Towards the Solid-State electrochromic devices: Platform based on transparent and flexible solid polymer electrolyte

Flexible electrochromic devices (ECDs) are garnering significant interest as they have potential applications in free-form displays and wearable electronics. Typically, liquid electrolytes were used to prepare ECDs. However, liquid electrolytes are susceptible to leakage, especially when cracks develop during their use. Solid electrolytes can avoid such leakage and use for the assembly of solid-state ECDs. This study aims to create a solid-state ECD with a solid polymer electrolyte (SPE) via a solution casting method. Tungsten trioxide (WO3), lithium bis ((trifluoromethyl)sulfonyl) azide (LiTFSI) and polyvinyl alcohol (PVA) were used as electrochromic (EC) material, ion source and solid-state ion source container, respectively. Furthermore, succinonitrile (SN) and phosphoric acid (H3PO4) were added into PVA/LiTFSI to increase the ionic conductivity, which allowed Li+ to move to the WO3 layer at a faster rate. The resulting SPE exhibited an electrochemical window of 4.0 V, room temperature ionic conductivity of 3.4 x 10-4 S/cm, a glass transition temperature of 38 degrees C, and high mechanical flexibility (500 % strain at a stress of 2 MPa). The assembled solid-state flexible ECDs demonstrated an optical contrast of 41 % at 650 nm, responses of coloration to 49 % in 17 s and bleaching to 90 % in 43 s, coloration efficiency of 70.12 cm2/C and less than 5 % degradation of optical contrast after 1000 cycles. This work presents a simple approach to fabricating flexible and transparent solid-state ECDs without the risk of liquid leakage.