Higher electrochemical stability and flexibility than ITO

We have succeeded in developing a flexible and transparent electrode, which has been considered the biggest challenge in manufacturing flexible wearable devices.

The Electronics and Telecommunications Research Institute (ETRI) has developed an electrochromic device that changes color in 0.5 seconds by stacking four layers of graphene material. This is expected to accelerate the wearability of various display devices.

ETRI previously developed electrochromic technology and created smart windows in 2013. Currently, this technology has been transferred and is being prepared for commercialization. It is expected to be applied to rearview mirrors to prevent glare caused by the lights of vehicles behind while driving.

At the time, the technology used indium tin oxide (ITO) on a substrate. However, since ITO is a rare material, it has lower electrochemical stability, reliability, and flexibility compared to graphene, which limited its applicability to wearable devices.

The research team created flexible, electrically conductive electrodes, which are an essential requirement for wearables. Through the introduction of these flexible electrodes, the possibility of applying existing glass-based displays to plastic-based wearable displays has been increased.

To be used as an electrode, it must have good electrical conductivity. Of course, for use as a display, the property of allowing light to pass through well and controlling it when placed on a transparent substrate is also crucial. Therefore, this achievement is highly significant in that it confirmed a transparent and easily bendable electrode utilizing graphene.

To meet these requirements, ETRI stacked graphene layers one million times thinner than the thickness of paper. In essence, they tackled the problem through stacking.

The research team transferred graphene by laminating a thermal transfer film with a single layer of graphene, provided by Hanwha Techwin, at a high temperature of 160°C. Through the transfer process, they succeeded in stacking up to six layers, and subsequently formed a device like a sandwich by placing a polymer-based electrochromic material on top.

ETRI explained that when a four-layer graphene electrode was applied, it demonstrated the best electrochemical stability and maintained a high transmittance of over 90%, while the discoloration speed was 10 times faster. Although the electrical resistance was at the 100 ohm (Ω) level, which is higher than that of existing ITO materials, the discoloration speed was significantly improved to less than 0.5 seconds, which is up to 10 times faster than existing methods. Therefore, it was confirmed that it can replace existing electrodes.

With this, the research team succeeded in fabricating a transparent electrochromic device measuring 2 mm in thickness and 2 x 3 cm. Moving forward, the team plans to scale it up to a larger surface area to enable its application in wearable devices. When electrodes were passed through the actual fabricated device, the bottom of the transparent component was clearly visible, while the bottom of the opaque component was not visible.

ETRI explained that in the future, this device could be utilized in energy-saving applications such as smart windows and automotive rearview mirrors, as well as for camouflage like a chameleon for soldiers or tanks, as it can automatically and rapidly change color according to the surrounding environment. Furthermore, they stated that it could be widely applied to information display devices used in signage, outdoor advertising, and displays.

ETRI plans to conduct additional research to develop perfect 'flexible display' technology in the future. Once the technology is developed, it is expected to bring significant changes to the design of various electronic devices that utilize displays, such as TVs, PCs, smartphones, and signage.

Kim Tae-yeop, Project Leader of the Realistic Display Research Group at ETRI, stated, “We plan to develop color and temperature variable elements using graphene and expand this into chameleon camouflage technology that protects friendly forces from the enemy by platforming military boots, helmets, and camouflage uniforms in personal combat systems.”