Energy underpins every aspect of modern human life. Despite substantial progress in renewable energy technologies such as wind and solar power, fossil fuels remain the leading energy resource in many countries, and demand is still increasing. However, fossil fuels are a finite resource, and the CO 2 and other emissions from extracting and burning them have serious impacts on the climate and on human health. To reduce our reliance on fossil fuels, it is critical both to develop a portfolio of clean energy technologies and also to improve the efficiency with which we use energy. At present, around 60% of the energy consumed globally is wasted as heat, which has led to substantial interest in thermoelectric generators (TEGs) to convert heat to electricity and thereby improve the efficiency of energy-intensive processes. TEGs are solid-state devices that extract electrical energy from a temperature gradient and are easily scalable to a range of applications including in the automotive, manufacturing and powergeneration industries. The performance of a thermoelectric material can be described by the dimensionless figure of merit ZT : 9