Synergistic Band and Defect Engineering Realize High‐Efficiency CuAgSe‐Alloyed Lead‐Free GeTe Thermoelectrics

Abstract Lead‐free GeTe is a promising thermoelectric material; however, its performance is hindered by intrinsically high carrier concentrations arising from Ge vacancies and relatively high lattice thermal conductivity. Here, a synergistic alloying strategy combining band engineering and defect modulation is proposed to simultaneously optimize electronic and phonon transport in CuAgSe‐alloyed GeTe. The delocalized Cu ions substantially enhance carrier mobility, while CuAgSe alloying induces band flattening and convergence that increase the effective mass and synergistically boost electrical transport. Furthermore, the introduction of hierarchical structural defects—including point defects, planar vacancies, and Cu‐rich nanoprecipitates—intensifies phonon scattering over multiple length scales, leading to a strong suppression of lattice thermal conductivity. Consequently, a peak zT of 2.02 at 603 K and an average zT of 1.22 (303–803 K) are achieved. A seven‐pair device exhibits a conversion efficiency of 6.02% at a temperature difference of 382 K. This work demonstrates an effective co‐optimization pathway toward high‐performance, lead‐free GeTe‐based thermoelectrics.