Graphene is an unusual form of pure carbon where the atoms are arranged in a honeycomb crystal lattice just one atom thick. If the term is unfamiliar, it may be because it was not proven to exist until 2004. As it exists today, Graphene is produced by breaking down the same nearly pure graphite commonly used for pencils into layers one atom thick. In this form, Graphene has some remarkable properties. It is more flexible and 200 times stronger than steel, making it the strongest material tested to date.
Undoped graphene’s properties have been described as neither a metal; a semiconductor; nor an insulator, but rather a semimetal. It conducts heat 10 times faster than copper and electrons can flow through it at very high speed – a 1,000 times the density of electric current through a copper wire. It is hoped that this "ballistic transport” of electric current will lead to a new generation of super fast, super-efficient electronics, including a new generation of PV cells. In fact, graphene's structure gives it electrical properties that scientists worldwide see as presenting possibilities for a number of new applications. However, in its pure form graphene lacks the ability to form a band gap needed for solar cells.
Applications for PVs
Graphene is experiencing a steep trajectory in terms of ongoing research and new discoveries, with nearly 200 companies involved, according to Georgia Tech's Enterprise Innovation Institute.
One promising application is for making PV cells that are inexpensive, lightweight and flexible through the use of organic compounds instead of the currently used and expensive highly purified silicon. But one problem that has slowed the development of organic PV cells is that researchers have had a hard time coming up with appropriate materials for the electron donor and acceptor electrodes to carry the current to and from the cells. It has been a challenge to make these electrodes using materials that can match the organic PV cells’ flexibility and transparency. The emerging commercialization of “organic” PVs have also been slowed due to the unviable cost of the materials currently used for the electrodes in these devices.
The standard material currently used for these electrodes is indium-tin-oxide (ITO). Indium is expensive and relatively rare, so a suitable replacement has been a goal of researchers for some time. Grapheme is a good candidate. Graphene is transparent, so that electrodes made from this material can be applied to the transparent organic PV cells without blocking any of the incoming sunlight. Graphene is also flexible like the organic PV cells themselves. So, unlike the stiff and brittle ITO, the grapheme electrodes would allow PV installations that require the panels to bend and follow the contours of a roof or structure.
The biggest problem with graphene as an electrode material for organic PVs has been finding a way to make the graphene adhere to the panels. Since graphene repels water, the standard procedure for creating the electrodes by depositing the material from a solution on the panel surface has failed to work. What does seem to work is “doping” the panel surface by applying a certain amount of impurities that allows the graphene to bond securely and even improve the electrical conductivity.
Organic PV cells may become practical with the development of transparent graphene electrode technology that is both cheaper and more robust than conventional metal oxides. Of course the widespread use of this technology will require new techniques for large-scale manufacturing of graphene, which is an area of very active research.
Large-Scale Manufacturing is Next Step
It was not until 2004 that scientists were able to document the electronic properties of graphene after pulling off flakes of graphite from a lead pencil using only pieces of tape. But making graphene in large quantities has continued to be a challenge for a large-scale, commercially viable supply.
Nature Nanotechnology journal recently published a paper describing a new method identified by Massachusetts Institute of Technology scientists to produce considerable quantities of two or three-layer graphene by adding such chemicals as iodine chloride or iodine bromide to cause the graphite atoms to flake off in much more even layers. When these layers are arranged in the proper manner, the structures give the graphene the needed band gap required for electronic devices and PV cells.
Graphene Quantum Dots
Like organic materials, quantum dots have also become attractive for PVs. But most quantum dots currently being tested are composed of toxic metals such as cadmium and lead, which can pose serious cradle-to-grave problems for large-scale manufacturing and applications. Yet, quantum dots made of graphene do not have the same hazardous nature. Most importantly, graphene has a higher charge mobility that enables it to more quickly transport current to the electrodes, which reduces current loss and so improves the PV cell efficiency.
Dr. Vinay Gupta and researchers from the Organic and Hybrid Solar Cell Group at the National Physical Laboratory in New Delhi, India, have fabricated graphene quantum dots (GQDs) blended with organic polymers for use as electron acceptors, which they believe could offer better performance at a lower cost. The PVs created from this new GQD-based material has also demonstrated to be more environmentally friendly and more stable than current materials, and the researchers predict that the performance of these GQD-based devices could be further improved by experimenting with other polymers.
New Applications
While graphene PV electrodes differ slightly from the ITOs they could replace, the flexibility and light weight of organic PVs with graphene electrodes could open up a wide variety of new applications that are not possible with current panels, such as irregular walls and rooftop surfaces. And because of their transparency, the graphene PVs could not only be applied directly to windows without the loss of the view, they could also be stacked on top of existing PV panels to increase the amount of power generated from any given area.
In the meantime, interest in grapheme is increasing. During the past year, Georgia Tech's Enterprise Innovation Institute has documented the steady increase of patents using graphene, including in EV battery cathode material, an alcohol fuel battery, organic single crystal transistors, controlled drug release systems, dye-sensitized solar cells, wastewater dyeing, water resistant fiber board, metallic foam, and a proton exchange membrane fuel battery.
