Structural and dynamic properties of hybrid organolead perovskites for photovoltaic applications
2020
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.
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