Dye-sensitized solar cells (DSSCs) have received considerable attention lately because they are cost-effective and environmentally friendly with efficiencies comparable to those of the traditional silicon-based cells 1 . Generally, granular titanium dioxide powder is commonly used in dye-sensitized solar cell light anode structure. The sol–gel method is used to produce porous film structure, but small pores form between particles of the transmission path of clutter, resulting in a more dye adsorption capacity and low clutter of electron transfer path. The path is too long and will make the leakage current and the probability of electron recombination, thus affecting the overall conversion efficiency of solar cells. The titanium dioxide nano-tubular structure of high surface area and large aspect ratio can be beneficial to the dye adsorption, and more rules of order can be reduced when the electron and hole in the transmission probability of recombination. TiO₂ nanotubes have been synthesized by various methods including hydrothermal method 2 , seeded growth 3 , template-assisted deposition 4 , and anodization 5 . Especially, anodization is a relatively simple method for synthesizing large-area and self-organized TiO₂ nanotube arrays 6 7 8 . In this paper, we used the electrochemical anodization to prepare TiO₂ nanotubes arrays with different thickness and applied them on the photoelectrode of dye-sensitized solar cells. The TiO₂ nanotubes and solar cells were investigated by field-emission scanning electron microscopy, X-ray diffraction (XRD), I V characteristic analyses, and electrochemical impedance spectroscopy (EIS) to study the effect of titanium tetrachloride (TiCl₄) treatment on the photoelectrode of TiO₂ nanotubes for dye sensitized solar cells.

In this study, the growth of nanotubes was anodized on Ti foils (purity of 99.6%, thickness of 0.2 mm) by constant current at 15 mA in the ethylene glycol solution containing 0.3 wt.% NH₄F and 2 vol.% deionized water kept at 20°C. The anodized TiO₂ nanotubes were annealed in oxygen at 450°C for 60 min. For the treatment of TiCl₄, TiO₂ nanotubes were immersed in 0.2 M TiCl₄ solution for 1 h and annealed in air at 350°C for 30 min. Pt counter electrodes were prepared by coating with a drop of H₂PtCl₆ solution and heating at 400°C for 15 min 9 . To adsorb N3 dye, TiO₂ nanotubes were immersed in 3×10⁻⁴ M solution containing N3 dye and ethyl alcohol at 45°C for 8 h in the oven. The working electrodes were then rinsed with ethanol. Electrolyte solution is adopted from Everlight Chemical Industrial Corporation (ESE-20). The electrode was assembled into a sandwich-type open cell using platinum plate as a counter electrode. Both electrodes were spaced by a kind of polymer films. The thickness was 60 μm, and the size of TiO₂ working electrode was 0.25 cm² (0.5 ×0.5 cm). The surface morphology of the TiO₂ nanotubes was observed by scanning field emission electron microscopy. Structural analysis was carried out by powder X-ray diffraction (XRD). The ultraviolet–visible absorption spectrum of the TiO₂ electrodes was observed by a UV–vis spectrophotometer. The current–voltage characteristics and impedance of samples were measured by Keithley 2400 source meter (Keithley Instruments Inc., Cleveland, OH, USA), and EIS was determined under simulated sunlight with white light intensity, P _L = 100 mW/cm².

Figure  1 shows the SEM images of the TiO₂ nanotubes before and after TiCl₄ treatment. Clearly, after the samples were treated with TiCl₄, the walls of TiO₂ nanotubes were coated with TiO₂ nanoparticles, which could increase the surface area of TiO₂. In order to explore the impact of annealing gas on the properties of TiO₂ nanotubes, the samples were carried out with XRD characterization. XRD patterns of TiO₂ nanotubes are shown in Figure  2. It is found that the as-formed TiO₂ nanotubes are amorphous and are converted to anatase after annealing. The oxygen annealed TiO₂ nanotubes have a better anatase phase than that annealed in air. After the treatment of TiCl₄, TiO₂ nanotubes also show a good anatase phase. Figure  3 shows the current–voltage characteristics of DSSCs with the electrodes of different lengths of TiO₂ nanotubes without TiCl₄ treatment. The parameters for the short-circuit current density (J _sc), the open circuit potential (V _oc), the fill factor, and the overall conversion efficiency (η) are listed in Table  1. From the results of Figure  3 and Table  1, it is found that the best conversion efficiency of DSSCs is 4.35%, while the length of TiO₂ nanotubes is 20 μm. The result of conversion efficiency is quite higher than the previous reports 10 11 12 . This may be due to the length of TiO₂ nanotubes in this study, which is quite longer than those of the previous reports. It is advantage to adsorb N3 dye on the TiO₂ nanotubes. Figure  4 shows the current–voltage characteristics of DSSCs with the electrodes of different lengths of TiO₂ nanotubes after TiCl₄ treatment. The parameters for the J _sc, the V _oc, the fill factor, and the η are listed in Table  2. From the results of Figure  4 and Table  2, it is found that the best conversion efficiency of DSSCs can be improved up to 6.58%, while the length of TiO₂ nanotubes is 20 μm.

In order to study the effect of TiCl₄ treatment on the transport properties of TiO₂ nanotubes, the analysis of EIS for TiO₂ nanotubes has been investigated. Figure  5 shows the spectra of EIS for the dye-sensitized solar cells with and without TiCl₄ treatment. The simulation of equivalent circuit is referred to the previous reports 13 14 15 . The parameter R _k, which is represented by charge transfer resistance related to recombination of electrons, is also listed in Table  3. The value of R _k decreases from 26.1 to 17.4 Ω after TiCl₄ treatment. These results indicate that the effect of TiCl₄ treatment on TiO₂ nanotubes can increase the surface area of TiO₂ and the adsorption of N3 dye, resulting in better transport properties of TiO₂ nanotubes and the improvement of conversion efficiency for DSSCs.

In summary, we prepared TiO₂ nanotube arrays by electrochemical anodization to apply on the electrode of dye-sensitized solar cell. After TiCl₄ treatment, the walls of TiO₂ nanotubes were coated with TiO₂ nanoparticles. It can increase the surface area of TiO₂ and the adsorption of N3 dye, resulting in better transport properties of TiO₂ nanotubes and the improvement of conversion efficiency of DSSCs.

DSSCs: Dye-sensitized solar cells; EIS: Electrochemical impedance spectroscopy; I-V characteristics: Current–voltage characteristics; J _sc: Short-circuit current density; η: Overall conversion efficiency; TiCl₄: Titanium tetrachloride; V _oc: Open circuit potential; XRD: X-ray diffraction.

The authors declare that they have no competing interests.

THM wrote this manuscript. YTJ and NYL carried out the preparation of samples. SMC and LWJ carried out the XRD measurements. JKT and TCW carried out the I-V measurements. WRC, WW, and CJH carried out the EIS measurements. All authors read and approved the final manuscript.