DFT Calculations and Experimental Studies of the Cu Doping Mechanism in Hot-Extruded Bi–Te–Se Bulk Thermoelectric Materials

Cu doping is an effective way to enhance the thermoelectric properties of n-type Bi₂Te₃-based materials, but there are conflicting views on the mechanism of Cu doping. In this work, a remarkable ZT value of 0.92 (T = 300 K) is achieved in a Cu-doped Bi₂Te_2.85Se_0.15 hot-extruded material. By combining DFT calculations with experimental characterization, the Cu doping behavior involving Cu atomic lattice occupation, the effect of Cu on the thermoelectric properties, and the carrier donor–acceptor mechanism of Cu are investigated. Cu atoms are mainly intercalated in the tetrahedral sites between Te(1)–Te(1) layers, causing lattice expansion along the c-axis. The interstitial Cu atoms form covalent bonds with Te atoms and inhibit the escape of the Te atoms. These interstitial Cu atoms act as carrier donors, providing free electrons, and also act as carrier acceptors, reducing free electrons through trapping Te atoms. The remarkable ZT value currently achieved is primarily attributed to the predominant carrier–acceptor mechanism of Cu. Furthermore, based on this carrier donor–acceptor mechanism, a valley response of carrier concentration with increasing Cu content is further proposed.