Indium is one of the rarest metals on Earth, but it’s a key ingredient in ubiquitous electronic devices. Engineers have now demonstrated a way to swap indium for graphene, to make devices with the exact same electronic properties.
Thin films of indium tin oxide (ITO) are highly electrically conductive and transparent to light, making them perfect for a variety of display technologies, including LCD, OLED, e-ink and touchscreens, as well as LED lighting, glass coatings and solar cells.
The problem is that indium isn’t exactly common. While it’s technically more abundant than things like gold and silver, indium rarely occurs in elemental form in the Earth’s crust, so it has to be extracted as a by-product of ores mined for other metals, usually zinc. As such, it shows up on Critical Raw Materials lists in Europe, the US, Australia and Japan.
It’s not surprising then that scientists are searching for more common alternatives. Previous work has found promising candidates in carbon nanotubes, copper nanowires, or new glassy polymers. In a new study, researchers at Paragraf and Queen Mary University of London may have found one, in the form of everybody’s favorite wonder material – graphene.
Composed of a sheet of carbon just one atom thick, graphene exhibits a variety of useful electronic and optical properties, and of course it helps that carbon is one of the most abundant elements on Earth.
The researchers started by depositing a layer of graphene onto a transparent substrate, using a technique called metal-organic chemical vapor deposition. The graphene is doped with nitric acid to boost its conductivity, and then etched with lasers into a specific pattern to make it an anode. The resulting graphene-based OLED device performs just as well as the old indium tin oxide ones, the team says.
While other studies have shown how graphene can be used as a replacement for indium, the resulting devices haven’t been as efficient and conductive as the original. The team says the key issue is that the graphene is usually laid down onto a metal catalyst using a different deposition method, then transferred to the transparent substrate, but this extra step can introduce impurities that affect performance.
“Because of its importance and scarcity there have been many attempts to replace ITO, but no material has been found to have a comparable performance in an electronic or optical device until now,” says Professor Colin Humphreys, co-corresponding author of the study. “Our paper is the first paper in the world to demonstrate that graphene can replace ITO in an electronic/optical device. We have shown that a graphene-OLED has identical performance to an ITO-OLED.”
But there’s still some pieces of the puzzle to be solved before the so-called wonder material could become the silver bullet scientists hope for. Despite its scarcity, indium is still relatively cheap, while graphene is currently costly to produce in bulk. This economic landscape is shifting though, with companies like Paragraf developing production methods to produce graphene on larger scales at cheaper cost.
The research was published in the journal Advanced Optical Materials.
Source: Queen Mary University of London