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New Nanomaterial Increases Yield of Solar Cells
Linked quantum dots – In the new nanomaterial two or more electrons jump across the band gap as a consequence of just a single light particle (arrow with waves) being absorbed. Using special molecules the researchers have strongly linked the …more
Researchers from the FOM Foundation, Delft University of Technology, Toyota Motor Europe and the University of California have developed a nanostructure with which they can make solar cells highly efficient. The researchers published their findings on 23 August 2Researchers from the FOM Foundation, Delft University of Technology, Toyota Motor Europe and the University of California have developed a nanostructure with which they can make solar cells highly efficient. The researchers published their findings on 23 August 2013 in the online edition of Nature Communications.
Smart nanostructures can increase the yield of solar cells. An international team of researchers including physicists from the FOM Foundation, Delft University of Technology and Toyota, have now optimised the nanostructures so that the solar cell provides more electricity and loses less energy in the form of heat.
A conventional solar cell contains a layer of silicon. When sunlight falls on this layer, electrons in the silicon absorb the energy of the light particles (photons). Using this energy the electrons jump across a 'band gap', as a result of which they can freely move and electricity flows.
The yield of a solar cell is optimised if the photon energy is equal to the band gap of silicon. Sunlight, however, contains many photons with energies greater than the band gap. The excess energy is lost as heat, which limits the yield of a conventional solar cell.
Nanospheres
Several years ago the researchers from Delft University of Technology, as well as other physicists, demonstrated that the excess energy could still be put to good use. In small spheres of a semiconducting material the excess energy enables extra electrons to jump across the band gap. These nanospheres, the so-called quantum dots, have a diameter of just one ten thousandth of a human hair.
If a light particle enables an electron in a quantum dot to cross the band gap, the electron moves around in the dot. That ensures that the electron collides with other electrons that subsequently jump across the band gap as well. As a result of this process a single photon can mobilize several electrons thereby multiplying the amount of current produced.
Contact between quantum dots
However, up until now the problem was that the electrons remained trapped in their quantum dots and so could not contribute to the current in the solar cell. That was due to the large molecules that stabilize the surface of quantum dots. These large molecules hinder the electrons jumping from one quantum dot to the next and so no current flows.
In the new design, the researchers replaced the large molecules with small molecules and filled the empty space between the quantum dots with aluminium oxide. This led to far more contact between the quantum dots allowing the electrons to move freely.
Using laser spectroscopy the physicists saw that a single photon indeed caused the release of several electrons in the material containing linked quantum dots. All of the electrons that jumped across the band gap moved freely around in the material. As a result of this the theoretical yield of solar cells containing such materials rises to 45%, which is more than 10% higher than a conventional solar cell.
This more efficient type of solar cell is easy to produce: the structure of linked nanospheres can be applied to the solar cell as a type of layered paint. Consequently the new solar cells will not only be more efficient but also cheaper than conventional cells.
The Dutch researchers now want to work with international partners to produce complete solar cells using this design.
Read more at: http://phys.org/news/2013-08-nanomaterial-yield-solar-cells.html#jCpn mobilise several electrons thereby multiplying the amount of current produced.
Read more at: http://phys.org/news/2013-08-nanomaterial-yield-solar-cells.html#jCp
Leave a comment Posted in Energy Independence, Environment, Nanotechnology, Renewable Energy Tagged Band gap, Delft University of Technology, Energy Independence, Environment, FOM Foundation, Nano Batteries, Nanotechnology, Photon, Quantum dot, Renewable Energy, Science, Solar cell, Solar Cells, Solar Energy, Toyota, University of California
Bursting through the Silicon Barrier: Developing Carbon-based Nanoelectronics with Graphene
On the road towards creating smaller and smaller electronic devices, silicon blocks the way by limiting the smallness of the electronic components that can be constructed with it. A promising way forward has been found by using carbon instead and its study has resulted in a rapidly growing field. In a work published in ACS Nano, using tools including those found at the Synchrotron Radiation Center, scientists have developed a process for making a never-before-seen, atomically thin, composite material containing ordered layers of graphene and nanocrystals of graphene monoxide.
Graphene, composed of an atomically thin layer of carbon, does not by itself have the necessary properties that lend itself for use in modern nanoelectronics. To achieve this, other elements need to be added to the mix. When oxygen is added chemically to graphene, for example, a property called the band-gap is created. The band-gap determines the electrical conductivity of a material, an important factor in creating useful electronic devices. However, at this stage, the mix is a disorganized arrangement of atoms, and results in poor electronic properties, including the band-gap. Because of this it can only be used in basic electronic devices such as supercapacitors, sensors, and flexible transparent conductive electrodes.
In this publication researchers describe a method for annealing (heating) the graphene and oxygen mix resulting in a previously unobserved atomic structure. It is comprised of layers of oxygen poor graphene sandwiched between layers of oxygen rich graphene (graphene oxide).
In the image, the number of rings corresponds to the complexity of the different structures in the Graphene-Oxide (G-O) compound. The left side of the image corresponds to the G-O compound before annealing (heating). The right side of the image, corresponding to the compound after annealing, shows additional rings indicating a more complex and ordered structure.
Scientists determined that the new carbon based structure shows promise allowing them to tailor it creating, for example, ideal "band gaps" for use in nanoelectronic devices such as sensors, transistors, and optoelectronic devices.
This work was published in the journal ACS Nano by SRC Users Eric Mattson (lead author), Michael Nasse, and Carol Hirschmugl. Additional members of the scientific team included faculty and students from the University of Wisconsin-Milwaukee and the University of Texas at Austin. This paper can be found online at: http://pubs.acs.org/doi/abs/10.1021/nn203160n
Leave a comment Posted in Energy Independence, Leadership, Nanotechnology, Renewable Energy Tagged Band gap, Carol Hirschmugl, Electronics, Energy Independence, Eric Mattson, Graphene, Graphite oxide, Nanotechnology, Printed Electronics, Renewable Energy, Science, Semi-Conductors, Solar Energy, Synchrotron Radiation Center
Why quantum dots can join every aspect of everyday life
Tiny bits of semiconductor crystals – so-called quantum dots – have such remarkable properties that scientists think they will soon be used in everything from light bulbs to the design of ultra-efficient solar cells. Photograph: Science Photo Library
The properties of a material were once thought to be defined only by its chemical composition. But size matters too, especially for semiconductors. Make crystals of silicon small enough – less than 10 nanometres – and their tiny dimensions can start to dictate how the atoms behave and react in the presence of other things.
These tiny bits of semiconductor crystals – so-called quantum dots – have such remarkable, novel properties that scientists think they will soon be used in everything from light bulbs to imaging of cancer cells or in the design of ultra-efficient solar cells.
Semiconductors such as silicon or indium arsenide are chosen to build electronic circuits because of the discrete energy levels at which they can give off electrons or photons. This makes them useful in building switches, transistors and other devices. It was once thought these energy levels – known as band gaps – were fixed. But shrinking the physical size of the semiconductor material to quantum-dot level seems able to change the band gaps, altering the wavelengths of light the material can emit or changing the energy it takes to change a material from an insulator to a conductor.
Instead of looking for brand new materials to build different devices, then, quantum dots make it possible to use a single type of semiconductor to produce a range of different characteristics. Researchers could tune dots made from silicon to emit a range of different colours in different situations, for example, instead of having to use a range of materials with different chemical
compositions.
"The main application for quantum dots at the moment is biological tagging of cells," says Paul O'Brien, a professor of inorganic materials at the University of Manchester and co-founder of Nanoco Technologies a quantum dot manufacturer also based in Manchester. They are used in the same way as fluorescent dyes, to label agents, he says, but with the advantage that a single laser source can be used to illuminate many different tags each with a specific wavelength.
By attaching different types of quantum dots to proteins that target and attach to specific cell types in the body, these bits of semiconductor can be used by doctors to monitor different kinds of cells. When a laser is then directed on to tagged cells, doctors can see what colour they glow.
The ability to shine also makes quantum dots well suited to produce white light. Existing white bulbs based on low energy light emitting diode (LED) technology tend to produce a garish and bluish form of light that notoriously feels cold, says O'Brien. This is because these LEDs use a phosphor that produces an artificial white light that contains less red wavelengths than natural white light. By embedding quantum dots into a film that is placed over a bulb containing blue LEDs, it is possible to get a much warmer colour of white light. The blue light from the LED stimulates the quantum dots which, in turn, emit light in a range of colours. Provided you have chosen your dots carefully, these will combine to form white light.
The first of these quantum dot lights hit the market in 2010, a partnership between QD Vision, an MIT spinout in Lexington, Massachusetts, and Nexxus Lighting of Charlotte, North Carolina.
Backlights for laptops, tablets and mobile devices are next in line, and they should appear in products before the end of 2012 says VJ Sahi, head of corporate development at materials design company Nanosys of Palo Alto, California. Besides the colour advantages, quantum-dot-based backlights can be three times more efficient than traditional backlights.
Eventually, says Sahi, quantum dots will do more than just light up displays. The long-term aim is use them to create each red, green and blue sub-pixel that makes up a coloured display. This should produce much brighter colours and consume less power than LCD or even the latest state-of-the-art organic LED (OLED) displays. They should also have no problems with viewing angles, he adds.
The interesting properties of quantum dots come from the fact that they behave like tuning forks for photons, a result of a phenomenon called confinement. At less than 10 nanometres in size – about 50 atoms – they fall within the dimensions of a critical quantum characteristic of the material known as the exciton Bohr radius. The energy levels of electrons within the material's atoms are constrained and, when a photon or electron hits an atom and excites it, the atom re-emits the energy as a photon of a very specific energy level.
Quantum dots also have another trick up their sleeve. Besides converting photons of one energy into photons of another, they can also be used to release electrons and create electrical currents: in other words they can be used to make solar cells. Arthur Nozik at the National Renewable Energy Laboratory in Boulder, Colorado, says that quantum-dot solar cells would be much more efficient at converting the energy from photons and therefore boost the amount of power they can produce.
Such applications are many years from becoming commercial reality. But they serve to demonstrate that no material technology stands still; sometimes all you have to do is cut it down to size.
Leave a comment Posted in Nanotechnology Tagged Band gap, breakthrough discovery, CQD, Nanotechnology, National Nanotechnology Initiative, National Science Foundation, Paul O'Brien, Quantum dot, Semiconductor, silicon semiconductors, Solar cell, University of Manchester
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