Enhanced thermoelectric performance in polymorphic heavily Co-doped Cu(2)SnS(3) through carrier compensation by Sb substitution

Heavily acceptor-doped Cu(2)SnS(3) (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 Cu(2)Sn(0.8)Co(0.2)S(3) 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(−1)K(−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(−1)K(−1) to ~1.2 Wm(−1)K(−1) at 323 K while maintained very low at 0.3–0.4 Wm(−1)K(−1) at 773 K. As a result, a peak ZT of ~0.88 at 773 K has been achieved for Cu(2)Sn(0.74)Sb(0.06)Co(0.2)S(3), 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.