Stabilized Cubic GeTe With Matched Grain-Boundary Networks and Band Convergence for High-Performance Dual-Mode...

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. AgSbTe₂ 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 ΔT = 470 K, while a cooling device achieves a maximum ΔT_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.