Fabrication of Complementary Electrochromic Device Based on Ag Deposition / Prussian Blue: Its Optical Modulating Properties and Memory Functions
Authors: Kimura Shunsuke , Nakamura Kazuki , Kobayashi Norihisa 
Published: December 2020

Electrochromism is a phenomenon of reversible color change induced by electrochemical redox reactions of functional materials. Simple structure, large viewing angle, and low power consumption are some of the advantages of electrochromic (EC) devices for optical modulation systems in comparison with the conventional systems such as liquid-crystal display systems with backlight or organic light-emitting diodes. Electronic papers, digital signages, and smart windows are examples of potential applications of the EC system. Recently, we have reported Ag deposition-based EC device based on reversible electrodeposition of Ag nanoparticles. The EC devices basically composed of a pair of transparent electrodes with a gel electrolyte containing Ag+ between them.1-3 The application of a negative voltage causes the Ag metal deposition on the electrode substrate to construct metal nanoparticles or thin film that alters the optical properties of the surface.
However, the Ag deposition-based EC device did not have enough color retention property. This is due to the oxidative dissolution of Ag deposits by Cu2+, mainly functioning as counter reaction material, contained in the electrolyte. To achieve color retention properties, we utilized Prussian blue (PB) instead of copper ions. The PB film was fabricated on the counter electrode as a counter reaction material to compensate for the reaction charge on the working electrode. We investigated fabrication condition of PB-modified electrode and EC properties of Ag deposition-based EC device incorporating the PB electrode.
PB-modified electrode was prepared by electrolytic deposition on the ITO electrode. The EC device was fabricated by sandwiching DMSO based gel electrolyte solution, containing AgNO3 as EC material and LiCl as supporting electrolyte, between ITO electrode and PB-modified electrode. EC properties of the device were investigated by applying constant voltages. Transmission spectra at both optical states by applying voltage of −1.0 V (colored state, black line) and +1.0 V (bleached state, dashed black line) are shown in Figure. The transmission spectrum of the as-prepared device before applying voltages is also shown in the Figure (dashed blue line). When we applied −1.0 V, the transmittance decreased according to electrochemical deposition of Ag metal, and the device tuned to mirror state (inset photo in the figure). On the other hand, by applying +1.0 V, the transmittance increased, and the device returned to a transparent state as the result of dissolution of Ag metal (inset). In this way, the device demonstrated distinct color changing between transparent and mirror state by applying constant voltage of ±1.0 V.
Then the color retention property at the mirror state of this device was evaluated. After applying the coloring voltage of -1.0 V, the change in transmittance at 600 nm was recorded. As a result, the device kept its colored state (transmittance below 20 ％) under the open-circuit condition for 5 h. As for the conventional device, the colored state of the device was completely returned to the initial transparent state in a few minutes due to the dissolution of Ag deposit by Cu2+. The replacement of counter reaction materials from copper ions dissolved in the electrolyte to PB layer fixed on the counter electrode successfully achieved this great improvement of color retention properties. This novel structure of Ag deposition-based EC device is expected to contribute to effective power savings of EC devices such as smart windows, digital signages, information displays, and e-papers.

References

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Figure 1