Controlling energy level alignment at a chromophore/TiO₂ interface using a helical peptide dipole

The sensitization of wide band gap transition metal oxide semiconductors by chromophores has become ubiquitous in the field of photovoltaics. 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.

We present two novel and versatile approaches of controlling this energy alignment: (1) 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, and (2) by preparing a homogeneous monolayer comprising ZnTPP-terminated peptide molecules. Using UV and X-ray photoemission spectroscopies, we find 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, and that this amount is nearly doubled for the homogeneous monolayer. These results are consistent with a simple parallel plate capacitor model of the embedded peptide dipole layer.