Cation Modulation in AgSbTe₂ Realizes Carrier Optimization, Defect Engineering, and a 7% Single-Leg Thermoelectric...

AgSbTe₂ plays a pivotal role in mid-temperature thermoelectric generators (TEGs). Leveraging the seminal advances in cation manipulation within AgSbTe₂, this study demonstrates an enhanced TE power factor (PF = S²σ) of 1.5 mWm^-1 K^-2 and a peak zT of 1.5 at 583 K in an off-stoichiometric Ag_1.04Sb_0.96Te₂ crystal. The introduction of Ge in place of Ag leads to an increased n_H as evidenced by the detection of trace Ge⁴⁺ through XPS analysis. Further chemical state analysis reveals the simultaneous presence of Ag⁺, Sb³⁺, and Ge⁴⁺, elucidating the effect of cation modulations. TEM characterizations validate the presence of superlattice structure, and the linear defects discerned within the AgSbTe₂ matrix. Consequently, the lattice thermal conductivity κ_L is substantially reduced in the Ag_1.02Ge_0.02Sb_0.96Te₂ 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_L, facilitated by cation modulation. Additionally, a single-leg TE device incorporating Ag_1.02Ge_0.02Sb_0.96Te₂ 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.