Enhanced Thermoelectric Performance in Ge_{0.955- x} Sb_x Te/FeGe₂ Composites Enabled by Hierarchical Defects

Manipulations of carrier and phonon scatterings through hierarchical structures have been proved to be effective in improving thermoelectric performance. Previous efforts in GeTe-based materials mainly focus on simultaneously optimizing the carrier concentration and band structure. In this work, a synergistic strategy to tailor thermal and electrical transport properties of GeTe by combination with the scattering effects from both Ge vacancies and other defects is reported. The addition of Fe in GeTe-based compounds introduces the secondary phase of FeGe₂ , synchronously increasing the concentration of Ge vacancies and arousing more Ge planar defects. These hierarchical defects contribute to a large scattering factor, leading to a significant enhancement of Seebeck coefficient and further a splendid power factor. Meanwhile, benefiting from the reinforced phonon scatterings by multiscale hierarchical structures, an extremely low lattice thermal conductivity is successfully achieved. With simultaneously optimized electrical and thermal transport properties, a maximum figure of merit, zT, value of 2.1 at 750 K and an average zT value of 1.5 in 400-800 K are realized in Ge_0.875 Sb_0.08 Te/1.5%FeGe₂ . This work demonstrates that manipulation of hierarchical defects is an effective strategy to optimize the thermoelectric properties.