3 research outputs found

    Hyperbranched Quasi-1D TiO<sub>2</sub> Nanostructure for Hybrid Organic–Inorganic Solar Cells

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    The performance of hybrid solar cells is strongly affected by the device morphology. In this work, we demonstrate a poly­(3-hexylthiophene-2,5-diyl)/TiO<sub>2</sub> hybrid solar cell where the TiO<sub>2</sub> photoanode comprises an array of tree-like hyperbranched quasi-1D nanostructures self-assembled from the gas phase. This advanced architecture enables us to increase the power conversion efficiency to over 1%, doubling the efficiency with respect to state of the art devices employing standard mesoporous titania photoanodes. This improvement is attributed to several peculiar features of this array of nanostructures: high interfacial area; increased optical density thanks to the enhanced light scattering; and enhanced crystallization of poly­(3-hexylthiophene-2,5-diyl) inside the quasi-1D nanostructure

    TiO<sub>2</sub> Nanotubes: Interdependence of Substrate Grain Orientation and Growth Rate

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    Highly ordered arrays of TiO<sub>2</sub> nanotubes can be produced by self-organized anodic growth. It is desirable to identify key parameters playing a role in the maximization of the surface area, growth rate, and nanotube lengths. In this work, the role of the crystallographic orientation of the underlying Ti substrate on the growth rate of anodic self-organized TiO<sub>2</sub> nanotubes in viscous organic electrolytes in the presence of small amounts of fluorides is studied. A systematic analysis of cross sections of the nanotubular oxide films on differently oriented substrate grains was conducted by a combination of electron backscatter diffraction and scanning electron microscopy. The characterization allows for a correlation between TiO<sub>2</sub> nanotube lengths and diameters and crystallographic parameters of the underlying Ti metal substrate, such as planar surface densities. It is found that the growth rate of TiO<sub>2</sub> nanotubes gradually increases with the decreasing planar atomic density of the titanium substrate. Anodic TiO<sub>2</sub> nanotubes with the highest aspect ratio form on Ti(−151) [which is close to Ti(010)], whereas nanotube formation is completely inhibited on Ti(001). In the thin compact oxide on Ti(001), the electron donor concentration and electronic conductivity are higher, which leads to a competition between oxide growth and other electrochemical oxidation reactions, such as the oxygen evolution reaction, upon anodic polarization. At grain boundaries between oxide films on Ti­(<i>hk</i>0), where nanotubes grow, and Ti(001), where thin compact oxide films are formed, the length of nanotubes decreases most likely because of lateral electron migration from TiO<sub>2</sub> on Ti(001) to TiO<sub>2</sub> on Ti­(<i>hk</i>0)

    Chemical Bonds and Charge-Transfer Dynamics of a Dye–Hierarchical-TiO<sub>2</sub> Hybrid Interface

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    The adsorption of Zn-tetraphenylporphyrin (ZnTPP) on nanoporous hierarchically organized anatase TiO<sub>2</sub> structures and the properties of the corresponding hybrid interface were studied by synchrotron radiation experiments. The molecular structure, electronic properties, and bonding with nanostructured TiO<sub>2</sub> surfaces were analyzed by photoemission (XPS and UPS) and X-ray absorption spectroscopy (XAS). The charge transfer at the interface was investigated by means of valence band resonant photoemission experiments (ResPES) at the C K-edge. We show that the charge-transfer dynamics between the photoexcited ZnTPP and TiO<sub>2</sub> is strongly influenced by the presence of defects on the TiO<sub>2</sub> surface. On a stoichiometric anatase nanostructure, ZnTPP bonding occurs primarily via carbon atoms belonging to the molecular phenyl rings, and this creates a preferential channel for the charge transfer. This phenomenon is reduced in the case of defective TiO<sub>2</sub> surface, where ZnTPP interacts mainly through the molecule macrocycle. Our results represent a surface science study of the dye molecule behavior on a nanoporous TiO<sub>2</sub> photoanode relevant to dye-sensitized or hybrid solar cell applications, and they show the importance of the surface oxidation state for the charge-transfer process