As discussed in several works,the overall efficiency-limiting factor in TiO2nanotube-based solar cells is specific dye loading; that is, the specific surface area of the tubes (BET30 m2g1) is considerably smaller than comparable nanoparticulate layers (BET80–100 m2g1). Severalstrategies to increase the specific surface area in tube systemshave been explored, such as the above-mentioned TiCl4 treatment or modifying the tube walls by creating bamboo-type structures or double-walled nanotubes. The geometry of the tube tops is very crucial (see for exampleRef. ), and several approaches have been reported for removing inhomogeneity (surface nanograss) from the tube tops.To further increase the efficiency, Tsuchiya et al.introduced a new concept by showing that using silver-decorated TiO2 nanotubes, an enhancement of the conversion efficiency for DSSCs can be obtained. This effect may either be due to plasmon enhancement or junction formation. Some variations in TiO2 nanotubes, such as that formed by rapid-breakdown anodization (RBA), show promising improvement regarding the photoconversion efficiency . Recently, an anodic self-organized TiO2 mesosponge/nanochannel layer was reported that has a significantly higher specific surface area than tubular layers, and it seems to be capable of outperforming nanotube layers in the field of TiO2-based solar cells and other applications.
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