Efficiency enhancement of electrochromic performance in NiO thin film via Cu doping for energy-saving potential

Abstract

Construction of low-cost and high-efficiency electrochromic electrodes with high color contrast and fast response time in electrochromic applications have led to a proliferation of studies in metal oxides. This research study presents a comprehensive overview of the electrochemical synthesis of nanoporous Cu-doped nickel oxide films on ITO-coated glass substrate and its electrochromic behavior in alkaline medium. X-ray diffraction analysis and scanning electron microscopy are used to determine the phase and morphology of the produced films. The films indicate uniform and good adhesion to the substrate. The electrochromic behavior of the deposited films is tested by means of cyclic voltammetry, chronocoulometry, repeating chronoamperometry, and electrochemical impedance spectroscopy measurements. The color change from dark brown to transparent appears reversibly well suitable under sequential potential from -0.2 to +1.0 V. After the Cu atoms incorporate into the host NiO matrix, noticeably enhancements are observed in optical modulation (57.1% at 550 nm), coloration efficiency (13.78 cm(2)/C) and response time (t(b) = 2.26 s and t(c) = 1.77 s) compared to its as-prepared NiO films. Energy level diagrams including fermi level, valance band edge and conduction band edge positions are presented based on the Mott-Schottky approximation and Tauc's method using absorbance data for the NiO and Cu:NiO films. The negative slope of the Mott-Schottky plot shows p-type conductivity for the films, and acceptor density is found as 4.44 x 10(19) cm(-3) and 5.41 x 10(19) cm(-3) for the NiO and Cu:NiO films, respectively. An equivalent electronic circuit is fitted using the measured Nyquist data of the NiO or Cu:NiO semiconductor/electrolyte system, and the calculated values of the circuit elements show significant sensitivity to the Cu doping. Therefore, our results make a major contribution to the research on energy-saving smart windows by demonstrating analyses in detail.