Durability-enhanced monolithic inorganic electrochromic devices with tantalum-doped nickel oxide as a counter electrode

Nickel oxide (NiO) is a well-known electrochromic (EC) material with anodic coloration. However, NiO degrades significantly upon cycling with Li+/H+-conducting electrolytes. Doping with various additives is the most effective method to enhance the cycling stability of NiO films. In this study, films of tantalum-doped nickel oxide, denoted as Ni1-xTax oxide, were deposited onto indium tin oxide (ITO)-coated glass substrates by reactive DC magnetron co-sputtering from Ni and Ta metal targets for use as counter electrodes in monolithic inorganic EC devices (ECDs). The influence of the Ta content on the composition, structure, optical properties and EC properties of NiO was investigated. It was found that the microstructure of the Ni1-xTax oxide films was closely related to the EC performance and cycling stability. With moderate optical modulation, the cycling stability and optical transmittance of the Ni1-xTax oxide film with x = 0.274 were increased. All-solid-state inorganic ECDs were fabricated with the configuration ITO/WO3/Ta2O5/Ni1-xTax oxide/ITO/glass. The full ECDs showed different EC behaviors compared to those of the Ni1-xTax oxide single layers, which was attributed to the different counter electrode/ion conductor interfaces of the solid–solid and solid–liquid interfaces, respectively. The ECD fabricated with the Ni1-xTax oxide film with x = 0.065 showed stable transmittance modulation up to 1000 cycles. We propose a degradation mechanism for monolithic all-solid-state inorganic ECDs based on the observed degradation in the EC performance, which can pave the way for highly durable ECDs for various optoelectronic devices.