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How Transparent Graphene Electrodes Could Aid Solar Cell Generation

How Transparent Graphene Electrodes Could Aid Solar Cell Generation
A new manufacturing process for graphene is based on using an intermediate carrier layer of material after the graphene is laid down through a vapor deposition process. The carrier allows the ultrathin graphene sheet, less than a nanometer (billionth of a meter) thick, to be easily lifted off from a substrate, allowing for rapid roll-to-roll manufacturing. These figures show this process for making graphene sheets, along with a photo of the proof-of-concept device used (b). Image Credit: MIT

A new technique of manufacturing graphene could revolutionize solar power by enabling the creation of ultra-lightweight, flexible solar panels. 

A novel technique developed by researchers at the Michigan Institute of Technology (MIT) that allows for the creation of large sheets of graphene — a layer of single carbon atoms extracted from graphite — could have a significant impact on the development of future electronic devices. 

In particular, the development could give a significant boost to the field of solar power where graphene is used as a replacement for indium tin oxide (ITO) in the creation of electrodes. The resultant transparent and light electrodes can bend up to 78 ⁰ — much more flexible than traditional ITO electrodes.

These electrodes are used in transparent conducting films (TCFs) — thin films that are electrically conductive and need to be as transparent to light as possible to be maximally efficient. TCFs are vital in a number of electronic devices including touchscreens, OLEDs, and crucially, optoelectronics — devices that source, detect, and control light. Of particular importance in this latter category; solar cells.

Ultra-lightweight graphene-based devices can pave the way to a new generation of applications.

Giovanni Azzellino, Postdoc, MIT

The team, led by Azzellino and fellow postdoc Mahdi Tavakoli, in addition to professors Jing Kong, Tomas Palacios, and Markus Buehler, solved a particular problem that causes tearing and wrinkling of atom-thin sheets of graphene, with the use of a ‘buffer’ material. 

Now we are able to reliably manufacture large-area graphene sheets, transfer them onto whatever substrate we want, and the way we transfer them does not affect the electrical and mechanical properties of the pristine graphene.

Giovanni Azzellino, Postdoc, MIT

The researchers describe their technique in a paper published in the journal Advanced Functional Materials.

Solving the ‘Open-Air’ Problem in Optoelectronics

One major problem that has dogged the development of optoelectronic devices has been the struggle to make thin, large area, transparent electrodes that are also stable in the open air. 

One solution to this problem that many researchers have decided to pursue is the replacement of indium tin oxide (ITO), a material that has several limitations, not the least of which is the fact that the elements that comprise it are restrictively expensive. 

A vast array of materials has been investigated as potential replacements for ITOs, with scientists testing both inorganic and organic substances. One major candidate for such a substitute is graphene. As an atom thin layer, in terms of flexibility, this material extracted from graphite is tough to beat. Also, as a sheet of graphene is just an atom-thick, in terms of transparency it is impressive too.

Researchers currently estimate that a single sheet of graphene — an allotrope of carbon — absorbs just 2% of the light that passes through it. This means that it takes literally hundreds of layers to make graphene opaque.

However, there are still some drawbacks to this wonder material. 

Copper Complications

Graphene is grown in large sheets in a process known as chemical vapor deposition (CVD), which utilizes copper as a ‘seed layer.’

The problem arises from the fact that the graphene can be difficult to ‘peel’ away from the copper substrate. The process of releasing the graphene — known as the graphene transfer process — can result in highly undesirable and costly wrinkling and tearing. These effects significantly reduce the efficiency of the graphene’s electrical conductivity. 

The team’s revolutionary suggestion is the use of a ‘buffer’ material. They tested such a material made from a thin polymer called perylene, which coincidentally is also made by a CVD process. The perylene better conforms to the atomic layer of graphene and thus allows it to be extracted from the copper substrate without tearing, thus leaving the continuity of the material undisturbed. 

Of three types of perylene currently manufactured, the team found that the version containing a relative abundance of chlorine atoms was the most effective in their process. Having this chlorine-rich perylene bonded to the graphene in an arrangement that is analogous as doping — the adding of impurities to a semiconductor — but for graphene, means that conductivity was improved and non-destructive over a larger surface area. 

This means that the technique allows the process of graphene production to be significantly scaled up. 

Proof of Concept and Future Applications

The team demonstrated the technique and its viability by creating solar cells with a graphene layer and perylene substrate comprising one of the two electrodes within the device. 

Using this ‘proof of concept’ cell the team report they were able to achieve a 90% optical transmittance. This is a massive improvement of around 36 times the transmission achieved by the current generation of ITO cells of a comparable scale.

Another massive advantage of the system over ITO is the fact that using this process consumes 1/200 of the material required in those devices, and as Azzellino points out, “graphene is virtually free.” The framework to deliver perylene also currently exists, with the material already widely used in microelectronics. 

However, before the widespread application of graphene-based transparent electrodes, there are other problems that have tackled. The production of graphene via CVD consumes far more copper than the raw materials that are used up in the manufacture of ITO-based electrodes. Thus, the rate of copper consumption required by the process is still currently unsustainable, according to a paper published in the journal Nature. 

Despite this, the MIT team has provided a major step forward in the deployment of cost-effective, flexible, transparent electrodes. Therefore, thanks to graphene, the future is very bright indeed for solar energy. 

Sources and Further Reading

Chandler. D.L, [2020], ‘Transparent graphene electrodes might lead to new generation of solar cells,’ MIT News Office [http://news.mit.edu/2020/transparent-graphene-electrodes-solar-cells-0605]

Adams. F, Barbante. C, [2015], ‘Optoelectronic Devices,’ Comprehensive Analytical Chemistry. 

Chun. A. L, [2015], ‘Life-cycle assessment: Should graphene replace ITO?’ Nature Nanotechnology. 

Nicol. W, [2019], ‘What is Graphene,’ Digital Trends, [https://www.digitaltrends.com/cool-tech/what-is-graphene/]

Robert Lea

Written by

Robert Lea

Robert is a Freelance Science Journalist with a STEM BSc. He specializes in Physics, Space, Astronomy, Astrophysics, Quantum Physics, and SciComm. Robert is an ABSW member, and aWCSJ 2019 and IOP Fellow.

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