TiO2 multilayer thick films (up to 4 μm) with ordered mesoporosity: influence of template on the film mesostructure and use as high efficiency photoelectrode in DSSCs

Mesoporous anatase thin films are very promising materials to act as electrode in dye-sensitized solar cell. Randomly oriented nanocrystalline TiO2 particles are usually used to prepare photoelectrodes with a thickness of 10-15 mu m. Template-assisted dip-coating techniques are used to obtain thin films with ordered porosity. However, monolayer films prepared by dip-coating from a solution suffer from a low quantity of active material with a limited surface area, leading to poor photovoltaic performances. Therefore a multilayer deposition process is needed to increase the film thickness along with the surface area. Multilayer dip-coating procedures have already been reported but are usually characterized by a lack of linearity in the evolution of parameters (roughness, surface area, PV performances) as the number of layer increases. In this paper, a dip-coating-based multilayer deposition technique is reported. First, the influence of the template on the film organization and porosity is studied in terms of long-range order, percentage of porosity, pore size and pores connectivity. Different techniques such as transmission electron microscopy (TEM), atmospheric poroellipsometry (AEP) and UV-visible absorption spectroscopy (UV-vis) have been used to describe the microstructural features of the films with a thickness of 1 mu m. The film exhibiting the highest dye loading was selected and its thickness gradually increased up to 4 mu m. Finally, the photovoltaic performances of the thick films (1 to 4 mu m) have been evaluated in combination with the N-719 dye and show excellent efficiency (6.1%) when compared to values reported in the literature. Such mesostructured films are compared in terms of photovoltaic performance with TiO2 nanoparticles films, generally used in DSSC.