the product of the hydrolysis of SnI 4 .
In situ Raman and optical images were also performed to further reveal the phase decomposition and precipitation processes of Cs 2 SnI 6 perovskite interacting with water. Figure 7a displays optical images of Cs 2 SnI 6 powders upon the addition of water, indicating that Cs 2 SnI 6 can decompose rapidly upon exposure to water. Cs 2 SnI 6 underwent a color change from black to a yellow solution and eventually a yellow slurry. The reversibility of the dissolution-precipitation process was further supported by these color changes, with the color change from yellow to black upon drying at room temperature (Fig. 7a). Figure 7b displays micro-Raman spectra obtained at various times after exposing Cs 2 SnI 6 powders to water. The starting Cs 2 SnI 6 powders had an average crystallite size around 15 µm with a well-defined octahedral shape, as shown in Fig. 7c. For the asprepared Cs 2 SnI 6 powders, a sharp band at 126 cm −1 and two weak bands at 77 and 248 cm −1 were observed. The band at 126 cm −1 can be attributed to the Sn-I stretching vibration ν (A 1g ). The low frequency band at 77 cm −1 can be ascribed to the δ (F 2g ) mode involving I-Sn-I asymmetric bending. 28 With the addition of several water droplets, the Cs 2 SnI 6 powders began to decompose and dissolve in water. The Raman spectrum obtained at 15 min shows broad bands at 108 and 580 cm −1 , which may be attributed to Sn(OH) 4 29 as a result of the hydrolysis reaction of SnI 4 . Further dissolution results in enhanced signal from decomposition products, further evidenced by a greater number of crystals formed at the bottom, consistent with the sharper peaks of Raman spectrum collected at 25 min. As water evaporated, Cs 2 SnI 6 crystals re-precipitated from the solution as shown in Fig. 7b. In addition, the strong peak at 145 cm −1 emerged due to the presence of SnI 4 , consistent with the findings from synchrotron XRD diffraction (Fig. 6a). The strong variation in color between the two compositions, the black crystal is Cs 2 SnI 6 and the orange particles are SnI 4 , enabling their identification via optical microscopy in Fig. 7f.
Optical images shown in Fig. 8a display the decomposition of Cs 2 SnI 6 in water, as evidenced by the white gel-like precipitates which are the result of a hydrolysis reaction forming tin (IV) hydroxide. Changes in the pH value suggest a strong acid solution arising from the formation of hydriodic acid (Fig. 8c) associated with the hydrolysis reaction (reaction 3). A similar phenomena was observed in a prior tin (IV) iodide hydrolysis experiment. 30 After 12 h, the transparent solution turned yellow, indicating the presence of molecular iodine in the solution (Fig. 8b). The formation of molecular iodine may be attributed to the decomposition product of hydrogen iodide. Based on the systematic in situ measurements of phases, morphology evolution and Fig. 5 a The partially dissolved Cs 2 SnI 6 crystal with a hollow structure, acting as nucleation sites. b The reprecipitates formed around the residual crystal. c The dissolved product precipitated and formed dendrites. d The needle-like structure formed from the dissolution products Fig. 6 a Phase identification of the re-precipitation products after dehydration and water evaporation by synchrotron X-ray diffraction at different locations (i.e. P1, P2, P3). b The synchrotron XRD 2D pattern displaying CsI precipitate solution chemistry, the dissolution and recrystallization mechanism of the lead-free all inorganic Cs 2 SnI 6 perovskite upon water interaction can be expressed as follows: Cs 2 SnI 6 $ SnI 4 þ 2CsI; (2) Finally, the degradation behavior of Cs 2 SnI 6 was compared to the organic-inorganic perovskite (MAPbI 3 or CH 3 NH 3 PbI 3 ) to further improve the general understanding of perovskite degradation mechanisms. For MAPbI 3 perovskite, during the reversible hydration process, water molecules are incorporated into the crystal structure, forming the monohydrate (CH 3 NH 3 PbI 3 ·H 2 O). 22,31,32 With prolonged water vapor exposure, it forms the dihydrate, (CH 3 NH 3 ) 4 PbI 6 ·2H 2 O), leading to an irreversible degradation of MAPbI 3 perovskite. 22,32 The water molecule intercalation into the crystal structure induces the distortion of the 3D MAPbI 3 perovskite structure and rearrangement, resulting in a separation of the [PbI 6 ] 4− octahedra to form 1D double-chains MAPbI 3 perovskite species or 0D frameworks. 22 However, in the Cs 2 SnI 6 perovskite no hydrated phases have been observed. This may be explained by the higher hydration energy of the Cs + cation (−67.7 kcal/mol), 33 as compared to the CH 3 NH 3 + cation (−75 kcal/mol). 34 In addition, due to the high ionic mobility of Cs + cation in water, 35 Cs + cations may diffuse out more rapidly upon exposure to water resulting in the dissociation of the Cs 2 SnI 6 perovskite crystal structure, rather than forming the hydrates. The Sn 4+ is stable in the ambient condition, but experiences the hydrolysis in the water. Therefore, the enhanced moisture stability of Cs 2 SnI 6 as compared with that of the organic-inorganic perovskite may be a result of synergistic effects from both Cs + and Sn 4+ , and more investigations are needed to determine the relative contributions of Cs, Sn, and I to the stability.
In summary, this work investigated the degradation process of Cs 2 SnI 6 perovskite under controlled RH environment or aqueous conditions via in situ measurements, such as in situ environmental SEM, in situ synchrotron XRD, and Raman spectroscopy. The results demonstrate that Cs 2 SnI 6 is stable below a critical relatively humidity of 80%, but experiences degradation when exposed to moisture in excess of 80% RH or aqueous conditions. The degradation process of Cs 2 SnI 6 was attributed to etch pit formation on the surface of crystals, resulting in the dissolution of the entire crystal as explained by the stepwave model. Through in situ synchrotron XRD and micro-Raman spectroscopy, it was determined that Cs 2 SnI 6 decomposes into CsI and SnI 4 in water as evidenced by the formation of the hydrolysis product Sn(OH) 4 in the solution. These results may elucidate the fundamental decomposition pathways of Cs 2 SnI 6 perovskite governing the stability of halide perovskite.

Cs 2 SnI 6 preparation
The method follows as previous reported. 16 Typically, CsI (1 mmol) was dissolved in 10 mL methanol and heated to~60°C in a water bath. In a separated 25 mL beaker, SnI 4 (0.5 mmol) was dissolved in 4 mL n-butyl acetate with addition of 2 mL hydroiodic acid. Addition of the acid SnI 4 solution to the warm CsI solution under vigorous stirring led to spontaneous precipitation of a fine black powder. The mixture was stirred for a further 30 min to ensure completion of the reaction in the 60°C water bath. Then the solid was washed by n-butyl acetate twice via centrifuging and dried in an oven at 80°C overnight at ambient conditions.
In situ environmental SEM test Environmental SEM (FEI, Versa 3D Dualbeam) was used for the in situ analysis of the dissolution and recrystallization process of Cs 2 SnI 6 via controlling RH. Water vapor was introduced into the chamber through a leak valve. Cs 2 SnI 6 crystals were placed on an Si substrate controlled at 15°C . For the in situ experiment, first the water vapor pressure was set to be around 600 Pa to achieve an RH of~35%. Then, the pressure was increased at a constant rate (20 Pa/min) to a value 1000 Pa (RH = 58.5%), followed by a rate at 10 Pa/min until reaching 1660 Pa. The pressure was then held for 20 min to allow crystal dissolution. After that, the water vapor pressure was reduced at a constant rate of 10 Pa/min to a value~715 Pa (RH = 41%). During the experiment, images were captured manually with a speed at every minute or every 30 s as appropriate for the reaction kinetics. Without further notification, each image is of the same magnification, taken at the same location.
In situ synchrotron powder diffraction data collection In situ synchrotron powder XRD was performed at the Advanced Photon Source on beamline 17-BM-B at Argonne National Laboratory using an Si (111) monochromator (λ = 0.39433 Å). The experiment was carried out by loading approximately 5 mg of powder sample in a Kapton tubing of 1.1 mm diameter. After the diffraction patterns of dry powders were taken, 250 µL water was added on the powder directly to monitor the degradation process of Cs 2 SnI 6 in aqueous condition. It should be noted that after water addition it takes about 5 min before the first pattern obtained (i.e. time between seventh and eighth diffraction pattern), due to the time for restarting the system. The diffraction pattern was recorded by an amorphous silicon-based area detector from PerkinElmer (2048 × 2048, 200 µm pixels). All data were collected at room temperature. To monitor the reaction at real time, the diffraction patterns were continuously collected with 60 s per frame. The obtained patterns were analyzed using the Rietveld refinement functions implemented in GSAS II. 36,37 In situ micro-Raman spectra were measured on a Renishaw Micro-Raman spectrometer equipped with an Ar+ ion laser (λ = 514.5 nm) at room temperature. The spectra were collected at every 5 min interval to monitor the interaction between Cs 2 SnI 6 and water. A typical spectrum was acquired at an exposure time of 30 s and two accumulations under a power of 20 mW.
To investigate the Cs 2 SnI 6 morphology changes before/after the water reaction, SEM was carried out on a Carl Zeiss Supra 55 field emission scanning electron microscope.

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