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Engineers create plastic with high transparency and conductivity

Image credit: Aleksey Popov/Dreamstime

Researchers at the University of Michigan (U-M) have developed a conductive plastic coating that is also transparent.

Developed by a team at U-M’s College of Engineering, the plastic conductive was created as part of an effort to improve large touchscreens, LED light panels and window-mounted infrared solar cells. 

The research allows for other researchers to find the best balance between conductivity and transparency by creating a three-layer anti-reflection surface. 

The conductive metal layer is sandwiched between two 'dielectric' materials that allow light to pass through easily. The dielectrics reduce the reflection from both the plastic and metal layer between them.

“We developed a way to make coatings with high transparency and conductivity, low haze, excellent flexibility, easy fabrication and great compatibility with different surfaces,” said Jay Guo, a professor of electrical engineering and computer science at U-M. 

Previously, Guo’s team had shown that it was possible to add a layer of metal onto a plastic sheet to make it conductive - a very thin layer of silver that, by itself, reduced the transmission of light by roughly 10 per cent.

Jay Guo holds a sheet of flexible transparent conductor on the University of Michigan’s College of Engineering North Campus.

Jay Guo holds a sheet of flexible transparent conductor on the University of Michigan’s College of Engineering North Campus.

Image credit: Robert Coelius/University of Michigan Engineering

According to the researchers, light transmission through plastic is a little lower than through glass, but its transparency can be improved with anti-reflection coatings. Guo and his colleague Dong Liu, a professor from Nanjing University of Science and Technology in China, found they could also make an anti-reflection coating that was conductive.

“It was taken for granted that the transmittance of the conductor is lower than that of the substrate, but we show that this is not the case,” said Chengang Ji, a PhD student who worked on the project.

For the conductive, the team used two dielectric materials: aluminium oxide and zinc oxide. On the side closest to the light source, the aluminium oxide reflects less light back to the source than the plastic surface would. 

The team said the conductive features a metal layer, composed of silver with a tiny amount of copper in it, just 6.5 nanometers thick. This allows for the zinc oxide to help guide the light into the plastic surface. 

Some light still gets reflected back where the plastic meets the air on the opposite side, but overall, the light transmission is better than the plastic alone, they explained, with the transmittance being 88.4 per cent - up from 88.1 per cent for the plastic alone.

Changyeong Jeong, a graduate student in electrical engineering and computer science, measures how well light passes through a sheet of flexible transparent conductor.

Changyeong Jeong, a graduate student in electrical engineering and computer science, measures how well light passes through a sheet of flexible transparent conductor.

Image credit: Robert Coelius/University of Michigan Engineering

With the results, the team anticipates that other researchers will be able to design similar sandwich-style, flexible, highly transparent conductors which allow even more light through than the plastic alone.

“We tell people how transparent a dielectric-metal-dielectric conductor could be for a target electrical conductance,” Liu explained. “We also tell them how to achieve this high transmittance step-by-step.”

According to the researchers, the trick is to select the right dielectrics and then figure out the right thickness for each to suppress the reflection of the thin metal.

In general, the material between the plastic and metal should have a higher refractive index, while the material nearest the display or light source should have a lower refractive index.

Guo is currently collaborating on a project that uses transparent conductors in solar cells for mounting on windows. These could absorb infrared light and convert it to electricity while leaving the visible spectrum to brighten the room. He also proposes large panel interactive displays and car windshields that can melt ice the way rear windows can.

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