Anisotropic and Ultralow Phonon Thermal Transport in Organic-Inorganic Hybrid Perovskites: Atomistic Insights into Solar Cell Thermal Management and Thermoelectric Energy Conversion Efficiency

Energy-related functionality and performance of organic-inorganic hybrid perovskites, such as methylammonium lead iodide (MAPbI(3)), highly depend on their thermal transport behavior. Using equilibrium molecular dynamics simulations, it is discovered that the thermal conductivities of MAPbI(3) under different phases (cubic, tetragonal, and orthorhombic) are less than 1 W m(-1) K-1, and as low as 0.31 W m(-1) K-1 at room temperature. Such ultralow thermal conductivity can be attributed to the small phonon group velocities due to their low elastic stiffness, in addition to their short phonon lifetimes (<100 ps) and mean-free-paths (<10 nm) due to the enhanced phonon-phonon scattering from highly-overlapped phonon branches. The anisotropy in thermal conductivity at lower temperatures is found to associate with preferential orientations of organic CH3NH3+ cations. Among all atomistic interactions, electrostatic interactions dominate thermal conductivities in ionic MAPbI(3) crystals. Furthermore, thermal conductivities of general hybrid perovskites MABX(3) (B = Pb, Sn; X = I, Br) have been qualitatively estimated and found that Sn- or Br-based perovskites possess higher thermal conductivities than Pb-or I-based ones due to their much higher elastic stiffness. This study inspires optimal selections and rational designs of ionic components for hybrid perovskites with desired thermal conductivity for thermally-stable photovoltaic or highly-efficient thermoelectric energy harvesting/conversion applications.