Stability of tin-containing hybrid perovskites: the thermal decomposition of formamidinium tin triiodide (FASnI3) investigated by thermogravimetry and effusion techniques

The gas-releasing thermal decomposition processes of formamidinium tin triiodide perovskite (CN2H5SnI3, usually denoted as FASnI3 or, briefly, FASI) were investigated in order to obtain information on the temperature at which decomposition begins and on the nature of the gas species emitted under heating. Results of thermogravimetry-differential thermal analysis (TG-DTA), thermogravimetry-differential scanning calorimetry (TG-DSC), and Knudsen effusion mass spectrometry (KEMS) experiments are presented. TG measurements showed that mass loss starts at temperatures much lower than those of the lead-based corresponding compound FAPbI3, with incipient loss at temperatures as low as 423 K. Unlike FAPbI3, practically no residue is left at T = 823 K. KEMS experiments showed a measurable release of SnI4(g) occurring from temperatures as low as 318 K, with a SnI4(g) pressure much lower than the vapor pressure of pure SnI4, suggesting the presence in the solid of Sn(IV) at reduced activity. At higher temperatures (T > 390 K), the release of SnI2(g) and various species coming from the organic portion, such as formamidine, hydrogen iodide, ammonia, triazine, and hydrogen cyanide, is observed. For the first time, thermal decomposition of FASI was shown to occur with release of both organic and inorganic (tin iodides) species, with the partial oxidation of Sn(II) to Sn(IV) significantly decreasing the decomposition onset temperature. Finally, based on TG measurements at various scan rates, a kinetic analysis of FASI decomposition was performed, using integral and incremental isoconversional methods, to derive parameters useful for lifetime prediction. Reasonable reaction time values to achieve a low degree of conversion (less than 0.25) were extrapolated at temperatures typically involved in the operation of photovoltaic devices (333 to 353 K).