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More Efficient, and Cheaper, Solar Cells

New manufacturing techniques could cut solar power costs by 20 percent.

Improvements to conventional solar cell manufacturing that could significantly increase the efficiency of multicrystalline silicon cells and bring down the cost of solar power by about 20 percent have been announced by startup 1366 Technologies of Lexington, MA.

Light trap: Incoming light reflects off grooves in a silver band and is redirected along a glass cover. This light, which is usually lost, can then be absorbed by the solar cell. The grooved band is one of three improvements that could significantly lower the cost of making solar power.

Such cost reduction would make solar power more competitive with conventional sources of electricity. In sunny environments, this could bring the cost of solar down to about 15 or 16 cents per kilowatt hour, says Craig Lund, 1366 Technologies’s director of business development. That’s cheaper than some conventional sources of electricity, especially those used during times of peak electricity demand.

1366 Technologies has developed three processes that can be incorporated into existing solar cell manufacturing lines to improve cell efficiency. It has shown that these technologies can be used to produce multicrystalline solar cells that are 18 percent efficient at converting sunlight into electricity. The current industry standard for such solar cells is 15 percent to 16 percent, according to Joonki Song, a partner with Photon Consulting, based in Boston, MA, although higher efficiencies have been reported. The company has demonstrated the new technologies before, but only with very small, experimental solar cells in a laboratory. Now it’s made full-size solar cells using the type of equipment used in large-scale manufacturing.

The key to the startup’s technologies, however, isn’t the efficiency that it’s achieved, but how little that efficiency costs. Lund says that the new processes add only a few cents per watt to the cost of fabricating solar cells, but this investment leads to much greater cost savings in the final product. Improving the amount of power each solar cell generates lowers materials costs, solar module manufacturing costs (in which cells are assembled into solar panels), and installation costs. In the end, Lund says, the cost of an installed solar panel will be reduced by 50 cents to 80 cents per watt.

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The new processes, which were invented by Emanuel Sachs, the company’s chief technology officer and a professor of mechanical engineering at MIT, all increase the amount of light that solar cells can absorb.

In a normal silicon solar cell, electrons generated in the silicon must make their way out of the material to produce an electrical current, traveling first to the top layer of the silicon and then along this layer to narrow silver lines called “fingers.” The fingers then conduct the electrons to the busbars, two or three prominent silver bands seen on the surface of most silicon solar cells. These bands shade the silicon under them, reducing the amount of light the cells can absorb.

The first new process developed by 1366 Technologies produces grooved busbars that prevent light from being reflected out of a solar panel. Instead, the grooves cause light to be redirected along the glass on top of solar panels. That light can then be absorbed by unshaded areas of the solar cell.

The second process improves the cell’s electron-conducting fingers. Although these silver lines are much narrower than the busbars, there are many more of them on a solar cell, and together they shade a significant portion of the silicon. Sachs developed a process for making much narrower lines without sacrificing their conductivity. Instead of using conventional screen-printing technology, his process involves etching troughs into the surface of the silicon and depositing silver particles into the troughs. Metal is then added to these particles via electroplating to build up the fingers. The trough keeps the lines narrow but allows the silver to be stacked relatively high, maintaining conductivity. Typically busbars and fingers shade 9 percent of a cell surface, 1366 Technologies says, but with the company’s new processes, this shading can be reduced to 2 percent. Others have developed techniques for reducing shading, but these have been expensive.

The third process decreases the amount of light reflected off the surface of the cell’s silicon by texturing its surface. This is an approach that’s been taken by others, but the texturing is done in a very regular pattern that creates less surface area than other approaches. Surface area is a problem in solar cells, because electrons are often trapped at the surface of materials, Sachs says.

Because 1366 Technologies’s processes can be incorporated into existing manufacturing lines, they could be adopted by solar cell manufacturers quickly and inexpensively, Sachs says. The company is working to further decrease the width of the silver fingers and improve the texturing, with the goal of reaching an efficiency of 19 percent.

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