An investigation on electron behavior employing vertically-alignedTiO2 nanotube electrodes for dye-sensitized solar cells

Abstract

Anodically grown TiO2 nanotubes(TONTs), approximately 13 µm 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, whichaffected the TONT surface morphology. The TONT (abbreviated as two-step TONT) grownon the pretreated substrate showed a uniform surface morphology with an interconnectednanotubular structure, while the surface morphology of the TONT (abbreviated asone-step TONT) formed on the bare substrate was quite rough. The photocurrent (8.4 mA cm−2) ofthe 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 theincreased light capturing efficiency, that is, light absorbance by a dye-sensitized TONTfilm. The discrepancy between the increasing light capturing yield (21%) and overallphotocurrent (14%) was attributed to the slower electron transport rate as aresult of the large surface area and lateral movement along the three-dimensionalnetwork. Therefore, the improved photocurrent of the two-step TONT-basedDSSC led to an enhancement (12.5%) of the overall power conversion efficiency.