Tuning Ag content to achieve high thermoelectric properties of Bi-doped p-type Cu₃SbSe₄-based materials

Cu₃SbSe₄, consisting of earth-abundant and inexpensive elements, is a promising p-type thermoelectric (TE) material. However, its thermoelectric performance is low in comparison with typical Pb-based materials. Here, we tune Ag content to optimize synergistically electrical and thermal transport properties for achieving high TE performance of Bi-doped Cu₃SbSe₄ materials. Dual-incorporation of Bi and Ag spontaneously generates multiple secondary phases involving CuSe and AgSbSe₂. The multiscale defects including point defects, dislocations, nanoprecipitates and grain boundaries can enhance phonon scattering, thereby obtaining a low thermal conductivity (κ) of ~ 0.67 W m⁻¹ K⁻¹ at 673 K. The carrier concentration is improved due to the enhanced Cu vacancies caused by softening of Cu 3d–Se 4p bond after Ag doping, leading to the obviously increased electrical conductivity (σ). Meanwhile, the heavy element dopants (Bi and Ag) can modify the band structure to enhance Seebeck coefficient (S) by enlarging band gap (E_g) and increasing carrier effective mass (m*). The simultaneously enhanced σ and S result in a large power factor (S²σ) of ~1200 μW m⁻¹ K⁻² at 673 K. Consequently, a maximum ZT of ~1.18 (~136% higher than that of pristine Cu₃SbSe₄) at 673 K and an averaged ZT of ~ 0.51 at 300–673 K are achieved for Cu_2.85Ag_0.15Sb_0.985Bi_0.015Se₄ sample due to a low κ and a high S²σ. This work offers a referential strategy to improve the comprehensive TE performance by dual-incorporation with isovalent heavy element doping for the diamond-like materials.