Band Engineering of the Second Phase to Reach High Thermoelectric Performance in Cu 2 Se-Based Composite Material.

Hitherto, Cu 2 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 Cu 2 Se 0.88 S 0.06 Te 0.06 has a highly similar electronic band structure to the Cu 2 Se matrix, which results in high carrier mobility and power factor in Cu 2 Se-based composite materials. Additionally, the dispersed second-phase Cu 2 Se 0.88 S 0.06 Te 0.06 , dislocations, and nanograins are observed in the Cu 2 Se/5 wt% Cu 2 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 Cu 2 Se in this work and comparable to the recently reported p-type Cu 2 Se with outstanding performance.