The most efficient solar cells currently on the market today are also the most expensive. Unlike traditional silicon cells commonly used in photovoltaic consumer power setups, NASA relies on multijunction germanium, gallium-indium-arsenide, and gallium-indium-phosphide cells due to their much higher efficiencies. Unfortunately germanium, a semiconductor like silicon, is far scarcer than silicon, with costs of about $680 per pound. Further, the traditional cutting processes for the germanium cells brass-coated, steel-wire saws typically waste much of the wafer due to cracking, as germanium is a hundred times weaker than silicon.
A new cutting method has been devised by University of Utah engineers that promises to cut costs. In the new approach, researchers used an electrified molybdenum wire 75 to 100 microns thick, often used in machining tools, to cut the cells (in contrast the typical saw is 170 to 180 microns thick). The result was that they were able to cut cells with virtually no cracking and even cut thinner cells, previously infeasible to process, with little to no cracking.
Eberhard "Ebbe" Bamberg, an assistant professor of mechanical engineering, describes, "The idea is to make germanium-based, high-efficiency solar cells for uses where cost now is a factor. You want to do it on your roof."
Currently, the 4-inch wafers in germanium solar cells cost $80 to $100 each. Grant Fines, chief technology officer for germanium wafer-maker Sylarus Technologies in St. George, Utah says the new processing method may reduce costs at his company by more than 10 percent. He states, "Anything that can be done to lower this cost ultimately will lower the cost of solar power per kilowatt-hour, which is beneficial. That's why this technology Ebbe has come up with is very intriguing. (It will) reduce the amount we have to recycle and increase the yield. It has the potential to give good savings, which helps enable this technology here on Earth."
The new method has been extended to a patent pending multiwire approach which Professor Bamberg compares to an egg slicer. This approach should help make the method ready for mass production.
Germanium cells have a maximum theoretical efficiency of over 50 percent. Using concentrators, efficiencies as high as 40 percent can be achieved. With traditional silicon solar cells, 20 percent conversion of sunlight to electricity is about the theoretical maximum.
Using current 300 micron thick wafers, the production yield was increased 30 percent. With thinner 100 micron wafers, production was up a whopping 57 percent. Perhaps most importantly, "kerf", or wasted germanium, was reduced by almost 22 percent.
The one remaining piece of the puzzle is to reduce the time of the cuts. Currently the method takes 14 hours, while traditional cutting takes 6 hours. While the new approach offers the possibility of crack free cutting, it currently has to be done a more ginger speed to avoid such cracking. However, Professor Bamberg and his fellow researcher Dinesh Rakwal, a doctoral student in mechanical engineering, are confident they can reduce the time to 6 hours before long. If this prediction holds true, this would be a very significant development to the solar power industry.
The new research appears in the Journal of Materials Processing Technology.