TiO₂ hollow spheres and their implication for nonvolatile electrolyte-based dye-sensitized solar cells

For practical application of dye-sensitized solar cells (DSSCs), it is prerequisite to ensure long-term stability. While the DSSCs employing volatile solvent electrolytes show a high efficiency of over 11%, they have a poor stability due to the evaporation of electrolyte or leakage of solvent. On the other hand, non-volatile electrolytes with high stability such as ionic liquids, oligomers, conducting polymers, and hole conductors have been reported as alternatives to volatile solvent, employing these materials to DSSCs has limitations in their efficiency compared with using volatile solvent electrolytes. Recently, the novel structures of a photoelectrode have been reported in the use of nanowires, nanotubes, and hollow spheres. In particular, the electrodes containing a hollow structure have shown significant potential to possess an outstanding light-harvesting efficiency. Moreover, the hollow structure of the electrodes would have an additional advantage owing to its large porosity - good permeability of non-volatile electrolytes with high viscosity. In this study, we report a novel approach preparing the electrodes consisting of TiO₂ hollow spheres. The TiO2 hollow spheres electrode shows a high area density and desirable thickness without cracks. Applying the TiO₂ hollow spheres electrode to the DSSCs with non-volatile electrolytes, the solar cells exhibit superior light-harvesting efficiency and long electron diffusion length, demonstrating that the structure of TiO₂ hollow spheres electrodes has a large advantage to improve the efficiency of DSSCs with non-volatile electrolytes.