Role of Solvent in the Energy Level Alignment of Dye-Sensitized NiO Interfaces

p-Type dye-sensitized solar cells (DSCs), based on hole injection from the photoexcited dye into the valence band of a semiconductor like NiO, represent an attractive device to exploit solar radiation. However, they perform rather poorly compared to n-type DSCs, for reasons that are currently heavily debated. In this work, we use simulations based on density functional theory and first-principles molecular dynamics to investigate the interface between NiO and C343, a widely used dye. We focus on the structure of the interface and on the energy level alignment. Accounting explicitly for the role of the solvent, we find that the NiO surface binds a monolayer of water molecules that, through the orientation of their dipoles, align favorably the dye's HOMO relative to NiO's valence band. We also show that an alignment consistent with the experimental evidence of ultrafast injection can only be obtained accounting explicitly for the role of the solvent.