Stabilized Cubic GeTe With Matched Grain‐Boundary Networks and Band Convergence for High‐Performance Dual‐Mode Thermoelectric Devices

ABSTRACT Lead‐free GeTe‐based thermoelectrics exhibit strong potential; however, device performance is limited by the intrinsic ferroelectric phase transition and structural instability. Here, we propose a cooperative structural‐electronic regulation strategy that stabilizes the cubic phase and simultaneously enhances the thermoelectric and mechanical properties. AgSbTe ₂ alloying completely suppresses the ferroelectric transition over the entire operating temperature range, while complementary I doping produces refined single‐phase domains with well‐matched multi‐degree‐of‐freedom grain boundary networks, together with a precisely tuned band structure. These synergistic effects intensify phonon scattering, optimize carrier transport, and increase the effective mass, thereby elevating the power factor and substantially reducing the lattice thermal conductivity ( κ _L ), thereby significantly improving the weighted mobility ( µ _w )‐to‐ κ _L ratio. The optimized composition achieves a room‐temperature zT of ∼0.7 and a peak zT of ∼2.2 at 773 K, demonstrating superior performance from near‐ambient to intermediate temperatures. Concurrently, grain refinement effectively enhances mechanical integrity, yielding a compressive strength of ∼248 MPa. A 7‐pair power‐generation device delivers ∼11% efficiency at Δ T = 470 K, while a cooling device achieves a maximum Δ T _max of ∼65.6 K at 350 K. This study establishes a generalizable framework for concurrent phase stabilization and performance optimization, enabling scalable, high‐reliability GeTe‐based dual‐mode thermoelectric devices.