Improved electrochromic performance of a radio frequency magnetron sputtered NiO thin film with high optical switching speed

Abstract

A nickel oxide (NiO) thin film with better reversibility, high optical modulation, and enhanced coloration efficiency with fast switching time was prepared using radio frequency (rf) magnetron sputtering technique. A cubic phase and spherical shaped crystal grain morphology of the NiO film have been determined from structural and surface morphological studies. A micro-Raman study showed two bands around 570 cm⁻¹ and 1100 cm⁻¹ corresponding to the vibrations of Ni–O. An X-ray photoelectron spectroscopic study confirmed the compositional purity and the presence of Ni²⁺ and Ni³⁺ ions in the film. In addition, the observed transmission threshold at about 368 nm revealed the presence of Ni vacancies or the presence of Ni³⁺ ions along with Ni²⁺ ions in the deposited film. The photoluminescence peak located at 368 nm in the UV region is attributed to band-edge emission, which may be due to the transition of Ni²⁺ ions from the top of the 3d⁸ valence band to the bottom of the 3d⁸ conduction band. The electrochromic studies reveal an excellent performance with a highest coloration efficiency of 235 cm² C⁻¹, reversibility of 64%, optical modulation of 58%, coloration time of 1.2 s and bleaching time of 1.1 s. An electrochemical impedance measurement indicates fast lithium ion transfer with fast switching time, which facilitates the improved electrochromic behavior.