Graphene – single atom thick sheets of carbon – is the key to entirely organic lighting devices that are both cheap to make and easy to recycle, scientists say. The work could lead to plastic panels that would produce light evenly across large areas of walls and ceilings.
Organic light-emitting diodes (OLEDs) are made from conductive polymers that release photons when holes combine with electrons from an electrical current. Compared with liquid crystal display screens, OLEDs offer thinner units, better picture quality and greater energy efficiency in TVs, computers and mobile phones.
But OLEDs remain expensive to make and difficult to recycle due to the rare metal indium tin oxide electrode.
The group of Swedish and US researchers has made a light-emitting electrochemical cell (LEC) with a graphene cathode and an anode of conductive organic dyes (ACS Nano doi: 10.1021/nn9018569). LECs are structurally similar to OLEDs, but use a conductive gel of light-emitting polymers in a liquid electrolyte, rather than a solid polymer matrix. The graphene LECs produced similar light intensity, at similar levels of energy comsumption, to polymer OLEDs, says coauthor Nathaniel Robinson from Linköping University in Sweden.
But they could be much cheaper for two reasons. First, graphene is cheaper than indium tin oxide. Second, they can be made from solutions and are relatively insensitive to variations in thickness, which means large-scale production could be based on efficient ‘roll-toroll’ printing processes.
LECs are unlikely to replace OLEDs in display screens with rapidly moving images, because of the time it takes to turn them on. ‘The LEC has mobile ions,’ Robinson explains. ‘These ions have to move around a bit when you turn it on, so it can take a tenth of a second. You’re not going to make a TV out of LECs.’
But they would lend themselves to light panels that could be rolled out on large areas of walls or ceilings like wallpaper, rather than point sources which, because of the extreme heat generated, would melt organic materials. But that could inspire all new lighting strategies. ‘The architects love these ideas,’ Robinson says.
Robinson has filed patents and set up a company to take the technology to market.
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