Band Engineering of the Second Phase to Reach High Thermoelectric Performance in Cu₂Se‐Based Composite Material

Abstract Hitherto, Cu₂Se 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 Cu₂Se_0.88S_0.06Te_0.06 has a highly similar electronic band structure to the Cu₂Se matrix, which results in high carrier mobility and power factor in Cu₂Se‐based composite materials. Additionally, the dispersed second‐phase Cu₂Se_0.88S_0.06Te_0.06, dislocations, and nanograins are observed in the Cu₂Se/5 wt% Cu₂Se_0.88S_0.06Te_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 Cu₂Se in this work and comparable to the recently reported p‐type Cu₂Se with outstanding performance.