Ordered networks of ZnO-nanowire hierarchical urchin-like structures for improved dye-sensitized solar cells.

Quasi-1D ZnO nanowires (NWs) ordered as patterned 3D hollow hierarchical urchin-like structures have been prepared on transparent conducting substrates by electrodeposition. The ZnO NWs have been grown on self-assembled ordered polystyrene microspheres with electrical charge densities ranging from 5 to 30 C cm(-2) and organized arrays of mono and multi-urchin layers have been built. These layers have been sensitized by the highly absorbing D149 indoline organic dye. The optical characterizations and dye titrations have shown a significant increase in the light scattering and absorption as well as dye loading for the organized structures compared to randomly vertically aligned ZnO NWs grown under the same conditions. The dye-sensitized solar cells (DSSC) prepared using the sensitized layers have been characterized by current-voltage (J-V) measurements, IPCE and by electrochemical impedance spectroscopy. We show that the best performances are obtained for the 3D urchin monolayer structures. The conversion efficiency is increased by up to 4 times compared to their counterparts made of randomly dispersed vertical ZnO NWs. Impedance spectroscopy results show a very fast charge transfer in the ZnO NWs and urchin monolayers and that the electron lifetime is in the 4-14 ms range.