Enhanced thermoelectric performance of famatinite double-doped with Ge and Se

Famatinite (Cu₃SbS₄) is a promising thermoelectric material with narrow bandgap. However, because its intrinsic carrier (hole) concentration is low, its electrical conductivity is low; thus, doping (partial substitution) is necessary to improve its thermoelectric performance. In this study, Cu₃Sb_1–xGe_xS_4–ySe_y (x = 0.04–0.06 and y = 0.25–0.50) famatinite compounds double-doped with Ge and Se are prepared through solid-state synthesis. Phase analysis and microstructural observations are conducted, and the charge-transport characteristics and thermoelectric properties are examined according to the doping levels. All specimens contain a single phase of famatinite and behave similar to degenerate semiconductors. As the Ge and Se doping amounts increase, the electrical conductivity increases (6.99 × 10³–1.23 × 10⁴ Sm⁻¹ at 323 K) because of the increase in the carrier concentration (1.28 × 10¹⁹ to 1.86 × 10¹⁹ cm⁻³) In contrast, the Seebeck coefficient decreases from 211 to 168 μV K⁻¹ at 323 K, exhibiting p-type conduction characteristics. The power factor is significantly increased by the double doping with Ge and Se, and Cu₃Sb_0.96Ge_0.04S_3.50Se_0.50 achieves a maximum power factor of 0.63 mW m⁻¹ K⁻² at 623 K. The thermal conductivity decreases with increasing temperature but increases with increasing doping amount. As a result, the dimensionless thermoelectric figure of merit (ZT) is improved by double doping with Ge and Se, especially at high temperatures, and a maximum ZT of 0.51 is achieved at 623 K for Cu₃Sb_0.96Ge_0.04S_3.50Se_0.50.