Phase Diagram, Electronic Structure, and Optical Properties of KCuTe_1–<i>x</i>Se_<i>x</i> Photocathodes: A Computational Study

Recent high-throughput computational studies of copper-based semiconductors have identified potassium-based copper chalcogenides, KCuA (A ∈ Te, Se, S) as good candidates for light absorbers in photovoltaic devices. KCuTe and KCuSe in particular exhibit high defect tolerance and attractive bandgaps. In this work, we investigate the applicability of KCuTe1–xSex as H2 evolution electrodes (photocathodes) in photoelectrochemical water-splitting devices using first-principles calculations. The calculated temperature–composition phase diagram predicts the formation of a solid solution of KCuTe1–xSex in the hexagonal structure beyond a maximum critical temperature of 322 K. Since Te and Se have a large size and electronegativity mismatch, their solid solution is expected to follow bandgap bowing. However, our calculations show that structure relaxation in the alloy competes with volume deformation of the parent lattice and charge exchange between (Te, Se) anions to produce a net linear variation in the bandgap with varying alloy concentration. The following results suggest that this alloy is a suitable photocathode: (i) a tunable bandgap in the 1.2–1.5 eV range, (ii) a lower effective mass, and hence a higher mobility of electrons in the alloy compared to the end compounds, (iii) an absorption coefficient of the order of 105 cm–1, and (iv) an appropriate alignment of the conduction band with respect to the hydrogen reduction reaction.