Applications like LEDs, solar cells, and touch screens and panels require flexible transparent electrodes, and the demand for them is quickly rising. However, the combination of high optical transparency and high electrical conductivity sets a strict requirement on electrodes based on metallic materials.
Now, researchers at the University of Michigan (UMICH) have made a plastic material both more transparent and conductive. In their research published in Nature Communications, the team presents a flexible dielectric-metal-dielectric-based electrode with an ~88.4% rate of absolute transmittance.
Jay Guo, a professor of Electrical Engineering and Computer Science at the University of Michigan (UMICH), holds his research team’s flexible transparent conductor sheet on UMICH’s College of Engineering North Campus. Image Credit: UMICH.
Transparent Electrodes Using Metal Networks
Transparent electrodes are widely used in optoelectronic applications like those relevant to photovoltaics and LEDs—with indium tin oxide (ITO) being the material of choice for the given electrode. This is due to ITO’s high visible transmittance and electrical conductivity. Indium itself is scarce, however, and this is a limiting factor that is hindering further innovation in the field.
What’s more, indium’s applications in emerging flexible optoelectronic devices are significantly hindered by the material’s poor flexibility and the high annealing temperature that is needed to reduce its resistivity. And while alternative materials such as graphene and carbon nanotubes have been investigated instead, neither have sufficient conductivity and they cost a lot to mass produce. To overcome these challenges, transparent electrodes that use metal networks have already been proposed, and this is what the Michigan researchers set out to investigate further.
Introducing a Conductive, Anti-reflective Coating
The result of the UMICH team’s research is a conductive, highly transparent plastic that consists of a three-layer anti-reflection surface. The material’s conductive metal layer is sandwiched between two dielectric materials that enable light to pass through. The dielectrics reduce the reflection from both the plastic and the metal layer that is sandwiched between them.
Said Jay Guo, a professor of electrical engineering and computer science at UMICH, who led the work: “We developed a way to make coatings with high transparency and conductivity, low haze, excellent flexibility, easy fabrication, and great compatibility with different surfaces.”
Although the light transmission through the sheet is a little bit lower than that of clear glass, the material can be improved with anti-reflection coatings. It was not until they applied the reflective coating that Guo and his colleagues realised that they could make an anti-reflection coating that was also 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, first author of the paper in Nature Communications.
The Materials Science Behind the Anti-reflective Coating
The team chose aluminium oxide and zinc oxide dielectrics. On the side closest to the light source, the former material reflects less light back to the source than that of the plastic surface. Then, the metal layer, made from silver with a tiny amount of copper, guides the light into the plastic surface.
A close-up of the University of Michigan’s (UMICH) flexible transparent conductor sheet, whose transparency allows a clear view of UMICH’s College of Engineering North Campus in the background. Image Credit: UMICH.
Although some light is reflected back where the plastic meets the air on the opposite side, the resultant transmission rate of 88.4% is still better than the 88.1% transmission rate of plastic alone.
Guo and colleagues are hopeful that other researchers will be able to design similar sandwich-style flexible and transparent conductors. “We tell people how transparent a dielectric-metal-dielectric conductor could be, for a target electrical conductance. We also tell them how to achieve this high transmittance step-by-step,” said Liu.
According to the team, the trick with achieving similar results will be to select the right dielectrics and then work out the right thickness for each one of them, effectively suppressing the reflection of the thin metal. In general, the material in-between the plastic and metal should have a higher refractive index, whereas the material closest to the light source should have a lower refractive index.
Guo and his colleagues are continuing to work on the technology by integrating it into further projects. One such project involves the use of transparent conductors in photovoltaics, which can be mounted on windows. Plus, another area of interest involves having the technology melt the ice away from a car’s rear window.