Boosting thermoelectric performance of Ga doped GeSb2Te4 single crystals by impurity level and defect engineering

The GeSb₂Te₄ compound, with a layered structure, shows potential for mid-temperature thermoelectric applications. In this study, we successfully synthesized Ga-doped GeSb₂Te₄ single crystals using the Bridgman method, resulting in a significant enhancement in thermoelectric performance. Ga doping at Ge sites optimized carrier concentration and introduced impurity levels in the bandgap, as confirmed by first-principles calculations, significantly enhancing the Seebeck coefficient of GeSb₂Te₄. Additionally, the introduction of the Ga element brought about point defects in the lattice, which strengthen phonon scattering. Due to the reduced thermal conductivity and increased power factor, the Ge_0.93Ga_0.07Sb₂Te₄ single-crystal sample achieved a peak zT of ∼0.87 at 773 K and an average zT of ∼0.6 over 323‒773 K, representing improvements of ∼34 % and ∼58 %, respectively, compared to the pristine GeSb₂Te₄ crystal. This research provides a new strategy for optimizing the thermoelectric performance of other layered compounds.