Moisture-Induced Amorphous to Crystalline Transition of (HOCH ₂ CH ₂ NH ₃ ) ₂ [PbI ₄ ] Perovskite

Hybrid perovskites are well-known to suffer from moisture instability, which leads to surface degradation. Two-dimensional (2D) layered organic-inorganic hybrid perovskites are more environmentally stable than the 3D family. However, we have found a unique moisture-induced crystallization phenomenon in 2D Ruddlesden-Popper perovskite (EOA)₂PbI₄ (EOA = HOCH₂CH₂NH₃⁺). The as-prepared amorphous film undergoes a rapid and irreversible transition to a crystalline hydrated phase, when exposed to a high relative humidity atmosphere (>70% RH). This transformation does not occur under dry conditions and remains stable after thermal treatment above 400 K. Remarkably, the crystallized hydrate exhibits a 2 orders of magnitude increase in electrical resistivity compared to the as-prepared amorphous state, suggesting significantly higher charge carrier concentration in the as-prepared film. To our knowledge, this is the first observation of humidity triggered amorphous to crystalline conversion in a 2D perovskite, providing novel insights into the role of moisture in perovskite phase evolution.