Cu₃SbSe₄, one of the ternary p-type semiconductor materials with chalcopyrite structure, has aroused much interest in thermoelectrics due to its inherent large effective mass and narrow bandgap. Therefore, many researches have been done, which cover the single and/or multi-element doping to manipulate its band structure and introduce the point defects. Although great achievements have been made in recent years, the mechanism in Cu₃SbSe₄ with respect to the phonon and electronic transport properties needs further investigating.

In this work, first, Sn and S are co-doped into Cu₃SbSe₄ and then the resulting compound is alloyed with Ga₂Te₃, to improve its TE performance and understand the mechanism by calculating the band structure and crystal structure. The calculation of band structure reveals that an impurity band is created within the bandgap after co-doping Sn and S due to their contributions to the density of the states (DOS), which is directly responsible for the significant improvement in carrier concentration (n_H) and electrical property. Therefore, the power factor (PF) is enhanced from 0.52 × 10^–3 to 1.3 × 10^–3 W·m^–1·K^–2.

Although the effect associated with the Ga (Te) residing at Sb (Se) sites on the band structure is limited due to the fact that both the single Ga- and single Te-doped band structure remain almost unchanged, the structural parameters (bond lengths and angles) of the polyhedrons SeCu₃Sb and SbSe₄ before and after Te and Ga residing at Se and Sb sites respectively change remarkably. This yields the significant distortion of local lattice structure on an atomic scale. Therefore, the phonon scattering is enhanced and the lattice thermal conductivity (κ_L) decreases from 1.23 to 0.81 W·K^–1·m^–1 at 691 K. The reduction in κ_L prevents the total thermal conductivity (κ) from being enhanced rapidly. As a consequence, the highest ZT value of 0.64 is attained, which is much higher than that of the pristine Cu₃SbSe₄ (ZT = 0.26). In addition, we not only present a synergistic strategy to separately optimize the phonon and electronic properties, but also fully elaborate its mechanism and better understand that this strategy is an effective way to improve the TE performance of the Cu₃SbSe₄-based solid solutions.