Elsevier

Applied Surface Science

Volume 253, Issue 15, 30 May 2007, Pages 6508-6511
Applied Surface Science

Structural properties of single and multilayer ITO and TiO2 films deposited by reactive pulsed laser ablation deposition technique

https://doi.org/10.1016/j.apsusc.2007.01.110Get rights and content

Abstract

Indium tin oxide (ITO) and titanium dioxide (TiO2) single layer and double layer ITO/TiO2 films were prepared using reactive pulsed laser ablation deposition (RPLAD) with an ArF excimer laser. The films were deposited on SiO2 substrates heated at 200 and 400 °C. ITO and TiO2 films with uniform thicknesses of about 400 and 800 nm, respectively, over large areas were prepared. X-ray diffraction (XRD) analysis revealed that the ITO films are formed of highly orientated nanocrystals with an average particle size of 10–15 nm. Atomic force microscopy (AFM) observations indicate rough ITO films surfaces with average roughness of 26–30 nm. Pores were also observed. TiO2 films deposited on the prepared ITO films result less crystalline. Annealing at 300 and 500 °C for three consecutive hours promoted formation of TiO2 anatase phase, with crystal size of ∼6–7 nm. From the scanning transmission electron microscope (STEM) images, it can be seen that the TiO2 films deposited onto the prepared ITO films present a relatively high pore sizes with an average pore diameter of ∼40 nm and excellent uniformity. In addition, STEM cross-sectional analysis of our films showed a columnar structure but no evidence of voids in the structure. Therefore, films exhibited large surface area, well suited for dye-sensitized solar cells (DSSC) applications.

Introduction

Dye-sensitized solar cells based on nanostructured TiO2 electrodes are of great interests due to their promising applications. One of the factors, which determine the performance of these cells, is the short circuit between the electrolyte and the conducting support, due to the porous nature of the TiO2 layer. This could not only lower the efficiency of solar cells but also contribute to their faster degradation [1]. Another important issue for DSSC is the use of a large surface area semiconductor to provide sufficient light absorption with only one adsorbed monolayer of dye. The Grätzel group reported on the details of the correlation between the microstructure of TiO2 film and preparation conditions [2]. It has been known that the higher the surface area of TiO2, the higher the current that is generated by the DSSC [3]. It has been demonstrated that the anatase TiO2 films show a very high efficiency due to their large internal surface [4]. On the other hand, the structural properties of the ITO conducting contact are also important both for the TiO2 properties layer and for the DSSC efficiency. This is why the deposition conditions play a fundamental role on the properties of DSSC photo-electrodes. Recently, Cameron et al [5] analyzed the behavior of a thin TiO2 blocking layer under the porous nanocrystalline TiO2 layer. They deposited them using spray pyrolysis following the method developed by Kavan and Grätzel [6]. Single layer deposition techniques have been reported for the ITO [7], [8], [9] and TiO2 films [10], [11], [12]. In this work, we report on the successful deposition of single and multilayer of ITO and anatase TiO2 using RPLAD technique. To the best of our knowledge, this is the first work reporting a RPLAD of double layer ITO/TiO2.

Section snippets

Experimental details

Single layer ITO and TiO2 films were deposited on SiO2 fused-quartz glass at 200 and 400 °C, respectively. The targets were a sintered ITO disk (90 wt.% In2O3 and 10 wt.% SnO2) and a TiO2 disk (99.6% purity). In the case of multilayer ITO/TiO2 thin films, the ITO film was first deposited on the SiO2. The deposited ITO film later acted as a substrate for the TiO2 film deposition. The substrate holder was equipped with an electric heater. The vacuum chamber was equipped with a rotating multi-target

XRD and Raman measurements

The XRD spectra of the single layer ITO films deposited at 200 and 400 °C are given in Fig. 1. The spectra show evident amorphous “humps” peaked around of 2θ = 21.9° which are characteristic of the glass substrate. In the sample prepared with a substrate temperature of 200 °C, Bragg reflections around 34.98°, 37.10°, and 60.00° are observed. The film grows preferentially oriented along the (1 0 0) direction and the mean grain size is evaluated to be ∼10 nm. When the substrate temperature is increased

Conclusion

We deposited high quality single layer ITO and anatase TiO2 thin films as well as multilayer ITO/TiO2 by RPLAD technique. We have determined the experimental conditions necessary to obtain highly orientated ITO film with their a-axis normal to the glass substrate surface. Anatase TiO2 films with high degree of porosity and roughness have been successfully obtained. The pores are expected not only to provide a huge surface, which can support a larger amount of functional materials of dye, but

Acknowledgements

F. Fotsa Ngaffo is very grateful and gives many thanks to Professor Armando Luches for his helpful suggestions and corrections. She gratefully acknowledges support of the TWOWS for the PhD research grant in the University of the Witwatersrand. The XRD measurements were performed in the University of Virginia (thanks to Professor R. Sean Agnew) while the STEM measurements were performed in the Bologna CNR labs. The support of the ICTP-TRIL is acknowledged with many thanks.

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