Energy band alignment at the heterointerface between a nanostructured TiO2 layer and Au22(SG)18 clusters: relevance to metal-cluster-sensitized solar cells

Abstract

This study is the first to quantify energy band alignments at a nanostructured TiO₂/Au₂₂(SG)₁₈ cluster interface using X-ray photoelectron spectroscopy. The d-band of Au clusters shows band-like character and occupied states at the Fermi level are not detected. The results provide evidence of the existence of a finite optical energy gap in Au₂₂(SG)₁₈ clusters and the molecular-like nature of these clusters. The pinning position of the Fermi energy level at the interface was determined to be 2.8 and 1.3 eV higher than the top of the TiO₂ valence band and the highest occupied molecular orbit level of the Au clusters, respectively. A diffuse reflectance and absorption analysis quantified a 3.2 eV bandgap of the TiO₂ layer and a 2.2 eV energy gap between the highest occupied molecular orbit (HOMO) and the lowest unoccupied molecular orbit (LUMO) levels of the Au clusters. Thus, a cliff-like offset of 0.5 eV between the LUMO level and the TiO₂ conduction band was determined. The cliff-like offset of 0.5 eV provides room for improving the efficiency of metal-cluster-sensitized solar cells (MCSSC) further by lowering the LUMO level through a change in the cluster size. The offset of 0.5 eV between the HOMO level and the 3I⁻/I⁻³ redox level yields a remarkable loss-in-potential, which implies the possibility of increasing the open-circuit voltage further by properly replacing the redox couple in the MCSSCs.