<p>Significantly enhanced thermoelectric performance is realized in Cu_2–<i>x</i>Ag_<i>x</i>Se via synergetic optimization of electrical and thermal transport properties. Positron annihilation lifetime (PAL) measurements reveal that Ag doping effectively suppresses excessive Cu vacancies, leading to a pronounced reduction in vacancy concentration, as evidenced by the decreased average positron annihilation lifetime. Simultaneously, the intrinsic hole carrier concentration is tuned into an optimal range of 2.4&#xa0;×&#xa0;10²⁰cm⁻³ by Ag incorporation, enabling coordinated improvement of both electrical and thermal transport properties in Cu₂Se. As a consequence, a high power factor (<i>PF</i>) of approximately 1.61 m W m⁻¹K⁻² is obtained, which originates from the optimized carrier concentration together with the enhanced weighted mobility. In addition, the introduction of Ag into the Cu₂Se matrix generates extra scattering centers, intensifying phonon scattering and thereby effectively reducing the thermal conductivity. Consequently, an elevated thermoelectric figure of merit <i>ZT</i> of approximately 1.14 is achieved at 773 K, along with an average <i>ZT</i> (<i>ZT</i>_<i>ave</i>) of 0.66 within the temperature range of 300–773 K. These results demonstrate a synergistic optimization strategy that concurrently enhances the power factor and suppresses thermal conductivity, offering an effective route for improving the thermoelectric performance of Cu₂Se-based materials.</p> Graphical Abstract <p></p>