Tunable optical and structural properties of Zn(S,O) thin films as Cd-free buffer layer in solar cells

Zn(S,O) thin films were grown by three step process: (1) the deposition of Zn layer on glass substrates by vacuum thermal evaporation, (2) the oxidation of the as-deposited Zn layer in air atmosphere and (3) the sulfurization of the oxidized samples in N2 atmosphere. The samples were analyzed for their microstructural, morphological and optical properties. XRD analysis revealed that the films were polycrystalline with a highly c-axis orientation. EDX measurements suggested that the sulfur content of the films could be inflected by the sulfurization temperature. The optical constants of the films were calculated from the analysis of the transmittance and reflectance data. The band gap energy varied from 3.27 to 3.08 eV depending on the sulfur content. The surface morphological studies revealed that the films had an average roughness between 13.1 and 16.9 nm. Zn(S,O) thin films exhibited n-type electrical conductivity.