A new method of making large sheets of high-quality, atomically thin graphene could lead to ultra-lightweight, flexible solar cells and to new classes of light-emitting devices and other thin-film electronics. The manufacturing process involves an intermediate “buffer” layer of material that 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.
Finding a way to make thin, large-area, transparent electrodes that are stable in open air has been a major quest in thin-film electronics in recent years for a variety of applications in optoelectronic devices — things that emit light like computer and smartphone screens or harvest it like solar cells. Today’s standard for such applications is indium tin oxide (ITO), a material based on rare and expensive chemical elements.
Graphene — a form of pure carbon whose atoms are arranged in a flat, hexagonal array — has extremely good electrical and mechanical properties yet is vanishingly thin, physically flexible, and made from an abundant, inexpensive material. Furthermore, it can be easily grown in the form of large sheets by chemical vapor deposition (CVD) using copper as a seed layer. For device applications, however, the trickiest part has been finding ways to release the CVD-grown graphene from its native copper substrate.
This release, known as graphene transfer process, tends to result in a web of tears, wrinkles, and defects in the sheets that disrupts the film continuity and therefore drastically reduces their electrical conductivity. But with the new technology, large-area graphene sheets can be manufactured and transferred onto any substrate; the way they are transferred does not affect the electrical and mechanical properties of the 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.
As a demonstration of this technology, the researchers 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.
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.
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