Properties of SnO2:F thin films prepared by using HF or NH4F after exposure to atmosphere

Nanocrystalline fluorine doped tin oxide (SnO 2 :F) thin films were produced by the spray pyrolysis technique (SP) on glass substrates at a substrate temperature T s = 450 °C. The hydrated stannous chloride (SnCl 2 .2H 2 O) was used as a precursor and ammonium fluoride (NH 4 F) was used as the doping compound for one set of films, and hydrofluoric acid (HF) was used as the doping compound to produce another set of films. After exposure to atmosphere for about one month, a comparison between the compositional, morphological and optical properties of the films obtained by using the two doping compounds was performed by using scanning electron microscope (SEM) observations, X-ray energy dispersive spectroscopy (EDS) and transmittance measurements. Both sets of films are found to be oxygen rich, but the films prepared by using NH 4 F have higher oxygen content. SEM images showed that the films prepared by using HF have larger grain size, and EDS analysis showed that these films have higher fluorine to oxygen ratio. In addition, the films prepared by using HF have lower transmittance and higher direct optical bandgap energy. These results are interesting for the use of SnO 2 :F  as a fore contact in CdS/CdTe solar cells.