POSTED 28 MAR 2002
Paul Alivisatos, (c) Science
Paul Alivisatos, (c) Science
Like Harold Gray, the creator of Little Orphan Annie, photovoltaic (PV) cells work by liberating small creatures from their "homes."
Like Annie, an electron that leaves home can roam the world to aid the innocent and nab the guilty. While Annie blew up Nazi submarines, electrons can power computers so you can visit Annie on the web.
Comic-strip creators can make an orphan with just pen and paper. But liberating electrons with PV requires special material and structures. Because electric current is simply moving electrons, a PV cell must remove electrons from their humdrum existence orbiting atoms, and then export them quickly. Otherwise, they can be grabbed by an "electron trap," and the current will cease.
When you consider the environmental advantages of PV cells, in an ideal world, they could be printed on plastic as cheaply as comic strips on newsprint. In the real world, PV cells are made of silicon, and the cost is closer to the price of Harold Gray's original artwork of the famous orphan.
Gadzooks! A better cell?
The solid semiconductor is cadmium selenide, formed into tiny rods, and the plastic is a polymer with the soft-and-cuddly nickname P3HT. The technique is still highly experimental, but it points to an era of low-price, high efficiency photovoltaics.
The hybrid cells produced by Paul Alivisatos and colleagues worked well under low light -- converting 6.9 percent of the energy in sunlight to electricity. That, says Alivisatos, a professor of chemistry and material science at UC, is close to the 10 percent to 12 percent efficiency of standard solid-state cells.
Under simulated sunlight, however, the efficiency plunged to 1.9 percent. The problem, Alivisatos says, is that the polymer and solid are in such close contact that electrons and holes (places that lack electrons) can reach the wrong conductor, nullifying the electrical current. "It's a tricky business," he says. "It turns out that in cells with interpenetrating networks, normally you'd like each material to attach to the correct electrode only." Intense light, he says, "allows some leakage in the wrong direction." However, he adds that it should be possible to raise efficiency in bright light by adding blocking layers to the cells.
In fact, such tinkering may allow the technology to outdo standard PV cells, which use only one material to absorb incoming photons. "If you absorb red light or blue light," Alivisatos says "you get the same power out per photon, even though the blue photon carries more energy."
More sophisticated and expensive cells, like the ones just bolted onto the Hubble Space Telescope, use more than one light-absorbing semiconductors, and thus can grab light more efficiently.
Even after six years of testing a fabrication technique intended to mass-produce solar cells, Alivisatos says the gadgets are being made not by the acre, but in one-inch squares. "We're at the testing stage, and it doesn't make any sense to make large areas."
The continuing interest in electricity from sunlight reflects PV's advantages: it is clean, silent and, theoretically, as abundant as sunlight itself. But until the price falls considerably, the dream of reducing the potential for global warming and other problems caused by fossil-fuels with carpets of solar cells will remain just that -- a dream.
If the hybrid technology -- or something similar -- ever bears fruit, however, PV will no longer be an orphan technology. And you will read about that on the front page, not the comics.
-- David Tenenbaum
Sun: SOHO Extreme ultraviolet Imaging Telescope (EIT) full-field He II 304 Å image from NASA Goddard Space Flight Center [2002/03/28 13:19:38]