XPS and UPS Characterization of the TiO₂/ZnPcGly Heterointerface:  Alignment of Energy Levels

Abstract

The electronic structure and the highest occupied molecular orbitals (HOMO)/the lowest unoccupied molecular orbitals (LUMO) alignment at the molecular semiconductor heterointerface of nanostructured TiO 2 /ZnPcGly dye sensitizer were characterized by X-ray and ultraviolet photoemission spectroscopy (XPS and UPS). The HOMO level of the dye ZnPcGly was determined to be located at 1.62 eV below the Fermi edge, and the corresponding LUMO level was estimated to be 0.10 eV above the conduction band of TiO 2 based on the HOMO/LUMO gap (1.82 eV) of ZnPcGly determined by optical absorption measurements. This energy level matching between the orbitals of the dye and the bands of TiO 2 can enable efficient electron transfer from photoexcited ZnPcGly to TiO 2, which is very important in photoinduced charge-transfer reactions and for applications in dye-sensitized solar cells.