Effect of In-doping on electrochromic behavior of NiO thin films

Undoped and Indium doped Nickel Oxide (NiO) thin layers with different doping content (2, 4, 6 at.%) were successfully deposited on glass and Indium-doped Tin Oxide (ITO) substrates using spray pyrolysis (SP) technique. The effect of indium on the microstructural, morphological, optical, and electrochromic behaviors of NiO films was investigated by X-ray diffraction (XRD), Raman spectroscopy, Scanning Electron Microscopy coupled with Energy Dispersive X-ray Spectroscopy, UV-Vis-NIR spectrophotometry. XRD examination shows that all NiO layers have a polycrystalline cubic phase with a preferred orientation along the (111) plane. Raman spectroscopy proves the XRD result. SEM images show nanograins formation in all the sprayed films. The EDS analysis and the elemental mapping reveal the homogeneous distribution of the elements. The optical analysis reveals an average transmission of about 72% in the visible light region and an optical band gap energy of 3.54 eV for the undoped sample. These values decreased with increasing Indium content to reach 65% and 3.46 eV, respectively for the 6 at.% In-doped sample. The electrochromic behavior of NiO films deposited on ITO substrates was measured by cyclic voltammetry in a 2 M KOH aqueous electrolyte. The results show that a specific capacitance of 66 F. g -1 at 5 mV.s -1, an optical density variation of 43%, and a coloration efficiency of 93.74 cm2/C were obtained for 2 at.% In doped NiO. These values are considerably higher than those obtained for the undoped films, indicating that the In doping improved the electrochromic performance of nickel oxide films.