Enhanced Thermoelectric Performance of Cu2−xRExSe (x = 0–0.2) by Doping with Rare Earth Elements Tm, Er, and Lu

Copper selenide (Cu₂Se) has attracted extensive research interest in the field of thermoelectricity due to its large abundance of constituent elements, environmental friendliness, and excellent thermoelectric properties. In this study, a facile and low-cost hydrothermal synthesis method combined with vacuum sintering was used for Cu₂Se-based sample preparation, and the possibility of improving the thermoelectric properties of Cu₂Se by doping with rare earth elements (Tm, Er, Lu) was explored using experimental tests and density functional theory calculations. The electronic structure calculations show that the electrical conductivity is increased due to the increased electron density after rare earth element doping. At the same time, the sharp increase in the density of states at the Fermi level after rare earth element doping enhances the Seebeck coefficient of the Cu₂Se matrix. The experimental results show that the thermoelectric properties of all the samples doped with rare earth elements (Tm, Er, Lu) were improved compared to the Cu₂Se matrix. The doping of rare earth elements led to an increase in point defects and the formation of pores in the samples, which play an important role in reducing the thermal conductivity. Among them, the Tm-doped Cu_1.995Tm_0.005Se sample reached a maximum ZT value of 0.80 at 773 K, which is a nearly 43% increase compared to the Cu₂Se matrix (ZT_max = 0.56).