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 Sn(2+) to Sn(4+) and poor film formability of the perovskite films. Herein, a synthetic strategy is developed for the fabrication of methylammonium tin iodide (MASnI(3)) film via ion exchange/insertion reactions between solid‐state SnF(2) and gaseous methylammonium iodide. In this way, the nucleation and crystallization of MASnI(3) can be well controlled, and a highly uniform pinhole‐free MASnI(3) perovskite film is obtained. More importantly, the detrimental oxidation can be effectively suppressed in the resulting MASnI(3) film due to the presence of a large amount of remaining SnF(2). This high‐quality perovskite film enables the realization of a PCE of 7.78%, which is among the highest values reported for the MASnI(3)‐based solar cells. Moreover, the MASnI(3) 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.