Effect of the grain size on the electrochromic properties of NiO films

The use of electrochromic materials is increasing, and nickel oxide is one of the most studied electrochromic materials. In this work, the effect of the current density and film thickness on the microstructure and electrochromic properties of a nickel oxide film was investigated. The number of cycles affected the optical modulation amplitude of the film. Ni thin films with different deposition parameters were prepared by electrodeposition, and NiO thin films were obtained by oxidization at 300 °C for the same time. When the thickness was 20 nm, an increase in the deposition current density from 1.25 mA/cm2 to 3.75 mA/cm2 increased the optical modulation amplitude by 25%, the colouring time by 2.1 s, and the bleached time by 1.2 s. With increasing current density, an inflection point representing the difference between the grain boundary fading rate and the grain fading rate appeared. When the thickness of the sample deposited at 2.50 mA/cm2 was increased from 10 nm to 50 nm, the optical modulation amplitude increased by 35%, and the cycle response time obviously increased. And the coloured state transmittance of the film could be reduced to 30%. Increasing the film thickness and deposition current density increased the grain boundary surface area of the NiO film. As a result, the optical modulation amplitude and cycle response time of NiO electrochromic films increased.