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The new manufacturing process of making large sheets of high-quality, atomically thin graphene developed by MIT researchers could lead to ultra-lightweight, flexible solar cells, and to new classes of light-emitting devices and other thin-film electronics.
The new method involves an intermediate “buffer” layer of material that is key to the technique’s success. The buffer allows the ultrathin graphene sheet, less than a nanometer (billionth of a meter) thick, to be easily lifted off from its substrate, allowing for rapid roll-to-roll manufacturing. The method should be relatively easy to scale up for industrial production as well.
New Method to Replace Indium Titanium Oxide
Many research groups have worked on finding a replacement for ITO, focusing on both organic and inorganic candidate materials. Graphene has extremely good electrical and mechanical properties, yet it is vanishingly thin, physically flexible, and made from an abundant, inexpensive material.
Graphene can be easily grown in the form of large sheets by chemical vapor deposition (CVD), using copper as a seed layer, as Kong’s group has demonstrated. However, for device applications, the trickiest part has been finding ways to release the CVD-grown graphene from its native copper substrate.
Manufacturing Large-area Graphene Sheets
This release, known as graphene transfer process, tends to result in a web of tears, wrinkles, and defects in the sheets, which disrupts the film continuity and therefore drastically reduces their electrical conductivity. With the new technology researchers can reliably manufacture large-area graphene sheets, transfer the sheets onto any substrate and the way of transfer, does not affect the electrical and mechanical properties of the pristine graphene.
The key is the buffer layer, made of a polymer material called parylene, that conforms at the atomic level to the graphene sheets on which it is deployed. Like graphene, parylene is produced by CVD, which simplifies the manufacturing process and scalability.
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The graphene and the parylene films are always face-to-face,” Azzellino says. “
So basically, the doping action is always there, and therefore the advantage is permanent.”
Proof-of-Concept Graphene-based Solar Cells
The team made proof-of-concept solar cells, adopting a thin-film polymeric solar cell material, along with the newly formed graphene layer for one of the cell’s two electrodes and a parylene layer that also serves as a device substrate. They measured an optical transmittance close to 90 percent for the graphene film under visible light.
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The prototyped graphene-based solar cell improves by roughly 36 times the delivered power per weight, compared to ITO-based state-of-the-art devices. It also uses 1/200 the amount of material per unit area for the transparent electrode. And, there is a further fundamental advantage compared to ITO: Graphene comes for almost free,” says Giovanni Azzellino MIT postdoc.
Ultra-lightweight graphene-based devices can pave the way to a new generation of applications. In terms of portable devices, the power per weight becomes a very important figure of merit. With some further developments, the new method could be used to deploy a transparent solar cell in a tablet that is able to power up the tablet itself.
Source: MIT