Effect of humidity on the orientational ordering of CH3NH+3 in methylammonium lead iodide

With high energy conversion efficiency and low-cost production, hybrid organic–inorganic perovskite solar cells (PSCs) have the potential to be alternative to silicon-based technology. However, there are concerns about their long-term stability and environmental friendliness, which must necessarily be addressed to enable large-scale commercialization of PSCs. Here, we use first-principles theory to determine and understand the effects of humidity on the T-dependent tetragonal to cubic structural transition in CH3NH3PbI3, which can impact the long-term stability of its properties. We show that ferroelectric vs. antiferroelectric structural ordering in CH3NH3PbI3 is influenced by humidity. Within first-principles density functional theory, we determine the lowest energy configurations of dipolar ordering in CH3NH3PbI3·xH2O and effects of their interaction with H2O molecules. Developing a simple effective Hamiltonian to model these configurations, we use Monte Carlo simulations to determine temperature-dependent structural phase transitions in CH3NH3PbI3. We establish ferroelectric ordering in MAPbI3 at low temperature, and demonstrate that it changes to antiferroelectric ordering of MA+ cations at x > 0.2 in CH3NH3PbI3·xH2O.