Controlling energy level alignment at a chromophore/TiO₂ interface using a co-adsorbed helical peptide

The sensitization of wide band gap transition metal oxide semiconductors by chromophores has become ubiquitous in the field of photovoltaic devices. Since performance hinges on the charge transfer across the interface, which in turn depends upon the alignment of the chromophore frontier orbitals with respect to the substrate band edges, it is crucial to find ways of controlling this energy alignment.

Here, we present a novel and versatile approach of controlling this energy alignment via a “mixed-layer” solution sensitization process, where a monolayer of a dipole containing helical peptide is prepared on a single crystal TiO₂(110) surface and successively exposed to a solution of a Zinc Tetraphenylporphyrin (ZnTPP) derivative. Using UV and X-ray photoemission spectroscopies, we find that (1) repeated exposure to the ZnTPP derivative progressively displaces the peptides on the surface, and (2) the highest occupied molecular orbital and N 1s core level of the ZnTPP derivative within the mixed layer are located 300 meV lower in energy than their respective counterparts on a pure ZnTPP-derivative monolayer. These results are consistent with a simple parallel plate capacitor model of the intermixed peptide dipole layer.