Anharmonic Semiconductors - Lessons Learned from Halide Perovskites
Omer Yaffe a
a Department of Materials and Interfaces, Weizmann Institute of Science, Rehovoth 76100, Israel.
Invited Speaker, Omer Yaffe, presentation 068
Publication date: 25th November 2019

Halide perovskite semiconductors exhibit a unique combination of very-low electronic defect density, self-healing properties, and low exciton binding. This set of properties is surprising for a solution-processed material and results in an excellent photovoltaic activity.

 I will show that the fundamental property that sets the halide perovskites apart from conventional semiconductors like silicon and GaAs is strongly anharmonic lattice dynamics.  Large amplitude, local polar fluctuations that, result from lattice anharmonicity, localize the electronic states and enhance the screening of electric charges within the material. In other words, in some aspects, halide perovskites behave more like a liquid than a crystalline solid.

First,  I will show that anharmonic lattice dynamics in methylammonium lead bromide lead to strong changes in its dielectric response with temeparture. Then, I will discuss the benefits of  Raman polarization-orientation measurements to investigate temperature-induced symmetry breaking. Finally, I will discuss in detail the mechanisms that lead to strong anharmonicity in methylammonium lead iodide. 


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