Promoteur: Rony Snyders
Résumé de la dissertation: It is nowadays accepted that one of the most important challenge that Humankind will faces in the next decades is related to the management of the climatic changes that Earth is starting to experience. To address this problem, it clearly appears that the development of technology allowing to efficiently utilize sustainable energy resources is a wise approach. Among the various sustainable energy resources available on Earth, the Sun energy is by far the most important. Therefore, the design of more effective solar cells is crucial for our future generations. Among all considered solar cells technologies, the so-called Dye-Sensitized Solar Cells (DSSCs) are especially promising for many reasons such as their low cost in comparison with the Si-based solar cells that are already installed on our roofs. Nevertheless, the large scale implementation of DSSCs is, among others, hindered by the modest charge transport properties of often used TiO2 nanoparticles-based photo-anode.
In this context, this work mainly aims to contribute towards the development of new TiO2-based charge carrier materials for the design of more efficient DSSCs. Based on well-established morphological and chemical concepts, we therefore developed and studied specifically designed semiconductor oxide nanomaterials that could potentially ensure better charge transport properties than the current TiO2 nanoparticles-based material. More specifically, we studied (i) the growth of 1D objects-based nano-sculpted anatase(core)-rutile(shell) TiO2 thin film and (ii) the chemistry and electronic properties of nitrogen-doped TiO2 films. Both approaches are indeed recognized as promising strategies to improve the charge transport properties of TiO2.
Our results demonstrate that the growth of anatase(core)-rutile(shell) TiO2 nano-sculpted structures is possible by using reactive magnetron sputtering strategies in glancing angle configuration. Indeed, our results reveal that 1D objects-based nano-sculpted anatase film can be generated by DC sputtering while, optimizing the magnetron process geometry and using a high power impulse regime, it is possible to, at least partially, cover this structure by a thin rutile layer.
On the other hand, we studied the chemical and structural features of nitrogen doped TiO2 nano-sculpted films by ion implantation. As a first step, we established a benchmark to assign the components of the experimental N1s region in XPS spectra to the position of N atoms into the TiO2 lattice. This has been unambiguously performed thanks to DFT calculations. Then, based on a parametric study including simulation data, we established the experimental window allowing for the nitrogen doping of TiO2 by ion implantation without affecting the structural properties of the material. It was shown that Doses ≤ 1016 ions/cm² of under-stoichiometric TiO2 allows to keep a rather high transmittance in the UV-visible region while promoting a substitutional doping defined as the ideal one for the foreseen application. Finally, it has also be demonstrated that using the ion implantation approach developed in this work, it is possible to tune the electrical behavior of the N-doped TiO2 from n-type to p-type which is a first step toward the development of efficient tandem DSSCs solar cells which are considered as the most promising concept in view of a large scale implantation of this solar technology.