O 1s core-level shifts at the anatase TiO2(101)/N3 photovoltaic interface: Signature of H-bonded supramolecular assembly

We here report an atomic-scale first-principles investigation of the O 1s core-level shifts at the interface between TiO2 and the dye N3 found in dye-sensitized solar cells. We first perform extensive validation of our computational setup in the case of small molecules containing carboxylic acid groups in the gas phase. Then we calculate the O 1s core-level shifts for a variety of atomistic models of the TiO2/N3 interface. We investigate in detail the effects of water contamination, dye packing density, exchange and correlation functionals, and hydrogen-bonding interactions on the calculated core-level spectra. The quantitative comparison between our calculated core-level shifts and measured photoemission spectra [Johansson et al., J. Phys. Chem. B 109, 22256 (2005)] leads us to propose a new atomic-scale model of the TiO2/N3 interface, where the dyes are arranged in supramolecular H-bonded assemblies. Our interface models describe dry TiO2/N3 films as in [Johansson et al., J. Phys. Chem. B 109, 22256 (2005)], and are of direct relevance to solid-state dye-sensitized solar cells. Our present work suggests that the adsorption energetics is not a reliable indicator of the quality of an interface model, and highlights the importance of combining experimental and computational spectroscopy for determining the atomic-scale structure of nanostructured solar cell interfaces.