Vibrational study of hybrid organic-inorganic perovskites*

Metal halide perovskites (MHPs) have achieved striking success as low-cost photovoltaic and light-emitting devices owing to their long carrier lifetimes, long carrier diffusion lengths, and exceptional carrier protection from defects. Our recent study on the three-dimensional (3D) hybrid organic-inorganic MHPs (MAPbI_3, MA=CH₃NH₃), has shown that the reorientation of the polarized molecules can assist the polaron formation, and thus prolong the charge carrier lifetime. Yet the purely inorganic MHPs without organic molecules can also achieve a moderate photovoltaic performance, which indicates that vibrations of the inorganic perovskite framework must also play indispensable roles in the optoelectronic properties of MHPs through their interactions with the charge carriers. In general, there are two scenarios for the phonon-mediated polaron formation: (1) polaron formation facilitated by optical phonons and (2) overdamping of acoustic phonons. Upon heating, acoustic phonons in the sample get overdamped which reduces their interactions with charge carriers and charge carriers can survive for a longer time. Additional question is which momentum directions such phonon melting, or overdamping would occur in MHPs. These questions call for detailed studies on phonon dispersion relations as a function of temperature on MHPs using inelastic neutron scattering. We would be presenting our results from inelastic neutron scattering measurements on partial-deuterated powder sample of MAPbI₃ and non-deuterated single crystal of MAPbBr₃. Both results showed that the well-defined optical phonons get overdamped in the material’s tetragonal phase (above 165 K for MAPbI₃ and 145 K for MAPbBr₃), which coincides with polaron formation in these materials.  We would also present our results from DFT calculations to show phonon modes along specific Brillouin zone directions as a function of temperature throughout all its structural phases.