Thermoelectric properties of Pnma and R3m GeS and GeSe

With similar to 60% of global energy lost as heat, technologies such as thermoelectric generators (TEGs) are an important route to enhancing the efficiency of energy-intensive processes. Optimising thermoelectric (TE) materials requires balancing a set of interdependent physical properties to meet efficiency, cost and sustainability requirements, and is a complex materials-design challenge. In this study, we demonstrate a fully first-principles modelling approach to calculating the properties and thermoelectric figure of merit ZT and apply it to the orthorhombic and rhombohedral phases of GeS and GeSe. While p-doped Pnma GeS and GeSe do not match the performance of the Sn analogues, due to a smaller electrical conductivity sigma and larger lattice thermal conductivity kappa(latt), we predict a large ZT(max) = 2.12 for n-doped Pnma GeSe at 900 K, which would make it a good match for p-type SnSe in a thermoelectric couple. Moreover, with n-type doping the sigma is largest along the layering direction and aligns with the minimum kappa(latt), and a much larger ZT(max) > 3 could potentially be accessible with control over the growth direction. We also predict that p-doped R3m GeS and GeSe can achieve an industrially-viable ZT > 1, through a high sigma counterbalanced by a large thermal conductivity, and experiments indicate this can be further improved by alloying. Our results therefore strongly motivate further study of the under-explored Ge chalcogenides as prospective TEs, with a particular focus on strategies for n-doping the Pnma phases.