Facile Fabrication of Trimodal Switchable Mirror Device with Zero Transmittance in the Black State

Abstract

Electrochromic material reversibly demonstrates the colored state and the bleached state by the electrochemical redox reaction upon application of external voltage. Tungsten oxide (WO3) is one of the most widely used electrochromic (EC) material. When negative voltage is applied, WO3 is colored with the intercalation of cations such as Li+ ions in the electrolyte. Among various types of EC devices, EC mirror device can demonstrate transparent, mirror, and black state. These EC mirror device contains Ag ions in its electrolyte, thus demonstrating the mirror state by the reversible electrodeposition of uniform and dense Ag film onto the flat surface of the electrode. In contrast, the black state is achieved by uneven electrodeposition of Ag NPs onto the rough surface of the EC layer. The device can achieve the black state using the localized surface plasmon resonance (LSPR) effect. The spatially separated Ag NPs with large size distribution results in scattering and diffraction of the incident light, increasing its absorbance. Conventionally, EC layer can be fabricated using solution based processes, such as sol-gel and electrodeposition, or sputtering method. However, since these methods are difficult to fabricate EC layer with rough surface, the device can not achieve the black state with zero transmittance. In this study, we have fabricated WO3 film with high surface roughness of 34.04±0.39 nm as EC layer using Nanoparticle Deposition System (NPDS). Since NPDS is a solvent-free thin film fabrication process based on dry deposition of powder directly onto the substrate, it is easy to control the thickness and roughness of the film, and thus the surface morphology. We have successfully demonstrated the trimodal switchable mirror device using the dry deposited WO3 film. The mirror state has achieved its reflectance over 70% throughout the visible light range, by the formation of smooth Ag film onto the flat electrode. In contrast, Ag NPs were electrodeposited onto the rough surface of the WO3 film, resulting in the complete blockage of the incident light showing its transmittance of nearly 0% throughout the entire visible light range. It was confirmed that the mirror state was formed by dense and uniform deposition of Ag NPs, so that the surface roughness and the size of the Ag NPs for mirror state were measured to be 8.85±0.33 nm and 27.7±5.42 nm, respectively. On the other hand, the LSPR effect caused by uneven electrodeposition of Ag NPs, was produced at its black state, so that its surface roughness and the size of Ag NPs for the black state were measured to be 59.50±4.60 nm and 109.62±47.52 nm, respectively. The electrochromic properties such as switching speed, have been evaluated. The switching speed for the mirror state and the black state was measured to be 35 seconds and 16 seconds, respectively. Moreover, the transmittance up to 100 cycles switching was measured at its wavelength of 680 nm. As a result, the transmittance for the transparent state was confirmed to be over 70% up to 100 cycles switching. The black and the mirror states were also confirmed to be stable, showing its transmittance of 7-8%. Moreover, the reflectance of the mirror state after 100 cycles switching was measured to be over 60%, indicating that the device is stable for 100 cycles switching. Moreover, the electrochemical stability was confirmed for the 100 cycles of switching based on cyclic voltammetry analysis. In order to identify the coloration mechanism of the device in relation to roughness, the device using flat WO3 film prepared by sol-gel method was fabricated in comparison with its surface roughness, which is the key parameter for LSPR production. The device using flat WO3 film demonstrated the black state with its transmittance of approximately 7~9%. Since the device using flat WO3 film could not achieve the complete black state unlike the device using rough film reported in this study, it was confirmed that the dry deposited WO3 film with rough surface is the key parameter to achieve nearly zero transmittance along with LSPR effect. Therefore, we have introduced dry deposition method to be the facile fabrication method of the trimodal switchable mirror device with high optical contrast, resulting in zero transmittance during black state.