Thermal stability of lead-free CH3NH3SnxI3 systems (0.9 ≤ x ≤ 1.1) for photovoltaics

Methylammonium-tin-iodide (MASnxI3, 0.9 ≤ x ≤ 1.1) systems were prepared by precipitation process in aqueous solutions. The “as prepared” MASnxI3 systems exhibited a tetragonal crystalline phase (space group I4cm) with polyhedral crystallites (length 50-400 µm). The as prepared samples were annealed at T = 150 °C for t = 8 h under nitrogen and synthetic air. Under nitrogen, the CH3NH3SnxI3 systems adopt nearly-cubic tetragonal structure (space group P4mm) with crystallites of 2-4 µm length whereas a degradation process with formation of non-crystalline phases occurred in air. The differential thermal analysis (DTA) profile in nitrogen revealed events at T = 247 °C, T = 297 °C (decomposition of CH3NH3SnxI3 systems into methylamine (CH3NH2), hydroiodic acid (HI) and SnI2), and in the range T = 342-373 °C (melting of SnI2) respectively. The thermal profile in air showed endothermic events at T = 139 °C and T = 259 °C with additional events at onset temperatures of T = 337 °C and T = 423 °C respectively which correspond to the formation of Sn(IV)-O binds and to the decomposition of methylamine. Static thermogravimetry analysis (TG), performed at T = 85 °C and T = 150 °C for t = 2 h, revealed a linear weight loss as a function of the time. The optical absorption spectra displayed absorbance edges in near infrared range, at 1107.0 nm (x = 0.9), 1098.6 nm (x = 1.0) and 1073.2 nm (x = 1.1) respectively.