Phonon, thermal, and thermo-optical properties of halide perovskites

Metal halide perovskites are semiconductors with many fascinating characteristics and their widespread use in optoelectronic devices has been expected. High-quality thin films and single crystals can be fabricated by simple chemical solution processes and their fundamental electrical, optical, and thermal properties can be changed significantly by compositional substitution, in particular halogen ions. In this perspective, we provide an overview of phonon and thermal properties of metal halide perovskites, which play a decisive role in determining device performance. After a brief introduction to fundamental material properties, longitudinal-optical phonons and unusual thermal properties of metal halide perovskites are discussed. Remarkably, they possess very low thermal conductivities and very large thermal expansion coefficients despite their crystalline nature. In line with these discussions, we present optical properties governed by the strong electron-phonon interactions and the unusual thermal properties. By showing their unique thermo-optic responses and novel application examples, we highlight some aspects of the unusual thermal properties.