Oriented hierarchical single crystalline anatase TiO2 nanowire arrays on Ti-foil substrate for efficient flexible dye-sensitized solar cells

Hierarchical anatase TiO2 nanowire (HNW) arrays consisting of long single crystalline nanowire trunks and short single crystalline nanorod branches have been synthesized on Ti-foil substrate via a two-step hydrothermal growth process. The formation of the HNW arrays based on anatase TiO2 nanowire (NW) arrays can be ascribed to the crystallographic relationship between trunk and branch. The power conversion efficiency of dye-sensitized solar cells (DSSCs) based on such a HNW photoelectrode (4.51%) shows a significant enhancement compared to TiO2 nanowire (NW) array photoelectrode (3.12%) with similar thickness (similar to 15 mu m in nanowire length), which can be attributed to more dye loading, superior light scattering ability and comparable electron transport rate for the former. Furthermore, flexible DSSC using TiO2 HNW arrays on Ti substrate as working electrode and transparent PEDOT/ITO-PET prepared via in situ electrodeposition as counter electrode shows a comparable photovoltaic performance to the rigid Pt/FTO-glass cell. A power conversion efficiency as high as 4.32% (J(sc) = 7.91 mA cm(-2), V-oc = 796 mV, FF = 0.69) is obtained for the first time for fully flexible DSSC based on hierarchical TiO2 nanowire arrays and Pt-free counter electrode.