Enhanced Anisotropic Thermoelectric Properties in GeBi₂Se₄-Based Single Crystals

Layered GeBi₂Se₄ exhibits pronounced anisotropic thermoelectric properties, yet its Seebeck coefficient is constrained by a low density of states effective mass. In this work, by capitalizing on its structural anisotropy through the growth of high-quality single crystals, we demonstrate superior thermoelectric performance along the c-axis compared to the ab-plane. Subsequent Te alloying at the Se site induces band convergence, increasing the density of states effective mass near the Fermi level, while simultaneously enhancing phonon scattering through mass and strain fluctuations. Coupled with optimized carrier concentration via Bi doping, these synergistic effects yield a peak zT of ∼0.9 at 675 K along the c-axis in n-type Ge_0.92Bi_2.08Se_2.4Te_1.6 single crystals, representing enhancements of ∼280% and ∼160% over pristine polycrystalline and single-crystalline GeBi₂Se₄, respectively. Our work underscores the potential of engineering anisotropy in single-crystal IV-V-VI chalcogenides for high-performance thermoelectrics.