High Thermoelectric Performance of Al₂X₂Se₂ (X = Cl, Br, I) Monolayers with Strong Anisotropy in Lattice Thermal Conductivity

Recently, a class of thermoelectric (TE) materials with a FeOCl-type layered structure have been reported, such as GaOI, InOI, and TaCX (X = Cl, Br, I). These reports simulated further research on screening excellent TE monolayers with FeOCl-type structures. In this work, we carry out a comprehensive study on exploring the thermoelectric properties of Al2X2Se2 (X = Cl, Br, I) monolayers. The heat transport properties of the three monolayers are calculated and compared. Specifically, the thermal conductivities of 7.94 (16.98) W m–1 K–1 and 5.01 (16.15) W m–1 K–1 for the Al2Cl2Se2 and Al2Br2Se2 monolayers in the x direction (y direction) are obtained, respectively, which are more than that (0.29 (0.70) W m–1 K–1) for the Al2I2Se2 monolayer. The significant difference in the lattice thermal conductivity κl of Al2X2Se2 monolayers is attributed to their different phonon anharmonicity. It is also noted that strong anisotropy exists in the κl between the x and y directions. Additionally, the electronic transport parameters (the Seebeck coefficient and electrical conductivity) are figured out. Thus, the optimal ZT values of Al2X2Se2 monolayers from 200 to 700 K are obtained. It is found that the highest ZT value of the Al2I2Se2 monolayer (3.24) in the x direction is almost two times as higher as that (1.78) in the y direction at 700 K. The maximum ZT values of the Al2Cl2Se2 (0.59) and Al2Br2Se2 (0.95) monolayers in the y direction are higher than those of the Al2Cl2Se2 (0.34) and Al2Br2Se2 (0.77) monolayers in the x direction at 700 K, respectively, which are smaller than that of the Al2I2Se2 monolayer along with whether the x-axis (3.37) or the y-axis (1.75). This work reveals that the three monolayers exhibit anisotropies in the κl and ZT. The Al2I2Se2 monolayer has better TE performance than the Al2Cl2Se2 and Al2Br2Se2 monolayers.