Revealing the Role of Methylammonium Iodide Purity on the Vapor‐Phase Deposition Process of Perovskites

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Vapor deposition processes, well established in the industry, are considered a cost‐effective manufacturing route for large‐area depositions of lead halide perovskites. However, the vapor deposition of perovskites is constrained by its low reproducibility, whereby in particular the evaporation of methylammonium iodide (MAI) is frequently reported as extremely challenging. Herein, the influence of impurities on the evaporation dynamics of MAI is analyzed. It is proved that MAI heated in a high vacuum, depending on the purity, either undergoes a sublimation with a subsequent desublimation or a chemical decomposition with a following reverse reaction, leading to solid MAI in both cases. In situ mass spectrometry enables us to show that low purity MAI containing water and methylammonium phosphite impurities mainly sublimes, which results in a directional and by crystal microbalances traceable evaporation. In contrast, high purity MAI sublimes at higher temperatures and tends to form decomposition products such as methylamine and hydrogen iodide, which omnidirectional diffuse and can react with lead iodide to perovskite. The findings reveal the reason for the reported challenges, serve as a guideline for future material selections, and can enhance the reproducibility of the evaporation process to achieve a more controlled solar cell fabrication.