The current global energy crisis has compelled users to seek alternative energy sources such as solar energy. It is generally cheap and renewable source thus a key consideration for sustainable future development. In particular, dye-sensitized solar cells (DSCs) has attracted significant attention due to their cost-effective, lightweight, and flexibility. The conversion efficiency is evaluated by the product of the short-circuit current density, open-circuit voltage and fill factor. Unfortunately, titanium oxide-based DCSs is leveled off in efficiency due to short electron diffusion length in the network. However, one-dimensional titanium oxide nanostructures like nanorods and nanotubes are promising solutions to solve this problem of DSCs.
Presently, various techniques like oxidation of titanium metal in an electrolyte are used to prepare the one-dimensional TiO2 nanostructures. Even though vertically aligned nanotubes to the conducting substrate exhibit high efficiencies, desired efficiency level have not been realized in TiO2 nanotube (TNT) arrays due to the limited dye loading surface area. Therefore, researchers have been looking for alternative methods for preparation of TiO2 nanotube arrays and have identified liquid phase deposition (LPD) method as efficient solution.
Shizuoka University researchers: Raimu Endo (currently a PhD candidate), Dr. Hirulak D. Siriwardena, Atsuyoshi Kondo, Chisato Yamamoto and Professor Masaru Shimomura fabricated TiO2 nanotubes arrays of different lengths using the liquid phase deposition method. Their main aim was to fabricate dye-sensitized solar cells using the resulting TiO2 nanotube arrays and to clarify the relationship between the surface characteristics and the energy conversion performance. Their research work is published in the journal, Applied Surface Science.
Briefly, the authors conducted their experimental work by employing a one-step zinc oxide nanorods (ZNR) template technique prepared at low temperatures in aqueous solution. Etching of ZNR arrays, however, occurred at the same time with the formation of one-dimension TiO2 nanotube arrays. The height of the template was controlled desirably to control the length of the nanotubes. Furthermore, they used X-ray photoelectron spectroscopy to analyses the influence of zinc species on TiO2 nanotube surface. Eventually, they investigated the relationship between the TiO2 nanotube surface properties with the characteristics of the dye-sensitized solar cells and resulting zinc residue.
The research team observed that some zinc residue species were attracted to the TiO2 nanotube array surface thus enhancing the open circuit voltage of dye-sensitized solar cells. For instance, the recorded maximum voltage of 0.876V is similar to the theoretical value of TiO2 based dye-sensitized solar cells. This was attributed to the dissolution of the template thus negatively shifting the quasi-Fermi TiO2 level. Consequently, a significant decrease was observed in the photocurrent density accompanied with a slight increase in the rate of dye adsorption especially for the sample with a high amount of zinc.
Professor Masaru Shimomura and colleagues successfully analyzed the effects of the efficiency of the dye-sensitized solar cells. The formation of fine particles TiO2 nanotubes improved the adsorption of the dye due to the increased surface area. Generally, the effect of zinc residue on the surface of TiO2 nanotube array is of great importance in determining the device efficiency. For instance, the fabricated device produced a maximum efficiency of 3.62% at TNT arrays thickness of 2.8µm. The study will, therefore, advance design and fabrication of highly efficient dye-sensitized solar cells.
Endo, R., Siriwardena, H. D., Kondo, A., Yamamoto, C., & Shimomura, M. (2018). Structural and chemical analysis of TiO2 nanotube surface for dye-sensitized solar cells. Applied Surface Science, 439, 954-962.Go To Applied Surface Science