Crystallographically preferred oriented TiO2 nanotube arrays for efficient photovoltaic energy conversion

We describe the fabrication of crystallographically preferred oriented TiO2 anatase nanotube arrays (p-NTAs) and the characterization of their photovoltaic properties. The preferred orientation to the (004) plane of the TiO2 nanotube array (NTA) was carefully controlled by adjusting the water content in the anodizing electrolyte; similar to 2 wt% of water yielded a p-NTA, whereas other contents of water yielded randomly oriented NTAs (r-NTAs). A structural analysis using X-ray diffraction and a high-resolution transmission electron microscope revealed that the p-NTA showed a preferred orientation along the [001] direction of the anatase crystal structure. When the NTAs were employed to dye-sensitized solar cells (DSSCs) as photoelectrodes, the p-NTA showed a similar electron lifetime to the r-NTA, which was an order of magnitude higher than that for a TiO2 nanoparticle (NP) film. Moreover, the p-NTA exhibited faster electron transport than the NP film, and even faster than the r-NTA. These properties resulted in a longer electron diffusion length of the p-NTA, compared to the r-NTA and NP film, thereby improving the charge collection property of the photoelectrode. The p-NTA exhibited superior photovoltaic energy conversion performance in the DSSC system, and showed a higher thickness for the optimal photovoltaic performance compared to the NP film, which were attributed to the excellent charge collection properties. Our results address that the crystallographic orientation of NTAs improves their charge transport properties, which can be applied to various optoelectronics, especially to solar-driven energy conversion devices.