Study of the anchoring process of tethered unsymmetrical Zn-phthalocyanines on TiO₂ nanostructured thin films

Thin nanoporous TiO₂ layers, deposited by dc reactive sputtering, have been functionalized with a novel unsymmetrical Zn(II) phthalocyanine (ZnPc-II) bearing a push-pull system, properly designed for application in dye sensitized solar cells. The anchoring process has been studied by combining visible absorption (vis), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM) in order to investigate the molecular assembly during the early stage of the sensitizing process. The vis and XPS measurements indicate that the ZnPc-II surface density (d_surf, molecules/cm²) and its binding geometry are strongly affected by the concentration of the grafting solutions. It has been observed that the ZnPc-II surface density monotonically increases by increasing the solution concentration up to a saturation of d_surf ∼ 1.0 × 10¹³ molecules/cm². Angle resolved XPS analyses indicate that at low molecular surface density a strong interaction between the ZnPc-II aromatic macrocycle and the TiO₂ surface occurs, suggesting a planar binding geometry. The increase of the molecular surface density is accompanied by a mitigation of the surface-molecule interaction that leads to a different ZnPc-II binding geometry. Conductive atomic force microscopy has demonstrated that the charge transport through the dye-TiO₂ interface strongly depends on the binding arrangements which exhibit different threshold values in the local I-V characteristics.