First Principles Phase Diagram and Electronic Structure Properties of Potassium-Based Copper Chalcogenide Alloys for Photocathode Applications

A recent high throughput study of copper-based semiconductors has identified potassium-based copper chalcogenides as optimal light absorbers in PV/PEC devices. In this work, we investigate the applicability of KCuTe_1-xSe_x and KCuSe_1-xS_x as photocathode materials. We first calculate the temperature-composition phase diagram of KCuTe_1-xSe_x and KCuSe_1-xS_x via solid solution model and cluster expansion-based Monte Carlo simulations. Our calculations predict a miscibility gap up to a maximum critical temperature of 243 K beyond which the alloy forms a solid solution in the hexagonal structure over the entire composition range. Similarly, KCuSe_1-xS_x forms a solid solution in orthorhombic structure, although ordered ground states at x = 0.25, 0.333, 0.5, 0.667, 0.75 and 0.833 are present at low temperatures. Based on the stable structure of the alloys we calculate the electronic structure properties via DFT. Unlike the bandgap bowing typical of highly mismatched alloys, predicted by band-anticrossing (BAC) model the electronic band gap of potassium-based copper chalcogenide alloys obeys Vegard’s law. The conduction band of these alloys is appropriately aligned with respect to the hydrogen reduction reaction and suggests the utility of these alloys as a photocathode.