  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 (this 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 in the range 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.
