Unveiling the ultralow in-plane thermal conductivity in 2D organic-inorganic hybrid perovskite (EA)₂PbI₄ single crystals

This study highlights the extremely low in-plane thermal conductivity of the two-dimensional organic-inorganic hybrid perovskite (OIHP) EA₂PbI₄ single crystal, which approaches its amorphous limit near 300 K. To elucidate the mechanism underlying this ultralow thermal conductivity, phonon dispersion relations of EA₂PbI₄ were directly measured using inelastic neutron scattering. Additionally, the Debye temperature (θ_D) of EA₂PbI₄ was 284 K, corresponding to an average phonon group velocity of 2284 m s⁻¹. The suppressed thermal transport efficiency is then attributed to the exceptionally short phonon mean free paths, which approach the bond lengths in the PbI₆ framework. Moreover, a low Einstein temperature (θ_E) of 45 K was identified through heat capacity fitting, indicating the presence of low-lying optical vibrational modes as revealed by detailed Raman scattering measurements. These softened phonons can readily engage with acoustic ones, creating a more complex scattering environment. This study also reports exceptionally low exciton binding energies of 5.3-6.4 meV in high-quality EA₂PbI₄ single crystals, the lowest among OIHPs. These findings not only unveil the distinctive thermal transport behavior and optical properties of EA₂PbI₄ but also emphasize the unique lattice dynamics arising from the orientational dynamics of EA molecules and their coupling with the PbI₆ octahedra.