Understanding Instability in Formamidinium Lead Halide Perovskites: Kinetics of Transformative Reactions at Grain and Subgrain Boundaries br

Transformative and reconstructive reactions impart sig-nificant structural changes at particle boundaries of hybrid perovskites,which influence environmental stability and optoelectronic properties ofthese materials. Here, we investigate the moisture-induced transformativereactions in formamidinium(FA)-based perovskites FAPbX3(X = I, Br)and show that the ambient stability of these materials can be adjustedfrom a few hours to several months. For FAPbI3, roles of water vapor,particle size, and light illumination on the kinetic pathways of the cubic(alpha) transformation to the hexagonal (delta) phase are analyzed by X-raydiffraction, optical microscopy, photoluminescence, and solid-state (ss)NMR spectroscopy techniques. The grain and subgrain boundariesexhibit different alpha-->delta-FAPbI3phase transformation kinetics. Our studysuggests that the dynamic transformation involves the local water-induced dissolution of the cubic phase occurring at the crystal surfacesfollowed by precipitation of the hexagonal phase. Insights into structures and dynamics of a kinetically trapped alpha-|delta-FAPbI3are obtained by1H,2H, and207Pb ssNMR spectroscopy.