Compromise and synergy in thermoelectric GeTe–CuSbS2 alloys

Abstract

GeTe-based thermoelectric materials are promising for mid-temperature applications. To achieve a high thermoelectric performance, pristine GeTe is typically alloyed with I–V–VI₂ tellurides or selenides, such as the well-known GeTe–AgSbTe₂ system used in radioisotope thermoelectric generators. In contrast, I–V–VI₂ sulfides remain underexplored, primarily due to concerns regarding their low carrier mobility. Here, we systematically investigate GeTe–CuSbS₂ alloys, elucidating the detrimental impact of sulfur on carrier mobility while exploring potential benefits which include the germanium vacancy suppression, optimized carrier concentration, and hierarchical lattice defects that enhance phonon scattering. Using analytical models and density-functional theory calculations, we predict the thermoelectric enhancement through the synergistic optimization of these effects. To validate our predictions, we synthesize samples with different alloying ratios and characterize them using synchrotron techniques and electron microscopy analyses. Notably, the GeTe–4% CuSbS₂ sample exhibits an ultralow lattice thermal conductivity of 0.30 W m⁻¹ K⁻¹, yielding the power factor of 46.43 μW cm⁻¹ K⁻² at 723 K. This study establishes a reasonable framework for alloying GeTe with I–V–VI₂ sulfides to achieve better thermoelectric performance.