University of Utah engineers have devised a way to slice thin wafers of germanium for use in what may become the most efficient type of solar power cell. The new method should lower the cost of such cells by reducing the waste and breakage of the brittle semiconductor.
Though silicon-based solar cells on Earth have a maximum efficiency of 20 percent, in space, germanium solar cells typically convert 28 percent of sunlight into electricity. But on Earth where solar concentrators are used, they can convert more than 40 percent of sunlight into electricity, and their theoretical efficiency exceeds 50 percent.
Now used mainly on spacecraft, germanium solar cells are expensive, but with the improved wafer-slicing method, the researchers expect to come up with a more efficient way of making germanium wafers for solar.
Until now brass-coated, steel-wire saws have been used to slice round wafers of germanium from cylindrical single-crystal ingots. But the brittle chemical cracks easily, requiring broken pieces to be recycled, and the width of the saws means a significant amount of germanium is lost — kerf loss — during the cutting process.

The new sawing method, known as wire electrical discharge machining (WEDM), 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. It has been used previously for machining metals during tool-making.
The study was funded by the National Science Foundation, The University of Utah Research Foundation and Sylarus Technologies.

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