Hydrogel-confined environment for aqueous-compatible perovskite crystallization

Halide perovskite synthesis generally uses toxic, high-boiling-point and strongly coordinating organic solvents, hindering scalable processing and crystallographic control in thick-film applications. Water is ideal as a solvent but incompatible with perovskite synthesis due to precursor insolubility and rapid hydrolysis in water. Here we introduce a hydrogel-confined reaction environment that overcomes this incompatibility by decoupling precursor solvation from lattice formation. Within a polymer–water network, PbI 2 and organic iodides are dispersed in a stabilized ionic state that suppresses aggregation and kinetically inhibits water-induced decomposition. Concurrently, this network anchors emerging nuclei and imposes spatial confinement, directing crystal growth into dense thick films. Spectroscopic and computational analysis reveals facet-specific coordination between polymer and perovskite, suppressing surface defect formation and preserving efficient charge transport despite the presence of the polymer matrix. The resultant thick hydrogel-grown films (180 µm) exhibit charge carrier mobilities of ∼10 cm 2  V −1  s −1 . When directly integrated into pixelated X-ray imaging sensors, the device achieves a high sensitivity-to-noise ratio of \(25,000\,\upmu {\text{C}}\,{{\text{Gy}}}_{{\text{air}}}^{-1}\,{{\text{cm}}}^{-2}\,{{\text{pA}}}^{-1}\) , with an ultralow detection limit of 19.8 nGy air  s −1 , enabling high-fidelity imaging with negligible signal crosstalk.