Double-Wall TiO2 Nanotubes for Dye-Sensitized Solar Cells: A Study of Growth Mechanism

One–dimensional TiO₂ nanotubes (TNTs) are promising semiconductor photoanodes for dye–sensitized solar cells (DSSCs) due to their rapid electron transportation property, however, the lack of high surface area restricts their application. Double–walled TiO₂ nanotubes (DWTNTs) were synthesized to compensate for the smaller surface area of single–walled TiO₂ nanotubes (SWTNTs) and the comprehensive growth mechanism of DWTNTs is clearly demonstrated in this study. The evolution of pores and pits in three dimensions causes the formation of DWTNTs. After optimizing the thickness of DWTNTs, the DSSC with DWTNTs on the flexible Ti substrate showed a higher power conversion efficiency (η) of 6.85%, as compared to that of the cell with SWTNTs (4.63%), owing to a significant increase in the short–circuit photocurrent density (J_SC) caused by the higher surface area for dye adsorption for the former case. Electrochemical impedance spectroscopy (EIS), incident photon–to–electron conversion efficiency (IPCE) curves, and UV–vis absorption spectra were conducted to sustain the explanations.