Advanced thermoelectric technology offers a potential for converting waste industrial heat into useful electricity, and an emission-free method for solid state cooling. Worldwide efforts to find materials with thermoelectric figure of merit, zT values significantly above unity, are frequently focused on crystalline semiconductors with low thermal conductivity. Here we report on Cu(2-x)Se, which reaches a zT of 1.5 at 1,000 K, among the highest values for any bulk materials. Whereas the Se atoms in Cu(2-x)Se form a rigid face-centred cubic lattice, providing a crystalline pathway for semiconducting electrons (or more precisely holes), the copper ions are highly disordered around the Se sublattice and are superionic with liquid-like mobility. This extraordinary 'liquid-like' behaviour of copper ions around a crystalline sublattice of Se in Cu(2-x)Se results in an intrinsically very low lattice thermal conductivity which enables high zT in this otherwise simple semiconductor. This unusual combination of properties leads to an ideal thermoelectric material. The results indicate a new strategy and direction for high-efficiency thermoelectric materials by exploring systems where there exists a crystalline sublattice for electronic conduction surrounded by liquid-like ions.

A new type of high performance thermoelectric material Cu2‐xS composed of non‐toxic and earth‐abundant elements Cu and S is reported. Cu2‐xS surprisingly has lower thermal conductivity and more strikingly reduced specific heat compared to the heavier Cu2Se, leading to an increased zT to 1.7.

Many IV-VI semiconductors tend to be good thermoelectric materials, these include all Pb chalcogenides as well as Pb-free SnTe: all of which crystallize in a NaCl cubic structure. Another group of IV-VI compounds form layered orthorhombic structures. SnSe is one of these compounds, whose transport properties as a polycrystalline thermoelectric material have rarely been studied. Here we present our study of p-type polycrystalline SnSe doped with Ag, prepared by melting and hot pressing. SnSe has anisotropic properties with hysteresis observed in resistivity between 300 and 650 K regardless of doping. Ag is not an ideal dopant but is able to increase the carrier density significantly, as a result a peak zT of 0.6 was observed at 750 K. Transport properties of doped SnSe can be explained with a single parabolic band model, which suggests promising potential for this compound together with its challenges.

CuGaTe(2) with a chalcopyrite structure demonstrates promising thermoelectric properties. The maximum figure of merit ZT is 1.4 at 950 K. CuGaTe(2) and related chalcopyrites are a new class of high-efficiency bulk thermoelectric material for high-temperature applications.