Properties of fluorine-doped SnO2 thin films by a green sol-gel method

Fluorine doped tin oxide (FTO) films were fabricated on a glass substrate by a green sol-gel dip-coating process. Non-toxic SnF2 was used as fluorine source to replace toxic HF or NH4F. Effect of SnF2 content, 0-10 mol%, on structure, electrical resistivity, and optical transmittance of the films were investigated using X-ray diffraction, Hall effect measurements, and UV-vis spectra. Structural analysis revealed that the films are polycrystalline with a tetragonal crystal structure. Grain size varies from 43 to 21 nm with increasing fluorine concentration, which in fact critically impacts resultant electrical and optical properties. The 500 °C-annealed FTO film containing 6 mol% SnF2 shows the lowest electrical resistivity 7.0×10−4 Ω cm, carrier concentration 1.1×1021 cm−3, Hall mobility 8.1 cm2V−1 s−1, optical transmittance 90.1% and optical band-gap 3.91 eV. The 6 mol% SnF2 added film has the highest figure of merit 2.43×10−2 Ω−1 which is four times higher than that of un-doped FTO films. Because of the promising electrical and optical properties, F-doped thin films prepared by this green process are well-suited for use in all aspects of transparent conducting oxide.