Unveiling the ultralow in-plane thermal conductivity in 2D organic–inorganic hybrid perovskite (EA) 2 PbI 4 single crystals

This study highlights the extremely low in-plane thermal conductivity of the two-dimensional organic–inorganic hybrid perovskite (OIHP) EA 2 PbI 4 single crystal, which approaches its amorphous limit near 300 K. To elucidate the mechanism underlying this ultralow thermal conductivity, phonon dispersion relations of EA 2 PbI 4 were directly measured using inelastic neutron scattering. Additionally, the Debye temperature ( θ D ) of EA 2 PbI 4 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 PbI 6 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 EA 2 PbI 4 single crystals, the lowest among OIHPs. These findings not only unveil the distinctive thermal transport behavior and optical properties of EA 2 PbI 4 but also emphasize the unique lattice dynamics arising from the orientational dynamics of EA molecules and their coupling with the PbI 6 octahedra.