Low resistivity and transparent Co-doped NiO thin films synthesized by the sol-gel method: Structural, morphology and photoluminescence studied for optoelectronic applications

Thin films of undoped nickel oxide (NiO) and cobalt-doped nickel oxide (NiO:Co) with different concentrations 1, 3 and 5 at.%, were deposited on glass substrates using the combined sol-gel method to the spin-coating technique. The effect of Co doping concentration on the structural, morphological, optical and electrical properties of NiO thin films was investigated. X-ray diffraction (XRD) analysis revealed that all the films were polycrystalline with cubic structure and have a preferential orientation in (111) direction. Scanning electron microscopy (SEM) analysis revealed that NiO:Co films exhibited a uniform surface morphology with spherical shaped nanocrystalline grains. The energy dispersive X-ray spectroscopy (EDS) spectra confirmed the presence of Ni, O and Co elements in the doped films. Atomic force microscopy (AFM) images revealed that the deposited films have a low roughness value varying between 3.04 and 5.76 nm. All the films showed a high transmittance, ranging from 70 to 85%, in the visible and near-infrared (NIR) regions and the band gap value was reduced from 3.80 eV to 3.76 eV after doping. The room temperature photoluminescence (PL) spectra showed the presence of defects such as nickel vacancies (VNi) playing an important role in improving the conductivity of the films. The lowest electrical resistivity value equal to 1.08 × 10−2 Ω cm and the highest values of figure of merit varying between 6.17 × 10−5 Ω-1 and 1.7 × 10−4 Ω-1 in the visible region were obtained for the sample doped with 3 at. % Co, which can be useful in various optoelectronic applications.