Nano solar cells &  graphene OLED 

Wednesday 18th January 2017
Nano solar and graphene OLEDS

 Kyoto University researchers develop nanoscale semiconductors that raise the energy conversion rate of solar cells to at least 40%, while Fraunhofer researchers have produced functional OLED electrodes from graphene. 

"Current solar cells are not good at converting visible light to electrical power. The best efficiency is only around 20%," explains Kyoto University's Takashi Asano, who uses optical technologies to improve energy production. Higher temperatures emit light at shorter wavelengths, which is why the flame of a gas burner will shift from red to blue as the heat increases. The higher heat offers more energy, making short wavelengths an important target in the design of solar cells.

"The problem," continues Asano, "is that heat dissipates light of all wavelengths, but a solar cell will only work in a narrow range. To solve this, we built a new nano-sized semiconductor that narrows the wavelength bandwidth to concentrate the energy."

Previously, Asano and colleagues of the Susumu Noda lab took a different approach. "Our first device worked at high wavelengths, but to narrow output for visible light required a new strategy, which is why we shifted to intrinsic silicon in this current collaboration with Osaka Gas," says Asano.

To emit visible wavelengths, a temperature of 1000˚C was needed, but conveniently silicon has a melting temperature of over 1400˚C. The scientists etched silicon plates to have a large number of identical and equidistantly-spaced rods, the height, radii, and spacing of which was optimised for the target bandwidth.

According to Asano, “The cylinders determined the emissivity," describing the wavelengths emitted by the heated device. Using this material, the team has shown in Science Advances that their nanoscale semiconductor raises the energy conversion rate of solar cells to at least 40%.

"Our technology has two important benefits," adds lab head Noda. "First is energy efficiency: we can convert heat into electricity much more efficiently than before. Second is design. We can now create much smaller and more robust transducers, which will be beneficial in a wide range of applications."

The paper "Near-infrared–to–visible highly selective thermal emitters based on an intrinsic semiconductor" appeared 23 December 2016 in Science Advances.  Using nanotech to boost solar output: A Kyoto University and Osaka Gas silicon device could double the energy conversion rate of solar cells. Each vertical rod measures about 500 nm in height. (Kyoto University/Noda Lab)

The Fraunhofer Institute for Organic Electronics, Electron Beam and Plasma Technology FEP,  Dresden, together with partners, has succeeded for the first time in producing OLED electrodes from graphene. The electrodes have an area of 2 × 1 square centimetres. "This was a real breakthrough in research and integration of extremely demanding materials," says FEP’s project leader (left) Dr. Beatrice Beyer. The process was developed and optimised in the EU-funded project "Gladiator" (Graphene Layers: Production, Characterisation and Integration) together with partners from industry and research.

Graphene considered new miracle material offers impressive advantages  being light, transparent and extremely hard with more tensile strength than steel. Flexible and extremely conductive for heat or electricity, graphene consists of a single layer of carbon atoms which are assembled in a honeycomb pattern. It is only 0.3 nanometers thick.. Graphene has a variety of applications – for example, as a touchscreen in smartphones.

Chemical reaction: copper, methane and hydrogen
The production of the OLED electrodes takes place in a vacuum. In a steel chamber, a wafer plate of high-purity copper is heated to about 800 degrees. The research team then supplies a mixture of methane and hydrogen and initiates a chemical reaction. The methane dissolves in the copper and forms carbon atoms, which spread on the surface. This process only takes a few minutes. After a cooling phase, a carrier polymer is placed on the graphene and the copper plate is etched away.

The Gladiator project launched in November 2013. The Fraunhofer team is working on the next steps until the conclusion in April 2017. During the remainder of the project, impurities and defects which occur during the transfer of the wafer-thin graphene to another carrier material are to be minimised. The project is supported by the EU Commission with a total of €12.4 million. Fraunhofer Institute’s important industrial partners are the Spanish company Graphenea S.A., which is responsible for the production of the graphene electrodes, as well as the British Aixron Ltd, which is responsible for the construction of the production CVD reactors.

"The first products could  be launched in two to three years", says a confident Beyer.  Due to their flexibility, the graphene electrodes are ideal for touch screens. They do not break when the device drops to the ground! as it is a transparent polymer film. Many other applications are also available as  in windows, transparent graphene could regulate the light transmission, or serve as an electrode in polarization filters. Graphene can also be used in photovoltaics, high-tech textiles and even in medicine.


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