Impact of Large Thermal Motion on Critical Properties of Perovskite Photovoltaic Systems

Solar cells based on hybrid perovskites have shown high efficiency while possessing simple processing methods. To gain a fundamental understanding of their properties on an atomic level, we investigate single crystals of CH3NH3PbI3 with transition T*~330 K. Temperature dependent structural measurements reveal a persistent tetragonal structure with smooth changes in the atomic displacement parameters (ADPs) on crossing T*. We find that the atomic displacement parameters for I ions yield extended flat regions in the potential wells consistent with the measured large thermal expansion parameter. Molecular dynamics simulations reveal that this material exhibits significant high asymmetries in the Pb-I pair distribution functions. We show that the intrinsic enhanced freedom of motion of the iodine atoms enables large deformations. This flexibility of the atomic structure results in highly localized atomic relaxation about defects and makes transport in these systems resilient in the presence of large defect densities.