Anionic regulation and valence band convergence boosting the thermoelectric performance of Se-alloyed GeSb2Te4 single crystal

GeSb2Te4, a layer-structured pseudo-binary chalcogenide, is a promising thermoelectric material with low lattice thermal conductivity ensured by the van de Waals gap. However, its excessive hole concentration leads to low Seebeck coefficient and limited thermoelectric performance. Herein, GeSb2(Te1-xSex)4 (x = 0, 0.05, 0.07, 0.1, and 0.15) single crystals were grown via Bridgman method for thermoelectric performance investigation, with single crystals to utilize the anisotropic low lattice thermal conductivity, and Se-alloying as the strategy to optimize the thermoelectric performance. It was found that Se alloying increases the Seebeck coefficients, deriving from the reduced carrier concentrations due to the enlarged formation energy of intrinsic GeSb(A1), GeSb(A2), and VGe(A2) defects, as well as the simultaneous enhancement of density-of-states effective mass from a facilitated valence band convergence. Besides, Se-alloying also contributes to the reduction of the lattice thermal conductivity. With the forementioned benefits, a high figure of merit of 1.02 is obtained at 723 K in the out-of-plane direction of the crystal sample GeSb2(Te0.9Se0.1)4, and a 74% improvement on average zT is gained. The study of anionic regulation and valence band convergence in GeSb2Te4-based single crystal provides an effective pathway for performance optimization in related layer-structured thermoelectric materials.