Resolving discrepancies in mechanical properties of hybrid perovskites: a DFT study

The mechanical properties of hybrid perovskite materials are important for the behavior of flexible devices, resistance to fracture, epitaxial growth, surface energetics of quantum dots, and induction or relief of stress in thin films due to thermal expansion and phase changes. These issues are particularly salient for solar cells in space applications. Nonetheless few studies are available on the mechanical properties of CH₃NH₃PbI₃ (MAPI). Experimental results are only available for the room-temperature tetragonal phase, which have significant variation.  Results from density functional theory (DFT) are available for all three phases but have even larger discrepancies from each other and from experiments. To bring order to the confusion in the literature, we have studied the elastic properties of all three phases in detail with DFT calculations. We have examined the effect of different aspects such as structure, exchange-correlation functionals, van der Waals corrections, pseudopotentials, and other considerations in calculation methodology. Our results provide accurate reference values and an appropriate general methodology for elastic properties of metal halide perovskites.