Structural, morphological, and optical properties of TiO2 thin films synthesized by the electro phoretic deposition technique
© Ghraïri and Bouaïcha; licensee Springer. 2012
Received: 27 April 2012
Accepted: 18 June 2012
Published: 1 July 2012
In this work, we report the structural, morphological, and optical properties of TiO2 thin films synthesized by the electro phoretic deposition technique. The TiO2 film was formed on a doped fluorine tin oxide (SnO2:F, i.e., FTO) layer and used as a photo electrode in a dye solar cell (DSC). Using spectroscopic ellipsometry measurements in the 200 to 800 nm wavelengths domain, we obtain a thickness of the TiO2 film in the range of 70 to 80 nm. Characterizations by X-ray diffraction and atomic force microscopy (AFM) show a polycrystalline film. In addition, AFM investigation shows no cracks in the formed layer. Using an ultraviolet–visible near-infrared spectrophotometer, we found that the transmittance of the TiO2 film in the visible domain reaches 75%. From the measured current–voltage or I-V characteristic under AM1.5 illumination of the formed DSC, we obtain an open circuit voltage Voc = 628 mV and a short circuit current Isc = 22.6 μA, where the surface of the formed cell is 3.14 cm2.
The EPD [9–12] is one of the colloid processes in ceramic production. It was discovered since two hundred years ago (1808) and patented in the USA in 1933. This technique was applied in multiple domains; ceramics, coatings, nanoscale assembly, etc [9–12]. It has many advantages such as the homogeneity of the formed film and its fast deposition velocity; it does not require any complex equipment and permits the deposition of thin films on different surface architectures  and different textures. The thickness of the film can be controlled via the applied voltage and the deposition duration .
In our case, TiO2 nanoparticles have a spherical shape in the suspension solution. When deposited by the EPD technique, the non violent arrangement of spherical particles of TiO2 on FTO creates void regions (inter spheres); which in turn, gives to the TiO2 film its porous structure, with a very high internal surface, which is very useful and may be crucial for the number of dye molecules that we can insert in a DSC and enhance its efficiency.
In this work, the thickness of the formed film is about several tenths of nanometers and was carried out using spectroscopic ellipsometry, as well as the refractive index, n, and the extinction coefficient, k, in the 200 to 800 nm wavelength range. The structural properties showing a polycrystalline aspect is carried out by X-Ray diffraction (XRD). Morphological investigations using atomic force microscopy (AFM) show no cracks in the film. In addition, the transmittance of the TiO2 film in the visible domain reaches 75%. After studying these properties, we fabricate a DSC on which we obtain an open circuit voltage Voc = 628 mV and a short circuit current Isc = 22.6 μA, where the surface of the cell is 3.14 cm2.
Thin TiO2 films are deposited on FTO substrates using the EPD technique (Figure 1). We used TiO2 powder from Aldrich (Sigma-Aldrich Corporation, St. Louis, MO, USA) where 99.7% of the particles have dimensions less than 25 nm. The electrolyte solution is composed of a mixture of 0.02 g of TiO2 nanopowder with 30 ml of isopropanol and 10 ml of acetone. Then, we added a solution of iodine (I2) dissolved in 5 ml of acetone and 0.5 ml of acetyl-acetone. The electrolyte was dried and ultrasonicated just before deposition. To enhance the TiO2 nanoparticles adhesion, the FTO film was first cleaned and exposed to UV-irradiation during 1 h. One of the electrodes is a transparent conductive oxide film, where we used FTO (SnO2:F). The latter was deposited on a glass substrate by the pyrolitic technique, and the second electrode is in aluminum (Al) deposited by thermal evaporation. A constant voltage of 80 V was applied between the two electrodes during 5 s. The used voltage and duration are optimized values. Hence, different biases and durations were used by studying the transmittance and the resistivity of the elaborated films. After deposition, the TiO2 film was annealed in air at 450 °C during 1 h to enhance the interconnection between nanoparticles. We notice that obtained film has a high adhesion to the FTO substrate. This may be explained by the generated bonding strength between the film and the substrate created during the UV-irradiation of the FTO substrate.
Results and discussion
Obtained thicknesses of mediums considered in Figure 4
Medium 1: TiO2/void
Medium 2: porous TiO2
Medium 3: SnO2/TiO2
Medium 4: SnO2:F
Medium 5: glass
where d is the thickness of the film (Table 1) and T is its transmittance.
It is established that TiO2 has direct and indirect band gaps . To determine values of these forbidden energies, we use the expression in Equation 2.
In this paper we give the structural, morphological, and optical properties of TiO2 films which are formed using the EPD technique. By analyzing the elaborated film by means of XRD, AFM, ellipsometry, and UV–vis-NIR spectrophotometry, we established that EPD technique permits the formation of a film that could be useful for DSC application. Hence, the XRD characterization shows polycrystalline films. The AFM investigations show no cracks in the film. The spectroscopic ellipsometry analysis permits us to deduce the thickness of the TiO2 film which we found in the range of 70 to 80 nm, as well as the refractive index, n, and the extinction coefficient, k, in the 200 to 800 nm wavelength range. In addition, we found that the transmittance of the TiO2 film in the visible domain reaches 75% in a large spectral range.
From the I-V characteristic measured at AM1.5 of the formed DSC, we obtain Voc = 628 mV and Isc = 22.6 μA. Though we remark that due to the fact that we used aluminum in the counter electrode instead of platinum, we observed the degradation on the counter electrode of the DSC. This degradation might create a leakage current, leading to a decrease of the shunt resistance of the cell, as it can be observed in the I-V curve for low voltage.
Authors would like to thank the reviewers for their critical revision and the editor for his contribution in improving the manuscript.
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