Silicon chips for optical quantum technologies

March 28, 2008

Generating and detecting single photons, by Carmel King, from the University of Bristol websiteDoes this have implications for solar physics? Some fascinating research from the University of Bristol: A team of physicists and engineers has demonstrated exquisite control of single particles of light – photons – on a silicon chip to make a major advance towards the long sought after goal of a super-powerful quantum computer.

Dr Jeremy O’Brien, his PhD student Alberto Politi, and their colleagues at Bristol University have demonstrated the world’s smallest optical controlled-NOT gate – the building block of a quantum computer. The team were able to fabricate their controlled-NOT gate from silica wave-guides on a silicon chip, resulting in a miniaturised device and high-performance operation. “This is a crucial step towards a future optical quantum computer, as well as other quantum technologies based on photons,” said Dr O’Brien. The team reports its results in the March 27 2008 Science Express – the advanced online publication of the journal Science.

Quantum technologies aim to exploit the unique properties of quantum mechanics, the physics theory that explains how the world works at very small scales. For example a quantum computer relies on the fact that quantum particles, such as photons, can exist in a “superposition” of two states at the same time – in stark contrast to the transistors in a PC which can only be in the state “0” or “1”.

Photons are an excellent choice for quantum technologies because they are relatively noise free; information can be moved around quickly – at the speed of light; and manipulating single photons is easy. Making two photons “talk” to each other to realise the all-important controlled-NOT gate is much harder, but Dr O’Brien and his colleagues at the University of Queensland demonstrated this back in 2003 [Nature 426, 264]. Photons must also “talk” to each other to realise the ultra-precise measurements that harness the laws of quantum mechanics – quantum metrology…

Generating and detecting single photons, by Carmel King, from the University of Bristol website

Thin-film solar cell now competitive with silicon

March 26, 2008

IndiumSolar Daily: Researchers at the U.S. Department of Energy’s National Renewable Energy Laboratory have moved closer to creating a thin-film solar cell that can compete with the efficiency of the more common silicon-based solar cell. The copper indium gallium diselenide (CIGS) thin-film solar cell recently reached 19.9 percent efficiency, setting a new world record for this type of cell.

Multicrystalline silicon-based solar cells have shown efficiencies as high as 20.3 percent. The energy conversion efficiency of a solar cell is the percentage of sunlight converted by the cell into electricity. “This is an important milestone,” said NREL Senior Scientist Miguel Contreras. “The thin film people have always looked for matching silicon in performance, and we are reaching that goal.”

CIGS cells use extremely thin layers of semiconductor material applied to a low-cost backing such as glass, flexible metallic foils, high-temperature polymers or stainless steel sheets. Thin-film cells require less energy to make and can be fabricated by a variety of processes. Because of this, they provide a promising path for providing more affordable solar cells for residential and other uses.

The CIGS cells are of interest for space applications and the portable electronics market because of their light weight. They are also suitable in special architectural uses, such as photovoltaic roof shingles, windows, siding and others. Researchers were able to set the world record because of improvements in the quality of the material applied during the manufacturing process, boosting the power output from the cell, Contreras said.

Members of the record-setting team at the National Center for Photovoltaics include Contreras, Ingrid Repins, Brian Egaas, John Scharf, Clay DeHart and Raghu Bhattacharya.

Electron shell diagram for indium, “Pumbaa” and Greg Robson, Wikimedia Commons 

Superconductivity at room temperature, with graphene

March 24, 2008

Graphene Graphene, a single-atom-thick sheet of graphite, is a new material which combines aspects of semiconductors and metals. University of Maryland physicists have shown that in graphene the intrinsic limit to the mobility, a measure of how well a material conducts electricity, is higher than any other known material at room temperature – and 100 times faster than in silicon. A team of researchers led by physics professor Michael S. Fuhrer of the university’s Center for Nanophysics and Advanced Materials, and the Maryland NanoCenter said the findings are the first measurement of the effect of thermal vibrations on the conduction of electrons in graphene, and show that thermal vibrations have an extraordinarily small effect on the electrons in graphene.In any material, the energy associated with the temperature of the material causes the atoms of the material to vibrate in place. As electrons travel through the material, they can bounce off these vibrating atoms, giving rise to electrical resistance. This electrical resistance is “intrinsic” to the material: it cannot be eliminated unless the material is cooled to absolute zero temperature, and hence sets the upper limit to how well a material can conduct electricity.

In graphene, the vibrating atoms at room temperature produce a resistivity of about 1.0 microOhm-cm (resistivity is a specific measure of resistance; the resistance of a piece material is its resistivity times its length and divided by its cross-sectional area). This is about 35 percent less than the resistivity of copper, the lowest resistivity material known at room temperature. “Other extrinsic sources in today’s fairly dirty graphene samples add some extra resistivity to graphene,” explained Fuhrer, “so the overall resistivity isn’t quite as low as copper’s at room temperature yet. However, graphene has far fewer electrons than copper, so in graphene the electrical current is carried by only a few electrons moving much faster than the electrons in copper.”

In semiconductors, a different measure, mobility, is used to quantify how fast electrons move. The limit to mobility of electrons in graphene is set by thermal vibration of the atoms and is about 200,000 cm2/Vs at room temperature, compared to about 1,400 cm2/Vs in silicon, and 77,000 cm2/Vs in indium antimonide, the highest mobility conventional semiconductor known.

“Interestingly, in semiconducting carbon nanotubes, which may be thought of as graphene rolled into a cylinder, we’ve shown that the mobility at room temperature is over 100,000 cm2/Vs” said Fuhrer (T. Dürkop, S. A. Getty, Enrique Cobas, and M. S. Fuhrer, Nano Letters 4, 35 (2004)).

Mobility determines the speed at which an electronic device (for instance, a field-effect transistor, which forms the basis of modern computer chips) can turn on and off. The very high mobility makes graphene promising for applications in which transistors much switch extremely fast, such as in processing extremely high frequency signals.

Mobility can also be expressed as the conductivity of a material per electronic charge carrier, and so high mobility is also advantageous for chemical or bio-chemical sensing applications in which a charge signal from, for instance, a molecule adsorbed on the device, is translated into an electrical signal by changing the conductivity of the device.

Graphene is therefore a very promising material for chemical and bio-chemical sensing applications. The low resitivity and extremely thin nature of graphene also promises applications in thin, mechanically tough, electrically conducting, transparent films. Such films are sorely needed in a variety of electronics applications from touch screens to photovoltaic cells.

Fuhrer and co-workers showed that although the room temperature limit of mobility in graphene is as high as 200,000 cm2/Vs, in present-day samples the actual mobility is lower, around 10,000 cm2/Vs, leaving significant room for improvement. Because graphene is only one atom thick, current samples must sit on a substrate, in this case silicon dioxide.

Trapped electrical charges in the silicon dioxide (a sort of atomic-scale dirt) can affect the electrons in graphene and reduce the mobility. Also, vibrations of the silicon dioxide atoms themselves can also have an effect on the graphene which is stronger than the effect of graphene’s own atomic vibrations. This so-called “remote interfacial phonon scattering” effect is only a small correction to the mobility in a silicon transistor, but because the phonons in graphene itself are so ineffective at scattering electrons, this effect becomes very important in graphene.

“We believe that this work points out the importance of these extrinsic effects, and creates a roadmap for finding better substrates for future graphene devices in order to reduce the effects of charged impurity scattering and remote interfacial phonon scattering.” Fuhrer said.

Article: J. H. Chen, C. Jang, S. Xiao, M. Ishigami, M. S. Fuhrer, ‘Intrinsic and Extrinsic Performance Limits of Graphene Devices on SiO2, Nature Nanotechnology published online: 23 March 2008 | doi:10.1038/nnano.2008.58

Graphene crystals, “Vinograd19,” from Wikimedia Commons

Scientists fabricate non-cryogenic superconducting material

March 21, 2008

The chemical structure of silaneThis could be huge. Imagine the ability to transmit power from remote solar and wind sites without huge powerlines. From Next Energy News: A new breakthrough superconducting material fabricated by a Canadian-German team has been made out of a silicon-hydrogen compound and does not require cooling. The implications of the discovery are enormous and could transform the way people live by cutting power usage from everything from refrigeration to cell phones.

Instead of super-cooling the material, as is necessary for conventional superconductors, the new material is instead super-compressed. The researchers claim that the new material could sidestep the cooling requirement, thereby enabling superconducting wires that work at room temperature.

“If you put hydrogen compounds under enough pressure, you can get superconductivity,” said professor John Tse of the University of Saskatchewan. “These new superconductors can be operated at higher temperatures, perhaps without a refrigerant.”

He performed the theoretical work with doctoral candidate Yansun Yao. The experimental confirmation was performed by researcher Mikhail Eremets at the Max Plank Institute in Germany.

The new family of superconductors are based on a hydrogen compound called “silane,” which is the silicon analog of methane–combining a single silicon atom with four hydrogen atoms to form a molecular hydride. (Methane is a single carbon atom with four hydrogens).

Researchers have speculated for years that hydrogen under enough pressure would superconduct at room temperature, but have been unable to achieve the necessary conditions (hydrogen is the most difficult element to compress). The Canadian and German researchers attributed their success to adding hydrogen to a compound with silicon that reduced the amount of compression needed to achieve superconductivity.

Tse’s team is currently using the Canadian Light Source synchrotron to characterize the high pressure structures of silane and other hydrides as potential superconducting materials for industrial applications as well as a storage mechanism for hydrogen fuel cells….

The chemical structure of silane,  SiH4, “Jrockley,” from Wikimedia Commons

Toward cheaper, robust solar cells, using organic dye

March 14, 2008

Polycrystalline silicon wafer, from Q-Cells. Photo by George Slickers, Wikimedia CommonsTechnology Review: Cheap and easy-to-make dye-sensitized solar cells are still in the early stages of commercial production. Meanwhile, their inventor, Michael Gratzel, is working on more advanced versions of them. In a paper published in the online edition of Angewandte Chemie, Gratzel, a chemistry professor at the École Polytechnique Fédérale de Lausanne in Switzerland, presents a version of dye-sensitized cells that could be more robust and even cheaper to make than current versions.Dye-sensitized solar cells consist of titanium oxide nanocrystals that are coated with light-absorbing dye molecules and immersed in an electrolyte solution, which is sandwiched between two glass plates or embedded in plastic. Light striking the dye frees electrons and creates “holes”–the areas of positive charge that result when electrons are lost. The semiconducting titanium dioxide particles collect the electrons and transfer them to an external circuit, producing an electric current.

These solar cells are cheaper to make than conventional silicon photovoltaic panels. In principle, they could be used to make power-generating windows and building facades, and they could even be incorporated into clothing. (See “Window Power” and “Solar Cells for Cheap.”) A Lowell, MA-based company called Konarka is manufacturing dye-sensitized solar cells in a limited quantity. But the technology still has room for improvement.

In existing versions of the solar cells, the electrolyte solution uses organic solvents. When the solar cells reach high temperatures, the solvent can evaporate and start to leak out. Researchers are now looking at a type of material that may make a better electrolyte: ionic liquids, which are currently used as industrial solvents. These liquids do not evaporate at solar-cell operating temperatures. “Ionic liquids are less volatile and more robust,” says Bruce Parkinson, a chemistry professor at Colorado State University.

New dyes are also being investigated. In commercial cells, the dyes are made of the precious metal ruthenium. But researchers have recently started to consider organic molecules as an alternative. “Organic dyes will become important because they can be cheaply made,” Gratzel says. In the long run, they might also be more abundant than ruthenium.

In the recent paper, Gratzel and his colleagues describe making a dye-sensitized solar cell that combines these two material advances. In their prototype cell, they use an ionic liquid as the electrolyte and a dye based on the organic compound indoline. The solar cells convert light to electricity with an efficiency of 7.2 percent. Ruthenium-based dyes get efficiencies of about 11 percent, says Gerald Meyer, a chemistry professor at Johns Hopkins University. But, he says, “to my knowledge, these are the highest efficiencies with organic [dyes].”

. In a dye-sensitized solar cell, electrons go to the titanium dioxide layer, while the holes go to the electrolyte. This separates the charges so that they do not recombine and reduce the current generated by the cell. Keeping the charges separated is the challenge with organic dyes. Gratzel and his colleagues attach long hydrocarbon chains to one end of the indoline-based dye molecule. These hydrocarbon chains, which do not conduct electrons, act as barriers between the titanium dioxide layer and the electrolyte. “It is like a molecular insulator that stops electrons from coming out and recombining with the positive charges in the ionic liquid,” Gratzel says.With this charge barrier in place, the researchers can make the titanium dioxide layer thinner. That shortens the distance that the electrons have to travel to get to the external circuit, increasing the cell’s efficiency.

Parkinson cautions, though, that work on organic-dye solar cells is still at a very early stage. Going from a laboratory prototype to a commercial module typically reduces efficiencies significantly. To capture a larger share of the solar-power market, dye-sensitized solar cells will require some more improvements. “We really need a breakthrough to get up to 15 percent efficiency in the lab,” Parkinson says.

Nickel oxide coating greatly improves solar cell performance

February 26, 2008

Solar Daily reports on some potentially significant photovoltaic research: …A team of Northwestern University researchers has developed a new anode coating strategy that significantly enhances the efficiency of solar energy power conversion. A paper about the work, which focuses on “engineering” organic material-electrode interfaces in bulk-heterojunction organic solar cells, is published online this week in the Proceedings of the National Academy of Sciences (PNAS).

This breakthrough in solar energy conversion promises to bring researchers and developers worldwide closer to the goal of producing cheaper, more manufacturable and more easily implemented solar cells. Such technology would greatly reduce our dependence on burning fossil fuels for electricity production as well as reduce the combustion product: carbon dioxide, a global warming greenhouse gas.

Tobin J. Marks, the Vladimir N. Ipatieff Research Professor in Chemistry in the Weinberg College of Arts and Sciences and professor of materials science and engineering, and Robert Chang, professor of materials science and engineering in the McCormick School of Engineering and Applied Science, led the research team. Other Northwestern team members were researcher Bruce Buchholz and graduate students Michael D. Irwin and Alexander W. Hains.

….The Northwestern researchers employed a laser deposition technique that coats the anode with a very thin (5 to 10 nanometers thick) and smooth layer of nickel oxide. This material is an excellent conductor for extracting holes from the irradiated cell but, equally important, is an efficient “blocker” which prevents misdirected electrons from straying to the “wrong” electrode (the anode), which would compromise the cell energy conversion efficiency.

In contrast to earlier approaches for anode coating, the Northwestern nickel oxide coating is cheap, electrically homogeneous and non-corrosive. In the case of model bulk-heterojunction cells, the Northwestern team has increased the cell voltage by approximately 40 percent and the power conversion efficiency from approximately 3 to 4 percent to 5.2 to 5.6 percent.

The researchers currently are working on further tuning the anode coating technique for increased hole extraction and electron blocking efficiency and moving to production-scaling experiments on flexible substrates.

Electron shell illustration of nickel atom, “Pumbaa,” Wikimedia Commons

Emissions from photovoltaic life cycles

February 23, 2008

Energy Blog: A new report has found that thin-film cadmium telluride solar cells have the lowest life-cycle emissions primarily because they consume the least amount of energy during the module production of the four types of major commercial PV systems: multicrystalline silicon, monocrystalline silicon, ribbon silicon, and thin-film cadmium telluride (CdTe).

The study, published in the Environmental Science & Technology journal, based on PV production data of 2004–2006, presents the life-cycle greenhouse gas emissions, criteria pollutant emissions, and heavy metal emissions of the four types of PV systems considered. Life-cycle emissions were determined by employing average electricity mixtures in Europe and the United States during the materials and module production for each PV system.

They found that thin-film cadmium-telluride solar cells had the best life-cycle profile. Even though the process emitted heavy metal cadmium, it still had a lower overall level of “harmful air emissions” than the other PV technologies in the study.

The report stated that “Overall, all PV technologies generate far less life-cycle air emissions per GWh than conventional fossil-fuel-based electricity generation technologies. At least 89% of air emissions associated with electricity generation could be prevented if electrity from photovoltaics displaces electricity from the grid.”

The fact that Cd-Te technology was found to have the lowest emissions profile is interesting, but the main point, to me, is that all technologies had low emissions profiles, that are insignificant when compared to the emissions of the fossil fuel technologies that they replace. While I do not find it surprising that all solar PV systems have a low emissions profile, I find it surprising that the authors did not include thin-film silicon or copper indium gallium selenide (CIGS) cells in their study. I assume the overall results would have been the similar, but it would have given a fairer comparison to the technologies now in use. One problem with scientific research is that it takes so much time to do the study and get it published that by that time the information is made public it is sometimes outdated.

Photo of PV array at Nellis Air Force Base, US Airforce, Wikimedia Commons