Hierarchical DSSC structures based on “single walled” TiO₂ nanotube arrays reach a back-side illumination solar light conversion efficiency of 8%

In the present work we introduce a path to the controlled construction of DSSCs based on hierarchically structured single walled, self-organized TiO₂ layers. In a first step we describe a simple approach to selectively remove the inner detrimental shell of anodic TiO₂ nanotubes (NTs). This then allows controlled well-defined layer-by-layer decoration of these TiO₂-NT walls with TiO₂ nanoparticles (in contrast to conventional TiO₂ nanotubes). We show that such defined multiple layered decoration can be optimized to build dye sensitized solar cells that (under back-side illumination conditions) can yield solar light conversion efficiencies to the extent of 8%. The beneficial effects observed can be ascribed to a combination of three factors: (1) improved electronic properties of the “single walled” tubes themselves, (2) a further improvement of the electronic properties by the defined TiCl₄ treatment, and (3) a higher specific dye loading that becomes possible for the layer-by-layer decorated single walled tubes.