Defect Chemistry Engineering Promotes the Dopability of Cu for an Extraordinary Thermoelectric Performance of Hole-Doped PbSe with Resonant States

Cu substitution for Pb (CuPb) has been theoretically predicted to introduce resonant states near the valence band edge of PbSe. However, experimentally, it has not been verified yet due to the extremely low solubility (∼0.3 atom %) of CuPb. In this study, we demonstrate that the solubility limit of CuPb is extended to at least 1.5 atom % in PbSe by pairing with an identical amount of Cu interstitials (Cui), while excess Cu precipitates out as a Cu2Se phase at grain boundaries. As a result of the increased dopability of CuPb, a significant enhancement of the Seebeck coefficient is realized in hole-doped PbSe because of CuPb-induced resonant states. This is also testified by first-principles calculations. Moreover, the rich point defects (CuPb, Cui), line defects (dislocations), and Cu2Se precipitates remarkably frustrate the phonon propagation of PbSe, leading to ∼45% reduction of lattice thermal conductivity (κlat) at room temperature. At elevated temperatures (>623 K), there is a dynamic migration of Cu atoms from Cu2Se precipitates to tetrahedral interstices of the PbSe matrix as evidenced by the temperature-variant Hall study and thermal expansion coefficient measurement. This diffusion-like process of Cu further drops the κlat to ∼0.27 W m–1 K–1 at 823 K. Consequently, a peak ZT of ∼1.8 at 873 K is achieved in the (Cui)0.01Pb0.97(CuPb)0.01Na0.02Se sample. This work highlights the potential of the defect structure design in innovating new functional materials, particularly high-performance thermoelectrics.