The effect of fluorine and tungsten co-doping on optical, electrical and structural properties of tin(IV) oxide thin films prepared by sol-gel spin coating method

Fluorine and tungsten co-doped tin(IV) oxide (WFTO) thin films have been prepared first time by a sol gel spin coating method. The effect of F and W co-dopant ratio on optical, electrical and structural properties of SnO2 was investigated. It was found that the optical properties of the films were obviously affected by the co-dopant ratio. When the F:W co-dopant ratio increases from 0.25:0.25 to 0.75:0.50, the transmittance in the short wavelengths slightly increases and transmittance edge shifts towards shorter wavelength. With an increase in the co-dopant ratio, the reduction and red shifts in the optical transmittance edge are very obvious at all wavelengths. Transmittance values at 550 nm for the films have varied between 79.64% and 50.20%. The indirect and direct band gap values for WFTO-1, WFTO-2, WFTO-3 and WFTO-4 samples were calculated to be 3.73, 3.79, 3.48, 3.40 eV and 4.03, 4.04, 3.98, 3.97 eV, respectively. The crystal structure of the films has been investigated by X-ray diffraction patterns. It has been observed that WFTO-1 and WFTO-2 samples have (111) preferential orientation corresponding to SnO2 cubic phase. This orientation almost disappears and changes to tetragonal phase (110) orientation for WFTO-3 and WFTO-4. To the best of our knowledge, this is the first cubic structure observation for SnO2 grown by the sol gel technique. The electrical properties were also changed with co-doping ratio. The best optical, electrical and structural properties were obtained for mole ratio 0.75:0.50 (F:W).