Highly durable electrochromic tungsten oxide thin films prepared by high rate bias-enhanced sputter deposition

Electrochromic (EC) materials are of interest for applications such as smart windows for increased energy efficiency, but their long-term durability remains an issue for widespread implementation and commercialization. In the present work, we compare traditional radio frequency (RF) magnetron-sputtered porous WO3 films produced at high pressures (20 mTorr) with films deposited at lower pressures (1 mTorr) under energetic ion bombardment due to intense substrate biasing. Our results show that above a specific threshold substrate bias voltage of similar to 400 V, the films exhibit not only comparable EC properties and colouration efficiencies with their higher-pressure porous counterparts, but also an exceptional long term durability in a 0.1 M H2SO4 electrolyte, while being deposited five times faster. The observed performance increase is ascribed to the high energy ion bombardment induced effects leading to the formation of a nanocrystalline WO3 matrix, while simultaneously generating the necessary porous microstructure for ionic diffusion. This novel fabrication approach is believed to pave the way towards more cost-effective fabrication of highly durable EC devices and enhancement of their application in different sectors. (C) 2018 Elsevier Ltd. All rights reserved.