Graphene Sees the Light: Sheets of Carbon Just One Atom Thick Could Be Used in Photovoltaic Cells


Dec. 19, 2013 — Sheets of carbon just one atom thick could make effective transparent electrodes in certain types of photovoltaic cells.

Graphene, a one-atom-thick sheet of carbon that is extremely strong and conducts electricity well, is the thinnest material ever made. Researchers believe that it could be used as a transparent electrode in photovoltaic cells, replacing a layer of indium tin oxide (ITO) that is brittle and becoming
increasingly expensive.

Wee Shing Koh of the A*STAR Institute of High Performance Computing in Singapore and co-workers have compared these two materials. They found that graphene outperforms ITO when used with solar cells that absorb a broad spectrum of light
The wavelengths of light from the Sun have a range of intensities and deliver varying amounts of power. To maximize a photovoltaic device's performance, its transparent electrode should have a low electrical resistance, while also transmitting light of the right wavelengths for the cells to absorb.
Square sheets of graphene produced by today's chemical vapor deposition technology have an electrical resistance roughly four times that of a typical 100-nanometer-thick layer of ITO. Although adding more layers of graphene reduces its resistance, it also blocks more light. Koh and his co-workers calculated that four layers of graphene stacked together had the best chance of matching ITO's performance.
Graphene has one key advantage over ITO: it allows more than 97% of light to pass through to the solar cell beneath, regardless of its wavelength. In contrast, ITO tends to block certain wavelengths more than others. Four-layer graphene is slightly more transparent at near-infrared wavelengths than ITO is, for example.
Koh and co-workers estimated how each material would affect a flexible organic solar cell that absorbs light with wavelengths of 350 to 650 nanometers. They found that four layers of graphene delivered only 92.3% of the power of an equivalent ITO electrode. When paired with another organic photovoltaic device that operates from 350 to 750 nanometers, thus making it more effective at absorbing near-infrared light, graphene almost matched ITO's capabilities.
The researchers concluded that graphene would be ideally suited to photovoltaic cells with a very broad absorption range, such as a recently developed organic solar cell that can harvest light from 350 to 850 nanometers.
"With the refinement in the graphene manufacturing process, it would be possible for the sheet resistance of graphene to be an order of magnitude lower than the current state of the art," says Koh. This would allow just one or two sheets of graphene to beat ITO on both conductivity and transparency, making graphene transparent electrodes much more widely applicable.
The A*STAR-affiliated researchers contributing to this research are from the Institute of High Performance Computing



How Graphene and Friends Could Harness the Sun's Energy Hitting Walls


May 2, 2013 — Combining wonder material graphene with other stunning one-atom thick materials could create the next generation of solar cells and optoelectronic devices, scientists have revealed.



University of Manchester and National University of Singapore researchers have shown how building multi-layered heterostructures in a three-dimensional stack can produce an exciting physical phenomenon exploring new electronic devices.
The breakthrough, published in Science, could lead to electric energy that runs entire buildings generated by sunlight absorbed by its exposed walls; the energy can be used at will to change the transparency and reflectivity of fixtures and windows depending on environmental conditions, such as temperature and brightness.
The isolation of graphene, by University of Manchester Nobel Laureates Professor Andre Geim and Professor Kostya Novoselov in 2004, led to the discovery of the whole new family of one-atom-thick materials.
Graphene is the world's thinnest, strongest and most conductive material, and has the potential to revolutionise a huge number of diverse applications; from smartphones and ultrafast broadband to drug delivery and computer chips. The isolation of graphene also led to the discovery of a whole new family of one-atom-thick materials.
Collectively, such 2D crystals demonstrate a vast range of superlative properties: from conductive to insulating, from opaque to transparent. Every new layer in these stacks adds exciting new functions, so the heterostructures are ideal for creating novel, multifunctional devices.
One plus one is greater than two -- the combinations of 2D crystals allow researchers to achieve functionality not available from any of the individual materials.
The Manchester and Singapore researchers expanded the functionality of these heterostructures to optoelectronics and photonics. By combining graphene with monolayers of transition metal dichalcogenides (TMDC), the researchers were able to created extremely sensitive and efficient photovoltaic devices. Such devices could potentially be used as ultrasensitive photodetectors or very efficient solar cells.
In these devices, layers of TMDC were sandwiched between two layers of graphene, combining the exciting properties of both 2D crystals. TMDC layers act as very efficient light absorbers and graphene as a transparent conductive layer. This allows for further integration of such photovoltaic devices into more complex, more multifunctional heterostructures.
Professor Novoselov said: "We are excited about the new physics and new opportunities which are brought to us by heterostructures based on 2D atomic crystals. The library of available 2D crystals is already quite rich, covering a large parameter space.
"Such photoactive heterostructures add yet new possibilities, and pave the road for new types of experiments. As we create more and more complex heterostructures, so the functionalities of the devices will become richer, entering the realm of multifunctional devices."
University of Manchester researcher and lead author Dr Liam Britnell added: "It was impressive how quickly we passed from the idea of such photosensitive heterostructures to the working device. It worked practically from the very beginning and even the most unoptimised structures showed very respectable characteristics"
Professor Antonio Castro Neto, Director of the Graphene Research Centre at the National University of Singapore added: "We were able to identify the ideal combination of materials: very photosensitive TMDC and optically transparent and conductive graphene, which collectively create a very efficient photovoltaic device.
"We are sure that as we research more into the area of 2D atomic crystals we will be able to identify more of such complimentary materials and create more complex heterostructures with multiple functionalities. This is really an open field and we will explore it."
Dr Cinzia Casiraghi, from The University of Manchester, added: "Photosensitive heterostructures would open a way for other heterostructures with new functionalities. Also, in future we plan for cheaper and more efficient heterostructure for photovoltaic applications."

Flaky Graphene Makes Reliable Chemical Sensors


Jan. 23, 2012 — Scientists from the University of Illinois at Urbana-Champaign and the company Dioxide Materials have demonstrated that randomly stacked graphene flakes can make an effective chemical sensor.


The researchers created the one-atom-thick carbon lattice flakes by placing bulk graphite in a solution and bombarding it with ultrasonic waves that broke off thin sheets. The researchers then filtered the solution to produce a graphene film, composed of a haphazard arrangement of stacked flakes, that they used as the top layer of a chemical sensor. When the graphene was exposed to test chemicals that altered the surface chemistry of the film, the subsequent movement of electrons through the film produced an electrical signal that flagged the presence of the chemical.
The researchers experimented by adjusting the volume of the filtered solution to make thicker or thinner films. They found that thin films of randomly stacked graphene could more reliably detect trace amounts of test chemicals than previously designed sensors made from carbon nanotubes or graphene crystals.
The results are accepted for publication in the AIP's journal Applied Physics Letters.
The researchers theorize that the improved sensitivity is due to the fact that defects in the carbon-lattice structure near the edge of the graphene flakes allow electrons to easily "hop" through the film.



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