Thermal Conductivity of CH₃NH₃PbI₃ and CsPbI₃: Measuring the Effect of the Methylammonium Ion on Phonon Scattering

Temperature-dependent thermal conductivity in the range between 7 and 300 K was measured for CH₃NH₃PbI₃ and CsPbI₃ and compared to a Debye model via the Callaway method. Thermal conductivity was found to be extremely low across the whole temperature range for both materials, with CH₃NH₃PbI₃ lower than CsPbI₃. Fitting analysis showed that a resonant phonon scattering term can account for the difference in thermal transport behavior between the perovskite with a methyl­ammonium (MA) ion versus a single cesium atom in the cationic A site of the lattice. The resonant frequency associated with this term is in the range of ∼15–30 cm^–1, pointing to the rotational degree of freedom of the organic ion. Analysis of the temperature dependence of the possible phonon scattering mechanisms showed that thermal conductivity of both CH₃NH₃PbI₃ and CsPbI₃ perovskites was dominated by Umklapp scattering at room temperature, and the rotation of the organic cation may be responsible for suppressing the thermal conductivity of CH₃NH₃PbI₃ in comparison with CsPbI₃, particularly at low temperatures, ∼25 K. This work presents the first determination of temperature-dependent thermal conductivity of CsPbI<sub>3