Ion Exchange/Insertion Reactions for Fabrication of Efficient Methylammonium Tin Iodide Perovskite Solar Cells

The low toxicity, narrow bandgaps, and high charge-carrier mobilities make tin perovskites the most promising light absorbers for low-cost perovskite solar cells (PSCs). However, the development of the Sn-based PSCs is seriously hampered by the critical issues of poor stability and low power conversion efficiency (PCE) due to the facile oxidation of Sn²⁺ to Sn⁴⁺ and poor film formability of the perovskite films. Herein, a synthetic strategy is developed for the fabrication of methylammonium tin iodide (MASnI₃) film via ion exchange/insertion reactions between solid-state SnF₂ and gaseous methylammonium iodide. In this way, the nucleation and crystallization of MASnI₃ can be well controlled, and a highly uniform pinhole-free MASnI₃ perovskite film is obtained. More importantly, the detrimental oxidation can be effectively suppressed in the resulting MASnI₃ film due to the presence of a large amount of remaining SnF₂. This high-quality perovskite film enables the realization of a PCE of 7.78%, which is among the highest values reported for the MASnI₃-based solar cells. Moreover, the MASnI₃ solar cells exhibit high reproducibility and good stability. This method provides new opportunities for the fabrication of low-cost and lead-free tin-based halide perovskite solar cells.