Effect of open- and close-ended TiO₂ nanotube arrays on transparent conducting substrates for dye-sensitized solar cells application

The application of freestanding TiO₂ nanotubes on the photovoltaic performance of dyesensitized solar cells utilizing front side illumination has been investigated. TiO₂ nanotubes were grown on Ti foil using the anodization process at a constant voltage of 60 V for various anodization times such as 2, 4, and 6 h. Subsequently, the formations of freestanding films of close-ended and open-ended TiO₂ nanotubes were carried out via a second anodization step at 40 and 80 V, respectively. X-ray diffraction analysis revealed the formation of amorphous structure TiO ₂ nanotubes before annealing and anatase phase after annealing at 450°C. The diameter (60-90-nm-inner diameter) and thickness (7-19 lm) of TiO₂ nanotubes were measured from scanning electron microscopy and transmission electron microscopy analysis. The detached TiO₂ nanotube films were affixed on FTO substrates employing anatase TiO₂ paste. The fixation of freestanding TiO₂ nanotubes on FTO substrates can be categorized as three parts namely: close-ended nanotubes downside, close-ended nanotubes upside, and open-ended nanotubes. Among the various anodization times, the cells fabricated using 6 h anodized TiO₂ nanotubes photoelectrode attained excellent photoelectric efficiencies of 6.91, 6.47, and 7.69% for close-ended downside, close-ended upside and open ended, respectively. The DSSCs based on open-ended photoelectrodes achieved the highest photoelectrical conversion efficiency are due to higher dye loading amount.