Impact of Coinage Metal Insertion on the Thermoelectric Properties of GeTe Solid-State Solutions

Abstract

A comprehensive study on the thermoelectric effect of Ag substitution in GeTe solid solutions, a congenital base for highly efficient TAGS-m [(GeTe)m(AgSbTe2)100–m] thermoelectric materials, was performed. First-principles calculations were carried out to probe the changes arising from doping on the electronic band structure of GeTe, which exhibits a rhombohedral (r) structure at temperatures lower than 700 K. Aliovalent Ag substitution in GeTe increases the hole concentration and decreases the thermoelectric figure of merit (zT) due to the reduction of the Seebeck coefficient, which is ascribed mainly to the lowering of the Fermi level together with the loss of band degeneracy. The band structure and effective mass calculations of these doped materials also point to a soaring contribution from several hole pockets in the valence band. First-principles calculations carried out with two other group 11 transition metals (Cu, Au) reveal that silver substitution has the lowest impact on the thermopower of r-GeTe. A mean zT of ∼0.85 at 773 K is achieved for Ge1–xAgxTe (x ≤ 0.04) solid solutions. The study highlights the limits in doping just a coinage metal to GeTe and recapitulates the need for pair substitution to enhance the thermoelectric properties of GeTe-based solid-state solutions.