Improvement of Color Purity in Silver Electrodeposition-Based Multicolor Electrochromic Device

Abstract

Electrochromism (EC) is defined as a reversible color change by electrochemical redox reaction and has several features such as color variation, memory property and so on. Multifunctional electrochromic materials which control multiple colors, various color density, and specular reflection are expected to be potential candidate for energy saving light-modulation device such as smart window and novel reflective display device such as e-paper.We reported silver electrodeposition–based EC device achieving three optical states -transparent, silver-mirror and black- in a single cell.1 Its mechanism was based on the electrodeposition of silver nanoparticles on two facing transparent electrodes, a flat ITO electrode and an ITO particle modified electrode. The EC material, gel electrolyte containing silver ion, was sandwiched by the two electrodes. The device’s default state was transparent, whereas, applying a negative voltage of -2.5 V to one or the other electrode causes the electrodeposition of silver on its surface. When silver was deposited on the flat ITO electrode, the device turns mirror. On the other hand, when silver was deposited on the rough ITO particle modified electrode, the device turns black. The image retention property (memory property) can be improved by inserting ion-exchange membrane2 or by using redox active counter electrode.3 Electrodeposition is an attractive method to create colors because silver nanoparticles exhibit various optical states based on their localized surface plasmon resonance (LSPR). LSPR bands of metal nanoparticles are affected by the size and shape.4 The control of LSPR, therefore, must enable dramatic changes in color for the surface where nanoparticles are deposited. We successfully demonstrate the first LSPR based multicolor EC device enabling reversible control of six optical states such as transparent, silver mirror, cyan, magenta, yellow and black, which is attractive for smart window as well as color e-paper application.5, 6 In this paper, we optimized composition of the EC solution and condition of step voltage application. As a result, we newly enabled silver electrodeposition-based multicolor EC device to achieve the improvement of color purity and green color representation in addition to magenta, cyan and yellow states (Figure 1). In order to discuss the relationship between the morphologies of the electrodeposited silver nanoparticles and the device color, we carried out field emission scanning electron microscopy (FE-SEM) analysis of silver-deposited electrodes and finite-difference time-domain (FDTD) calculation of silver nanoparticles’ models.