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Monday, 30 January 2012

Startup Makes Peel-Off Solar Cells: Wafer-making method could mean cheaper solar power

Technology Review
Jan 30, 2012

Solar peel: This 25-micrometer film of crystalline silicon, deposited on a layer of metal, was produced using a new technique. Credit: Astrowatt

Today, most solar cells are made with a process that turns almost half of the raw material—highly refined and processed crystalline silicon—into sawdust. A new process developed by startup Astrowatt aims to eliminate most of this waste while making solar cells more efficient.

Conventional solar manufacturing requires sawing a block of crystalline silicon into wafers about 180 micrometers thick. As the saw cuts through the silicon, it turns almost the same amount of silicon (a layer 100 to 150 micrometers thick) into sawdust that can't typically be reused.

With the conventional process, a millimeter of silicon can produce about three solar-cell wafers. Astrowatt says it can make five or more wafers from the same amount of material by mostly replacing the sawing with a technique that allows it to peel thin layers of silicon away from a thick silicon wafer.

Astrowatt is one of several companies hoping to substantially reduce the amount of silicon needed to make solar cells. Although the price of silicon has dropped in recent years, it's still the most expensive item in solar-panel manufacturing.

The Astrowatt process begins by sawing a block of silicon into relatively thick wafers, each nearly a millimeter thick. The company then modifies the top of each wafer so that it can act as the back of a solar cell—a process that ends with depositing a layer of metal onto the wafer.

Next, the wafer is heated, causing stress within the material because the metal and silicon expand at different rates. Applying a wedge to the edge of the stressed silicon starts a crack that propagates from one edge to the other, allowing the engineers to finally peel away the metal film along with a thin, 25-micrometer layer of silicon. Crucially, the crystalline structure of silicon allows the crack to propagate evenly across the entire wafer, and the silicon is flexible, so it won't shatter as it's peeled off.
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