Enhanced Thermoelectric Performance of Bi–Se Co-Doped Cu_1.8S via Carrier Concentration Regulation and Multiscale Phonon Scattering

Copper sulfides (Cu2–xS) have become potential thermoelectric (TE) materials because of their high element abundance, low toxicity, and high performance. A series of Cu1.8–2xBi2xS1–3xSe3x (0 ≤ x ≤ 0.03) bulks were fabricated using mechanical alloying and spark plasma sintering. The main Cu1.8S phase was obtained in all compositions of 0 ≤ x ≤ 0.03, and marginal Cu1.96S and Cu2S phases were detected at 0.02 ≤ x ≤ 0.03, which is attributed to the volatilization of sulfur and selenium. Benefiting from the introduced extra electronics by Bi3+ doping, the carrier concentration was optimized in 2.31×1021 cm–3. Multiscale defects introduced by Bi–Se co-doping, including secondary phases, micropores, and point defects (BiCu••, SeS×, and VS••), strongly scattered the phonons, leading to a drastically decreased thermal conductivity from 2.71 W m–1 K–1 for Cu1.8S at 773 K to 0.80 Wm–1 K–1 for Cu1.74Bi0.06S0.91Se0.09. A maximum ZT of 0.78 was achieved for Cu1.74Bi0.06S0.91Se0.09 at 773 K, which is 144% higher than that of Cu1.8S (0.32). The current stress test confirms that Bi doping could improve the stability of Cu1.8S by suppressing the Cu ion migration. Our work demonstrates that Bi–Se co-doping is an effective way to enhance the TE properties for Cu1.8S.