Reducing costly waste

University of Utah engineers have devised a new way to slice thin wafers of germanium that they claim could lower the cost of solar cells based on the semiconductor material.

The expensive solar cells now are used mainly on spacecraft, but with the improved wafer-slicing method, germanium-based, high-efficiency solar cells could be used for generating solar power on Earth, said Eberhard Bamberg, an assistant professor of mechanical engineering at the university.

Brass-coated, steel-wire saws are currently used to slice round wafers of germanium from cylindrical single-crystal ingots. But the brittle chemical element cracks easily, requiring broken pieces to be recycled, and the width of the saws means a significant amount of germanium is lost during the cutting process. The sawing method was developed for silicon wafers, which are roughly 100 times stronger.

The new method for slicing solar cell wafers - 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.

Germanium serves as the bottom layer of the most efficient existing type of solar cell, but is used primarily on NASA, military and commercial satellites because of the high expense - raw germanium costs about $680 (£369) 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 per cent, said Grant Fines, chief technology officer for germanium wafer-maker Sylarus Technologies in St George, Utah.

Fines added: '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. That's why this technology is very intriguing.'

Sylarus Technologies is considering using the new method, but must determine if it can be scaled up so wafers can be mass-produced in a commercially viable manner, Fines said.

A patent is pending on a way of using the new method so that multiple parallel electrically charged wires could be used to cut the germanium wafers in a production line environment.