Solvent Esterification and Stoichiometric Control in Ambient-Grown FAPbI ₃ Single-Crystal Solar Cells

Inverse temperature crystallization (ITC) is widely used to grow high-quality perovskite single crystals, yet prolonged thermal exposure during this process can introduce chemical instabilities that hinder controlled crystal growth. In this work, we uncover unexpected solvent-dependent redissolution of formamidinium lead iodide (FAPbI 3 ) during ITC. While a mixed γ-butyrolactone (GBL) and 2-methoxyethanol (2ME) solvent system beneficially enables α-FAPbI 3 growth in ambient conditions, we discover that FAPbI 3 facilitates an esterification reaction of the two solvents that modifies the coordination environment and destabilizes perovskite during extended heating. Lead iodide (PbI 2 ) deficiency effectively delays the redissolution process, resulting in reduced δ-FAPbI 3 formation and stabilized growth of thin α-FAPbI 3 single crystals. Single-crystal solar cells based on phase-pure α-FAPbI 3 crystals achieve 22.99% power conversion efficiency, setting a record for such devices fabricated under ambient air conditions. These results reveal an overlooked solvent-precursor interaction during ITC and demonstrate stoichiometry control as a practical strategy for stabilizing α-FAPbI 3 single crystals for high-performance photovoltaic applications.