We use a data-driven approach to discover the influence of trace amounts of water on perovskite crystal formation. Statistical analysis of 8,470 inverse-temperature crystallization lead iodide perovskite synthesis reactions, performed over 20 months using a robotic system, revealed discrepancies between the empirical crystal formation rate in experiments conducted under different ambient relative humidity conditions. We used the robotic system to conduct 1,296 controlled interventional experiments in which small amounts of water were deliberately introduced to the reactions. Addition of trace amounts of water promotes crystal formation for 4-methoxyphenylammonium lead iodide and iso-propylammonium lead iodide and inhibits crystal formation for dimethylammonium lead iodide and acetamidinium lead iodide. We also performed thin-film syntheses of these four materials and determined the grain size distributions using scanning electron microscopy. Addition of water results in smaller grain sizes for dimethylammonium and larger grain sizes for isopropylammonium, consistent with earlier or delayed nucleation, respectively.
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