SYNTHESIS AND CHARACTERIZATION OF IRON-DOPED TITANIUM DIOXIDE THIN FILMS

Abstract

Titanium dioxide (TiO2) is a very promising thin film photovoltaic (PV) material since it is abundant, chemically stable and non-toxic. It has however, a comparatively large bandgap (3.0–3.2 eV), which only allows absorption within the ultraviolet region, making it less effective for use in solar energy conversion. The wide study of Fe is caused by its potential to enhance the optical characteristics of TiO2 by the creation of impurity energy levels that aid in the absorption of visible light. In this work the 3 mol% iron doped titanium dioxide (Fe: TiO2) thin films were deposited on clean soda￾lime glass substrates by spin coating method. The influence of the volume of precursor solution, spinning time and number of deposited layers on the structural and optical properties were investigated and the stability of the optimized films were examined for 28 days. The X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and UV-Vis spectroscopy with Tauc plot analysis were used to characterize the deposited films, after annealing at 450 °C for 2 h. All samples showed the formation of mixed phases of anatase and rutile TiO2 by XRD analysis. The best volume of precursor solution was 1.0 mL, which had the largest crystallite size of 4.107 nm and a smaller full width at half maximum (FWHM) value of 1.992°. By changing the spinning time, it was found that 30 s gives the optimum crystallinity, the crystallite size was 5.206 nm and the FWHM was 1.571° which is the best crystal growth from the uniform film formation. Optimum layer number was 5 and resulted in high crystalline films with better uniformity of the structure. FTIR spectra showed the presence of both Ti–O–Ti and Ti–O–Fe bonds, which confirmed the incorporation of Fe in the TiO2 lattice. The results of the UV–Vis analysis revealed that the optical band gap depended on the deposition parameters. The band gap of 2.96 eV obtained for the optimum spinning time of 30 s was lower than the typical band gap of 3.2 eV of undoped TiO2, which was attributed to the Fe induced impurity energy levels that narrowed the band gap and increased the visible light absorption. The optimized films showed only minor changes in the peak position, FWHM and crystallite size for the 28-day time period, which gave evidence of good structural stability. The results of this study show that the parameters of spin coating can be optimized to obtain the best structural and optical characteristics for 3 mol% Fe doped TiO2 thin films. Specifically, films with improved crystallinity, lower band gaps and enhanced stability were generated after spinning for 30 s using 1mL precursor solution volume and 5 layers were deposited onto the surface, which are promising materials for use in better PV devices.