Enhancing thermoelectric performance of Cu₃SnS₄-based solid solutions through coordination of the Seebeck coefficient and carrier concentration

Improving the thermoelectric (TE) performance of Cu₃SnS₄ is challenging because it exhibits a metallic behavior, therefore, a strategy should be envisaged to coordinate the carrier concentration (n_H) and Seebeck coefficient (α). The coordination in this work has been realized through the Fermi level (E_f) unpinning and shifting towards the conduction band (CB) via addition of excess Sn in Cu₃SnS₄. As a result, the solid solution Cu₃Sn₁+_xS₄ (x = 0.2) has a moderate α (178.0 μV K⁻¹) at 790 K and a high n_H (1.54 × 10²¹ cm⁻³) value. Along with the lowest lattice thermal conductivity κ_L (0.39 W K⁻¹ m⁻¹) caused by the increased phonon scattering by carriers, the highest ZT value of 0.75 is attained at ∼790 K. This value is 2.8 times that of the stoichiometric Cu₃SnS₄, and stands among the highest for ternary Cu–Sn–S sulfide thermoelectrics at the corresponding temperatures. More importantly, this approach used in the case of ternary Cu₃SnS₄ provides a guidance or reference to improve the TE performance of other materials.