University of Utah engineers have a devised a less wasteful way to slice wafers of germanium for solar power cells.

Currently, germanium is restricted to high-end dual or triple junction solar cells, but the researchers have found a way to cut 10% from the price.

"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 mechanical engineer Ebbe Bamberg.

According to the University, wafers today are sawn from single crystal ingots of germanium using a brass-coated steel wire.

"The sawing method was developed for silicon wafers, which are roughly 100 times stronger," said the University. "But germanium is brittle and 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," said Utah.

Adapting 'wire electrical discharge machining' (WEDM), a method previously used for machining metals during tool making, Bamberg's technique makes a thinner cut and puts less stress on the wafer.

It uses a thin molybdenum wire connected to a pulsed power supply.

With the ingot resting horizontally, the wire is lowered onto the germanium where tiny sparks erode the ingot and the wire, the latter being continuously replaced from a spool as it wears.

Thin synthetic oil injected along the wire increases the erosion effect and flushes away waste.

The process is slow. "Wire electrical discharge machining takes 14 hours to cut a single wafer, said the University. "We hope 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 are around 175µm thick whereas the molybdenum wire used at Utah is 75 to 100µm.

Mechanical engineers Dinesh Rakwal and Ebbe Bamberg watch as an electrified molybdenum wire cuts a wafer from a germanium block with less waste.

The study found that a 100µm electrified wire significantly reduced the waste and increased the number of wafers that could be made from a germanium ingot, and 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 ingot."

The researchers cut 66mm-diameter wafers with a thickness of 350µm.

Germanium serves as the substrate and bottom junction material in triple junction cells, topped with junctions made in gallium-indium-arsenide and gallium-indium-phosphide.

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