Degradation mechanism of hybrid tin-based perovskite solar cells and the critical role of tin (IV) iodide

Tin perovskites have emerged as promising alternatives to toxic lead perovskites in next-generation photovoltaics, but their poor environmental stability remains an obstacle towards more competitive performances. Therefore, a full understanding of their decomposition processes is needed to address these stability issues. Herein, we elucidate the degradation mechanism of 2D/3D tin perovskite films based on (PEA)0.2(FA)0.8SnI3 (where PEA is phenylethylammonium and FA is formamidinium). We show that SnI4, a product of the oxygen-induced degradation of tin perovskite, quickly evolves into iodine via the combined action of moisture and oxygen. We identify iodine as a highly aggressive species that can further oxidise the perovskite to more SnI4, establishing a cyclic degradation mechanism. Perovskite stability is then observed to strongly depend on the hole transport layer chosen as the substrate, which is exploited to tackle film degradation. These key insights will enable the future design and optimisation of stable tin-based perovskite optoelectronics.

Supplementary Note 1: I2-degraded perovskite films (SnI4-rich) exposed to ambient air for further 10 minutes result in toluene extractions showing stronger absorption at ~300 and ~500 nm relative to samples exposed to I2 vapour only ( Figure 4c); this observation being consistent with the rapid evolution (< 10 min) of SnI4 to I2 via Reactions 3 and 4.
Supplementary Figure 1. a. Powder X-Ray diffraction (XRD) patterns of a (PEA)0.2(FA)0.8SnI3 film degraded in ambient air for 0 min, 30 min, 60 min and 24 h (black dots). Patterns are deconvoluted into Voigt peak functions (colour lines: cumulative fittings; red lines: single peak functions) to estimate the percentage of perovskite signal loss (relatively to signal at 0 min) and double perovskite signal gain (normalised to signal at 24 h). b. Powder XRD pattern of a (PEA)0.2(FA)0.8SnI3 thin film degraded in ambient air for 24 h. Peaks are assigned to the vacancy-ordered double perovskite derivative FA2SnI6. We note that the absence of SnO2 reflections is likely owed to the highly disordered/amorphous character of this degradation product, in agreement with previous work. 1 Figure 2. SnI2 purification heat treatment. a. Image of the treatment setup: commercial SnI2 is placed inside a covered Petri dish and heated at 250ºC for 1h in a N2-filled glovebox. SnI4 is removed from SnI2 via sublimation and deposited on the inner side of the lid. The purification of SnI2 via sublimation of SnI4 exploits the much higher volatility of the latter, being a molecular solid in contrast with ionic SnI2. b. Image of vials containing SnI2 before (left) and after (right) the purification treatment.

Supplementary
Supplementary Figure 3. Thermogravimetric analysis (TGA) of SnI2 precursors and (PEA)0.2(FA)0.8SnI3 perovskite registered under N2. a. TGA curves of control SnI2 and purified SnI2. Inset: magnification of main graph at 40-350ºC temperature range. The initial decrease in mass in control SnI2 is 0.5% approximately and starts at relatively low temperatures (~100ºC), while purified SnI2 only exhibits a mass drop from ~300ºC attributed to SnI2 evaporation. b. TGA curves of (PEA)0.2(FA)0.8SnI3 perovskite made with control SnI2 and purified SnI2. The curves shows two distinctive mass losses corresponding to i) the evaporation of the organic salts and ii) the sublimation of SnI2 at higher temperatures, in good agreement with previous reports. 2 We note that perovskite made with control SnI2 shows mass losses at lower temperatures throughout the scan, being this difference more accentuated in the second mass loss step in consistence with SnI2 richer in more volatile Sn 4+ impurities.
Supplementary Figure 4. a. UV-Visible absorbance spectra of impurities dissolved in ultradry toluene (SnI4 is highly soluble in non-polar solvents like toluene, unlike SnI2) and extracted from 0.05 g control SnI2 (red; 3 mL toluene used), 0.05 g purified SnI2 (black; 3 mL toluene used) and the solidified phase eliminated from control SnI2 via sublimation (blue). These absorption spectra match the one recorded for a 0.216 mM SnI4 reference solution in toluene (dashed orange line), allowing to identify the impurity as SnI4. b. UV-Visible absorbance spectra of reference SnI4 solutions in toluene at different concentrations. c. Absorbance (λ = 365 nm) vs concentration plot obtained from data in Supplementary Figure 4b and its linear fitting (ε(SnI4) = 9934 M -1 cm -1 ). We obtain the mass percentage of SnI4 in the precursor before and after the heat treatment by applying the Beer-Lambert Law (A = εcl; l = 1 cm) to calculate the concentration of SnI4 in the toluene extractions (Supplementary Figure 4a) from their absorbance at 365 nm. Control SnI2 contains 0.65% SnI4 in mass, while only 0.06% SnI4 is found in purified SnI2.
Supplementary Figure 5. a. Powder XRD patterns of glass/(PEA)0.2(FA)0.8SnI3 and FASnI3 thin films made with control/purified SnI2. The indexed diffraction peaks reveal that perovskites possess an orthorhombic crystal structure (Amm2 space group), consistent with the α-phase of FASnI3. 3 In contrast with archetypal 3D FASnI3 film, peaks in patterns of (PEA)0.2(FA)0.8SnI3 films provide much higher diffraction intensity, indicating that the addition of PEAI increases the crystallinity of the final perovskite. Furthermore, this also causes (h00) reflections in the diffractogram to clearly become more prominent (particularly (100) and (200)), which suggests that perovskite crystals preferentially grow with their {h00} planes oriented parallel to the substrate. 4,5 Diffraction patterns of (PEA)0.2(FA)0.8SnI3 do not present significant changes when the control or purified SnI2 is employed, confirming that the perovskite crystalline structure is not altered as a consequence of the purification of the precursor. b. Magnified X-ray diffraction patterns of (100) and (200) main reflections acquired from (PEA)0.2(FA)0.8SnI3 and FASnI3 films made with control/purified SnI2. Peaks from perovskites made with purified SnI2 are shifted to higher diffraction angles, indicative of smaller lattice parameters. We suggest that reducing the incorporation of Sn 4+ into perovskite may result in less Sn 2+ /Ivacancies in the lattice and therefore more compact unit cells.  We consider any major contribution from SnI4 or FA2SnI6 to the Sn 4+ component unlikely due to i) SnI4 being highly volatile and rapidly evaporating from the film at vacuum levels used in XPS (10 -6 mbar or lower) and ii) the Iratios being close to 3 in consistence with the perovskite stoichiometry (Iratios expected to be significantly higher if Sn 4+ components were caused by SnI4 or FA2SnI6).  Supplementary Figure 10. Contour graphs representing (PEA)0.2(FA)0.8SnI3 normalised absorbance (Abs(t)/Abs(0)) as a function of time and wavelength in films made with a. purified SnI2, b. control SnI2 and c. control SnI2 + 2 mol% SnI4.
Supplementary Figure 11. Downfield proton nuclear magnetic resonance ( 1 H-NMR) spectra of DMSO-d6, FAI, PEAI, fresh (PEA)0.2(FA)0.8SnI3 and (PEA)0.2(FA)0.8SnI3 aged for 24h in ambient air. Peaks in FAI ad PEAI spectra are assigned to protons in their respective molecules (see inset). The observed increase in multiplicity in degraded perovskite peaks assigned to FAI is attributed to the inhibition of the dynamic proton exchange between water and FA + upon H2O protonation. 6 This is attributed to the presence of SnI4 and its reaction with H2O to form HI.
Supplementary Figure 12. a. Image of a vial filled with degassed deionized water after the addition of SnI4. The white precipitate at the bottom corresponds to the formation of SnO2 via Reaction 3 in the main paper. This experiment was carried out under inert N2 atmosphere to rule out the involvement of O2 in the reaction. b. pH test of deionized water vial before and after SnI4 addition under N2 atmosphere. The acidification of water upon SnI4 addition is indicative of the formation of HI via hydrolysis (Reaction 3). c. Image of the SnI4-treated vial after 4h exposure to ambient air. The change in colour from transparent to yellow/light brown indicates the presence of I3species. d. UV-Visible absorbance spectra of water before SnI4 addition (blank), after SnI4 addition/before air exposure and after SnI4 addition/after 4h air exposure. The feature at ~ 250 nm is assigned to UV absorption by SnO2 crystals (formed via Reaction 3). After air exposure, HI in solution is oxidized to I2, which combines with remaining Ianions to give I3 -(peaks at ~285 and ~350 nm).
Supplementary Figure 13. Absorbance time evolution of a SnI4 reference solution in toluene left in air for 6 hours and spectrum of an I2 reference solution in toluene. The decrease of the absorption band at ~360 nm, the rise of spectral features at ~300 nm and ~500 nm and a colour change from orange to purple confirm the evolution of SnI4 to I2 in solution. Spectral changes over time are detected after only 30 min of ambient air exposure, suggesting the degradation of SnI4 to I2 is a relatively fast process.
Supplementary Figure 14. Images of (PEA)0.2(FA)0.8SnI3 perovskite thin films made with purified SnI2 before degradation (left) and after being exposed to I2 vapours in a N2filled glovebox for 3 minutes (right).