Electrochromic nickel oxide thin films by a simple solution process: Influence of post-treatments on growth and properties

Nickel oxide is one of the few inorganic compounds that combines p-type behavior and large band gap making it very attractive for numerous optoelectronic applications. A very simple and reproducible method to grow nickel oxide thin films with accurate control of their structural, optoelectronic and electrochromic properties is reported in this work. The procedure consists in spin coating a nickel (II) 2-ethylhexanoate solution, followed by an ultraviolet (UV) treatment and a thermal annealing at moderate temperature (T_anneal < 300 °C). The impact of the UV irradiation and thermal annealing post-treatments on the film formation mechanism and on the morphological, crystalline and optoelectronic properties of the material was explored. Following careful optimization of the post-treatment conditions, the formation of p-type nickel oxide films with high transparency in visible light range, band gap values between 3.6 and 3.8 eV and a doping level up to 8 × 10²⁰ cm⁻³ (as determined by Mott-Schottky measurements) has been achieved. Finally, NiO_x layers presented efficient electrochromic behavior with >500 coloration/bleaching cycles showing negligible degradation.