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

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 materials in the homologous series KCu₃Se_2(1-x)S_2x (0 ≤ x ≤ 1) as proton reduction photocathodes. We first calculate the temperature-composition phase diagram of KCu₃Se_2(1-x)S_2x through solid solution models and Monte Carlo simulations based on cluster expansion. Our calculations predict that the alloy forms a solid solution in the monoclinic structure over the entire composition range although ordered ground states at x = 0.125 and 0.5 are present at low temperatures. On the bases of 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 poly-chalcogenide alloys obey the simple rule of mixtures. The optical absorption coefficient, mobility, and band alignment of the alloy with water redox potentials suggest its effectiveness as a photocathode.