Exceptional Grain Growth in Formamidinium Lead Iodide Perovskite Thin Films Induced by the δ-to-α Phase Transformation

The annealing of fine-grained (∼0.175 μm) "yellow" δ-FAPbI3 nonperovskite thin films at 100 °C in dimethyl sulfoxide vapor for a few minutes results in exceptional grain growth (∼5 μm) induced by their transformation to the desirable "black" α-FAPbI3 perovskite phase. Mechanistic insights into this unprecedented phenomenon are provided.