Massive reduction in lattice thermal conductivity and strongly enhanced thermoelectric properties in Ge- and Se-doped CoSbS

Abstract

The thermoelectric figure-of-merit (ZT) can be significantly enhanced through the introduction of substitutional point defects of different elements, leading to pronounced scattering of phonons and consequently reducing lattice thermal conductivity. In this study, we deliberately induced atomic disorder in the paracostibite-structured CoSbS by substituting Sb with Ge. This approach was guided by density functional theory calculations, which revealed that the 12.5% Ge (1/8) substituted CoSbS exhibited characteristics of a degenerate p-type semiconductor; the Fermi level shifted within the valence band, creating hole pockets and flat energy bands. Experimentally, single-phase Co(Sb_1−xGe_x)S samples could be synthesized up to x = 0.1. For these samples a massive reduction in lattice thermal conductivity due to softening of the low energy acoustic phonon modes and strong scattering of phonons from defects could be realized. Moreover, we investigated the effects of additional Se-for-S substitution for Co(Sb,Ge)(S,Se). This synergistic co-substitution approach allowed – along with the remarkably reduced thermal conductivity – a substantial enhancement in electrical conductivity owing to the increased charge carrier concentration. Notably, we achieved a ZT value as high as 0.10 at 400 K for the Co(Sb_0.9Ge_0.1)(S_0.95Se_0.05) sample. This novel co-substitution scheme thus outlines a prominent avenue for the CoSbS-based materials towards true applications in thermoelectric devices.