Excellent thermoelectric properties of the Tl₂S₃ monolayer for medium-temperature applications

Exploring materials with high thermoelectric (TE) performance can alleviate energy pressure and protect the environment, and thus, TE materials have attracted extensive attention in the new energy field. In this paper, we systematically study the TE properties of Tl₂S₃ using first-principles combined with Boltzmann transport theory (BTE). The calculation results show an excellent power factor (1.12 × 10^-2 W m^-1 K^-2) and ultra-low lattice thermal conductivity (k_l = 0.88 W m^-1 K^-1) at room temperature. Through analysis, we attribute the ultra-low k_l of Tl₂S₃ to the lower phonon group velocity (v_g) and larger phonon anharmonicity. Meanwhile, discussion of chemical bonding showed that the filling of the anti-bonding state leads to the weakening of the Tl-S chemical bond, resulting in low v_g. Furthermore, this research also investigates the scattering processes (the out-of-plane acoustic mode (ZA) + optical mode (O) → O (ZA + O → O), the in-plane transverse acoustic mode (TA) + O → O (TA + O → O), and the in-plane longitudinal acoustic mode (LA) + O → O (LA + O → O)), from which we find that 2D Tl₂S₃ possesses strong acoustic-optical scattering. Based on the analysis of electron transport properties under electron-phonon coupling, 2D Tl₂S₃, as a novel TE material, exhibits a ZT value as high as 2.8 at 400 K. Our calculations suggest that Tl₂S₃ is a potential TE material at medium temperature.