Although electrochromic (EC) “smart” windows are recently making inroads into consumer applications, any large-scale usage needs further maturity in balancing their cost, processability and usability. A flexible and mountable EC device that can be refitted with a normal glass pane has potential to expand the market for existing buildings, by eliminating the requirement of replacing the entire window. Any EC based tunable windows have several functional components such as active/counter electrodes, conducting back panel and electrolyte. One must adopt a viable low temperature based fabrication process for the transition metal oxide (TMO) films with sufficient cycling stability for the intended application. At the same time, a mechanically robust semi-solid/solid polymer electrolyte (SPE) with adequate thermochemical range must be developed for operational safety. In this work, we demonstrate the use of ultraviolet (UV, 365 nm) photoannealing process for TMOs specially optimized to support flexible ECD operations [Figure-1]. This inexpensive and facile process step enabled us to manipulate nanoscale transport properties to extract preferred functionalities. A significant network densification is observed as photo-decomposition of bridging ligands facilitated direct interactions of surface dipoles in those films that produced ~57% improvement in the optical density (OD) [1]. The free-standing SPEs was designed by dispersing Li-salts into polymer matrix followed by thermochemical crosslinking of the host [2,3]. The assembled ECDs were investigated using electrochemical and spectroelectrochemical techniques to identify the sources of device degradations. A typical device fabricated with optimized components showed remarkably high OD (~0.65) and coloration efficiency (CE) ~122 cm²/C at 600 nm.

Keywords: Electrochromic devices, photo-annealing, polymer electrolyte, mountable foils

[1] Gayathri P.T.G., Sajitha S., Vijitha I., Shaiju S.S., and Biswapriya Deb “Tuning of Physical and Electrochemical Properties of Nanocrystalline Tungsten Oxide through Ultraviolet Photoactivation”, Under Revision (2017)

[2] Gayathri P.T.G., Remya R., Biswapriya Deb, “A pragmatic approach to methyl methacrylate based solid polymer electrolyte processing: A case study for electrochromism”, Sol. Energ. Mat. Sol. Cells 140 17–24 2015

[3] Gayathri P.T.G. and Biswapriya Deb “Designing an All-Solid-State Tungsten Oxide Based Electrochromic Switch with a Superior Cycling Efficiency” Adv. Mater. Interfaces 2017, 1700124

Figure-1: TMO based flexible ECD fabrication; optimization of components through electrochemistry and spectroelectrochemistry studies.