16 September 2008
Solar power by the slice
A new way to slice thin wafers of the chemical element germanium may soon lead to a more efficient use of solar power cells.
This new method should lower the cost of such cells by reducing the waste and breakage of the brittle semiconductor, said engineers at the University of Utah.
The expensive solar cells now see use mainly on spacecraft, but with the improved wafer-slicing method, “the idea is to make germanium-based, high-efficiency solar cells for uses where cost now is a factor,” particularly for solar power on Earth, said Eberhard “Ebbe” Bamberg, an assistant professor of mechanical engineering. “You want to do it on your roof.”
“We’re coming up with a more efficient way of making germanium wafers for solar cells—to reduce the cost and weight of these solar cells and make them defect-free,” said Dinesh Rakwal, a doctoral student in mechanical engineering.
Brass-coated, steel-wire saws now slice round wafers of germanium from cylindrical single-crystal ingots. But the brittle chemical element cracks easily, requiring users to recycle broken pieces, and the width of the saws means a significant amount of germanium is lost during the cutting process. The engineers developed a sawing method for silicon wafers, which are roughly 100 times stronger.
The new method for slicing solar cell wafers, known as wire electrical discharge machining, wastes less germanium and produces more wafers by cutting even thinner wafers with less waste and cracking. The method uses an extremely thin molybdenum wire with an electrical current running through it. They used the method previously for machining metals during tool-making.
Germanium serves as the bottom layer of the most efficient existing type of solar cell, but its use mainly goes to NASA, military, and commercial satellites because of the high expense as raw germanium costs about $680 per pound. Four-inch-wide wafers used in solar cells cost $80 to $100 each, and the new cutting method may reduce the cost by more than 10%, said Grant Fines, chief technology officer for germanium wafer-maker Sylarus Technologies in St. George, Utah.
“Anything that can be done to lower this cost ultimately will lower the cost of solar power per kilowatt-hour, which is beneficial,” and will encourage wider use of solar power, he said. “That’s why this technology Ebbe has come up with is very intriguing.”
Sylarus is considering using the new method but must determine if he can scale it up so he can mass produce wafers in a commercially viable manner, Fines said.
Bamberg’s method would “reduce the amount we have to recycle and increase the yield,” he said. “It has the potential to give good savings, which helps enable this technology here on Earth.”
Germanium is a semiconductor at the bottom of “multijunction” solar cells. Above it are layers of gallium-indium-arsenide and gallium-indium-phosphide. The layers work together to capture different wavelengths of sunlight, and the germanium also serves as the substrate upon which the solar cell “grows.”
When sunlight hits a solar cell, the energy converts to a flow of electrons in the cell, namely, electricity.
Silicon-based solar cells on Earth have maximum efficiency of 20%, Fines said. In space, germanium solar cells typically convert 28% of sunlight into electricity, but on Earth where researchers use solar concentrators, they can convert more than 40% of sunlight into electricity, and their efficiency theoretically exceeds 50%, he said.
Despite the greater efficiency of germanium-based solar cells, a 2005 survey found 94% of solar cells made for non-space uses ended up silicon-based because silicon is much cheaper and less fragile than germanium, the Utah researchers said.
Bamberg said germanium-based solar cells go on most spacecraft because they are more efficient and lighter than silicon-based solar cells. By making it more attractive economically to use efficient germanium solar cells on rooftops, the weight and size of solar panels can go down “so it doesn’t bother you aesthetically,” he said.
The new method may make germanium-based solar cells competitive with less efficient but less expensive silicon-based solar cells for uses on Earth, Bamberg said.
In the new method, the molybdenum wire essentially is an electrode, and it connects to a pulsed power supply that charges the wire during the cutting process.
A cylinder-shaped germanium ingot rests on a horizontal support, and the wire lowered into the ingot as new wire pulls out continuously from a supply spool to replace the cutting wire as it wears. Thin, synthetic oil injects along the wire, to increase the electrical charge on the wire and to flush away material that melts during the cutting process.
The process is slow. Wire electrical discharge machining takes 14 hours to cut a single wafer. Bamberg said the electrified wire method has to go gently to avoid cracking the germanium, but he hopes to increase the speed to the six hours it now takes to cut a wafer using a wire saw.
Wire saws made of brass-coated steel have a thickness of about 170 or 180 microns (millionths of a meter). The Utah researchers used molybdenum wire 75 to 100 microns thick, a bit thicker than a human hair. They use less germanium during the slicing process because the electrified cutting wire is thinner.
The study found a 100-micron-thick electrified wire significantly reduced the waste and increased the number of wafers coming from a germanium ingot, but a thinner 75-micron-wide wire did even better.
“At the current standard wafer thickness of 300 microns, you can produce up to 30% more wafers using our method” with a 75-micron-wide wire, Bamberg said. “Since we produce them crack free, we can also make them thinner than standard techniques. So if you go down to a 100-micron-thick wafer, you can make up to 57% more wafers [from the same germanium ingot]. That’s a huge number.”
For related information, go to www.isa.org/manufacturing_automation.
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