First principles phase diagram and electronic structure estimation of ZnO1-xSex photoanodes

Abstract

Terminal solid solutions in the ZnO1−xSex system (0≤x≤0.15,0.95≤x≤1) exhibit extreme bandgap reduction attributable to band anti-crossing (BAC). In this work, we perform a theoretical investigation of alloying in this system (0≤x≤1). The temperature-composition phase diagram of ZnO1−xSex is obtained via first principles and cluster expansion-based Monte–Carlo simulations. For 0≤x≤0.05, a solid solution in the wurtzite structure and for 0.5≤x≤1, a solid solution in the sphalerite structure is obtained. The alloy system exhibits a miscibility gap in the range of 0.05≤x≤0.5. Only the solid solutions are seen to obey bandgap reduction predicted by BAC. The bandgap of the alloys, calculated using the Δ-sol method, shows a bowing behavior as predicted by the BAC model. Difference in the electronegativities of O and Se atoms in the lattice leads to hybridization of O-2p and Se-4p electronic states. Interaction between these electronic states also leads to a split in the valence band edge at the O-rich end and a split in the conduction band edge at the Se-rich end. The effective mass, estimated from the density of states, of holes at the O-rich end and that of electrons at the Se-rich end, increases with alloying. These fundamental insights should help in choosing suitable alloy compositions for optimal photocurrent density when these materials are used as photoanodes.