Elastic softness of hybrid lead halide perovskites
Afonso da Cunha Ferreira a b, Antoine Létoublon b, Serge Paofai c, Stéphane Raymond d, Claude Ecolivet e, Benoit Rufflé f, Stéphane Cordier c, Claudine Katan c, Makhsud Saidaminov g, Ayan Zhumekenov g, Osman Bakr g, Philippe Bourges a, Jacky Even b
a Laboratoire Léon Brillouin, CEA-CNRS, Université Paris-Saclay, CEA Saclay, 91191 Gif-sur-Yvette, France
b Fonctions Optiques pour les Technologies de l’Information (FOTON), Institut National des Sciences Appliquées (INSA) de Rennes, CNRS, UMR 6082, Rennes, France
c Institut des Sciences Chimiques de Rennes, CNRS, Université de Rennes 1, Ecole Nationale Supérieure de Chimie de Rennes, INSA Rennes, Rennes, France
d Univ. Grenoble Alpes, CEA, INAC, MEM, 38000 Grenoble, Francia, Grenoble, France
e Institut de Physique de Rennes, CNRS, Université de Rennes 1, Rennes, France
f Laboratoire Charles Coulomb, UMR 5221, Université de Montpellier, 34095 Montpellier, France
g King Abdullah University of Science and Technology (KAUST) - Saudi Arabia, 4700 King Abdullah University of Science and Technology (KAUST), Thuwal, Saudi Arabia
NIPHO
Proceedings of International Conference on Perovskite Thin Film Photovoltaics, Photonics and Optoelectronics (ABXPV18PEROPTO)
Perovskite Thin Film Photovoltaics (ABXPV18). 27-28 Feb
Rennes, France, 2018 February 27th - March 1st
Organizer: Jacky Even
Poster, Afonso da Cunha Ferreira, 006
Publication date: 11th December 2017

Hybrid organic-inorganic perovskites have been intensively studied as promising materials for photovoltaic applications, taking advantage of their high efficiency and low cost processing. Although much recent attention has also been devoted towards unraveling their microscopic optoelectronic properties, the origin of their softness, by comparison to classic semiconductors, is currently still lacking a comprehensive understanding and systematic experimental studies. Here we investigate by coherent inelastic neutron scattering spectroscopy and Brillouin light scattering, low frequency acoustic phonons in four different hybrid perovskite single crystals: MAPbBr3, FAPbBr3, MAPbI3 and α-FAPbI3. We report a very small shear C44 elastic constant for all the compounds and a considerable elastic anisotropy. The extremely low bulk modulus and negative C12 in α-FAPbI3 substantiates its very unstable nature and in FAPbBr3, a tendency towards an incipient ferroelastic transition, is interpreted as further evidence of the influence of plasticity in hybrid perovskites. We observe a systematic lower sound group velocity in the technologically important iodide-based compounds compared to the bromide-based ones. The findings suggest that low thermal conductivity and hot phonon bottleneck phenomena are expected to be enhanced by low elastic stiffness, particularly in the case of the ultrasoft α-FAPbI3. This project has received funding from the European Union’s Horizon 2020 programme, through a FET Open research and innovation action under the grant agreement no 687008.

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