Uniquely anisotropic mechanical and thermal responses of hybrid organic-inorganic perovskites under uniaxial strain

Thermal Transport Properties of Phonons in Halide Perovskites

Halide perovskites have emerged as promising candidates for various applications, such as photovoltaic, optoelectronic and thermoelectric applications. The knowledge of the thermal transport of halide perovskites is essential for enhancing the device performance for these applications and improving the understanding of heat transport in complicated material systems with atomic disorders. In this work, the current understanding of the experimentally and theoretically obtained thermal transport properties of halide perovskites is reviewed. This study comprehensively examines the reported thermal conductivity of methylammonium lead iodide, which is a prototype material, and provides theoretical frameworks for its lattice vibrational properties. The frameworks and discussions are extended to other halide perovskites and derivative structures. The implications for device applications, such as solar cells and thermoelectrics, are discussed.

Origin of Ultralow Thermal Conductivity in Metal Halide Perovskites

Resulting from their remarkable structure-property relationships, metal halide perovskites have garnered tremendous attention in recent years for a plethora of applications. For instance, their ultralow thermal conductivities make them promising candidates for thermoelectric and thermal barrier coating applications. It is widely accepted that the "guest" cations inside the metal halide framework act as "rattlers", which gives rise to strong intrinsic phonon resistances, thus explaining the structure-property relationship dictating their ultralow thermal conductivities. In contrast, through systematic atomistic simulations, we show that this conventionally accepted "rattling" behavior is not the mechanism dictating the ultralow thermal conductivities in metal halide perovskites. Instead, we show that the ultralow thermal conductivities in these materials mainly originate from the strongly anharmonic and mechanically soft metal halide framework. By comparing the thermal transport properties of the prototypical fully inorganic CsPbI₃ and an empty PbI₆ framework, we show that the addition of Cs⁺ ions inside the nanocages leads to an enhancement in thermal conductivity through vibrational hardening of the framework. Our extensive spectral energy density calculations show that the Cs⁺ ions have well-defined phase relations with the lattice dynamics of the "host" framework resulting in additional pathways for heat conduction, which is in disagreement with the description of the individual "rattling" of guests inside the framework that has been widely assumed to dictate their ultralow thermal conductivities. Furthermore, we show that an efficient strategy to control the heat transfer efficacy in these materials is through the manipulation of the framework anharmonicity achieved via strain and octahedral tilting. Our work provides the fundamental insights into the lattice dynamics that dictate heat transfer in these novel materials, which will ultimately help guide their further advancement in the next-generation of electronics such as in thermoelectric and photovoltaic applications.