Unveiling disparities and promises of Cu and Ag chalcopyrites for thermoelectrics

Cu chalcopyrites exhibit excellent thermoelectric performance because of their high thermopower and low thermal conductivity. Conversely, despite that Ag chalcopyrites have even lower thermal conductivity than the Cu-based ones, they generally display lower thermoelectric performance. The underlying physics for the thermoelectric disparity between Cu- and Ag-based materials remains unclear. In this work we investigate thermal transport and thermopower of ternary $AM{\mathrm{Se}}_{2}$ chalcopyrites $(A=\mathrm{Cu}/\mathrm{Ag}; M=\mathrm{Ga}/\mathrm{In})$ using first-principles methods. We reveal that strong anharmonicity from $s\ensuremath{-}d$ coupling leads to low thermal conductivity in Cu as well as Ag chalcopyrites, whereas weaker bond strength and heavier atomic mass contribute to even lower thermal conductivity in Ag chalcopyrites than the Cu ones. On the other hand, Cu chalcopyrites show superior thermopower owing to their higher band degeneracy and stronger $p\ensuremath{-}d$ coupling, which leads to enhanced mobility. We conclude that if Ag chalcopyrites could reach comparable hole concentrations as in Cu chalcopyrites, high thermoelectric performance in Ag chalcopyrites can also be achieved because of their low thermal conductivity. Understanding the disparity between Cu and Ag chalcogenides from a mechanistic perspective is important for future design of superior thermoelectric materials.