Chemical Composition and Resistivity of Sprayed CuInS2 Thin Films for Solar Cells

CuInS₂ thin films were prepared on glass substrates with the spray pyrolysis technique. The films were sprayed at 390degC using different ratios of x = [Cu] / [In] in the aqueous solutions with the objective of studying their compositional, structural, optical and electrical properties. The values of the Cu/In ratio in the aqueous solutions have been changed in the range 0.66 - 1.5. SEM photographs show that the surface of films prepared with low values of x were smooth, formed by small crystallites, while the films grown with higher values of x present rough surfaces constituted by larger crystallites. The X-ray diffraction show that films grown with low x values are composed by CuInS₂, In₆S₇, In₂S₃ and CuIn₅S₈ phases, while the Cu-rich films present only the CuInS₂ phase with the sphalerite structure. The chemical composition has important influence in the grain size of the films since it is observed that the grain size increases as copper concentration in the films increases. The optical band-gap of the Cu-poor films is larger than for Cu-rich films possibly by the quantum confinement of carries inside the small crystals. The electrical resistivity for the Cu-poor films is high, around 10⁶ Omega.cm, while the Cu-rich films present relatively small values, around 0.06 Omega-cm. This result can also be explained as due to the difference in grain size for the different layers grown from different precursor solutions. The above results will allow us to select the most appropriate conditions for preparing CuInS₂ thin films by spray pyrolysis for their use in hetero-junction solar cells.