Engineering Electronic Structure and Lattice Dynamics to Achieve Enhanced Thermoelectric Performance of Mn–Sb Co-Doped GeTe

GeTe, as a p-type semiconductor, has been intensively studied in recent years as a promising lead-free mid-temperature-range thermoelectric (TE) material. Herein, we report an improved energy conversion efficiency (η) using a two-step TE properties optimization in Mn–Sb co-doped GeTe by engineering electronic structure and lattice dynamics. Mn–Sb co-doping enhances the TE properties of GeTe, as evidenced from both experiments and first-principles-based theoretical calculations. The density functional theory (DFT) calculations indicate that Mn–Sb co-doping improves the band convergence and optimizes the Fermi level position. This in turn helps in enhancing the Seebeck coefficient (α). As a result of the optimized Seebeck coefficient and electrical conductivity (σ), an enhanced power factor (α²σ) is obtained for the Mn–Sb co-doped system. Moreover, a significant reduction in the phonon (lattice) thermal conductivity (κ_ph ∼ 0.753 W/mK) at 748 K is observed for Ge_0.87Mn_0.05Sb_0.08Te, attributed to the point-defect scattering and reduced phonon group velocity. The synergistic improvement in α and reduction in κ_ph result in a maximum figure-of-merit (zT) of 1.67 at 773 K, with an average zT (zT_av) of ∼ 0.9 for Ge_0.87Mn_0.05Sb_0.08Te over a temperature range of 300–773 K, leading to an η of ∼12.7%.