Enhanced thermoelectric performance in polymorphic heavily Co-doped Cu₂SnS₃ through carrier compensation by Sb substitution

Heavily acceptor-doped Cu₂SnS₃ (CTS) shows promisingly large power factor (PF) due to its rather high electrical conductivity (σ) which causes a modest ZT with a high electronic thermal conductivity (κ_e ). In the present work, a strategy of carrier compensation through Sb-doping at the Sn site in Cu₂Sn_0.8Co_0.2S₃ was investigated, aiming at tailoring electrical and phonon transport properties simultaneously. Rietveld analysis suggested a complex polymorphic microstructure in which the cation-(semi)ordered tetragonal phase becomes dominant over the coherently bonded cation-disordered cubic phase, as is preliminarily revealed using TEM observation, upon Sb-doping and Sb would substitute Sn preferentially in the tetragonal structure. With increasing content of Sb, the σ was lowered and the Seebeck coefficient (S) was enhanced effectively, which gave rise to high PFs maintained at ~10.4 μWcm^-1K^-2 at 773 K together with an optimal reduction in κ_e by 60-70% in the whole temperature range. The lattice thermal conductivity was effectively suppressed from 1.75 Wm^-1K^-1 to ~1.2 Wm^-1K^-1 at 323 K while maintained very low at 0.3-0.4 Wm^-1K^-1 at 773 K. As a result, a peak ZT of ~0.88 at 773 K has been achieved for Cu₂Sn_0.74Sb_0.06Co_0.2S₃, which stands among the tops so far of the CTS-based diamond-like ternary sulfides. These findings demonstrate that polymorphic microstructures with cation-disordered interfaces as an approach to achieve effective phonon-blocking and low lattice thermal conductivity, of which further crystal chemistry, microstructural and electrical tailoring are possible by appropriate doping.