Complex lattice occupation of copper leading to enhanced thermoelectric performance in n-type PbSe

Among mid-temperature thermoelectric materials, PbSe is deemed as a natural replacement of PbTe owing to the much larger abundance of selenium than tellurium. Usually, extra Cu can enter the tetrahedral interstices of PbSe as an effective n-type dopant and significantly enhance its thermoelectric properties; however, anomalously low lattice thermal conductivity as reported due to extra Cu doping gave rise to controversies about the significance of the achieved thermoelectric performance. In this work, we introduced another method of Cu incorporation, i.e., Cu₂Se alloying, and systematically discussed its distinction from the method of extra Cu doping. Cu₂Se alloying was found to be able to complicate the lattice occupation of Cu in PbSe by introducing Cu_Pb-Cu_VF pair defects while offering no free electrons. Ultimately, by combining the two methods of Cu incorporation, we achieved a superior peak figure of merit (ZT) value of 1.56 at 773 K and an average ZT value of ∼1.01 at 323-773 K in the composition of PbSe-3% Cu₂Se-0.8% Cu. Notably, the enhancements were achieved without any lattice thermal conductivity anomaly as seen in the cases using extra Cu doping alone.