An investigation on electron behavior employing vertically-aligned TiO2 nanotube electrodes for dye-sensitized solar cells

Anodically grown TiO2 nanotubes (TONTs), approximately 13 mu m thick, were prepared on an ethylene glycol-based electrolyte containing 0.25 wt% NH4F with extremely small amounts of water. A Ti substrate was pretreated electrochemically, which affected the TONT surface morphology. The TONT (abbreviated as two-step TONT) grown on the pretreated substrate showed a uniform surface morphology with an interconnected nanotubular structure, while the surface morphology of the TONT (abbreviated as one-step TONT) formed on the bare substrate was quite rough. The photocurrent (8.4 mA cm(-2)) of the two-step TONT-based dye-sensitized solar cell (DSSC) was improved by 14% compared to that (7.2 mA cm(-2)) of the one-step TONT-based DSSC. This improvement was attributed mainly to the increased light capturing efficiency, that is, light absorbance by a dye-sensitized TONT film. The discrepancy between the increasing light capturing yield (21%) and overall photocurrent (14%) was attributed to the slower electron transport rate as a result of the large surface area and lateral movement along the three-dimensional network. Therefore, the improved photocurrent of the two-step TONT-based DSSC led to an enhancement (12.5%) of the overall power conversion efficiency.