Direct Observation of Reversible Transformation of CH3NH3PbI3 and NH4PbI3 Induced by Polar Gaseous Molecules.

Despite its competitive photovoltaic efficiency, the structural transformations of the prototypical hybrid perovskite, methylammonium lead iodide, are facilitated by interactions with polar molecules. Changes in optical and electronic properties upon exposure to ammonia potentially can enable the use of hybrid perovskites in gas-sensing applications. We investigated the effects of ammonia on CH3NH3PbI3 by exposing perovskite films to a wide range of vapor pressures. Spectroscopic analyses indicated that ammonium cations replaced the methylammonium cations in the perovskite crystal, thereby resulting in the formation of NH4PbI3. The transformation of CH3NH3PbI3 to NH4PbI3 caused distinct changes in the morphology of the film and its crystalline structure; however, the introduction of CH3NH2 gas reversed these changes. An in-depth understanding of the reversible chemical and structural alterations resulting from exposure to polar molecules can advance the development of hybrid perovskite sensors and provide insight into mechanisms by which perovskites convert due to interactions with polar molecules.