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) 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 (theta(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 PbI6 framework. Moreover, a low Einstein temperature (theta 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 PbI6 octahedra.