Towards superior nickel oxide electrochromic films using Si and Li co-doping and rapid thermal annealing

Highlights: • High quality electrochromic nickel oxide film is realized by Si and Li co-doping. • The optical modulation of Si-LiNiO_x thin film can reach 37% after RTA treatment. • The Si-LiNiO_x thin film after RTA has rapid bleaching and coloring response time. • The Si-LiNiOx thin film maintains high EC stability after RTA treatment. • The Si-LiNiO_x with superior performances is promising for high quality EC devices. -- Abstract: Herein, we firstly report a novel technical routine to improve the electrochromic (EC) performance of frequently-used nickel oxide counter-electrode films by RF magnetron sputtering using a Si and Li co-doping oxide target in Ar atmosphere and subsequently rapid thermal annealing (RTA) at 400 °C in air. The effects of co-doping and RTA treatment on the phase structure, microstructure, chemical states of Ni element, and EC properties of films were comprehensively investigated using X-ray diffraction, scanning electron microscope, X-ray photoelectron spectroscopy and electrochemical analysis. Comparing with the undoped and the as-deposited films, it is found that the mutual effects of Si and Li co-doping and RTA treatment can promote a preferential growth in (111) direction of nickel oxide, enhance the relative atomic concentration of Ni³⁺ to 60% in the total Ni element, and help to form a smooth and compact structure with uniformly distributed fine pinholes. The doped and annealed film behaves with the highest bleached transmittance of 93.3% (at 550 nm), largest optical modulation of 37.0% (at 550 nm), biggest charge capacity of 14.8 mC• cm⁻², best electrochemical stability to bear more than 100 cycles, and rapid bleaching and coloring response times of 2.4 and 8.8 s. These superior performances make this kind of material promising for future application in high quality EC devices.