Plastics for Large Touchscreens, LED Light Panels Made with High Transparency & Electrical Conductivity

Researchers at the University of Michigan, led by Jay Guo, professor of electrical engineering and computer science, mechanical engineering, macromolecular science & engineering, and applied physics, have developed a method to make plastic more transparent and electrically conductive for large touchscreens, LED light panels and window-mounted infrared cells.

The team, which also has a start-up company, has zeroed-in on a way to get the best balance between conductivity and transparency through the creation of 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.

First, Guo’s team showed that it is possible to add a very thin layer of silver metal (which by itself reduced the transmission of light by roughly 10%) onto a plastic sheet to make it conductive. They then came up with a way to make coatings with high transparency and conductivity, low haze, excellent flexibility, easy fabrication and great compatibility with different surfaces. 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 visiting professor at U-M from Nanjing University of Science and Technology, realized that they could make an anti-reflection coating that was also conductive.

Dielectrics aluminum oxide and zinc oxide were used by the research team. On the side closest to the light source, the aluminum oxide reflects less light back to the source than the plastic surface would. Then comes the thin (only 6.5 nanometers thick) metal layer of silver with a tiny amount of copper, followed by the zinc oxide, which helps guide the light into the plastic surface. Although some light still gets reflected back where the plastic meets the air on the opposite side, the light transmission is still better than the plastic alone—transmittance is 88.4%, up from 88.1%.

Gau tells Plastics Technology that the substrate is typically PET, COP (cyclo-olefin polymer) or CPI (colorless polyethermide/PEI)—materials used for electronics/displays and flexible touch panels. The sandwich structure is done via a sputtering process rather than extrusion. Sputtering is a physical vapor deposition vacuum process used to deposit very thin films onto a substrate for a wide variety of commercial and scientific purpose; (e.g., it’s used extensively in the semiconductor industry to deposit thin films of various materials in integrated circuit processing). According to Gau, the structure has been manufactured at his startup company.  

Guo and team are moving forward to advance the technology including a current project that utilizes transparent conductors in solar cells for mounting on windows. Such units could absorb infrared light and convert it to electricity while leaving the visible spectrum to brighten the room. Also being explored are large panel interactive displays and car windshields that can melt ice the way rear windows can.