  In the present work we grow anodic TiO$_2$ nanotube layer with tube diameter
~ 500 nm and an open tube mouth. We use this morphology in dye-sensitized solar
cells (DSSCs) and show that these tubes allow the construction of hybrid
hierarchical photoanode structures of nanotubes with a defined and
wall-conformance TiO2 nanoparticles decoration. At the same time, the large
diameter allows the successful establishment of an additional (insulating)
blocking layer of SiO$_2$ or Al$_2$O$_3$. We show that this combination of
hierarchical structure and blocking layer significantly enhances the solar cell
efficiency by suppressing recombination reactions. In such a DSSC structure,
the solar cell efficiency under back side illumination with AM1.5 illumination
is enhanced from 5% neat tube to 7 %.
