First principles investigation of the effect of excess carriers and illumination on the band edge energies of semiconducting transition metal dichalcogenides*

Photovoltage, or the change in surface potential  of a material  under illumination, is a central quantity for characterizing complex semiconductors and for optoelectronic applications such as photocatalysis. In this work we use first principles calculations to understand photovoltage in the limit of well-defined semiconducting two-dimensional transition metal dichalcogenides, materials with strong light-matter interactions that have been reported to be photocatalytically active. We model the effect of excess free carriers using density functional theory (DFT), computing shifts in the absolute band edge energies as a function  of  carrier density. We use DFT calculations, as well as ab initio many-body perturbation theory calculations using the GW-Bethe-Salpeter equation approach, to study the effects of illumination on the band edges, to establish limits on possible photogenerated carrier concentrations, and to understand the maximum photovoltage that could be achieved in these systems.