Two-dimensional Al₂I₂Se₂: A promising anisotropic thermoelectric material

Recently, a high-throughput computational screening of two-dimensional layered materials for thermoelectrics predicted that the layered FeOCl-type compounds with well-known photovoltaic and topological properties show the potential for thermoelectric (TE) applications. In this vein, further investigation and verification of the high TE performance in the FeOCl-type compounds should be carried out, especially the corresponding thin films. In this work, we comprehensively study the TE performance including both electrical and thermal transport properties of a novel FeOCl-type monolayer, Al₂I₂Se₂, by first-principles calculations. As compared with other FeOCl-type monolayers, Al₂I₂Se₂ monolayer exhibits significant lattice anharmonicity (with a Grüneisen parameter up to 4.8) and higher frequency phonon scattering because of the suitable element combination, and thus the ultra-low thermal conductivity. Moreover, its TE parameters show strong anisotropy originating from the unique electronic and phonon properties, which provides an opportunity to optimize the TE performance by using designed transport directions. Consequently, at the temperature of 700 K, both an ultra-low lattice thermal conductivity (κ_l) of 0.12 W m⁻¹ K⁻¹ and a satisfactory optimal power factor (PF) of 4.56 mW m⁻¹ K⁻² are found along the x-axis, resulting in a very high figure of merit (ZT) of 3.37, which is approximately 2 times higher than the value of 1.75 along the y-axis. The results achieved in this work suggest that Al₂I₂Se₂ monolayer is promising for high-performance TE applications and our study provides useful information for future studies on the TE performance of FeOCl-type monolayers.