Structural and dynamic properties of hybrid organolead perovskites for photovoltaic applications

This project focuses on the study of halide organolead perovskite (HOP) compounds, namely MAPbBr3, MAPbI3, FAPbBr3 and α-FAPbI3. To do this, we rely heavily on neutron inelastic scattering spectroscopy (INS), because it allows to systematically probe the structural properties of these materials. As a result, we are able to present a comprehensive investigation of lattice excitations (i.e. phonons) in the four of the most technologically relevant HOP compounds in the photovoltaics field. By measuring dispersion curves of acoustic phonons we give a clear picture of the difference in softness between FA and MA based compounds and how it relates to their structural stability and their ultralow thermal conductivities. We also present here an extensive comparison of optical phonon excitations in the four different hybrid, in which we carefully discuss mode attribution to the respective structural vibrations. In contrast to theoretical expectation and classical behavior in standard semiconductor compounds, the phonon modes show no dispersion, suggesting strong anharmonic behavior and localization effects. This behaviour puts into question the validity of the quasiparticle picture used for phonon simulation and the present understanding of the Frohlich interaction for carrier mobilities. This may help in solving the apparent paradox of acoustic-like temperature dependence of the charge carrier mobilities and dominant direct processes expected to be related to optical phonons. Our results also highlight the role of the strong acousto-optical anharmonic coupling (responsible for the characteristic low elastic stiffness) in the glassy-like thermal conductivities and hot-phonon bottleneck effect in HOPs. This experimental study could also provide a solid starting point for further theoretical calculations to understand the fundamental properties of these materials.