Long K-doped titania and titanate nanowires on ti foil and fluorine-doped tin oxide/quartz substrates for solar-cell applications

Potassium-doped titania and titanate nanowires are fabricated by moisture-assisted direct oxidation of titanium. The influence of the fabrication conditions on nanowire structure and morphology is investigated. It is shown that the presence of potassium is essential for nanowire formation, while the nanowire structure and morphology are strongly dependent on the fabrication temperature. The longest nanowires (ca. 10 μm) are obtained at 650 °C. At this substrate temperature, nanowires could be produced over a large substrate area both by oxidation of the Ti foil as well as by depositing a Ti film on the substrate (quartz or fluorine-doped tin oxide (FTO)/quartz). Photovoltaic cells based on these nanowires are fabricated. The cell performance is dependent on the nanowire fabrication temperature and the substrate used, as well as on the annealing environment. Short-circuit current densities of I_sc = 3.05 mA cm^-2 and I_sc = 4.97 mA cm^-2 could be obtained for Ti foil and FTO/quartz substrates, respectively, while the corresponding power-conversion efficiencies are η = 0.93% and η = 1.88% (under AM 1.5 illumination, 100 mW cm^-2; AM: air mass).