In situ observation of ultra-giant spherulites in perovskite hydrates

Spherulitic growth—commonly observed in polymers—is shown here to be reproducible in lead-halide perovskite hydrates on glass substrates, yielding ultra-giant spherulites spanning the mm–1 cm scale under specific conditions. Here I introduce Hygro-Triggered Giant Spherulite Formation (HT-GSF), an equipment-light process that transforms a routine spin-coated MAPbI₃₋ₓClₓ precursor film into a ~1 cm-scale giant spherulite using only standard glovebox and hot-plate steps. A 3:1 MAI/PbCl₂ precursor (40 wt% in DMF) is spin-coated on glass and annealed at 60 °C for 10 min under N₂ to produce a Cl-rich seed layer containing needle-like precursor crystallites. Subsequent exposure to ambient air (60–65% RH) reproducibly initiates a single surface nucleus that dominates growth and drives rapid two-dimensional radial expansion, completing within ~2 h. Real-time optical microscopy shows an advancing spherulite front that incorporates surrounding particles, consistent with a humidity-enabled dissolution–recrystallization pathway and local solute depletion near the growth interface. Post-annealing at 90 °C for 4 h converts the white precursor network into black perovskite while preserving a pronounced out-of plane (110) texture. Optical microscopy confirms centimeter-scale, single-nucleus spherulites with strong preferred orientation and far fewer macroscopically visible high angle grain boundaries than conventional spin-coated films, with 100% yield across >50 trials. Under ambient-air ageing tests, HT-GSF films exhibit only weak hydrate-related signatures after one week, whereas control films develop strong hydrate peaks and visible degradation within hours. HT-GSF provides a simple and reproducible route to mm–cm scale, highly textured perovskite spherulites on glass, offering a platform for mechanistic studies and future evaluation of device relevance.