Electronic and thermoelectric properties of semiconducting Bi₂SSe₂ and Bi₂S₂Se monolayers with high optical...

Bismuth telluride (Bi₂Te₃) and its derivatives are often focused on as thermoelectric materials around room temperature. In this work, we theoretically predicted two new types of Bi₂Te₃-based two-dimensional materials Bi₂SSe₂ and Bi₂S₂Se using density functional theory (DFT) combined with Boltzmann transport theory. The thermal, dynamic, and mechanical stabilities of Bi₂SSe₂ and Bi₂S₂Se monolayers are confirmed using ab initio molecular dynamics (AIMD) simulations, phonon dispersion, and elastic constant calculations. The phonon transport properties, including lattice thermal conductivity, group velocity, Grüneisen parameter, converged scattering rate, and phonon lifetimes contributed by different branches, are systematically investigated. The electronic and thermoelectric properties, including carrier mobility (μ), Seebeck coefficient (S), electrical conductivity (σ), power factors, and figure of merit (zT) along the zigzag and armchair directions as a function of carrier concentration at different temperatures, are also investigated. It is found that the Bi₂SSe₂ and Bi₂S₂Se monolayers have moderate indirect band gaps (0.92 eV and 1.08 eV at the PBE level, respectively) and low lattice thermal conductivities (4.35 and 5.37 W m^-1 K^-1 at 300 K, respectively). The largest zT values of Bi₂SSe₂ and Bi₂S₂Se monolayers are 0.50 and 0.28 at 300 K and 1.39 and 0.93 at 700 K for p-doping types, respectively. The Bi₂SSe₂ and Bi₂S₂Se monolayers are predicted to show high optical absorption peaks at 8 × 10⁵ cm^-1 in the visible and near-UV light region, respectively. Our results indicate that both Bi₂SSe₂ and Bi₂S₂Se could be promising candidates in energy conversion, solar cells, and optoelectronic devices.