Fabrication of dye-sensitized solar cells using a both-ends-opened TiO₂ nanotube/nanoparticle hetero-nanostructure

We fabricated a hetero-structure photoanode with both-ends-opened $TiO_2$ nanotubes and $TiO_2$ nanoparticles for dye-sensitized solar cells (DSSCs). Two-step anodization and selective etching process were used to obtain the both-ends-opened $TiO_2$ nanotubes. A heat treatment was employed to obtain anatase crystal-structured nanotubes. The effect of the both-ends-opened $TiO_2$ nanotubes on the photoelectric conversion efficiency (PCE) of the DSSCs was evaluated by positioning the $TiO_2$ nanotubes on $TiO_2$ nanoparticle layers and by sandwiching them between the $TiO_2$ nanoparticles. It is suggested that both-ends-opened $TiO_2$ nanotubes serve as incident light scattering layers that lead to superior light harvesting efficiency of DSSCs. This increases the electron lifetime and results in the recycling of more light by dye molecules, thus increasing the photocurrent density. Consequently, a high PCE of 7.74% was obtained when the $TiO_2$ nanotubes were placed on the $TiO_2$ nanoparticles and a PCE of 8.56% was obtained when the $TiO_2$ nanotubes were placed between the $TiO_2$ nanoparticles.