Compositing Effect Leads to Extraordinary Performance in GeSe‐Based Thermoelectrics

Abstract Rhombohedral GeSe has attracted extensive attention due to its facile fabrication, low toxicity, and greater affordability compared with popular GeTe‐based thermoelectrics. However, its thermoelectric properties require further optimization for practical applications. Here, a peak figure‐of‐merit of 1.31 at 623 K is reported for p‐type polycrystalline (GeSe) 0.9 (AgBiTe 2 ) 0.1 ‐1.5 mol.% SnSe, ranking among the highest reported values. AgBiTe 2 alloying induces a phase transition in GeSe from orthorhombic to rhombohedral while compositing with SnSe, which is known for its low thermal conductivity, establishes interfaces with strong phonon scattering and weak electron scattering. This strategy effectively suppresses thermal transport properties while maintaining exceptional electrical transport properties. Structural analyses reveal that multiscale defects, including intensive point defects (Ag Ge , Bi Ge , and Te Se ), linear defects (dislocation arrays), planar defects (grain boundaries and phase boundaries), and volume defects (SnSe and Ag 2 Te phases), result in an ultra‐low lattice thermal conductivity of 0.26 W m −1 K −1 at 623 K, approaching the amorphous limit. Density functional theory calculations and nanostructure characterizations indicate that the strong bonding between the SnSe phase and the matrix, coupled with the minimal electronegativity difference between Sn and Ge, minimizes carrier scattering and sustains high electrical performance.