Ultralow thermal conductivity in n-type Ge-doped AgBiSe₂ thermoelectric materials

The n-type I-V-VI₂ AgBiSe₂ features intrinsically low κ due to the anharmonicity of chemical bonds. Experimentally-determined isothermal section guides the starting compositions for the following AgBiSe₂-based alloys. Among the undoped alloys, the Ag₂₅Bi₂₅Se₅₀ exhibits a highest peak of zT∼0.75, and yet the neighboring Ag₂₀Bi_27.5Se_52.5, which involves a Se-rich liquid phase, has a much lower zT∼0.3 at 748 K, respectively. With the incorporation of Ge, the (GeSe)_0.03(AgBiSe₂)_0.97 exhibits an ultralow κ∼0.3 (W/mK), owing to the formation of Bi₂Se₃ nano-precipitate in the size of 20–40 nm. Additionally, the moiré fringes with a periodicity of 0.25 nm are observed in the Bi₂Se₃ nano-precipitate, implying the presence of local mass fluctuation and superlattice, which could further lead to enhancing phonon scattering and reduced κ. As a result, the ultra-low κ∼0.3 (W/mK) boosts the peak of zT up to zT∼1.05 in n-type (GeSe)_0.03(AgBiSe₂)_0.97, which shows a 140% enhancement compared with that of the undoped AgBiSe₂.