The challenge of achieving high thermoelectric (TE) performance is mainly from the entanglement among Seebeck coefficient ($S$), electrical conductivity ($\sigma$), and lattice thermal conductivity (${\kappa }_{\mathrm{L}}$). In this work, we propose a synergetic strategy of enhancing power factor (PF, $S^2\sigma$) and suppressing ${\kappa }_{\mathrm{L}}$ by applying a biaxial tensile strain in two silver chalcogenides ${\mathrm{Sc}\mathrm{Ag}\mathrm{Se}}_2$ and ${\mathrm{Tm}\mathrm{Ag}\mathrm{Te}}_2$ with ${\mathrm{Tl}\mathrm{Cd}\mathrm{S}}_2$-type structure. The forbidden $p$-$d$ orbital coupling at the $\mathrm{\Gamma }$ point and allowed $p$-$d$ orbital coupling at the $A$ point and the middle of $\mathrm{\Lambda }$ line leads to high electronic band dispersion along the $\mathrm{\Gamma }$-$A$ direction and a high-degeneracy valence-band valley (${\mathrm{\Lambda }}_2$). The elongation of the $\mathrm{Ag}$-$\mathrm{Se}$ bond under tensile strain weakens the orbital coupling between $\mathrm{Ag}$-$d$ and $\mathrm{Se}$/$\mathrm{Te}$-$p$ orbitals and reduces the band energy at the $A$ point, which aligns the valence band and achieving a high band degeneracy. Concurrently, the weaker $\mathrm{Ag}$-$\mathrm{Se}$/$\mathrm{Ag}$-$\mathrm{Te}$ bond under a small tensile strain leads to lower phonon group velocity and strong three- and four-phonon scatterings, leading to lower ${\kappa }_{\mathrm{L}}$. Our first-principles calculations combined with electron-phonon coupling analysis as well as phonon and electron Boltzmann transport equations show that applying a 3% (2%) tensile strain can enhance the PF along the $c$ axis of ${\mathrm{Sc}\mathrm{Ag}\mathrm{Se}}_2$ (${\mathrm{Tm}\mathrm{Ag}\mathrm{Te}}_2$) by 243% (246%) at a carrier concentration of $3\times {10}^{20}{\mathrm{cm}}^{-3}$ and reduce the ${\kappa }_{\mathrm{L}}$ by 37% (26%) at 300 K. Consequently, 2–4 times of $ZT$ enhancement is obtained by 3% (2%) tensile strain in ${\mathrm{Sc}\mathrm{Ag}\mathrm{Se}}_2$ (${\mathrm{Tm}\mathrm{Ag}\mathrm{Te}}_2$) at 300 K, achieving a maximum $ZT$ of 3.10 (3.40) at 800 K. Our material design strategy based on molecular orbital analysis reveals an effective route to boosting TE performance, and can be extended to other systems as well.

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• Lattice thermal conductivity
• Straintronics
• Thermal conductivity
• Thermoelectric effects
• Valleytronics
• Semiconductor compounds
• Thermoelectrics
• First-principles calculations