The rational design of two-dimensional thermoelectric materials demands the simultaneous optimization of electronic and phonon transport, a challenge that Janus monolayers address by coupling structural asymmetry with tunable chalcogen chemistry. Here, a comprehensive first-principles study of the thermoelectric performance of Janus γ-Sn2XY (X = S, Se; Y = Se, Te; X ≠ Y) monolayers is presented, combining HSE06+SOC electronic structure calculations, iterative phonon Boltzmann transport theory, and carrier scattering analysis. All Janus monolayers are predicted to be dynamically, thermally, and mechanically stable with an indirect band-gap energy of 1.01, 1.21, and 1.12 eV for γ-Sn2SSe, γ-Sn2STe, and γ-Sn2SeTe, respectively. A camel-back valence band topology generates nearly degenerate multi-valley states that enhance the Seebeck coefficient while maintaining favorable hole transport. Simultaneously, progressive Te substitution drives ultralow lattice thermal conductivities of 0.84, 0.42, and 0.41 Wm−1K−1 at 300 K through bond softening, reduced phonon group velocities, and enhanced anharmonicity. These cooperative effects yield peak p-type figures of merit (ZT) of 0.73, 2.13, and 2.62 at 900 K. Conversion efficiency analysis based on the temperature-averaged figure of merit further predicts that p-type γ-Sn2SeTe and γ-Sn2STe achieve maximum efficiencies of 21.99% and 18.72%, respectively, over a 300–900 K operating range, highlighting the strong potential of the γ-Sn2XY family for high-performance two-dimensional thermoelectric applications.
