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Copper sulfides (Cu2–x S) have become potential thermoelectric (TE) materials because of their high element abundance, low toxicity, and high performance. A series of Cu1.8–2x Bi2x S1–3x Se3x (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.

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Cu_1.8S-based materials have become potential thermoelectric materials due to their rich raw material reserves, low toxicity, and excellent electrical and thermal properties. In this study, a series of Cu_{1.8–<i>x</i>} Sb<i>_x</i> S (<i>x</i> = 0, 0.005, 0.02, 0.03, 0.04) bulk materials is synthesized by using mechanical alloying combined with spark plasma sintering process. This preparation method can shorten the preparation cycle of materials, and effectively improve the research and development efficiency of thermoelectric (TE) materials due to its simple process. The effects of different Sb doping amounts on the structure, micromorphology, and thermoelectric transport properties of Cu_{1.8–<i>x</i>} Sb<i>_x</i> S phase are investigated. The results show that when 0 ≤ <i>x</i> < 0.02, the bulk samples are single-phase Cu_1.8S. With the further increase of Sb doping to 0.02 ≤ <i>x</i> ≤ 0.04, the second phase CuSbS₂ is formed when Sb content exceeds the solid solubility limit of <i>x</i> = 0.02 in Cu_1.8S, all Cu_{1.8–<i>x</i>} Sb<i>_x</i> S bulk samples exhibit p-type conductivity characteristics. Benefitting from the synergistic phonon scattering effect by multiscale defects, such as point defects (<inline-formula><tex-math id="M1">\begin{document}${\rm{Sb}}_{{\rm{Cu}}}^{ \bullet\bullet }$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20201852_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20201852_M1.png"/></alternatives></inline-formula>, <inline-formula><tex-math id="M2">\begin{document}$ {\rm{V}}_{\rm{S}}^{ \bullet \bullet } $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20201852_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20201852_M2.png"/></alternatives></inline-formula>), nanopores, secondary phases (CuSbS₂), and dislocations, the thermal conductivity <i>κ</i> declines significantly from 1.76 W·m^–1·K^–1 (<i>x</i> = 0) to 0.99 W·m^–1·K^–1 at 723 K for the Cu_1.76Sb_0.04S sample. Finally, the peak dimensionless TE figure of merit (<i>ZT</i> ) value of 0.37 is achieved at 723 K for Cu_1.77Sb_0.03S resulting from a low thermal conductivity of 1.11 W·m^–1·K^–1 combining an appropriate power factor of 563 μW·m^–1·K^–2, which is 12% higher than that (0.33) of pristine Cu_1.8S. Although the Sb doped Cu_1.8S-based samples have lower thermal conductivity <i>κ</i>, the reduced power factor cannot be offset by reducing the thermal conductivity <i>κ</i>, so the TE figure of merit (<i>ZT</i> ) value is not significantly improved. Therefore, there is still much room for improving the performance of Sb doped Cu_1.8S-based thermoelectric material, and its thermoelectric performance can be further optimized through nano-second phase recombination, energy band engineering, and introducing multi-scale defects, etc. Our results suggest that the introduction of Sb into thermoelectric materials is an effective and convenient strategy to improve <i>ZT</i> value by reducing thermal conductivity <i>κ</i>.

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