Transparent graphene electrodes could lead to new generation of solar cells

A new way 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 new manufacturing process, which was developed at MIT and should be relatively easy to scale up for industrial production, 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.

This process is detailed in a paper published in Advanced Functional Materials, by MIT postdocs Giovanni Azzellino and Mahdi Tavakoli; professors Jing Kong, Tomas Palacios, and Markus Buehler; and five others at MIT.

 

The state-of-the-art

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 either 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.

Many research groups have worked on finding a replacement for ITO, focusing on both organic and inorganic candidate materials. Graphene, a form of pure carbon whose atoms are arranged in a flat hexagonal array, has extremely good electrical and mechanical properties, yet it 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, 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.

 

The new process

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. But with the new technology, Azzellino says, “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.”

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 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.

New manufacturing process for graphene - MIT
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). Credit: MIT

 

Possible interesting applications

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,” Azzellino says.

Ultra-lightweight graphene-based devices can pave the way to a new generation of applications,” he says. “So, if you think about portable devices, the power per weight becomes a particularly important figure of merit. What if we could deploy a transparent solar cell on your tablet that is able to power up the tablet itself?” Though some further development would be needed, such applications should ultimately be feasible with this new method, he says.

 

Parylene’s advantages

The buffer material, parylene, is widely used in the microelectronics industry, usually to encapsulate and protect electronic devices. So, the supply chains and equipment for using the material already are widespread, Azzellino says. Of the three existing types of parylene, the team’s tests showed that one of them, which contains more chlorine atoms, was by far the most effective for this application.

The atomic proximity of chlorine-rich parylene to the underlying graphene as the layers are sandwiched together provides a further advantage, by offering a kind of  “doping” for graphene, finally providing a more reliable and nondestructive approach for conductivity improvement of large-area graphene, unlike many others that have been tested and reported so far.

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.”

The research team also included Marek Hempel, Ang-Yu Lu, Francisco Martin-Martinez, Jiayuan Zhao and Jingjie Yeo, all at MIT. The work was supported by Eni SpA through the MIT Energy Initiative, the U.S. Army Research Office through the Institute for Soldier Nanotechnologies, and the Office of Naval Research.

 

Featured Image – Credit: Scitechdaily.com

Source: MIT


Leggi anche

Una nuova tecnologia rivoluzionaria sviluppata dal National Composites Center (NCC) e dalla Oxford Brookes University consente ora di separare (o smantellare) le strutture in materiale composito in modo rapido ed economico utilizzando una semplice fonte di calore. Questa ricerca potrebbe trasformare la progettazione, l’uso e il riciclaggio a fine vita di un’ampia gamma di prodotti, tra cui automobili, aeromobili e turbine eoliche…

Leggi tutto…

Il progetto NEMMO ha l’obiettivo di ridurre i costi di manutenzione e aumentare la resa delle turbine mareomotrici e più in generale, di migliorare l’efficacia in termini di costi dell’energia delle maree. Una delle fasi centrali del progetto è la creazione di nuovi rivestimenti e materiali per le pale delle turbine per ridurne l’usura. Proprio in quest’ottica, di recente, sono stati installati una serie di pannelli provenienti da pale per turbine mareomotrici realizzati in fibra di vetro e con un rivestimento in gel-coat che resteranno immersi per sei mesi per determinare il livello di biofouling sulla superficie…

Leggi tutto…

Uno studio dell’Istituto di tecnologia chimica Fraunhofer prevede che soltanto in Germania entro il 2024 dovranno essere sostituite 15.000 pale di generatori eolici, alle quali se ne aggiungeranno altre 72.000 nei tre anni successivi. Esistono già metodi ecologici per lo smaltimento dell’acciaio e del calcestruzzo nei generatori eolici, ma il riciclaggio delle pale del rotore rimane problematico. Per questo i ricercatori del Fraunhofer Institute for Wood Research, Wilhelm-Klauditz-Institut, WKI hanno sviluppato una soluzione: hanno usato una nuova tecnica di riciclaggio per recuperare il legno di balsa contenuto nelle pale del rotore, reimpiegandolo per esempio in tappetini isolanti per edifici…

Leggi tutto…

The Composites Institute e UK Research and Innovation’s (UKRI) Innovate UK hanno annunciato sette nuovi progetti di ricerca e innovazione che serviranno a sviluppare nuovi materiali compositi in grado di far avanzare la produzione di componenti in una serie di diversi settori industriali, come la produzione aerospaziale, automobilistica e di energia rinnovabile…

Leggi tutto…

Centri di lavoro best-in-class a 5 assi ed alta velocità per la lavorazione di materiali comositi, alluminio e metallo

CMS SpA produce centri di lavoro multiassi a controllo numerico, termoformatrici e sistemi di taglio a getto d’acqua che permettono all’azienda di servire molti settori industriali: aerospaziale, automotive, nautica, ferroviario e molto altro. CMS offre, insieme a qualità e precisioni, soluzioni innovative, capaci di coprire le diverse fasi del processo produttivo o le specifiche esigenze. …

Leggi tutto…