Nanostructured materials for photovoltaics and microwave electronics: methods and applications
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Nanoscale particles and thin layers are today used for a variety of applications to make industrial products lighter, stronger or more conductive. This study investigates synthesis, characterization and application of nanostructured materials for “low-cost” photovoltaics and high frequency (microwaves) analog electronics. Indeed, the two areas investigated demonstrate the enormous potential for diverse applications of nanostructured materials. The initial research stage focused on the optimization of titanium dioxide morphology for Dye Sensitized Solar Cells (DSSCs) applications. Four different combined sol-gel/solvothermal synthetic approaches were adopted and six different anatase phase mesoporous titanias were prepared and tested as photoanodes in DSSCs. Superior light scattering properties, stemming from their sub-micrometric mesoporous structure, were proved for TiO2 beads synthesized by using the exadecylamine method. An energy conversion efficiency ŋ = 7.0 % was achieved. The second stage of the research aimed at further DSSC performances’ improvements by introducing trivalent rare earth dopants (Pr, Nd, Sm, Gd, Er and Yb) into the “standard” TiO2 beads. Rare earth ions’ dimension was found to affect TiO2 average crystallites size through nucleation control and ultimately electron recombination lifetime through TiO2 natural oxygen defectivity and electron-impurity scattering control. While the largest (i.e. the lightest) ions of the lanthanide series (Pr3+ and Nd3+) suppress DSSCs performances, cations heavier than Sm 3+ produce an energy conversion increase compared to pure anatase. The best performances were obtained for a rare earth dopants concentration of 0.2 % erbium atoms (ŋ = 8.7 %, 20% efficiency improvement compared to un-doped TiO2). The third research step investigated materials capable of modulating TiO2 optoelectronic and interfacial properties at once, e.g. chirality separated Single Walled Carbon Nanotubes (SWCNTs). Chirality selection allowed for tuning the energy barrier at the TiO2/SWCNTs/FTO interface, electronic conductivity enhancement and reduced SWCNTs-Ruthenium dye competition for light absorption resulting in a 81 % ŋ improvement compared to mixed chirality cells. Besides, certain SWCNTs chiralities were ruled out as useful materials for DSSCs applications. By taking advantages of high porosity TiO2 beads, titania affinity for phosphate based moieties and group 8 metals coordination behavior toward pyridyl ligands (like Ruthenium in the N-719 dye used for DSSCs), a high sensitivity (0.3 ppm by naked eye), reusable, Fe2+ colorimetric sensor was developed. A terpyridine based ligand, L (2,2’:6’,2”-terpyridin-4’-phosphonic acid), was used as iron sensitive molecule, whereas screen printed TiO2 beads as scaffold material. Furthermore, L-functionalized Indium Tin Oxide (ITO) thick films were proved to be reliable electrochromic materials and possible candidates for trivalent iron detection predicated on microwaves antennas’ resonant frequency shift. Finally, Multiwalled Carbon Nanotubes (MWCNTs) and Graphene films for resonant frequency tuning of copper etched printed circuit boards antennas were produced and electrically characterized over a large range of operating frequencies.