Effect of Concentration of Reactants and Deposition Temperature on the Optical Properties of Iron-Doped Cadmium Stannate Thin Films Deposited On Glass Substrates by Spray Pyrolysis

Abstract

Thin films (TFs) have numerous applications in the modern technological world, ranging from energy production in TF solar cells/ photovoltaic (PV) cells to liquid crystal displays (LCDs) in television screens and in smartphone touch screens as transparent conducting oxides (TCOs). The most commonly used TCO material is indium tin oxide (ITO). However, ITO has two main drawbacks. Firstly, indium is expensive and secondly, it is rare. These drawbacks call for relatively cheaper compounds like cadmium stannate (Cd2SnO4). The TFs of Cd2SnO4 have been researched on and proved to possess excellent optical properties (transmittance, absorption coefficient and band gap), which are superior to most of the conventional TCO materials, including cadmium telluride (CdTe), copper-indium-gallium-diselenide and the ITO. The study of optical properties of Cd2SnO4 doped with metals such as zinc and yttrium has been done. However, the study of optical properties of Cd2SnO4 doped with Fe has not been done. This research thus studied the optical properties of Cd2SnO4 doped with Fe, deposited on glass substrates at temperatures of 350 °C, 400 °C and 450 °C. The specific objectives of the study were to determine the effect of (i) concentration of reactants and (ii) deposition temperature on the optical properties of Cd2SnO4: Fe. The Cd2SnO4 was prepared by mixing solutions of cadmium acetate and tin II chloride, both dissolved in ethanol at ratios of 0.1:0.1M, 0.2:0.2M and 0.3:0.3M, forming a white precipitate. 3ml of 2M hydrochloric acid was then added to the precipitate, after which it dissolved to form the colourless solution of Cd2SnO4. The Cd2SnO4 solution was then mixed with iron III chloride (the dopant) dissolved in ethanol at 0-8% concentration by volume of Cd2SnO4 and then sprayed onto the preheated microscope glass substrates by spray pyrolysis. The optical reflectance and transmittance were measured using spectrophotometer in the ultraviolet-visible-near-infrared wavelength range of 300-1100 nm, from which the optical constants were determined. It was found out that an increase in the concentration of the reactants reduced transmittance and band gap, but increased absorption coefficient, extinction coefficient and refractive index, which was. An increase in the deposition temperature increased transmittance and band gap but decreased absorption coefficient, extinction coefficient and refractive index, which was attributed to increase in crystallinity of the TFs. Doping reduced transmittance and band gap, but increased refractive index, absorption coefficient and extinction coefficient, which was attributed to increase in the free charge carriers. At the upper end of the visible spectrum, the values of the optical constants obtained were: transmittance of up to 78.45%, absorption coefficient of 4.022-5.522 x 104 cm-1, extinction coefficient of 0.188-0.258, refractive index of 1.77-2.12 and band gap of 3.5-3.9 eV. The results obtained in this study provide a good alternative antireflective material for use in: front panel of TF solar cells, smartphone touch screens and television LED screens, since the TFs have high transmittance of ~ 80%, a high absorption coefficient of x 104 cm-1 and a large band gap of up to 3.9 eV, which are the ideal properties of TCO materials