Sn-based perovskite thin films and solar cells (PSCs) are of great interest to overcome the toxicity issue posed by Pb-based perovskite. Several methods to synthesize Sn-based perovskite by solution and vacuum processes were already reported [1, 2]. 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 growth and to prevent the oxidation of Sn²⁺ into Sn⁴⁺, which are known to hamper the performance of the solar cells. Note that very few reports have also demonstrated that CH₃NH₃SnI₃ thin films can be produced by the reaction of Sn thin metallic films in the presence of MAI in a solution or in a gas phase [3, 4]. Based on this information and on our previous experience with the synthesis of CH₃NH₃PbI₃ (MAPbI₃) by reaction of Pb thin film in presence of CH₃NH₃I+I₂ [5], we address the synthesis of CH₃NH₃SnI₃ from the layers of metallic Sn.

Here, we report that CH₃NH₃SnI₃ thin film can be spontaneously formed by heating CH₃NH₃I powder source (at around 145 ^oC, 0.1 Atm) placed near a Sn thin film pre-deposited on glass subtract (at around 90 ^oC), as shown in the figure. We observed that the synthesis of CH₃NH₃SnI₃ thin films occurs by one-step chemical vapor reaction of Sn thin film in the presence of CH₃NH₃I vapor and/or its byproducts due to thermal decomposition, without the formation of SnI₂ intermediate compound. Based on the x-ray diffraction study (as shown in the figure) and the thermogravimetry/mass spectroscopy data, we will discuss several reaction mechanisms such as in eq. (1). Note that the chemical nature of the intermediate and byproducts is still highly debated.

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

In the course of this study, we noticed that the synthesis of CH₃NH₃SnI₃ thin films does not require exposure to I₂ vapor. Indeed, if we perform the reaction as shown in eq. (2) in a very similar way as described in ref. 5, we observed that the reaction mainly leads to (CH₃NH₃)₂SnI₆ and SnI₄ (only containing Sn⁴⁺) rather than the expected CH₃NH₃SnI₃ (only containing Sn²⁺), because I₂ is a very strong oxidant.

Sn + CH₃NH₃I + I₂ → CH₃NH₃SnI₃ ....... (CH₃NH₃)₂SnI₆ and SnI₄   (2)

In conclusion, we have developed a one-step chemical vapor reaction process to synthesize CH₃NH₃SnI₃ by directly converting Sn thin films in the presence of CH₃NH₃I vapor at moderate temperature, which also can be applied to synthesize various types of Sn-based perovskite such as FASnI₃.