The National Renewable Energy Laboratory has developed a means to screen dud silicon wafers prior to the manufacture of photovoltaic cells, a technique that the lab claims could save the solar industry billions of dollars.

A wafer is a fragile, 180-micrometer layer of semiconductor substrate which is doped with impurities to create the crucial p-n junctions that enable the flow of electrical current. The problem, according to the NREL, is that between 5 and 10 percent of wafers break during the PV cell manufacturing process. Failure is caused by minuscule cracks that occur as the wafers themselves are made, an intensive procedure that begins with raw silica, and involves furnaces, chemical purification, and multi-wire sawing, among other things.

The cracks leave some wafers too fragile to survive the expensive process of fabricating PV cells, in which the wafers are doped, fitted with electrical contacts, coated, and sealed. Even a process as simple as moving a wafer from one stage of cell manufacture to the next can prove fatal. The later the stage of failure, the more expensive it is to the manufacturer.

Dennis Schroeder

Dennis Schroeder

Dennis Schroeder

The NREL's idea is to weed out flawed wafers before they enter the PV cell manufacturing stage. To that end, it has created the Silicon Photovoltaic Wafer Screening System (leaving us with the less-than-compelling acronym SPWSS), a furnace designed to fit into existing factory assembly lines. The machine thermally stress-tests wafers by sequentially heating narrow strips of each to temperatures of about 500°C (the precise temperature can be calibrated to suit the needs of different manufacturers). NREL scientist Bhushan Sopori likens the process to filling a glass with very hot water. The idea is that, if the wafer is to break, it should break inside the new machine.

NREL is claimed that the system is extremely efficient, using "nearly 100 percent" of the input energy, keeping the cost of using the system down to what Sopori puts at "some fraction of a penny per wafer." (According to NREL, making solar cells costs approximately 15 cents per watt of capacity.) The lab estimates that by eliminating the 5 to 10 percent of cells with dud wafers, the solar industry could save billions of dollars. It hopes that this will be a tangible benefit to manufacturers in the US, where the share of the global PV market has fallen from 42 percent in 1997 to 4 percent in 2011.

The SPWSS will come in manual and automatic versions, both capable of processing 1,200 wafers per hour, which the NREL thinks is quick enough to keep pace with the production lines of most manufacturers. The basic SPWSS costs $60,000, but it doesn't automatically remove broken wafers; the automated $100,000 SPWSS-A does. Whether broken wafers are separated by machine or by hand, they can be recycled into new ones.

Listing image by USFWS Mountain Prairie.

Listing image by USFWS Mountain Prairie