Researchers make plastic coatings with high transparency and conductivity

Jay Guo holds a sheet of flexible transparent conductor on the University of Michigan’s College of Engineering North Campus. The material sandwiches a thin layer of silver between two “dielectric” materials, aluminum oxide and zinc oxide, producing a conductive anti-reflection coating on the sheet of plastic. Photo Credit: Robert Coelius/University of Michigan Engineering, Communications & Marketing

In a project designed to improve large touchscreens, LED light panels, and window-mounted infrared solar cells, researchers at the University of Michigan in Ann Arbor have made plastic conductive while also making it more transparent.

The team, led by Jay Guo, U-M professor of electrical engineering and computer science, has come up with a recipe to assist other scientists to strike the best balance between transparency and conductivity by making a three-layer anti-reflection surface. The conductive metal layer is inserted between two “dielectric” materials that enable light to pass through easily. The dielectrics decrease the reflection from both the metal layer and plastic between them.

The transmission of light via plastic is comparatively lower compared to that of glass. However, it is feasible to enhance the transparency of plastic with anti-reflection coatings. Guo and his collaborator Dong Liu, a visiting professor at the University of Michigan from Nanjing University of Science and Technology, identified that they could create an anti-reflection coating that was also conductive.

The dielectrics that they selected for this study are zinc oxide and aluminum oxide. On the side nearest to the light source, less light is reflected back to the source by the aluminum oxide compared to the plastic surface. What follows is the metal layer, which is made up of silver with a small amount of copper in it, with a thickness of just 6.5 nm. It also includes zinc oxide that helps direct the light into the surface of the plastic.

A part of the light still gets reflected back where the plastic comes into contact with the air on the opposite side. However, the overall light transmission is better compared to the plastic alone. The transmittance of light is 88.4 per cent, up from 88.1 per cent for the plastic alone.

According to Guo and Liu, the success of the project hinged on selecting the right dielectrics and then figuring 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, they said, while the material nearest the display or light source should have a lower refractive index.

From the theoretical outcomes, the researchers expect that other scientists will be able to design similar sandwich-style flexible, highly transparent conductors, which permit even more light to pass through compared to the plastic alone.

Guo and Liu are continuing to move the technology forward, 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. They also propose large panel interactive displays and car windshields that can melt ice the way rear windows can.

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

Source: University of Michigan