Suite aux recommandations sanitaires actuelles et aux limitations de déplacement, la thèse sera soutenue par visio-conférence. / Following current health recommendations and travel restrictions, the thesis will be defended by videoconference.
Thèse préparée en collaboration entre les laboratoires :
There is an ever-growing need to address both energy and environmental issues, product of generations of over-exploitation of fossil fuel sources and the increased associated industries, which represent one of the most beneﬁcial and, at the same time, injurious aspects of modern times. Solar energy emerges as a renewable and cleaner alternative energy source, with still a lot of possible room for improvement. Photovoltaic (PV) technology requires materials with high efficiencies, simple processability, low cost and abundant availability on Earth. In the effort of developing new generation PV cells, hybrid metal halide perovskites (HOPs) represent a promising opportunity for incorporating all these requirements.
This thesis focuses on the study of these 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 quasi-particle picture used for phonon simulation and the present understanding of the Fröhlich 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.
Keywords: Hybrid Perovskites; Photovoltaics; Spectroscopy; Neutron diffraction.