Sn-based perovskite solar cells (PSCs) are of great interest to overcome the problem of toxicity of Pb-based PSCs. Several methods to synthesize Sn-based perovskite films by solution and vacuum processes have already been reported. For instance, CH₃NH₃SnI₃ (MASnI₃) are often synthesized by the reaction of SnI₂ and CH₃NH₃I (MAI) dissolved in organic solutions including various additives to control the crystallinity and to prevent the oxidation of Sn²⁺ into Sn⁴⁺, which are known to hamper the performance of the solar cells [1]. Note that very few reports have also demonstrated that CH₃NH₃SnI₃ thin films can be synthesized by the reaction of Sn thin films in the presence of MAI in solution or in gas phase [2, 3]. Based on this information and on our previous experience with the synthesis of CH₃NH₃PbI₃ (MAPbI₃) by reaction of Pb thin film in the presence of CH₃NH₃I+I₂ [4], we studied the synthesis of CH₃NH₃SnI₃ from Sn thin films.

Here, we report that CH₃NH₃SnI₃ thin film can be readily synthesized by heating Sn thin film pre-deposited on glass subtract (at around 90 ^oC) and CH₃NH₃I powder source (at around 145 ^oC, 0.1 Atm), in a 2-zone furnace as shown in figure 1. We observed that the synthesis of CH₃NH₃SnI₃ thin films occurs by a one-step chemical vapor reaction of Sn thin film in the presence of CH₃NH₃I vapor (and/or the byproducts due to thermal decomposition). According to x-ray diffraction study and thermogravimetry/mass spectroscopy data, we will discuss several reaction mechanisms (such as in eq. (1)). Note that the chemical nature of the intermediate phase and the byproducts are still highly debated.

Sn + 3CH₃NH₃I → CH₃NH₃SnI₃ + (2CH₃NH₂ + H₂)      (1)

Interestingly, the above-mentioned reaction does not require any additive (such as SnF₂) to prevent the formation of Sn⁴⁺, because the reaction Sn⁴⁺ + Sn → 2Sn²⁺ will take place until all the Sn is completely consumed. Note that in the solution process, Sn has also been recently used to synthesize high-purity SnI₂ by the reaction Sn +I₂, which also prevents the formation of Sn⁴⁺ and SnI₄.

In conclusion, we have developed a one-step chemical vapor reaction process to synthesize CH₃NH₃SnI₃  (MASnI₃) by directly converting Sn thin films in the presence of CH₃NH₃I (MAI) vapor at moderate temperatures. This method can be applied to synthesize various types of Sn-based perovskite (e.g., FASnI₃).