Enhanced Thermoelectric Properties of Cu₃SbSe₄ Compounds by Isovalent Bismuth Doping

Cu₃SbSe₄, featuring its earth-abundant, cheap, nontoxic and environmentally friendly constituent elements, can be considered as a promising intermediate temperature thermoelectric (TE) material. Herein, a series of p-type Bi-doped Cu₃Sb_1−xBi_xSe₄ (x = 0–0.04) samples were fabricated through melting and hot pressing process, and the effects of isovalent Bi-doping on their TE properties were comparatively investigated by experimental and computational methods. TEM analysis indicates that Bi-doped samples consist of Cu₃SbSe₄ and Cu_2−xSe impurity phases, which is in good agreement with the results of XRD, SEM and XPS. For Bi-doped samples, the reduced electrical resistivity (ρ) caused by the optimized carrier concentrations and enhanced Seebeck coefficient derived from the densities of states near the Fermi level give rise to a high power factor of ~ 1000 µWm⁻¹ K⁻² at 673 K for the Cu₃Sb_0.985Bi_0.015Se₄ sample. Additionally, the multiscale defects of Cu₃SbSe₄-based materials involving point defects, nanoprecipitates, amorphous phases and grain boundaries can strongly scatter phonons to depress lattice thermal conductivity (κ_lat), resulting in a low κ_lat of ~ 0.53 Wm⁻¹ K⁻¹ and thermal conductivity (κ_tot) of ~ 0.62 Wm⁻¹ K⁻¹ at 673 K for the Cu₃Sb_0.98Bi_0.02Se₄ sample. As a consequence, a maximum ZT value ~ 0.95 at 673 K is obtained for the Cu₃Sb_0.985Bi_0.015Se₄ sample, which is ~ 1.9 times higher than that of pristine Cu₃SbSe₄. This work shows that isovalent heavy element doping is an effective strategy to optimize thermoelectric properties of copper-based chalcogenides.