Microstructure and electro-optical properties of Cu–Ni co-doped AZO transparent conducting thin films by sol–gel method

Cu–Ni co-doped Zn_1−xAl_xO (AZO; Al/Zn = 1.5 at.%) films with fixed Ni concentration at 0.5 mol% and different Cu concentrations (0–2.0 % mole ratio) were synthesized on glass substrates using a sol–gel method. The effects of the Cu composition on the structure, electrical and optical properties were examined. X-ray diffraction (XRD) of the Cu–Ni co-doped AZO (CuNi:AZO) films revealed a polycrystalline ZnO phase with a hexagonal wurtzite structure. The stress variation of the CuNi:AZO films were analyzed from the XRD pattern. XPS spectra indicated the existence of two valence states of Cu atoms in Cu⁺ and Cu²⁺ states after N₂/H₂ (95/5) forming gas heat treatment for CuNi:AZO films. Scanning electron microscopy showed that all the films were smooth with a good packing density. The particle size was calculated by both XRD and SEM analysis, and the difference between them has been discussed in detail. Hall measurements indicated that the lowest resistivity of the CuNi:AZO film is approximately 1.16 × 10⁻³ Ω cm at a 1.0 mol% Cu content, which is one order of magnitude lower than that of AZO film (1.01 × 10⁻³ Ω cm) and 43.9 % lower than that of Ni-doped AZO film (2.07 × 10⁻³ Ω cm). All the films exhibited high transmittance in the visible region and showed sharp absorption edges in the UV region. The optical band gap shifted from 3.44 to 3.35 eV with increasing Cu content. This study provides a simple and efficient route for preparing low resistivity and high transparency CuNi:AZO films for optoelectronic applications.