Ultrathin sputtered NiO films for enhanced electrochromic performance in smart windows

Abstract

The performance of electrochromic (EC) devices heavily relies on the properties of both anodic (NiO) and cathodic (WO₃) materials, particularly in achieving superior opaqueness. While extensive research has focused on enhancing the cathodic component (WO₃), the optimization of the anodic component (NiO) has been relatively neglected, resulting in moderate overall device performance at increased processing costs. Here, we address this concern by optimizing the thickness of the NiO anodic layer to an ultrathin 50 nm using an industrially compatible sputtering technique for superior EC performance. During optimization, the NiO thin film samples were deposited using various oxygen and argon ratios for optimizing their EC properties. Furthermore, the effect of thickness on the electrochromic properties of the optimized NiO was studied. Notably, these films exhibited more than 50% optical modulation at the wavelength of 550 nm with a coloration efficiency of 23.7 cm² C⁻¹. Structural, morphological, and optical properties were correlated with the electrochromic properties. Electrolytic studies employing different solutions, such as lithium perchlorate in propylene carbonate, aqueous KOH, and LiOH, revealed the superior electrochromic performance of NiO thin films in aqueous LiOH. Furthermore, the dual functionality of the NiO film is demonstrated by showing its charging and discharging capacity in an ITO/aqueous LiOH/NiO/ITO structure with a voltage of >1.8 V in the charged state. Integration of electrochromic capabilities with energy storage highlights the versatility of these films for advanced smart windows. This work may pave the way towards the commercialization of affordable electrochromic smart windows by enhancing the electrochromic contribution from a complementary anodic NiO film of only 50 nm.