Centimeter-Scale Giant-Grain Growth: Radial Expansion of MAPbI3-xClx Precursor Single Crystals from a Single Nucleus on Glass

Here I report a novel crystal growth phenomenon termed “radial Giant-Grain Growth” (r-GGG), which transforms spin-coated polycrystalline MAPbI3−xClx thin films into macroscopic single-nucleus domains spanning millimeters to centimeters. Although governed solely by substrate temperature and ambient humidity, r-GGG is characterized by the extreme suppression of secondary nucleation, allowing a single nucleus—either a hydrate or a perovskite phase—to dominate growth over extensive areas. Specifically, a Cl-rich seed layer is prepared by spin-coating a precursor solution (MAI:PbCl2 = 3:1 molar ratio in DMF), followed by brief annealing at 60°C under an inert atmosphere. Subsequent exposure to air (65–70% relative humidity) initiates r-GGG. At room temperature, densely packed, needle-like hydrate single crystals propagate radially from a single nucleus at an isotropic growth rate, resulting in films composed of needle-crystal aggregates with millimeter- to centimeter-scale domains—a morphology unprecedented in conventional perovskite fabrication. Conversely, substrate temperatures exceeding 50°C induce the direct lateral growth of large-grain MAPbI3−xClx. Notably, thermal annealing of the r-GGG-derived hydrate films at 90°C yields highly textured MAPbI3−xClx films with pronounced (110) preferred orientation (strong (110) texture), as evidenced by the dominance of the (110)/(220) reflections and a narrowed rocking-curve width relative to conventional thin films. These (110)-textured films exhibit exceptional stability, showing negligible degradation even after one week of ambient exposure. These findings challenge the long-standing dogma that moisture must be strictly excluded during perovskite fabrication, demonstrating instead that controlled humidity can serve as a potent driving force for forming macroscopically coherent crystal domains. Elucidating the physical mechanisms underlying r-GGG could enable the fabrication of giant-grain thin films with long-range, periodically spaced nucleation sites across diverse material systems, offering a new conceptual framework for crystal growth science.