Effect of Li Doping on Structure and Optical Energy Gap of NiO Films

Abstract

This research deals with the study of the structural and optical properties of pure and Li doped NiO films with different ratios ( 0.03,0.06,0.09 mol %) . Thin films of NiO were deposited on glass substrates at temperature 298oK and thickness 200 nm by sol–gel spin coating technique. X-Ray diffraction(XRD) showed that the films are polycrystalline and have cubic structure with a preferred orientation along (111). The Li doping by 0.03,0.06 & 0.09 mol % didn't change the preferable orientation. The average grain sizes of the crystallites (mean crystallites diameter) estimated from XRD data was found to lie in the range of 20.11 – 24.2 nm. Data of atomic force microscope (AFM) indicate that the surface of films is highly smooth.
The optical transmittance value of undoped NiO film reaches to 72% in the Visible and NIR regions, which is important for its applications as window layers in solar cells. The optical transmittance of the films deposited with various concentration of Li shows slightly lower transmittance. The values of optical energy gap for undoped NiO & Li doping by 0.03,0.06,and 0.09 mol % were equal to 3.9, 3.60, 3.54 and 3.47 eV respectively. Other important optical properties such as refractive index, extinction coefficient, , dielectric constant and optical conductivity have been calculated and were found to be increase with increasing doping concentration and thickness.