Simultaneously enhancing the power factor and suppressing lattice thermal conductivity is the primary challenge in designing high-performance thermoelectric materials. In tetradymite topological insulators ${\mathrm{Bi}}_2{X}_{2}Y$, engineering Janus configurations offers a route to tune both electronic and phonon transport. First-principles calculations reveal that in Janus $\beta \text{−}{\mathrm{Bi}}_2{\mathrm{Se}}_2\mathrm{Te}$, Rashba splitting induced by broken inversion symmetry acts in concert with topological band inversion to enhance the effective band edge density of states, resulting in an improved power factor relative to its centrosymmetric $\alpha$-phase counterpart. Concurrently, the structural asymmetry of the Janus configuration enhances anharmonicity and expands the phonon scattering phase space, reducing the lattice thermal conductivity below that of the corresponding $\alpha$ phase across the full temperature range. This synergistic effect of both electronic and thermal transport enables Janus ${\mathrm{Bi}}_2{X}_{2}Y$ compounds to achieve theoretical figure of merit ($zT$) values exceeding 1 over broad carrier-concentration ranges at medium and high temperatures, with $\beta \text{−}{\mathrm{Bi}}_2{\mathrm{Se}}_2\mathrm{Te}$ reaching a peak $zT$ of 2.75. Our work demonstrates an effective optimization strategy within Janus tetradymite topological insulators and identifies Janus ${\mathrm{Bi}}_2{X}_{2}Y$ as promising thermoelectric candidates.

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• Rashba coupling
• Thermoelectrics
• Janus materials
• Topological materials
• Density functional calculations