TiO2 nanotube arrays on substrates (either Ti or transparent conducting glass) provide a superior platform for building up ordered heterojunctions for enhanced charge separation and electron transportation. The parameters of the TiO2 nanotubes can be finely tuned, allowing
the optimization of device fabrication for different types of solar cells, such as DSSCs and TiO2 nanotube/polymer solar cells. Anodization conditions are being further optimized for desired structures, for example, tunable nanotube morphology, diameter, thickness and inter-tubular spacing, towards perfect alignment by the formation of top porous nanotubes. In terms of the surface area, TiO2 nanotubes are still incomparable with TiO2 nanoparticles. Different strategies have been developed to enhance the surface area, such as double-walled tubes or bamboo-type nanotubes, the creation of secondary structures on smooth tube walls (TiO2 nanoparticle decoration) or the fabrication of nanotubes on micro patterned substrates etc. A number of
synthetic strategies have been developed to introduce nanocrystals or nanocrystal combinations onto TiO2 to extend the spectrum absorption of TiO2 ; to create chemical bonding between p-type conducting polymers and the TiO2 surface. As some of the approaches have begun to
establish, the next step would be to use them for targeted device design with interesting materials. With the advance of novel sensitized dyes with high extinction constants and good stability, these dyes deserve trials on novel TiO2 nanotubular structures, so that their promising solar cell efficiencies can be explored. The deposition of p-type materials inside nanoporous TiO2 NTs can form perfectly ordered p–n heterojunctions, which induces amazing flexibility in the TiO2 NTs/polymer hybrids and is capable of fabricating flexible device in a single step. How to control the regularity during electrodeposition to enhance the conduction mobility remains a challenge and will require extensive trials.
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