Tuning Energy Level Alignment At Organic/Semiconductor Interfaces Using a Built-In Dipole in Chromophore–Bridge–Anchor Compounds

A chromophore–bridge–anchor molecular architecture is used to manipulate the molecular level energy position, with respect to the band edges of the substrate, of a chromophore bound to a surface via an anchor group. An energy shift of the chromophore’s frontier orbitals is induced by the addition of an oriented molecular dipole into the bridge part of the compound. This principle has been tested using three Zinc Tetraphenylporphyrin derivatives of comparable structure: two of which possess a dipole, but pointing in opposite directions and, for comparison, a compound without a dipole. UV–vis absorption and emission spectroscopies have been used to probe the electronic structure of the compounds in solution, while UV photoemission spectroscopy has been used to measure the relative position of the molecular levels of the chromophore with respect to the band edges of a ZnO(11–20) single crystal substrate. It is shown that the introduction of a molecular dipole does not alter the chromophore’s HOMO–LUMO gap, and that the molecular level alignment of the compounds bound to the ZnO surface follows the behavior predicted by a simple parallel-plate capacitor model

A Sensitized Nb₂O₅ Photoanode for Hydrogen Production in a Dye-Sensitized Photoelectrosynthesis Cell

Orthorhombic Nb₂O₅ nanocrystalline films functionalized with [Ru­(bpy)₂(4,4′-(PO₃H₂)₂bpy)]²⁺ were used as the photoanode in dye-sensitized photoelectrosynthesis cells (DSPEC) for hydrogen generation. A set of experiments to establish key propertiesconduction band, trap state distribution, interfacial electron transfer dynamics, and DSPEC efficiencywere undertaken to develop a general protocol for future semiconductor evaluation and for comparison with other wide-band-gap semiconductors. We have found that, for a T-phase orthorhombic Nb₂O₅ nanocrystalline film, the conduction band potential is slightly positive (<0.1 eV), relative to that for anatase TiO₂. Anatase TiO₂ has a wide distribution of trap states including deep trap and band-tail trap states. Orthorhombic Nb₂O₅ is dominated by shallow band-tail trap states. Trap state distributions, conduction band energies, and interfacial barriers appear to contribute to a slower back electron transfer rate, lower injection yield on the nanosecond time scale, and a lower open-circuit voltage (<i>V</i>_oc) for orthorhombic Nb₂O₅, compared to anatase TiO₂. In an operating DSPEC, with the ethylenediaminetetraacetic tetra-anion (EDTA^4–) added as a reductive scavenger, H₂ quantum yield and photostability measurements show that Nb₂O₅ is comparable, but not superior, to TiO₂