By Kris Walker
Flexible Solar Panels
Pros and Cons of Graphene Solar Cells
In 2004 Andre Geim and Konstantin Novoselov separated carbon layers and isolated the one-atom-thick material known as graphene from graphite, using sticky tape. In 2013, the number of research studies associated with the use for graphene in the field of materials science is rising.
Initial samples of graphene were very small, only a couple of square millimeters in size, but large enough for analysis. As graphene is only one-atom thick, it is considered to be a two-dimensional material.
Comparitively, electrons in graphene do not scatter when in motion as much as other materials such as silicon which has enabled researchers to make graphene-based transistors that are twice as fast as traditional silicon transistors.
While its first commercial application involves thin, flexible gadgets, current research reveals a wide range of possible applications including flexible solar panels, batteries for electric vehicles and enhanced natural gas production.
Flexible Solar Panels
Graphene has attracted several researchers who are trying to develop new, lightweight, and flexible solar panels that could be used to cover the outside surface of a building, in addition to roofs.
It is almost transparent to visible light, and to other forms of electromagnetic radiation including infrared light and ultraviolet. It absorbs only 2% of the visible, ultraviolet and infrared light or all the wavelengths in between, falling on it.
In 2008, a research team at the USC Viterbi School of Engineering reported the large-scale production of highly transparent graphene films by chemical vapor deposition technique.
The researchers created ultra-thin graphene sheets by depositing carbon atoms from methane gas in the form of graphene films on a nickel plate.
The films were then covered by a protective thermoplastic layer, and the nickel underneath was dissolved in an acid bath. Finally, they attached the plastic-protected graphene to a flexible polymer sheet that can be incorporated into an organic photovoltaic cell (OPV). Graphene/polymer sheets have been produced in a size range of 150 cm2 for developing dense arrays of flexible OPV cells.
These lightweight and flexible solar panels could be molded to suit an automobile body or wrapped around clothing or furniture. They have the potential to collect light and generate power when added to any surface.
Pros and Cons of Graphene Solar Cells
Unlike the indium-tin-oxide cells, the flexibility of graphene OPV cells enables them to be operational after repeated bending. In addition, they are easily available, electrode/organic film compatible, stable, conductive and at a low cost.
However, traditional silicon solar cells are more efficient as 14 W of power will be generated from 1000 W of sunlight where as only 1.3 W of power can be generated from graphene OPV cells.
The efficiency of graphene OPV cells to convert sunlight into electricity becomes lower than the conventional solar panel. Therefore, it is difficult to generate enough power for equipments using graphene solar cells.
While most of today's solar cells are made of silicon, they remain expensive as silicon is highly purified and made into thinly sliced crystals. Many researchers are developing alternatives, such as nanostructured or hybrid solar cells that rely on a material called indium tin oxide.
Recently, a research team at MIT has developed a new kind of photovoltaic cell that is made up of flexible graphene sheets coated with a layer of nanowires, and a cheaper alternative to indium tin oxide. They believe that this advancement could lead to low cost, transparent and flexible solar cells that can be deployed on windows, roofs or other surfaces.
This technology could also provide other advantages like chemical robustness, mechanical strength, light weight and flexibility. They have used a series of polymer coatings to modify the properties of graphene as building semiconducting nanostructures directly on it without impairing its electrical and structural properties would be challenging due to its stable and inert structure.
Another piece of research carried out by a team at the ICFO - The Institute of Photonic Sciences in Spain has revealed that graphene is more efficient at converting light into electricity than was previously known. Graphene can convert a single photon of light into multiple electrons to drive an electric current.
The discovery was made during an experiment that involves sending an exact quantity of photons with different energies onto a monolayer of graphene. Results showed that high energy photons are converted into a larger number of excited electrons when compared to low energy photons.
This relation between the photon energy and the number of generated excited electrons proved that graphene converts light into electricity at a high efficiency level of 60%. This discovery is exciting for the next-generation solar cells, and the other light-detecting and, light-harvesting technologies.
Graphene solar cells poses an interesting alternative for cheaper, durable solar power cells. Conventional materials like silicon and gallium arsenide that turn light into electricity generate a single electron for each photon absorbed. Since a photon contains more energy than one electron, a large amount of energy contained in the incoming light is lost as heat.
Research reveals that when graphene absorbs a photon it generates multiple electrons capable of driving a current. This means that graphene devices could be more efficient in converting light to electricity than the ones commonly used today ensuring that graphene can be used in advanced products such as night vision glasses, cameras and, eventually solar cells.
With graphene being so durable and so thin, we could even see at time whereby solar power technology is used in smartphones giving extended use of the device without running out of power.