Obtaining Enhanced Photocurrent by Beneficial Surface Treatment of Self-aligned TiO2 Nanotubes for Dye-sensitized Solar Cells

Abstract

Abstract Electrochemically anodic TiO2 nanotube (TONT) arrays were prepared in ethylene glycol solvent containing 0.25 wt % NH4F and 2 wt % water under constant voltage (60 V) for 1 h. As-grown TONT arrays exhibited an amorphous phase, and thermal treatment was performed at 450 °C for 3 h under air gas to improve the crystallinity. In addition, to develop water-treated TONT arrays, as-grown TONT arrays were directly placed in water for 3 days. After removing the sample from water, thermal treatment was performed under the same conditions. Further, to prepare the TiCl4-treated TONT arrays (denoted as TiCl4/TONT), the annealed TONT arrays were placed in 0.02 M TiCl4 aqueous solution in a 70 °C electric oven for 30 min. Three types of TONT arrays were prepared and characterized: TONT, TiCl4/TONT, and water/TONT. In the case of TiCl4/TONT arrays, the TiCl4 treatment has a well-known ability to create TiO2 nanoparticles on the TiO2 surface, increasing the surface area for dye uptake and beneficial charge-transport properties. Here, the TiCl4/TONT arrays showed high photocurrent due to the high dye uptake, resulting in 18% improvement of photoelectric conversion efficiency (η). Meanwhile, water/TONT arrays provide the largest surface area due to the small nanoparticles composed of the surface in the nanotubular structure. The beneficial electron-transport properties of water/TONT system through c axis led to an enhancement of up to 40%, approaching η of 2.65%, compared to 1.87% for TONT-based dye-sensitized solar cells.