Modeling with Hybrid Density Functional Theory the Electronic Band Alignment at the Zinc Oxide-Anatase Interface

The band alignment at semiconductor interfaces can be theoretically computed using periodic slab models together with hybrid functional density functional theory methods in which Hartree-Fock exchange mixing coefficients are properly chosen (as justified by their relationship with the dielectric constant), and the calculated electrostatic potential inside each slab is used as reference for the band-edge energies. This principle is applied here to the interface between wurtzite-type ZnO and anatase-type TiO2, two oxides with nearly identical band gap widths. According to the results, in a composite of both materials the conduction and valence bands of ZnO will lie ca. 0.3 eV lower in energy than those of anatase, influencing the way in which photogenerated electrons and holes will be routed in photocatalytic or photovoltaic systems which include interfaces between these two oxides. The performance improvement observed in dye-sensitized solar cells based on a nanostructured ZnO electrode when the surface of the latter is covered by a thin TiO2 layer is thus justified. The system may serve as an example for the estimation of the band alignment in other semiconductor junctions of interest in different kinds of devices.