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INSIDE SCIENCE RESEARCH --- PHYSICS NEWS UPDATE The American Institute of Physics Bulleting of Research News Number 876, October 24, 2008 www.aip.org/pnu

USING SUNLIGHT MORE EFFICIENTLY

Researchers at the National Renewable Energy Laboratory (NREL) in Golden, Colo have developed a way for low-cost solar cells to more efficiently convert sunlight into electricity.The research, which increases the "lifetime" of electrons created in a solar cell so they can make more electricity, is a possible step in the direction of bringing down the relatively high cost of solar cells. Reducing cost while sustaining efficiency is the big factor in determining how soon solar power will become a major player in the energy business. Generally you could have good efficiency or low cost but not both. Efficiency refers to the fraction of the sunlight falling on the solar panel that actually gets converted into useable electricity. And cost refers to the expense of mass-producing the panels in large sheets. Solar cells have been used in niche markets, such as for powering remote sensors or spacecraft, and are increasingly used for homes and utility applications.

Most of these solar cells are made from crystalline silicon. But for large-scale adoption to occur, the price will have to come down. Currently the cost-per-kilowatt-hour for solar-generated power is several times higher than for generating that power with fossil fuels. Solar cells mimic nature in the way that it converts sunlight into useful energy. In a green leaf, for example, the incoming sunlight liberates an electron in a molecule of chlorophyll. The electron (and its energy) gets passed from one molecule, eventually being incorporated into building up larger molecules such as a carbohydrate. In a solar cell the incoming sunlight liberates an electron from a piece of semiconductor. This "excited" electron, if it stays excited, can be incorporated into an electrical current feeding into an external circuit, where it can flow into a battery or the electric grid. The longer the lifetime of the excited electron, the better the efficiency of the solar cell. Unfortunately, electrons tend to lose their energy when they meet a defect or boundary in the crystals that make up a solar cell. Until now to get a better excitation lifetime and better efficiency, solar cells needed to be made of higher-priced single crystal materials like silicon or gallium arsenide. These solar cells need lots of complex processing to build, and these costs are not likely to be reduced. Meanwhile, lower-priced solar cells made from thin layers of multi-crystalline materials, such as compounds made of the atoms copper, indium, gallium, and selenium (CIGS), haven't been nearly as efficient.

The research focused on improving electron lifetimes in solar cells made from multi-crystalline CIGS, and in their research paper, NREL scientists Wyatt Metzger, Ingrid Repins, and Miguel Contreras announced they have achieved an electron lifetime of 250 billionths of a second. It sounds like a short time, but it is long enough for more electrons to contribute to the cell's electricity, making it dramatically more efficient, yet still low in cost when compared to the high-efficiency silicon solar cells. The results were recently published in the journal Applied Physics Letters.

(Phillip F. Schewe)

BUCKY BEAMS

Once nanochip manufacturers have made their multi-layered structures it is necessary also for them to verify precisely that the layers are composed in the proper way. One way of doing this is to shoot beams of ions which, like meteorites striking the Moon, eject material from below, providing information about subsurface layering. The ejected material is characterized using mass spectrometry. It seems that to do this large molecules or clusters of atoms are better than single-atom ions since the clusters excavate more cleanly and provide more unambiguous signs of deep structure in the sample being imaged. The lab of Nick Winograd (nxw@psu.edu) of Penn State has pioneered the use of beams of carbon-60 molecules (buckyballs). (See this site for pictures illustrating the difference between single atom probes and C60 beams: http://nxw.chem.psu.edu/nxw/pdf%5C327.pdf ). Recently Winograd and his students have greatly improved the sensitivity of detection of the ejected material by using an infrared laser for photoionization prior to analysis by the mass spectrometer. The infrared laser is effective since electrons can be removed from molecules with high efficiency via tunneling and without significant photofragmentation. (Results presented this week at the AVS meeting in Boston, http://www.avssymposium.org/overview.asp, Paper AS-TuM10)

TRAPPING SINGLE MOLECULES

at room temperature, and studying their properties has been accomplished by Adam Cohen and his colleagues at Harvard. Pinning down one molecule at a time is difficult at low temperatures, much less at warm temperatures, where the molecules are more agitated. The feat was carried off by using an Anti-Brownian Electrokinetic (ABEL) trap. In this device the fluorescently labeled molecule is tracked in a fluorescence microscope and its instantaneous motion slowed by the application of carefully timed bits of electricity applied to electrodes that surround the sample. Actually, the electrodes are kept at some distance from the molecule, the better not to pollute the local aqueous environment with chemical effects. The electric kicks are imparted to the molecule along micro-channels in an underlying chip. The faster this feedback process can be applied the better the trapping. An ABEL trap can hold smaller samples at room temperature than any other trap scheme. To hold a molecule to the same tiny volume of solution with laser light alone, enormous power would be needed, and
this would “cook” the object rather than trap it. The ABEL trap is gentle, and requires mere microwatts of laser power. Cohen (cohen@chemistry.harvard.edu) talked about the application of this process to the dynamics of membrane proteins at this week’s AVS meeting. (http://www.avssymposium.org/overview.asp Cohen website https://www2.lsdiv.harvard.edu/labs/cohen/ Paper IPF-MoM1 )



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