TiO2 nanotube formation by Ti film anodization and their transport properties for dye-sensitized solar cells
In this paper, we present the synthesis of TiO2 nanotube (NT) arrays formed by anodization of Ti film deposited on a fluorine-doped tin oxide-coated glass substrate by direct current magnetron sputtering. NH4F/ethylene glycol electrolyte was used to demonstrate the growth of stable nanotubes at room temperature. TiO2 NTs as long as 4.8 μm with the high expansion factor to the initial sputtered Ti film (2 μm) were obtained, showing little undesired dissolution of the metal in the electrolyte during anodization. The average pore size and wall thickness of NTs were about 70 and 30 nm, respectively. Structural investigations on the transparent NT arrays reveal the presence of anatase phase after annealing. The NTs were sensitized by the N719 complex and the resultant photoelectrodes were incorporated into dye sensitized solar cells (DSSCs). The conversion efficiency of 1.97 % was obtained under AM 1.5 illumination and the open circuit voltage, short circuit current density and the fill factor were 0.59 V, 6.71 mA/cm2 and 0.50, respectively. Investigation of the electron transport of the DSSCs by electrochemical impedance spectroscopy showed that the electron diffusion length (8.6 μm) was higher than NTs’ height. It was also observed that the electron transport resistance measured in NT DSSC was lower compared to the reported TiO2 nanoparticle one.
KeywordsTiO2 Electrochemical Impedance Spectroscopy Field Emission Scanning Electron Microscopy Image Short Circuit Current Density Electron Diffusion Length
Titanium dioxide nanotubes have proven to be a highly promising material for application in various fields, such as sensing, photocatalysis and photovoltaics [1, 2, 3]. For instance, when used as one-dimensional nanostructure photoanodes in dye-sensitized solar cells (DSSCs), they were found to show higher light harvesting and charge-collection efficiencies compared to their nanoparticle-based counterparts owing to their faster electron transport and slower charge recombination properties [4, 5].
A suitable method for fabrication of TiO2 nanotubes is anodization of metallic Ti. Commonly, high purity Ti foils are used and anodized in aqueous HF-containing solutions or non-aqueous electrolytes containing fluoride ions [6, 7]. However, especially for photovoltaic devices, TiO2 nanotubes on conducting glass are of particular interest. Structures on conducting glass substrates allow front side illumination, i.e. illumination from the substrate side, resulting in reduced loss of incident light intensity compared to cells fabricated from foils .
In this study, we fabricated an anodic TiO2 nanotube film on a transparent conductive oxide (TCO) layer by anodization of a sputtered Ti film in an ethylene glycol solution of ammonium fluoride (NH4F). The structure and morphology of prepared samples were characterized by X-ray diffraction and field emission scanning electron microscopy (FESEM). Finally, the photovoltaic performance of the prepared sample as the DSSC photoanode was investigated and the electron transport and recombination properties of TiO2 NTs were characterized using electrochemical impedance spectroscopy (EIS).
2 Experimental details
2.1 Deposition of Ti films on FTO substrate
The 2 μm titanium films were deposited on FTO coated glass (TEC-15, 15 ohm per square) using DC magnetron sputtering from a 99.99 % pure titanium target. The sputtering chamber was pumped down to the pressure of 2 × 10−3 mTorr before argon gas was introduced. Sputtering was operated in pure argon (23 sccm) and the working pressure was kept at 1.3 mTorr with the cathode power of 100 W. The substrate temperature was 300 °C, and the deposition time was about 150 min.
2.2 Preparation of TiO2 nanotube arrays on FTO substrate by anodization
TiO2 nanotube arrays were fabricated via electrochemical anodization in a two-electrode cell setup using 1.5 × 1 cm2 sample of Ti sputtered FTO/glass as a working electrode and stainless steel foil as the counter electrode. Anodization was performed in the ethylene glycol based electrolyte solution containing NH4F (0.5 wt%) at 50 V. The resulting TiO2 nanotubes on the FTO substrate were rinsed with DI water and ethanol, and then dried in air. Anatase TiO2 was obtained by annealing the anodized samples at 450 °C for 30 min, in air ambient.
2.3 Dye sensitized solar cell fabrication
The DSSCs were fabricated by soaking the nanotube array films in 0.4 mM N719 dye (Solaronix) in acetonitrile for 20 h. Counter electrode was prepared by deposition of Pt on FTO/glass by coating with a drop of H2PtCl6 solution (5 mM in isopropyl alcohol) with repetition of the heat treatment at 400 °C for 15 min. The dye-covered TiO2 electrode and Pt-counter electrode were assembled into a sandwich type cell and sealed with 30 µm surlyn (Dyesol) spacer which was annealed at 120 °C for a few seconds. The cells were filled with a redox electrolyte (I−/I3 −).
2.4 Sample characterization and device measurements
The crystal structure and morphology of as-grown TiO2 nanorods were investigated by X-ray diffraction (XRD) on a Philips X’pert Pro (the Netherlands) using Cu-kα radiation (λ = 1.54 nm), and field emission scanning electron microscopy (FESEM, Hitachi S-4160).
Photovoltaic measurements were performed using simulated AM 1.5 sunlight with an output power of 100 mW/cm2. Electrochemical impedance spectroscopy (EIS) was performed with a potentiostat/galvanostat (PGSTAT 30, Autolab, EcoChemie, the Netherlands) under illumination. The frequency range was explored from 0.01 Hz to 0.1 MHz. The applied bias voltage and ac amplitude were set at open-circuit voltage of DSSCs and 10 mV.
3 Results and discussion
Regular morphology of TiO2 nanotube arrays are revealed in our study, as observed from bottom view of the anodized layer (Fig. 3c). Thus, we can conclude that ordered array of nanotubes are formed beneath this upper nanoporous layer. It can be seen from Fig. 3c that the resulting NTs have average pore size and wall thickness of about 70 and 30 nm, respectively. Cross-sectional view of FESEM images (Fig. 3b) shows that the thickness of TiO2 NTs is about 4.8 μm. The tubes are 2.4 times longer than the thickness of the sputtered Ti film, showing little undesired dissolution of the metal in the electrolyte during anodization. Pilling–Bedworth ratio (“expansion factor”) of a TiO2 grown film to an initial sputtered Ti film is reported to be close to 2.4–2.7 for the TiO2 NTs anodization in viscous electrolyte . Digital images in Fig. 3d clearly show the change of titanium sputtered on FTO/glass substrate after anodization. The anodized area becomes almost transparent after anodization.
Cell performances and electron transport properties of DSSC, using the prepared TiO2 nanotube sample with thickness of 4.8 μm as photoelectrode
Electron transport properties
Compared to the traditional high efficiency (>10 %) TiO2 nanoparticle DSSCs [19, 20], the fabricated TiO2 NT DSSC in this work showed lower power conversion efficiency. The major factor that limits the power conversion efficiency here is the low photocurrent due to limited thickness of TiO2 layer. The Jsc increases with increasing the NTs’ length which can be possibly attributed to the increase in the number of adsorbed dye molecules, and therefore, a higher number of photogenerated electrons. Moreover, the long NTs would also improve the light-harvesting efficiency, which could also be a result of stronger light-scattering effects [5, 21]. However, it should be noted that further increase in the nanotube length may reduce the Jsc due to increase the photoanode thickness beyond the electron diffusion length and hence increase the electron recombination .
Another factor that limits the DSSC efficiency is low fill factor which is reduced with increasing series resistance. Poor contact between the NTs and the FTO substrate is a reason of high series resistance. Modification of annealing step and TiCl4 post-treatment of the NTs would improve the fill factor. In addition, Jsc can be enhanced by TiCl4 post-treatment [20, 22].
By modeling and fitting the EIS data of DSSCs, we can obtain the electron transport parameters in the photoanode, including the electron transport resistance Rt = rtL in the photoanode, the charge transfer resistance Rct = rct/L related to recombination of an electron at the TiO2/electrolyte interface, and the chemical capacitance Cµ = cµL of the semiconductor, where L refers to the film thickness. Moreover, the effective diffusion coefficient Dn = L2/RtCµ of an electron, the recombination lifetime τn = RctCµ and the effective electron diffusion length Ln = sqrt(Dnτn) in the photoanode of the DSSCs were further estimated (Table 1).
From EIS measurement, it is found that the electron diffusion length (Ln) of NT based DSSC, which depends on both the τn and Dn, is around 8.6 μm (Table 1), which is higher than the thickness of the TiO2 film (4.8 μm). However, Wang et al.  reported that the Ln of a DSSC needs to be at least three times the thickness of the TiO2 film in order to collect most of the photogenerated electrons. Electron life time (τn) was obtained as high as 68.7 ms which is comparable with literature values for NT photoanodes .
Tsai et al.  reported the electron transport resistance of 19.2 Ω for the DSSC fabricated using 12.7 μm TiO2 nanoparticle film as the photoanode. In this study, TiO2 NT photoanodes with thickness of 4.8 μm showed lower transport resistance (Rt = 18.87 Ω).
Furthermore, recombination resistant, Rct, was obtained 55.03 Ω that is comparable with literature data , implying that recombination of electrons through the electrolyte is not the dominant effect limiting photovoltaic performance.
In summary, TiO2 NT arrays with a thickness of 4.8 μm and a pore diameter of 70 nm were successfully grown on transparent conductive FTO substrates by anodizing the sputtered Ti films. The anodization was carried out under ethylene glycol based electrolyte containing 0.5 wt%. NH4F at a voltage of 50 V with the high ratio of the thickness of the anodic film to the thickness of the oxidized titanium layer (2.4). The as-grown NTs were amorphous and crystallize mainly in the anatase form by annealing at 450 °C. Then the prepared sample was used as phtoanode to assemble the DSSC, and the open circuit voltage of 0.59 V and a short circuit current density of 6.71 mA/cm2 were obtained. The fill factor and the conversion efficiency were 0.50 and 1.97 %, respectively. The relatively low photocurrent is attributed to limited thickness of NTs. Electrochemical impedance spectroscopy (EIS) showed that the electron diffusion length (8.6 μm) was higher than NTs’ height. It was also observed that the electron transport resistance measured in NT DSSC was lower compared to the reported TiO2 nanoparticle one.
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