Halide- and Temperature-Dependent Anharmonic Phonon Dynamics in Formamidinium Lead Halide Perovskites

Student’s name: Matthew J. Silverstein Home Institution: University of Michigan - Ann Arbor NNCI Site: RTNN @ Duke University REU Principal Investigator: Dr. Olivier Delaire - Thomas Lord Department of Mechanical Engineering and Materials Science, Department of Chemistry, and Department of Physics, Duke University REU Mentors: Patrick Postec - Thomas Lord Department of Mechanical Engineering and Materials Science, Duke University; Chengjie Mao - Department of Chemistry, Duke University Abstract: Phonons, sound quanta in the form of collective vibrations in a periodic crystal lattice, significantly affect the thermal and electrical transport properties of materials. The halide perovskite (HP) family of materials with the formula ABX3, where A is a monovalent cation, B is a divalent metal, and X is a halogen anion, boast long charge carrier lifetimes and high photovoltaic efficiency. Previous studies have shown significant electron-phonon coupling, which suggests that investigating the phonon dynamics of these materials is key to understanding their optoelectronic properties. At high temperatures, HPs adopt a cubic structure, but diffuse scattering experiments have shown that there are localized regions of correlated tilts of the BX6 octahedra, corresponding to strongly anharmonic phonon modes. We hypothesized that changing the chemical environment of the octahedral lattice, via substitutional disorder on the halide site, should affect the phonon dynamics. We studied FAPbBr3-xClx (FA = formamidinium) with Raman spectroscopy to elucidate the compositional and temperature-dependence of phonon modes. Doping the halide sites causes increased energy and broadening of certain phonons, while leaving other modes relatively unchanged. We found two vibrational modes of interest, one of which varied with temperature, and the other which coupled strongly to composition, but neither coupled to both variables. We found these phonons to be strongly anharmonic as evidenced by peak broadening with increasing temperature. Based on literature findings, we determined that these are molecular modes, vibrations of covalent bonds in organic compounds. The changes in one mode’s energy due to modification of the inorganic lattice suggests a coupling to formamidinium. These results can be used for rational design of photovoltaic materials, by helping us better understand the mechanism by which hybrid organic-inorganic materials derive their favorable optoelectronic properties.