Solar Cells Info

By LEAH KRAUSS, UPI Energy Correspondent
TEL AVIV, Israel, Nov. 16, 2006  (UPI)

Source: United Press International
http://www.upi.com/Energy/view.php?StoryID=20061116-050127-6744r

Sharp’s announcement this week that it has higher-efficiency thin film solar cells underlines the potential of this technology to take over the solar market. “The adoption of an amorphous/microcrystalline thin-film tandem cell design, which uses stacked layers of amorphous silicon and microcrystalline silicon achieves a conversion efficiency of 8.5 percent — 40 percent higher than conventional amorphous solar cells — among the industry’s highest levels for thin-film silicon-based solar cells currently in volume production,” Sharp said in a statement Wednesday.

By way of comparison, regular photovoltaic panels usually run around 17 percent efficiency. Representatives from Sharp did not respond by press time to questions about the cost of the 90-watt units.

Amorphous silicon is a material with “a random arrangement of atoms that operate at lower temperatures” than other forms of silicon, solar expert Martin A. Green, a professor at Australia’s University of New South Wales, told the Solar Power 2006 conference in October.

Green said that thin film technologies in general are the second generation of solar innovation, and offer several advantages over conventional photovoltaic panels: Generally cheaper, they also provide units with automatically interconnected cells for ease of installation; thin film is also more aesthetic and demonstrates a “potential for ruggedness.”

Furthermore, Green said, thin film “is already at a similar cost to crystalline with much lower production (cost) levels.” In short, the technology “has the potential for driving PV cost down rapidly,” Green said. The upper limit for solar cell efficiency is about 74 percent, according to Green, and ideas around today could bring the technology close to this high efficiency.

Stacking cells with different characteristics — somewhat similar to Sharp’s idea of combining amorphous and microcrystalline silicon — could do this, Green said. Such a tandem that is then stacked on top of silicon could “get the efficiency quite close to the limit,” Green continued. “It’s really quite neat.”

Thin film PV cell technology has improved in recent years. They “started off as amorphous silicon (and) suffered from two problems: that their starting efficiency was low — typically 5 percent, compared with 10 percent for crystalline silicon — and they were unstable, degrading by typically 5 percent per year of their starting efficiency,” the director of Israel’s National Solar Energy Center, David Faiman, told United Press International earlier this year. Faiman cited efficiency percentages based on his own experience testing many such panels.

“Recently some more promising thin-film materials have appeared that could replace amorphous silicon,” Faiman said. “The most interesting that I have tested is Copper-Indium-Diselenide, which had relatively high starting efficiency of about 8 percent and was stable for the nine or so months I was able to test it.”

Faiman said extremely thin films of crystalline silicon were also available “that should have efficiency and stability comparable with conventional silicon panels.” Economic predictions for thin film solar are difficult to make, Faiman said.

“About 10 years ago, when crystalline silicon panels were advertised at $5 per watt peak, amorphous silicon modules could be had for as cheaply as $3.50 per watt peak,” he said, referring to the direct current watts output of a solar module as measured before it leaves the factory.

“However, both of those prices were artificial,” Faiman said. “Since then, intense competition, driven by artificially high feed-in tariffs in Europe, particularly in Germany and Spain, have driven the price of conventional panels sky-high.”

The price for the conventional panels “is still around $5 per watt peak, instead of having continued to fall, and they are very hard to get hold of: typical quoted delivery times for small purchasers (are) around two years,” Faiman said. “This may be an opportunity for amorphous silicon, but caveat emptor!” Faiman said.

Venture capitalist Vinod Khosla — a prominent investor in the solar energy industry and the founding CEO of Sun Microsystems — doesn’t see silicon replacements’ price advantage as a long-term value. “Reducing cost may not be the right thing to do … multijunction thin film can (already compete with conventional energy forms) unsubsidized,” Khosla told the San Jose conference.

Khosla said his goal is to see solar energy compete with fossil fuel-fired energy on its own, without help from government subsidies and incentives. He cited higher efficiency solar cells as something “we need to work on.”
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