DOPING EFFECTS IN TITANIUM OXIDE FILMS: applications in photovoltaic solar cells

To develop more efficient thin films solar cells, new technologies for a better exploitation of the solar spectrum and the use of transparent electrodes are required. The need for low-cost transparent conducting oxides to replace indium-based oxides led to a considerable interest in transparent Ti-based materials and in new doping routes. Rare earth (RE)-activated nano-structured TiO2 films have also attracted a great interest in fundamental and technological fields. In this respect, the aim of the present work was to study doping effects and optimize the doping processes to produce transparent conductive niobium-doped TiO2 films and neodymium-doped TiO2 films. In the latter case, aim was to obtain photoluminescence in the useful absorption region of silicon. TiO2 nano-particles show strong absorbance above the optical bandgap of ~3.4-3.6 eV, a region in which the spectral response of a silicon solar cell is poor. A non-radiative transfer of the absorbed energy to RE ions produces a spectrum down-shift through photon conversion. This may satisfy one of the main requirements for solar cell efficiency improvement. To accomplish doping of the oxide matrix, different gas mixtures (Ar, Ar-H2, Ar-O2) were explored together with the incorporation of Nb in the matrix for transparent conductive films and of Nd for photoluminescence, by RF co-sputtering. The effects of the process parameters (nature of the gas mixtures, concentration of dopants, annealing conditions) on the structure, the valence band characteristics and the optical and electrical behaviour of the obtained oxide films were investigated. A combined use of optical spectroscopy and x-ray photoemission spectroscopy provided important information about the electronic properties of the films.