Enhanced thermoelectric properties of Cu-Se system via bond-structure adjustment by Ag-doping

Thermoelectric devices have shown attractive application prospects with the emergence of high performance thermoelectric materials in recent years. In the Cu-Se system, bond-structure adjustment was designed to improve thermoelectric properties through simple silver doping, which induces change of phase structure, energy band structure, and thermoelectric transport nature. The polycrystalline Cu _2−xAg _x Se bulk samples were prepared by the spark plasma sintering (SPS) following vacuum melting and grinding. Appropriate addition of silver makes α-Cu ₂Se lattice expansion, and β-Cu ₂ Se becomes the room temperature stable phase with further increase of the silver content (>=3%). Ag doping results in significant changes in valence band spectra, which improves thermoelectric transport behave in higher operation temperature. Due to the synergetic effect from electrical conductivity and Seebeck coefficient, the maximum power factor reaches 1456 μWm ⁻¹K ⁻² at 773 K, which increases by 10% compared with the undoped sample. It is notable that the Cu _1.99Ag _0.01 Se bulk sample achieve an low thermal conductivity of 1.1 W m ⁻¹K ⁻¹ at 773 K, which can be attributed to enhancement of phonon scattering at different frequencies due to the diversified bond lengths and bond energy induced by Ag-doping. Benefiting from reduction of lattice thermal conductivity and enhancement of powder factor, the Cu _1.99Ag _0.01Se bulk sample shows the better TE transport properties and exhibits the highest ZT value ∼1.12 at 773 K.