Thursday, August 14, 2008

More-Efficient Solar Cells

Original Article

A new solar panel could lower costs and improve efficiency.

Better cells: A new design for solar
panels (top) improves their
efficiency. Each panel is
made of arrays of square
solar cells. A conventional
solar cell (bottom) requires
thick silver contacts that
block light and reduce cell
performance. The new design
uses a novel electrode that
eliminates these silver contacts.
Credit: Day4 Energy
By changing they way that conventional silicon solar panels are made, Day4 Energy, a startup based in Burnaby, British Columbia, has found a way to cut the cost of solar power by 25 percent, says George Rubin, the company's president.

The company has developed a new electrode that, together with a redesigned solar-cell structure, allows solar panels to absorb more light and operate at a higher voltage. This increases the efficiency of multicrystalline silicon solar panels from an industry standard of about 14 percent to nearly 17 percent. Because of this higher efficiency, Day4's solar panels generate more power than conventional panels do, yet they will cost the same, Rubin says. He estimates the cost per watt of solar power would be about $3, compared with $4 for conventional solar cells. That will translate into electricity prices of about 20 cents per kilowatt-hour in sunny areas, down from about 25 cents per kilowatt-hour, he says.

Other experimental solar technologies could lead to much lower prices--indeed, they promise to compete with the average cost of electricity in the United States, which is about 10 cents per kilowatt-hour. But such technologies, including advanced solar concentrators and some thin-film semiconductor solar cells, probably won't be available for years. Day4's technology could be for sale within 18 months, the company says.

In conventional solar panels, the silicon that converts light into electricity is covered with a network of silver lines that conduct electrons and serve as connection points for soldering together the individual solar cells that make up a panel. The network consists of rows of thin silver lines that feed into thicker wires called bus bars. Day4 replaces these bus bars with a new electrode that consists of rows of fine copper wires coated with an alloy material. The wires are embedded in an adhesive and aligned on a plastic film. The coated copper wires run on top of and perpendicular to the thin silver lines, connecting them to neighboring cells. The new electrode conducts electricity better than the silver lines, resulting in less power loss. It also covers up less of the silicon than the bus bars, leaving more area for absorbing light.

What's more, the new electrode allowed Day4 to redesign solar cells to absorb more of the solar spectrum and convert this light into electricity more efficiently. Solar cells comprise two layers of silicon. For light to be converted into electricity, it has to pass through the first layer and reach the second. The thinner the top layer, the more light reaches the second layer to be converted into electricity. In a conventional cell, the silver lines are deposited and then heated to high temperatures, which causes the metal to diffuse into the silicon. The top layer must be thick enough that the silver does not diffuse through it and create a short circuit between the layers of the solar cell. By replacing the large bus bars with the new electrode, Day4 was able to make the top layer of the solar cells thinner, increasing the amount of light that can be converted into electricity. Also, since the silver can damage the silicon, replacing it with the new electrode increases the solar cell's power output.

The technology "sounds pretty exciting," says Travis Bradford, a solar-industry analyst with the Prometheus Institute for Sustainable Development, an energy research firm based in Cambridge, MA. The question, Bradford says, is whether the company can translate the latest advances from its lab to large-scale production without increasing costs.

Day4 has already started producing solar panels using its new electrode material--though not its new solar-cell designs. The company recently announced that it has the capacity to produce enough solar panels every year to generate 47 megawatts of electricity. These first-generation panels, which use conventional solar cells, have an efficiency of 14.7 percent. The company's next step is to put its new cell design into production and incorporate these cells into its solar panels, with the goal of improving their efficiency to 17 percent.

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