Morphological studies of vertically aligned TiO2 nanotubes for high efficiency solar cells

TiO₂ nanotubes (TiNT) were fabricated for different anodization time viz. 1, 2, 3, 4 and 5 hours at a constant DC voltage (60V). Effect of anodization time on the tube parameters such as diameter, length, effective surface area, tube density and diameter distribution profile were studied. It was observed that increase in anodization time leads to an increase in diameter and length of nanotubes. It was also observed that there is a decrease in number of tubes per unit area i.e. tube density with increased anodization time. Increase in diameter and length reflects that there should be an increase in overall effective surface area. In contrast, decrease in tube density is suggestive that surface area should decrease as there is less number of tubes per unit area. However, calculation of effective surface area per projected area showed an overall increase in the surface area. This is conclusive that effect of increased diameter and length plays dominant role over decreased tube density. PV results also indicated that longer anodization time leads to better photovoltaic performance. Enhancement in PV performance was attributed to the dominant effect of increased effective surface area leading to high dye uptake and hence increased charge injection efficiency. Also, increase in fill factor (FF) was observed at longer anodization time which can be attributed to low recombination due to decreased surface states. The decrease in surface states can however be attributed to decrease in tube density at longer anodization time. This result is representative that higher anodization time can lead to high injection efficiency with reduced recombination leading to better photovoltaic performance.