Cation Modulation in AgSbTe Realizes Carrier Optimization, Defect Engineering, and a 7% Single-Leg Thermoelectric Efficiency.

AgSbTe plays a pivotal role in mid-temperature thermoelectric generators (TEGs). Leveraging the seminal advances in cation manipulation within AgSbTe, this study demonstrates an enhanced TE power factor (PF = Sσ) of 1.5 mWm K and a peak zT of 1.5 at 583 K in an off-stoichiometric AgSbTe crystal. The introduction of Ge in place of Ag leads to an increased n as evidenced by the detection of trace Ge through XPS analysis. Further chemical state analysis reveals the simultaneous presence of Ag, Sb, and Ge, elucidating the effect of cation modulations. TEM characterizations validate the presence of superlattice structure, and the linear defects discerned within the AgSbTe matrix. Consequently, the lattice thermal conductivity κ is substantially reduced in the AgGeSbTe crystal, yielding a peak zT of 1.77 at 623 K. This notable advancement is attributed to the counterbalance achieved between the enhanced PF and the reduced κ, facilitated by cation modulation. Additionally, a single-leg TE device incorporating AgGeSbTe demonstrates a conversion efficiency of 7% across a temperature gradient (ΔT) of 350 K. This study corroborates the efficacy of cation modulation through thermodynamic approaches and establishes a relationship between transport properties and the presence of defects.