Entropy-engineered Ge-Sb-Se-Te alloys: Phase stability, thermoelectric properties and conversion efficiency

Germanium telluride (GeTe) is a promising mid-temperature thermoelectric material, but its rhombohedral-to-cubic phase transition near 700 K causes lattice strain and volumetric mismatch at module junctions, particularly on the hot side under large temperature gradients. This structural instability limits device reliability. To address this, we stabilize the cubic phase through entropy engineering by incorporating substantial amounts of Sb and Se into the GeTe lattice. The resulting Ge_0.5-xSb_xSe_0.18Te_0.32 (x = 0.08–0.15) alloys maintain the cubic phase from room temperature upward, eliminating the detrimental phase transition. These alloys exhibit Seebeck coefficients of 200–250 μV K⁻¹ and an ultralow thermal conductivity of 0.68 W m⁻¹ K⁻¹ at 300 K. A single-leg module fabricated from the optimized alloy achieves 4.7 % conversion efficiency under a 350 K temperature gradient, with electrical and thermal contact resistances of approximately 10⁻⁴ Ω cm² and 10⁻⁴ m² K W⁻¹, respectively. This work presents a practical strategy to enhance the phase stability and thermoelectric performance of GeTe-based alloys, while elucidating the impact of contact resistances on module conversion efficiency. These results underscore the promise of entropy-engineered Ge-Sb-Se-Te systems for scalable thermoelectric power generation in waste heat recovery applications.