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The Crystalline Nature of Colloids in Methylammonium Lead Halide Perovskite Precursor Inks Revealed by Cryo-Electron Microscopy.

Metal halide perovskites have generated interest across fields for the impressive optoelectronic properties achievable in films produced using facile solution processing techniques. Previous research has revealed the colloidal nature of perovskite precursor inks and shown a relationship between colloid distribution and film optoelectronic quality. Yet, the identity of colloids remains unknown, hindering our understanding of their role in perovskite crystallization. Here, we investigate precursor inks of the prototypical methylammonium lead triiodide perovskite using cryo-electron microscopy (cryo-EM) and show for the first time that the colloids are neither amorphous nor undissolved lead iodide as previously speculated, but are a crystalline non-perovskite material. We identify this as a perovskite precursor phase and discuss this as a potential means to understanding the role of chloride in processing. This establishes cryo-EM as a viable technique to elucidate the nature of colloids in perovskite inks, a vital step towards a fundamental understanding of thin-film crystallization.