Large-Diameter TiO 2 Nanotubes Enable Wall Engineering with Conformal Hierarchical Decoration and Blocking Layers for Enhanced Efficiency in Dye-Sensitized Solar Cells (DSSC).

Herein, we grew anodic TiO2 nanotube layer with a 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-conformal TiO2 nanoparticle decoration. At the same time, the large diameter allows the successful establishment of an additional (insulating) blocking layer of SiO2 or Al2 O3 on the tube wall. We showed 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 was enhanced from 3 % for the neat tube to 5 % for the hierarchical structure to 7 % when additionally a blocking layer was attached.