Solution-processable electrochromic materials and devices: roadblocks and strategies towards large-scale applications

Electrochromic devices (ECDs) that modulate optical transmission or reflection in a controllable manner are attractive in energy-saving and colour-tuning applications. In this review, we discuss the key components, fundamental working principles and performance metrics of ECDs using layman's terms. We also highlight recent materials development of each component and illustrate how the structure-property relationships impact their processability, device performance and stability. We further introduce emerging materials that exhibit electrochromic properties, and discuss the transition from laboratory-scale, half-cell-based characterisations of electrochromic materials to the fabrication of solid-state electrochromic devices and evaluation of their performances. Finally, we lay out major challenges for fabricating large-scale and cost-effective ECDs, particularly the materials design and fabrication strategies for achieving chemically and electrochemically stable, high performance ECDs that can be produced by roll-to-roll techniques.