TiO2/AZO bilayer thin films by magnetron sputtering as transparent electrodes for electrochromic devices

Transparent electrodes are dominated by a single material indium tin oxide (ITO). Indium has been classified as a critical raw material by the EU, thus some alternative transparent conductor materials are urgently required. Aluminum zinc oxide (AZO) has emerged as promising substitute due to its high transmittance and low resistivity as well as its low cost and resource availability. However, manufacturing processes for some optoelectronic devices such as electrochromic devices or organic light emitting diodes, chemically attack the AZO layers due to their instability against acidic and basic solutions. Chemical and environmental stability of AZO coatings must be improved to guarantee long-term stability of the devices. In this paper, electrochromic devices based on viologen modified nanostructured TiO2 layers have been manufactured on bilayer electrodes formed by sputtering TiO2 protective films on AZO layers. A thin sputtering TiO2 layer does not electrically insulate the AZO but improves its stability and protects it from being attacked during the deposition of the nanostructured sol-gel TiO2, enabling the fabrication of EC devices that switch properly between a transparent, colorless and a green coloured state. Optical contrast (ΔT %) values up to 40% have been reached with an operating voltage of −1.5 V.