A new method of high quality and high atomic thickness may bring ultra-light and flexible solar cells, as well as new light-emitting devices and other thin-film electronic products.
The new manufacturing process developed by the Massachusetts Institute of Technology should be easy to expand the scale of industrial production. He contains an intermediate "buffer" material layer, which is the key to successful technology. The buffer layer can easily lift ultra-thin graphene sheets with a thickness of less than 1 nanometer (one billionth of a meter) from the substrate, thereby achieving rapid roll-to-roll manufacturing.
In an article published in "Advanced Functional Materials", the authors are postdoctors Giovanni Azzellino and MahdiTavakoli, and the professors are JingKong, Tomas Palacios and Markus.
In recent years, the main goal of thin-film electronics is to find a large area, thin and transparent electrode that can work stably in an open environment. They are used in various applications of optoelectronic devices. These devices can not only emit light, such as computers And smartphone screens can also collect light, such as solar cells. The standard for this application today is indium tin oxide (ITO), a material based on rare and expensive chemical elements.
Many research groups are trying to find ways to replace ITO, focusing on organic and inorganic candidate materials. Graphene is a form of pure carbon. His atoms are arranged in a flat hexagonal array. He has excellent electrical and mechanical properties. His thickness is thin and has physical elasticity. He is made of cheap materials. Made. In addition, as demonstrated by the Kong team, he can easily grow into a large sheet with chemical vapor deposition (CVD) using copper as the seed layer. However, for equipment applications, the most difficult part is to find a way to release the CVD of graphene growth from its native copper substrate.
The release of this transfer process, called graphene, usually results in network tears, wrinkles, and defects in the film, thereby disrupting the continuity of the film, thereby greatly reducing its conductivity. Azzellino said: "Now with the new technology, we can reliably manufacture large-area graphene sheets and transfer them to any substrate we want. The transfer method will not affect the electrical and mechanical properties of the original graphene sheets. ."
The key is the buffer layer, which is composed of a polymer material called para-xylene, and his atomic level is consistent with the graphene block deployed on it. Like graphene, p-xylene is also produced by CVD, which simplifies the manufacturing process and scalability.
As a demonstration of this technology, the research team produced a proof-of-concept solar cell, using a thin-film polymer solar cell material, and a newly formed graphene layer as one of the two electrodes of the battery, and a para-xylene layer as a device Substrate. They measured the transmittance of graphene films under visible light close to 90%.
Compared with the latest ITO-based equipment, the output power per unit weight of the graphene-based solar cell prototype has increased by approximately 36 times. The material per unit area is also 1/200 of the transparent electrode. In addition, compared to ITO, graphene has a basic advantage: graphene is almost free, Azzellino.
He said: "Ultra-light equipment based on graphene can be used to pave the way for a new generation of applications." "So if you consider portable devices, the power per unit weight will become a very important number. If we can deploy a transparent solar cell on your tablet to power the tablet itself? Although further development is needed, But this application should eventually adopt a new method."
The buffer material p-xylene is widely used in the microelectronics industry, and is usually used for packaging and protection of electronic devices. Therefore, supply chains and equipment using this material have become very common, Azzellino. Among the three existing para-xylenes, the team’s testing showed that one of them has more polychlorine atoms and para-xylene is the most effective for this application.
When each layer of graphene is sandwiched together, chlorine-rich paraxylene has a further advantage on the atoms near the bottom graphene, which provides a "doping" for graphene, and finally a large area of graphene The improved conductivity provides a more reliable and non-destructive method, which is different from many other methods that have been tested and reported so far.
"Graphene and p-xylene film are always face to face," said Azzellino. "So basically, stimulants always exist, so the advantage is permanent."
The title of the paper is "Synergistic Roll to Roll Transfer and Doping of CVD Graphene Using Parylene for Ambient Stable and Ultra Lightweight Photovoltaics".