Microstructure control of dual-phase inkjet-printed a-WO3/TiO2/WOX films for high-performance electrochromic applications

The microstructural aspects related to crystalline or amorphous structure of as-deposited and annealed films of sol-gel-derived WO3 are shown in the literature to be critical for electrochromic (EC) performance. In consideration of ion insertion materials, there is a need for developing light and at the same time nanocrystalline structures to improve both coloration efficiency and switching kinetics. By controlling microstructure and morphology, one could design a material with optimal EC performance. This report compares the microstructural and morphological characteristics of standard WO3 wet deposition techniques versus inkjet printing technology (IPT), correlating these features with their optical and electrochemical performances, emphasizing the importance of the dual-phase a-WO3/TiO2/WOX film composition proposed in this work for high-performance EC applications. The effect of the type and content of metal oxide nanoparticles in the precursor sols formulated in various peroxopolytungstic acid (PTA) and oxalic acid (OAD) proportions on film properties is comprehensively studied using multi-factorial design of experiment (DOE). To the authors' knowledge, no other report on sol-gel deposition of inorganic EC materials via the inkjet printing technique exists, in which furthermore the film crystallinity can be controlled under low-temperature process conditions. The proposed method enables development of EC films which irrespective of their composition (a-WO3, a-WO3/TiO2 or a-WO3/TiO2/WOX) outperform their amorphous or nanocrystalline analogues presented as the state-of-the-art due to their superior chemical and physical properties.