Modulating The Thermal Transport of MAPbI₃ Hybrid Perovskites and The Effect on its Population-/Coherence-Channel

Hybrid halide perovskites, with their favorable carrier recombination time and ultrashort phonon mean-free paths, are potential candidates for numerous energy conversion applications like photovoltaics and thermoelectrics. The origin of the ultralow thermal conductivity in the prototypical methylammonium lead triiodide (MAPbI₃) is of intense research interest for improving and modulating its energy conversion performance.[1] So far, such an understanding remains elusive in the MAPbI₃ at above room temperatures (that exists as a cubic phase) despite numerous efforts[2] and under external modulation fields.<br/>Using molecular dynamics and the Wigner transport equation,[3] we first report the discovery of several cubic-phase MAPbI₃ (c-MAPbI₃) local energy minimum structures.[4] These stable structures are amenable to lattice dynamics-based calculations to produce positive phonon dispersions. Our results reveal a coherence-channel-dominated thermal transport mechanism in the c-MAPbI₃ crystals. Interestingly, an inter-conversion between the population- and coherence-channel occurs when the c-MAPbI₃ changes across these energetically equivalent structures at the same temperature. Such an effect is yet to be observed in simple atomic crystals.<br/>Next, we looked at how external pressure fields and electric fields affect the thermal transport of MAPbI₃. An external pressure increases the c-MAPbI₃ thermal conductivity of the phonon population-channel contribution, similar to trends in many crystals. However, little effect on the coherence-channel contribution is observed. This unusual trend impedes the thermal conductivity increase for this c-MAPbI₃ and imparts a trend that mirrors that of a diamond[5]. On the other hand, the electric field has little effect on c-MAPbI₃ although the polar MA cations are affected by the electric field.<br/>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 can affect the interpretation of other phonon-mediated processes in MAPbI₃ and other hybrid perovskites.<br/><br/><b>References:</b><br/>[1] M.A. Haque, S. Kee, D.R. Villalva, W. Ong, D. Baran, Halide Perovskites: Thermal Transport and Prospects for Thermoelectricity, Adv. Sci. 7 (2020) 1903389. https://doi.org/10.1002/advs.201903389.<br/>[2] T. Zhu, E. Ertekin, Mixed phononic and non-phononic transport in hybrid lead halide perovskites: Glass-crystal duality, dynamical disorder, and anharmonicity, Energy Environ. Sci. 12 (2019) 216–229. https://doi.org/10.1039/c8ee02820f.<br/>[3] M. Simoncelli, N. Marzari, F. Mauri, Unified theory of thermal transport in crystals and glasses, Nat. Phys. 15 (2019) 809–813. https://doi.org/10.1038/s41567-019-0520-x.<br/>[4] J. Yang, A. Jain, W.L. Ong, Inter-channel conversion between population-/coherence-channel dictates thermal transport in MAPbI3 crystals, Mater. Today Phys. 28 (2022) 100892. https://doi.org/10.1016/j.mtphys.2022.100892.<br/>[5] J. Yang, A. Jain, L. Fan, Y.S. Ang, H. Li, W.L. Ong, Anomalous Pressure-Resilient Thermal Conductivity in Hybrid Perovskites with Strong Lattice Anharmonicity and Small Bulk Modulus, Chem. Mater. 35 (2023) 5185–5192. https://doi.org/10.1021/acs.chemmater.3c00935.