Band Engineering of the Second Phase to Reach High Thermoelectric Performance in Cu₂ Se-Based Composite Material

Hitherto, Cu₂ Se incorporated with a dispersed second phase shows extremely low thermal conductivity and excellent thermoelectric properties. However, the significant mismatch in electronic band structure between the second phases and the matrix often causes a deterioration of carrier mobility. In this work, based on density functional theory (DFT) calculations, the electronic band structure of the second phase is adjusted through doping S and Te. It is found that Cu₂ Se_0.88 S_0.06 Te_0.06 has a highly similar electronic band structure to the Cu₂ Se matrix, which results in high carrier mobility and power factor in Cu₂ Se-based composite materials. Additionally, the dispersed second-phase Cu₂ Se_0.88 S_0.06 Te_0.06 , dislocations, and nanograins are observed in the Cu₂ Se/5 wt% Cu₂ Se_0.88 S_0.06 Te_0.06 product, which leads to a substantial reduction in the thermal conductivity. Finally, high figure of merit (zT) values of 2.04 (by Dulong-Petit heat capacity) and 2.34 (by Differential Scanning Calorimetry (DSC) measured heat capacity) are achieved at 850 K, which are about 65% higher than that of Cu₂ Se in this work and comparable to the recently reported p-type Cu₂ Se with outstanding performance.