Metal halide perovskites (MHPs) are revolutionizing the solar cell research field - after only about five years since the first report in 2009, the record power conversion efficiency of MHPs based solar cells has reached 20.1%. This represents the highest efficiency among all solution processable materials and the fastest rate of efficiency improvement in the history of all photovoltaic materials. Based on this trend, MHPs have been called the “next big thing in photovoltaics” and worldwide research efforts have recently experienced explosive growth.
Among various MHPs being studied, methylammonium lead iodide (MAPbI3) is currently the champion solar cell material and is being most actively studied by the research community. Despite the impressive solar cell performance, there are several optoelectronic properties of MAPbI3 that are not yet well understood. For example, there currently is an active discussion on possible presence of ferroelectricity in MAPbI3 which may originate from alignment of methylammonium (MA+) ions with dipole. Formation of ferroelectric domains within MAPbI3 will result in internal electric field that can assist separation and transport of photogenerated electrons and holes and may be a major contributor to the high solar cell efficiency. In addition, MAPbI3 exhibits a giant increase in dielectric constant upon light illumination as well as extremely slow photoconductivity response. These effects are also hypothesized to be due to MA+ ion rotation and associated structural fluctuation. All of these behaviors have direct consequences for light absorption, charge separation and transport and therefore solar cell performance. To understand and ultimately control these behaviors, it is critically important to characterize the structure and dynamics of MA+ ions in MAPbI3to obtain deeper insight on the structure-property-performance relationships.
Here we investigate the structure and rotational dynamics of MA+ ions in MAPbI3 perovskite using neutron diffraction and quasielastic scattering techniques. Over the temperature range of 70K to 370K, the hydrogen nuclei on MA+ ions exhibit three distinctive types of movements - (1) rotation around C-N axis, (2) rotation of the MA+ ion through jumping across preferential orientations and (3) isotropic rotation. Each of these motions is dominant at different temperature regimes. Activation energies and rates of the rotational motions were determined. Our results provide deeper insights on the cation rotational dynamics in metal halide perovskites and have major implications for understanding optoelectronic properties relevant for solar cell performance.