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The sun may be the only energy source big enough to wean us offfossil fuels. But harnessing its energy depends on silicon wafers that must be produced by the same exacting process used to make computer chips. The expense of the silicon wafers raises solar-power costs to as much as 10 times the price of fossil fuel generation—keeping it an energy source best suited for satellites and other niche applications.


Paul Alivisatos, a chemist at the University of California, Berkeley, has a better idea: he aims to use nanotechnology to produce a photovoltaic material that can be spread like plastic wrap or paint. Not only could the nano solar cell be integrated with other building materials, it also offers the promise of cheap production costs that could finally make solar power a widely used electricity alternative.

Alivisatos's approach begins with electrically conductive polymers. Other researchers have attempted to concoct solar cells from these plastic materials . . . but even the best of these devices aren't nearly efficient enough at converting solar energy into electricity. To improve the efficiency, Alivisatos and his coworkers are adding a new ingredient to the polymer: nanorods, bar-shaped semiconducting inorganic crystals measuring just seven nanometers by 60 nanometers. The result is a cheap and flexible material that could provide the same kind of efficiency achieved with silicon solar cells. Indeed, Alivisatos hopes that within three years, Nanosys — Palo Alto, CA, startup he cofounded — will roll out a nanorod solar cell that can produce energy with the efficiency of silicon-based systems.

The prototype solar cells he has made so far consist of sheets of a nanorod-polymer composite just 200 nanometers thick. Thin layers of an electrode sandwich the composite sheets. When sunlight hits the sheets,


they absorb photons,exciting electrons in the polymer and the nanorods, which make up 90 percent of the composite. The result is a useful current that is carried away by the electrodes.

Early results have been encouraging. But several tricks now in the workscould further boost performance. First, Alivisatos and his collaborators have switched to a new nanorod material, cadmium telluride, which absorbs more sunlight than cadmium selenide, the material they used initially. The scientists are also aligning the nanorods in branching assemblages that conduct electrons more efficiently than do randomly mixed nanorods. "It's all a matter of processing," Alivisatos explains, adding that he sees "no inherent reason" why the nano solar cells couldn't eventually match the performance of top-end, expensive silicon solar cells.

The nanorod solar cells could be rolled out, inkjet printed, or even painted onto surfaces, so "a billboard on a bus could be a solar collector," says Nanosys's director of business development, Stephen Empedodes. He predicts that cheaper materials could create a $10 billion annual market for solar cells, dwarfing the growing market for conventional silicon cells.

Alivisatos's nanorods aren't the only technology entrants chasing cheaper solar power. But whether or not his approach eventually revolutionizes solar power, he is bringing novel nanotechnology strategies to bear on the problem. And that alone could be a major contribution to the search for a better solar cell. "There will be other research groups with clever ideas and processes — maybe something we haven't even thought of yet," says Alivisatos. "New ideas and new materials have opened up a period of change. It's a good idea to try many approaches and see what emerges."



Others in Nano Solar Cells
Richard Friend University of Cambridge Fullerene-polymer composite solar cells
Michael Gratzel Swiss Federal Institute of Technology Nanocrystalline dye-sensitized solar cells
Alan Heeger University of California, Santa Barbara Fullerene-polymer composite solar cells
N. Serdar Sarkiftci Johannes Kepler University Polymer and fullerene-polymer composite solar cells



Thanks to nanotechnology, those new ideas and new materials could transform the solar cell market from a boutique source to the Wal-Mart of electricity production.

Eric Scigliano


LESSONS 48, 49

Script of the text for listening (VOA)

The picture that astronomers are creating of our universe has gotten a little more detailed. Last month, three teams of astronomers reported making direct images of planets orbiting other stars. Their observations are changing the way space scientists look at the universe.

Technological progress has made finding planets far from our own solar system common. The first discovery was confirmed in nineteen ninety-four. Today, more than three hundred twenty planets have been discovered beyond our solar system.

To make these discoveries, scientists looked for small movements in nearby stars. The movements show the gravitational influence of another object on the star.

Some planets have been found because they pass in front of the stars they orbit. This limits the amount of light seen on Earth for a short time.

What is new about the recent observations is that they are direct images of distant planets. That is something astronomers once thought would never be possible.

One team was led by Christian Marois, an astronomer at the Herzberg Institute of Astrophysics in Victoria, Canada. His group found three planets orbiting a star called HR Eight Seven Nine Nine. The star is about one hundred thirty light years from Earth in the constellation, or group of stars, known as Pegasus.

The team used the Keck Two and Gemini telescopes in Hawaii to produce the image showing the planets. The scientists also used a special technology called adaptive optics.

It changes the shape of a telescope's mirror to cancel the effects of the Earth's atmosphere. This permits clearer, higher quality images to be made.

The planets are huge. Two of them are ten times the mass of Jupiter - the largest planet in our solar system. This puts them near the mass, not of planets, but of failed stars called brown dwarfs. However, there is evidence that supports the idea that they are planets.

Bruce Macintosh of Lawrence Livermore National Laboratory helped make the discoveries. He says all three objects orbit their star in the same direction and plane -- the way planets do.

A second team announced the discovery of one planet orbiting the star Fomalhaut. Fomalhaut is the eighteenth brightest star in the sky. Paul Kalas of the University of California led a team that used the Hubble Space Telescope to find the planet now called Fomalhaut b. It took several years of observations to confirm its existence.

The planet is so distant from its star that it takes eight hundred seventy-two years to orbit Fomalhaut once.

Mister Kalas says Fomalhaut b orbits its star at the inside edge of a huge flattened ring of ice and dust. This disk is similar to the Kuiper Belt, which surrounds our own solar system.

A week after the announcements by the two teams, a French team announced that it had directly imaged a planet.

Ann-Marie Lagrange of the University of Grenoble led a team that used the Very Large Telescope operated by the European Southern Observatory in Chile. The astronomers found the planet orbiting Beta Pictoris, a star sixty-three light years away.

The planet has not been seen long enough for astronomers to know its orbital period. But the researchers believe they have confirmed its planetary nature. They believe it is about eight times the mass of Jupiter.



Date: 2015-12-17; view: 306

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