To optimise the ZT , the power factor should be maximised and the thermal conductivity minimised. S, σ , and κ el depend on the electronic structure and are typically related through the carrier concentration n such that increasing n increases the σ and κ el and reduces the S. Materials-engineering strategies such as "band convergence" can in some case enhance the S without degrading the conductivity, but aside from these optimising the electrical properties entails balancing the σ , S and κ el by optimising n, and the best balance is typically found in heavily-doped semiconductors. On the other hand, the lattice thermal conductivity is independent of the electrical properties and depends on the material structure and chemical bonding. The links between struc-ture/composition and κ latt are at present not well understood, but low κ latt is often associated with materials composed of heavy elements, with weak chemical bonding, and/or with stronglyanharmonic lattice dynamics arising from e.g. displacive phase transitions and active lone pairs. The current industry standard TEs are Bi 2 Te 3 for applications around room temperature (ZT ≈ 1 from 350-450 K) and PbTe for high-temperature applications (ZT = 2.2 at 915 K), due to their favourable electronic structure and intrinsically low thermal conductivity. However, the low elemental abundance of Te and the environmental toxicity of Pb all but rule out widespread deployment, restricting Bi 2 Te 3 -and PbTe-based TEGs to niche applications. Mass production of TEGs thus requires alternative candidate materials to be found that balance high performance against sustainability and cost requirements.