Novel Janus 2H-Tl₂SSe Monolayer with Ultralow Lattice Thermal Conductivity and Outstanding Thermoelectric Performance

Understanding the lattice dynamics from the perspective of chemical bonds and phonon transport is essential for finding and designing high-efficiency thermoelectric (TE) materials and achieving applications. In this work, we have constructed a novel two-dimensional (2D) Janus 2H-Tl2SSe from a 2D 2H-TlS monolayer for the first time. The intrinsic low lattice thermal conductivity of the Janus 2H-Tl2SSe monolayer is attributed to the coexistence of weak chemical bonding and strong phonon anharmonicity, combining first-principles calculations and Boltzmann transport theory. The crystal orbital Hamilton population (COHP) analysis reveals that the weak Tl–S and Tl–Se chemical bonding, stemming from the filled antibonding orbitals, results in a low average phonon group velocity (vg) and acousto–optic coupling. Through further investigating the scattering rates of various scattering channels, we have demonstrated that acousto–optic coupling plays a crucial role in providing important scattering channels for three-phonon scattering. Furthermore, the Janus structure of the 2H-Tl2SSe monolayer breaks the original symmetry, resulting in additional anharmonicity. The low vg and strong phonon scattering enable the Tl2SSe monolayer to exhibit a low room-temperature lattice thermal conductivity (κL) of 1.88 W mK–1. With the further inclusion of electron–phonon (el–ph) scattering, our research reveals that the Tl2SSe monolayer presents a high ZT of 2.31 at 700 K, attributed to its low κL and excellent electronic transport performance for n-type doping. Our work demonstrates that Janus Tl2SSe monolayer has been identified as a novel thermoelectric material for promising medium-temperature applications.