18 July 2012
A team from the University of Exeter, United Kingdom, may have invented the most transparent, lightweight and flexible material ever for conducting electricity. Called GraphExeter, the material could revolutionise wearable electronic devices, such as electronic clothing, and pave the way to much more efficient photovoltaics.
“Graphene is the thinnest flexible transparent conductive material nowadays — a just one-atom-thick layer of carbon,” says University of Exeter engineer Dr Monica Craciun, who also is a co-author of the paper “Novel Highly Conductive and Transparent Graphene-Based Conductors” published in the journal Advanced Materials. “However, graphene as such is not nearly conductive enough to replace indium tin oxide (ITO), the most wide-spread transparent conductor in optoelectronic applications.” So Craciun and her team at Exeter’s Centre for Graphene Science created a new material adapted from graphene, GraphExeter, by sandwiching ferric chloride molecules between flexible, transparent layers of graphene. The conductive molecules enhance the electrical properties of the graphene sheets without affecting the material’s transparency. The effect: GraphExeter “outperforms ITO and any other known carbon-based transparent and flexible conductor currently used in optoelectronics,” Craciun says. What’s more, ITO is becoming increasingly expensive as this finite resource is expected to run out in 2017.
Future photovoltaic devices require materials that are mechanically flexible, lightweight and low-cost, in addition to being electrically conductive and optically transparent. Could GraphExeter — dubbed by Craciun as “the best transparent electric conductor known by humanity” — be that holy grail? “Typically, good conductors of electricity that are also transparent are brittle, therefore not suitable for flexible devices. At the same time, transparent and flexible materials, such as polymers, are bad conductors of electricity,” Craciun explains. GraphExeter overcomes both these problems, making it a choice material for a breakthrough in wearable solar devices.
Adapting graphene for flexible electronics has been a challenge because of its sheet resistance, which limits conductivity. GraphExter seems to solve this issue by introducing ferric chloride between the graphene layers. Craciun reports, “A record low sheet resistance of 8.8 Ohms/square is attained together with an optical transmittance higher than 84% in the visible range.” In addition to these unique electro-optical properties, GraphExeter has technological advantages, too. It is relatively inexpensive to make and easily scalable to industrial production of large area electrodes. Plus, the Exeter team found their material to be air-stable for at least one year and, says Craciun, the manufacturing is easily achieved without using (extremely toxic) Chlorine gas, which ensures environmental friendly industrial processing. Furthermore, the low temperature required in the processing allows the use of a wide range of transparent flexible substrates that are compatible with existing transparent electronic technologies.
In solar technologies, GraphExeter has the potential to revolutionise conversion efficiencies. “GraphExeter is transparent in the range where indium tin oxide is opaque,” Cracium explains. “Therefore, if implemented in a solar panel, GraphExeter would allow to harvest energy over a much wider range of light than what is currently exploited, boosting the efficiency of solar panels significantly. This wider range of light corresponds to an enhance efficiency of solar panels of more than 30%.”
The British research team is now developing a spray-on version of GraphExeter, which could be applied straight onto fabrics, mirrors and windows.
Written by Sandra Henderson, Research Editor, Solar Novus Today