The Influence of the Substrate Temperature on the Optical Parameters of Spray-Deposited Fluorine-Doped Tin Oxide (FTO) Thin Films

Tin dioxide (SnO₂) is a transparent conducting oxide (TCO) of several applications, not limited to its use as an electrode in optoelectronic devices and solar cells. It is well known that the optical properties of the TCO affect the conversion efficiency of the solar cell, and that the optical parameters are crucial factors in determining the final device parameters. This work aims to study the effect of substrate temperature (T_s) on the optical parameters of fluorine-doped tin oxide (FTO) thin films, where polycrystalline FTO thin films were fabricated by spray pyrolysis method on glass substrates at different substrate temperatures (T_s = 360–480)°C. The microstructure and composition of the films were explored using X-ray diffraction (XRD), and scanning electron microscopy–energy dispersive spectroscopy (SEM–EDS). Transmittance of the films was recorded at room temperature using ultraviolet–visible (UV–vis) spectroscopy in the wavelength range of 290–1100 nm. Transmittance was used to deduce the absorption coefficient, direct bandgap energy, extinction coefficient, Urbach tail, and reflectance. Then, reflectance was used to calculate the refractive index, real and imaginary parts of the complex dielectric constant, and energy loss. The dependence of these optical parameters on the substrate temperature was investigated. It is found that all these parameters show dependence on substrate temperature.