Paper SE+EM+EN-MoA3
Synthesis of Crystallized and Nanostructured TiO₂ Thin Films by Reactive Magnetron Sputtering for Application as Photoanode in Dye Sensitive Solar Cells

Monday, October 19, 2015, 3:00 pm, Room 212A

Nowadays, the efficient use of renewable energies, and more specifically of solar energy, represents a major economic and environmental issue. Among the potential solutions, the Dye Sensitive Solar Cells (DSSC) present many advantages. In order to improve the efficiency of DSSC, TiO₂ nanoparticles, which are usually used as photo-anode, could be replaced by nanostructured TiO₂ thin films. Indeed, a photo-anode of ordered porous nano-columnar TiO₂ would provide large surface area for dye absorption, fast electron transfer path, enhanced light trapping, and tight interfaces to conducting electrodes and contributes to a high fill factor and an overall higher cell efficiency. In view of this application, the anatase phase of TiO₂ is usually the preferred polymorph as electron acceptors in DSSCs even if a synergistic effect exists between anatase and rutile with an optimal rutile content of around 13 wt%.

In this work, nanostructured and crystallized TiO₂ thin films are synthesized by reactive magnetron sputtering combined with Glancing Angle Deposition (GLAD). The substrate temperature, the substrate bias voltage and the rotation speed were varied in order to determine the best experimental conditions leading to (nano-)porous films with anatase TiO₂ columns. The chemical composition, the crystalline structure and the microstructure of the films were analyzed by XPS, XRD, SEM and TEM, respectively while the surface area is evaluated by the BET method.

It is demonstrated that many type of microstructures (tilted columns, straight pillars, chevrons,…) are obtained by combining the GLAD parameters and the sputtering conditions. On the other hand, depending on these growth conditions, the phase constitution can be tuned from amorphous to pure rutile or anatase phases including mixtures of both polymorphs. The surface area of the synthesized layer strongly depends on the experimental conditions and on the associated microstructure. The highest obtained value is of ~ 140 m²/g for a tilted columnar amorphous/anatase sample which is significantly better than the values reported for TiO₂ nanoparticules systems (~ 60 m²/g). On the other hand, a clear correlation between the surface area and the dye absorption is demonstrated revealing a good impregnation of the layer. It is also demonstrated that this impregnation behaviour is depending on the size of the dye molecule revealing different populations of pores as a function of their size. This is supported by TEM and modelling data using NASCAM, a 2D-3D Kinetic Monte Carlo code for the simulation of deposition, diffusion, nucleation and growth of films on a surface.