Electrochromic Device for Reversible Color Change Three Different Operating States
Authors: Jeong Kiryong , Lee Jong-Lam 
Published: February 2020

1. Objective
The electrochromic (EC) device can operate reversible color changes that are induced by electrochemical redox reactions of materials.[1] The obtained optical states are based on a change in the electronic state of a material caused by electron transfer between the EC material and an electrode. These EC devices offer many advantages over comparable conventional displays, including low operating voltages, memory effects and color variation. Therefore, EC devices are expected to achieve applications in information displays or in light-modulating devices such as smart window, switchable mirror, electronic paper and chemical sensors.[2,3] The many research of the EC device has been conducted due to these many advantages. However, previously reported inorganic EC device, such as MoO3, Ir(OH)3, NiO and other materials, has the low optical properties, and it happens degradation by repeating stability test. To overcome imperfect deposited states and low stability properties, we focused on the EC device by using novel metal deposition on ITO electrode such as Ag metal. Its underlying mechanism is based on the electrodeposition of silver particles on two facing transparent electrode that sandwich a gel electrolyte in which electrochromic material is dissolved. As a result, there are a number of advantages for employing Ag metal as the electrochromic material, namely the easy fabricating for using the EC device, good optical properties such as deep black state, high cyclic stability.
2. Experiments
The EC device was fabricated by sandwiching the electrolyte between two different transparent electrodes, with interelectrode distance of maintained with Kapton spacer. The solution of electrolyte is consist of silver nitrate (AgNO3), copper chloride (CuCl2), tetra-n-butylammonium bromide (TBABr), poly (butyral) (PVB) and dimethyl sulfoxide (DMSO). The ITO electrode was cleaned with acetone, isopropyl alcohol, and deionized water. The prepared electrolyte was coated on to the ITO electrode with a kapton spacer and a device size of 1 × 1 cm2, and then assembled with another ITO electrode.
3. Results and Discussion
Our EC device can operate three states in a single device. Flat ITO electrode and rough ITO electrode were used as the working electrode and counter electrode respectively. The default state is transparent. That is, the electrolyte solution is transparent color because Ag+ in the electrolyte is dissolved state. When Ag+ is deposited on the flat ITO electrode by applying a negative voltage (-2.5), the EC device is change to the mirror state (Fig. 1a). Subsequently, mirror state returned to the transparent state by applying oxidation voltage (0.5V) of deposited silver metal (Fig. 1b). Conversely, when Ag+ is deposited on the rough ITO electrode by applying positive voltage (2.5V), the EC device is change to the black state. The oxidation voltage (-0.5) is also applied to dissolve the Ag deposition. Therefore, these operating mechanism between the transparent, mirror and black states was reversible.
As a result, we obtained the transmittance and reflectance spectra of the EC device based on the two different electrodes. In the transparent state, the average transmittance in the visible region (400-700nm) of the EC device was 73.53%. The average transmittance of mirror and black states were 1.66% and 2.6% respectively, due to the deposited Ag of two electrode. The average reflectance of the mirror, black and transparent states were 79.0%, 5.26% and 8.36%, respectively. Finally, we carried out a repetition stability test of the EC device based on the two different ITO electrode, switching between three states by the sequential application of the following biases: -2.5V (20 s), 0.5V (40 s), 2.5V (15 s), -0.5V (35 s). The transmittance changes at 600nm during 10,000 cycles are performed by potentiostat and UV-Vis spectroscopy measurement. The transmittance was changed from 73.58% to 75.57% in the transparent state, from 11.09% to 15.7% in the mirror state and from 10.62% to 28.53% in the black state, respectively.
4. Conclusion
We demonstrated the operation of EC device based on two different ITO electrode that can control of three different optical states in response to a change in the applied voltage. The optical properties and stability of the EC device based on flat and rough ITO electrodes could obtain high performance. These EC device could find numerous applications, such as smart windows for energy saving buildings and automobiles, multifunctional display and light modulator.
5. Reference
[1] S. Araki, et al. Ad. Mater. 2012, 24, OP122.
[2] David R. Rosseinsky, et al. Adv. Mater. 2001, 13, 783.
[3] Dane T. Gillaspie, et al. J. Mater. Chem. 2010, 20, 9585.

Figure 1