Role of Cation Vacancies in Cu₂SnSe₃ Thermoelectrics

In this study, a series of Cu_2-SnSe₃ (x = 0.075-0.175) and Cu₂Sn_1-Se₃ (y = 0.06-0.1) compounds were synthesized by self-propagating high-temperature synthesis combined with plasma-activated sintering. The effects of different cation vacancies (Cu vacancies and Sn vacancies) on the thermoelectric properties are systematically studied. Both Cu vacancies and Sn vacancies enhance the carrier densities and move the Fermi level deeply into the valence band, promoting the multiband from Γ and S points involved in the electrical transport and increasing the effective mass, which is further corroborated by the theoretical calculation. Due to the stronger carrier scattering caused by Cu vacancies, the mobilities of samples with Cu deficiencies reduce leading to decreased power factors. The power factors of samples with Sn deficiencies increase owing to the increased carrier concentration and attaining a maximum power factor of 10.17 μW cm^-1 K^-2 at 800 K. Besides, the deficiencies of both Cu and Sn strengthen the phonon scattering, and samples with Cu deficiencies obtain lower thermal conductivity than samples with Sn deficiencies due to the lower electronic thermal conductivity. All the samples with cation deficiencies have improved thermoelectric properties. For Cu_1.875SnSe₃, ZT reaches 0.95 at 800 K, which is 83% higher than those of undoped samples, while for Cu₂Sn_0.93Se₃, ZT reaches 0.87 at 800 K, which is a 67% improvement.