Efficient dye-sensitized solar cells based on TiO₂ nanoparticles and skein-like nanotubes: Effect of arrangement modes of the layers and TiCl₄ treatment

Herein, we report a double-layer photoanode electrode composed of TiO₂ nanoparticles active layer and a mixture of nanoparticles and newly proposed skein-like nanotubes (NTs) top layer. Such structure performs well in solar light harvesting due to first, the top layer exhibits superior light scattering and second, the active layer with high dye-loading can make better use of the back-scattered light. The results reveal that 30-40 nm nanoparticles have anatase structure, whereas 5-7 μm skein-like NTs show a mixture of anatase and rutile phases. Photovoltaic measurements demonstrate that by tuning mass ratio between nanoparticles and skein-like NTs, the cells based on single-layer photoanode without pre- and post-treatment with TiCl₄ solution exhibits an optimum solar efficiency of 6.99%. Furthermore, the cells based on double-layer photoanodes show faster interfacial electron transfer and slower charge recombination process than that based on single-layer photoanodes. These advantages incorporated with employment of TiCl₄ treatment render the cells based on double-layer photoanode exhibiting superior performance under AM1.5G simulated solar irradiation. As an example, TiCl₄ pre-treated cell composed equal content of nanoparticles and skein-like NTs in the top layer and pure nanoparticles in the active layer shows cell efficiency of 7.88%.