DOPING EFFECTS IN WIDE GAP OXIDE FILMS

Oxide-based films and nanostructures have emerged as important and promising materials for a wide range of applications such as photovoltaics, photocatalysis, optoelectronics, gas sensing and electronics. Many of these technologies require the use of transparent electrodes and charge collectors. At present, indium tin oxide (ITO) is the commonly large scale manufactured and used material. Many materials are under study to replace the costly and scarce ITO. The doping effects of cheaper and more abundant materials Niobium-doped TiO2 films were produced by RF co-sputtering of a TiO2 in argon, argon-H2 and argon-O2 plasmas, followed by vacuum thermal annealing. In view of application in the photovoltaics field, new materials are also explored and developed to increase the conversion efficiency of solar cells through the increase of the most efficient and useful fraction of the solar spectrum which hits the cells, thanks to spectrum conversion processes such as frequency down-shifting conversion. In that sense, the development of oxides doped with rare earths offers new opportunities to realize active materials for the efficient absorption and conversion of the solar radiation in solar cell. In this view, wide band gap oxide films doped with rare earth ions able to insure such processes were synthesized. To develop an appropriate understanding of the properties of the doped oxide films, it is necessary to address the material preparation methods and also structural and defect issues. The effect of doping on the optical properties of the films was studied. A combined use of optical and x-ray photoemission spectroscopies provided information about the electronic properties of the films. Fundamental properties regarding the valence band, the Fermi level and the work function of the un-doped and doped films were studie

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

Experimental Study of the Freezing Point of γ-Al2O3/Water Nanofluid

Nanofluids are colloidal suspensions made of nanometer-sized particles dispersed in a conventional fluid. Their unusual thermal properties explain intensive investigations for several thermal and industrial applications. In this work, an experimental investigation was performed to measure the freezing point and to study the supercooling point made of alumina γ-Al2O3 nanoparticles with 30 nm diameter size and deionized water. Particles' volume fraction used in this work is ranging from 1% to 4%. The T-historic method based on the measurement of the point of inflexion was performed to measure the thermal properties such as the freezing point and the latent heat of solidification of the nanofluids for different concentrations. The results show that the supercooling degree decreases for the high particles volume concentrations and that the agglomeration does not influence the temperature of the freezing point. However, it makes the freezing process longer