Chain-Length Dependence of Thermal Conductivity in 2D Alkylammonium Lead Iodide Single Crystals

In 2D organic-inorganic hybrid perovskite materials, layers of conducting inorganic material are separated by insulating organic spacers whose length and composition can be tuned. We report the heat capacity and cross-plane thermal conductivity of 2D alkylammonium lead iodide single crystals with increasing chain length, (CH₂NH₃)₂PbI₄ (n = 4-7). The measured thermal conductivities are some of the lowest ever recorded for single crystals, with averages in the range k = 0.099-0.125 W/m K. Although a model based on independent interface resistances between adjacent layers predicts an increase in thermal conductivity with a chain length of more than 30%, experimentally we find that the thermal conductivity is nearly independent of chain length and possibly decreases. We hypothesize that phonons carry an appreciable portion of the heat across the interface coherently, rather than being limited by individual weak interfaces.