Showing papers by "Ramón Tena-Zaera published in 2013"

ZnO/ZnO Core–Shell Nanowire Array Electrodes: Blocking of Recombination and Impressive Enhancement of Photovoltage in Dye-Sensitized Solar Cells

Abstract: A multistep wet-chemistry route was developed, by combining electrodeposition, colloidal synthesis and spin coating, to obtain arrays of ZnO nanowires (NWs) coated by a ZnO nanocrystalline layer (i.e., ZnO/ZnO core–shell NWs). They were integrated as anodes in dye-sensitized solar cells. With the use of an iodide-based electrolyte, photovoltages as impressive as 870 mV were obtained with an enhancement of more than 250 mV with respect to devices based on bare NWs. A comprehensive device characterization study by means of impedance spectroscopy (EIS) and intensity-modulated photovoltage spectroscopy (IMVS) reveals a significant blockage of recombination upon NW shell deposition. To study the generality of this multistep method, electrodes of core–shell nanostructures based on commercial ZnO nanoparticles were also prepared. A decrease of recombination rate is also detected, although it is much more moderate than the observed for nanowire-based electrodes. The present ZnO synthetic approach allows obtaining...

ZnO–ionic liquid hybrid films: electrochemical synthesis and application in dye-sensitized solar cells

Abstract: An innovative ionic liquid (IL)-based synthesis route was developed to obtain ZnO-based hybrid nanostructured films with a modified surface. In particular, ZnO–IL hybrid films, with thickness from 1.5 to 4.5 μm and an appealing sponge-like morphology, were obtained from the electrochemical reduction of NO3− in ionic liquid-based electrolytes containing Zn2+. The presence of the ionic liquid moieties and/or derivatives in the as-deposited films was demonstrated by Fourier transform infra-red spectroscopy and energy dispersive X-ray spectroscopy. However, a relatively soft thermal annealing (i.e. 1 hour in air at 350 °C) was proved to be an effective way to remove the ionic liquid content from the samples, leading to porous ZnO films with high specific surface area. In comparison to ionic liquid-free samples, a blue shift of ∼40 meV in the absorption onset is detected for the ZnO-PYR14TFSI hybrid films, which suggests a modification of the bandgap. Both kind of films (i.e. ZnO–IL and ZnO) were sensitized with an indoline dye, coded D358, and evaluated as photoanodes in Dye-sensitized Solar Cells (DSCs). Although less D358 was adsorbed onto ZnO–IL samples (suggesting co-adsorption of the IL and D358), a clear enhancement (by a factor of ca. 2) of the power conversion efficiency was detected in DSC based on ZnO–IL hybrid films. This improvement was mainly due to a huge increase (∼180 mV) in the photovoltage, which reaches values of up to 780 mV. The DSC characterization, by electrochemical impedance spectroscopy and open circuit voltage decay techniques, indicates that the photovoltage improvement is likely due to a negative displacement of the conduction band in hybrid films. Therefore, the use of metal oxide–IL hybrid anodes appears to be a promising strategy to increase the open circuit voltage of the DSCs.

Nanomorphology influence on the light conversion mechanisms in highly efficient diketopyrrolopyrrole based organic solar cells

Abstract: In this work, diketopyrrolopyrrole-based polymer bulk heterojunction solar cells with inverted and regular architecture have been investigated. The influence of the polymer:fullerene ratio on the photoactive film nanomorphology has been studied in detail. Transmission Electron Microscopy and Atomic Force Microscopy reveal that the resulting film morphology strongly depends on the fullerene ratio. This fact determines the photocurrent generation and governs the transport of free charge carriers. Slight variations on the PCBM ratio respect to the polymer show great differences on the electrical behavior of the solar cell. Once the polymer:fullerene ratio is accurately adjusted, power conversion efficiencies of 4.7% and 4.9% are obtained for inverted and regular architectures respectively. Furthermore, by correlating the optical and morphological characterization of the polymer:fullerene films and the electrical behavior of solar cells, an ad hoc interpretation is proposed to explain the photovoltaic performance as a function of this polymer:blend composition.

Dense TiO 2 films grown by sol–gel dip coating on glass, F-doped SnO 2 , and silicon substrates

Abstract: Exceptionally dense titanium dioxide (TiO2) films were prepared via dip coating from a sol containing poly(hexafluorobutyl methacrylate) as the structure-directing agent. The films were grown on glass, F-doped SnO2, and crystalline silicon (111) faces, either pure or with a thin layer of SiO2. The TiO2 films cover perfectly even rough surfaces, which was ascribed to thixotropic properties of the precursor gel. The films provide antireflection function to crystalline Si wafers for photovoltaic applications. The optical reflectance in visible to near-infrared (NIR) wave lengths region is considerably smaller for Si wafers covered by TiO2/SiO2 film compared with that of SiO2/Si. The dense TiO2 films are amorphous with small amount of anatase and monoclinic TiO2(B). These two phases withstand calcination at 900 °C in films deposited on Si. For comparison, porous TiO2 films were grown by the same dip-coating protocol, but with alternative organic additives, either polymers or ionic liquids.

Face-Selective Etching of ZnO during Attachment of Dyes

Abstract: Scanning force microscopy and near-edge X-ray absorption spectroscopy are used to study the attachment of Protoporphyrin IX dye molecules to the low-index single-crystal faces of ZnO, namely, the polar (0001) face and the nonpolar (1010) face. We found that surface etching depends on the crystal face for various dye immobilization procedures: whereas the polar (0001) surface remains nearly unaltered, the nonpolar (1010) surface is strongly etched by acidic solutions; these results demonstrate the selective attachment of dye molecules to ZnO surfaces, showing that the dye attachment is extremely sensitive to both the exposed facet and the immobilization protocol. This could be a factor in the surprisingly poor efficiency observed for ZnO-based dye-sensitized solar cells, especially those using ZnO nanorod arrays.