The preparation of high-performance FTO thin film by Sol-Gel-evaporation method

Abstract

The purpose of this work is to prepare the high-performance transparent conductive thin films of fluorine-doped tin oxide (FTO) by using a simple technological process. The FTO thin films are formed in the period of calcination process combined with the advantages of sol-gel method and chemical vapor deposition method, which not only avoids the shortcomings of film cracking in sol-gel coating process, but also reduces the cumbersome traditional dip-coating method and spin-coating method on glass substrates, largely simplifying process and cutting costs. The FTO thin films are deposited onto glass substrates by the sol-gel-evaporation method with SnCl45H2O as a tin source, and SnF2 as a fluorine source. The effects of F-doping content and the structure of the film on the properties of FTO film are mainly studied. The prepared films are characterized by IR, DTA-TG, XRD, TEM, SEM, etc. The results show that the maximum performance index (TC) of the FTO film, the lowest surface resistance of 14.7 cm-1, and the average light transmittance of 74.4% when F/Sn=14 mol% are achieved under the conditions of the reaction temperature of 50 ℃, the reaction time of 5 h, sintering or evaporation temperature of 600 ℃ for 2 h. It is indicated that part of O is replaced by F, and SnO2-xFx crystal structure is formed. It reveals that the crystal structure is polycrystalline and has a preferential orientation along the (110) direction and the spacing between the lattice fringes is about 0.33 nm in the FTO film. And the particles in the FTO film present a tetragonal rutile phase with an average size of 20 nm and a film thickness of 1.22 m. Fractal dimension of image by dealing with SEM image of FTO film shows that the surface resistance decreases with the decreasing of fractal dimension, which in fact critically demonstrates the lower barrier. The lower the barrier, the smoother the surface of the thin films is. So the fluorine concentration is the main factor affecting the properties of FTO thin film. Too much or too less fluorine is not conducive to the growths of SnO2-xFx crystals. And then the three-dimensional information such as structure, particle shape and size of the FTO thin film is also the factor influencing the FTO film properties. The analysis of SEM shows that the surface morphology of the thin film is in the pyramid-shaped structure, which is beneficial to improving the utilization of photons, and well used in the optoelectronic devices.