Fabrication And Characterization Of Tin Oxide-based NFTO-TiO₂ Composite Transparent Conductive Films

Although SnO₂-based transparent conductive films have been extensive used in the fields of low-emissivity glass?Low-E?and thin film solar cells,it is a very urgent task to increae its optical and electrical properties at present.In this paper,some researches have been done in order to further reduce its emissivity.The processing parameters of F doped SnO₂?SnO₂:F,FTO?films were optimized by adjusting the starting materials?fluorine source?,temperature and film thickness.The electrical properties of FTO films were improved by co-doping with Nb and F.Based on first-principles,the cell structure,charge density difference,Bader charge,band structure and electronic density of states of Nb and F co-doped SnO₂?NFTO?were calculated.The NFTO/r-TTO composite film was fabricated by aerosol-assisted chemical vapor deposition?AACVD?with NFTO film as the functional layer and rutile TiO₂?r-TiO₂:Sn,r-TTO?as the transition layer.Low-E glasses with lower emissivity were obtained.Aiming at the problems existing in the application of front electrode of thin film solar cell,a TiO₂ passivation layer was deposited on the FTO glass,which improved the anti-reduction ability of FTO film.A nano-flower shaped surface morphology with higher haze was obtained also.The structure,morphology,optical and electrical properties of prepared thin films were studied by X-ray diffractometer,transmission electron microscope?TEM?,field emission scanning electron microscope,atomic force microscope,ultraviolet-visible spectrophotometer,Fourier infrared spectrometer,four-probe square resistor test and Hall effect meter.The distribution and chemical status of elements in films were analyzed by X-ray energy dispersion spectrum attachments in TEM,X-ray photoelectron spectroscopy and glow discharge spectrometer.The work function and energy level distribution of thin films were studied by ultraviolet photoelectron spectroscopy.The main results are as follows:The best optical and electrical properties of FTO film are obtained when using SnF₂as the fluorine source,substrate temperature is controlled at 500oC,and the film thickness is maintained at 648 nm.The average transmittance in visible region,resistivity and the figure of merit are 76%,3.89×10^-4?·cm and 17.9×10^-3?^-1,respectively.A highly dispersed conduction band bottom is formed in SnO₂ by doping with Nb and F together,Fermi moves up and more delocalized charges appeare around O and F.In addition,Nb and F co-doped SnO₂ film has a reduced lattice distortion compared with the FTO film,the grain size and preferred orientation are largely increased.When the concentration of F in precursor solution is 20 at.%,and the concentration of Nb is 1 at.%,the NFTO film obtain the highest carrier concentration of 7.463×10²⁰ cm^-3 and the lowest resistivity of 3.71×10^-4?cm,the figure of merit of 22×10^-3?^-1 is obtained.The average transmittance of NFTO films in the visible region is about 80%,and the infrared reflectance is about 92-96%.The crystallinity and degree of order of NFTO film are improved by fabrication of NFTO/r-TTO composite film,the mismatch between NFTO film and glass substrate is reduced,and the lattice distortion,structural defects and residual stress in the film are significantly reduces.The work function of r-TTO and NFTO are matched,and the heterogeneous structure formed between them is conducive to the electrons in TiO₂transfer to NFTO film,which increase the carrier concentration of NFTO film.Compared with FTO film,the carrier concentration and Hall mobility of NFTO/r-TTO composite film are obviously improved,the sheet resistance?5?/??and resistivity(2.82×10^-4?)are reduced by 40%and 37.2%,respectively.The average transmittance of the composite film is above 78%,and the reflectance in the mid-far infrared region is significantly improved,emissivity of 0.08 is obtained for NFTO/r-TTO film,which is reduced 33%compared with FTO film.TiO₂ passivation layer with a certain thickness can effectively prevent the underlying FTO film from being reduced by hydrogen plasma.The anti-reductive properties of TiO₂/FTO composite film has significantly improved,presenting a nano-flower shaped appearance,obtaining high surface roughness and haze.A high haze value of 36.7%is obtained when the approximate thickness of the prepared TiO₂ layer is 213 nm.