Study of the Anchoring Process of Tethered Unsymmetrical Zn-Phthalocyanines on TiO₂ Nanostructured Thin Films

Abstract

Thin nanoporous TiO 2 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/cm 2 ) 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 × 10 13 molecules/cm 2 . Angle resolved XPS analyses indicate that at low molecular surface density a strong interaction between the ZnPc-II aromatic macrocycle and the TiO 2 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−TiO 2 interface strongly depends on the binding arrangements which exhibit different threshold values in the local I−V characteristics.

Multi-Scale-Porosity TiO2 scaffolds grown by innovative sputtering methods for high throughput hybrid photovoltaics

“…From the comparison it emerges that the gig-lox layer annealed at 500 °C accommodates the largest amount of molecules, likely due to the crystallographic improvement of the surface exposed by the TiO 2 rods edges with respect to the starting material, comprising: a) the faceting of the external surfaces (see TEM in Fig. 5) and b) a likely increased reactivity of the anatase surfaces in comparison to less ordered surfaces342563. Note that a value of ∼1 × 10 20 molecules/cm 3 is consistent with a complete pore filling as calculated by taking into account the full volume of the pores and the N-719 size (the volume occupied by each molecule was ∼3.8 nm 3 ).…”

“…More recently, hybrid solar cells replacing the dye with hybrid Perovskites (PSC) materials1819202122 as sensitizer allowed getting efficiency values as high as 22.1% using advanced mixed perovskites2324. In the standard scheme of the photo-anode of a DSC4 or PSC18, a mesoporous thin film of nanosized TiO 2 crystals is deposited on a Transparent Conductive Oxide (TCO), annealed for grains sintering and anatase crystallization (typically at 500 °C), and subsequently imbued with a photoactive dye2526272829 or infiltrated with perovskite30. In both cases, mesoporous TiO 2 in the anatase polymorphism has emerged as the most appropriate choice for cell scaffold15212331.…”

Recent Advances in Phthalocyanine‐Based Sensitizers for Dye‐Sensitized Solar Cells

“…Another related example, in which the carboxylic acid group is connected to the Pc core through a phenylethynyl linker, was reported by Zanotti et al43 The role of the concentration of this ZnPc in the anchoring process was studied by UV/Vis, XPS and AFM, revealing strong interaction between the Pc and the TiO 2 surface at low molecular surface density 44. In addition, even though DFT calculations showed that the introduction of a phenylethynyl unit provides considerable electronic coupling between the dye's LUMO and the TiO 2 3d orbital, the power conversion efficiency remained lower than 1 % 44…”

Surface chemistry of porphyrins and phthalocyanines

“…Silanization using a linker with a -Si(OR) 3 end group was used to anchor a Zn porphyrin on a TiO 2 surface [465]. In a study by Pellegrino et al, a ZnPc derivative with a benzoic acid side group was used for covalent tethering on TiO 2 films [108,466]. The electron transfer dynamics of a porphyrin with the benzoic acid side group anchored on a ZnO surface was subject of a time-dependent DFT study [467].…”