Impurity band engineering and hierarchical defect scattering enable high zT in Co-doped AgSbTe₂

AgSbTe₂ is a promising p-type thermoelectric material for mid-temperature applications. However, its performance is limited by its low electrical conductivity and the presence of an n-type Ag₂Te secondary phase. In this study, a series of AgSb_1−xCo_xTe samples were synthesized by conventional melting followed by spark plasma sintering. Co doping suppresses the formation of the detrimental Ag₂Te phase and introduces an impurity band above the valence band maximum, as revealed by the calculated electronic structures. Moreover, Co incorporation generates multi-scale lattice defects that act as efficient phonon scattering centers, resulting in remarkably low thermal conductivity. Consequently, a peak thermoelectric figure of merit of 1.6 was achieved at 648 K. These results demonstrate a practical doping strategy that preserves favorable charge transport in AgSbTe₂ while strongly reducing its thermal conductivity, offering a clear pathway toward improving mid-temperature thermoelectric modules.