Inter-channel conversion between population-/coherence-channel dictates thermal transport in MAPbI3 crystals

Abstract

Hybrid halide perovskites, with their favorable carrier recombination time and ultrashort phonon mean-free paths, are leading candidates for various energy conversion applications like photovoltaics and thermoelectrics. The origin of the ultralow thermal conductivity of the prototypical methylammonium lead triiodide (MAPbI3) is of intense research interest as it is critical for improving its energy conversion performance. So far, such an understanding remains elusive in the MAPbI3 above room temperatures (c-MAPbI3) despite numerous efforts due to its unstable phonon modes. Here, we report the discovery of several c-MAPbI3 local minimum energy structures that produce stable phonon dispersions to reveal an amorphous-like coherence-channel thermal transport mechanism in these crystals. Interestingly, an inter-channel conversion between the population- and coherence-channel of the phonons occurs when the c-MAPbI3 changes across these different structures at the same temperature. Such an effect is not yet observed in simple atomic crystals. Our work also shows that existing thermal transport intuitions based on the phonon gas model can be misleading in such hybrid crystals. Further, the dominance of the non-traditional coherence-channel of phonons will affect the interpretation of other phonon-mediated processes in MAPbI3 and other hybrid perovskites.