Electrochemical design of ZnO hierarchical structures for dye-sensitized solar cells

ZnO single-crystalline nanowires (NWs) are highly relevant 1D-nanostructures for efficient charge collection in dye sensitized solar cells (DSSCs). In a first part of the paper, ZnO-NWs have been grown by electrochemistry on various fluorine-doped transparent oxide (FTO) coated glass substrates. The effects of substrate treatments and of the presence of an underlayer of TiO2 or ZnO on the cell performance are reported. In particular, we show that the presence of a TiO2 underlayer gives rise to a dense array of ZnO-NWs with a high aspect ratio (>30) and to the cells with the best performances. However, the roughness of the NW arrayed films were not sufficient for the efficient harvesting of sunlight and low fill factors were found. In a second step, hierarchical structures have been designed by growing electrochemically a secondary nanoporous epitaxial ZnO phase on the wire surface. We have found that the cell performances were significantly improved in that case due to a much larger specific surface area and to a much better fill factor. We also show that the cell efficiencies of hierarchical structures did not follow the performances of the starting ZnO-NWs. The volume occupied by the second phase, its thickness and its connection to the wires are important parameters for cell optimizing.