where S is the Seekbeck coefficient, σ is the electrical conductivity, S 2 σ is the power factor (PF), and κ el and κ latt are the electronic and lattice (phonon) components of the thermal conductivity κ. S, σ and κ el are related through the carrier concentration n, which can be controlled by chemical doping and is typically optimised in heavily-doped semiconductors. κ latt is independent of the electronic transport and must be minimised to achieve a high ZT . Typical materials engineering strategies to optimise ZT include doping and alloying, which can both improve the electrical properties (e.g. by increasing n or modulating the electronic structure to induce "band convergence") and/or reduce the κ latt . A variety of materials have been studied as prospective TEs including chalcogenides, skutterudites, half-Heuslers, zintl compounds, clathrates, oxides, and oxychalcogenides (Ref. and references therein). The current industry-standard materials for low-and high-temperature applications are based on Bi 2 Te 3 (ZT ≃ 1 from 350-450 K, ZT up to 1.9 reported for nanostructured (Bi 1-x Sb x )Te 3 alloys ) and PbTe (ZT up to 2.2 at 915 K with endotaxial nanostructuring ). However, the low abundance of Te precludes widespread adoption, and thus alternatives composed of more abundant elements are critical. The discovery of high ZT in orthorhombic SnSe (up to 2.6 in single crystals and 3.1 in polycrystalline samples ) has led to considerable interest in the group IV-VI sulphides and selenides. The favourable electrical properties of these materials are well established from the use of SnS as a photovoltaic (PV) absorber material, and SnS and SnSe also show unusually low κ latt associated with the high-temperature Pnma → Cmcm phase transition. However, the "headline" ZT in orthorhombic SnSe is only achieved at high temperature, whereas many potential applications of TEs require the recovery of low-grade waste heat, and so SnSe cannot replace Bi 2 Te 3 .