Some 5 to 10 percent of photovoltaic cells shatter at some point during fabrication, thanks to microscopic cracks in the silicon wafers on which they’re built. The fabrication processes—oxidation, annealing, purification, diffusion, etching, and layering—account for about half of the solar cell’s total installed cost, so this breakage wastes significant money, time, and material.
Researchers at the National Renewal Energy Laboratory (NREL) have built a device that identifies the friable wafers before they go into fabrication. The Silicon Photovoltaic Wafer Screening System (SPWSS), developed by Bhushan Sopori’s NREL team, uses intense light from an optical cavity furnace to heat narrow bands on the wafer up to 500ºC. This produces strong thermal gradients that splinter the already-cracked wafers while leaving solid wafers intact.
The SPWSS is built around the technology Sopori’s group used to develop the innovative optical cavity furnace, originally designed to support high-temperature photovoltaic-cell fabrication operations. The heart of the SPWSS is a chamber roughly 40 cm on a side lined with a highly insulating, reflective ceramic and lit by high-intensity lamps (the patent mentions three to five computer-controlled 1600-Watt tungsten-halogen lamps). The ceramics retain the radiation almost perfectly, capturing the light and diffusing it into a highly uniform thermal glow. By cutting a 15-millimeter slit in the bottom of the chamber, the group produced a very uniform light source for localized heating. Then they added a wire-mesh conveyor (think of the toaster at a breakfast buffet), computer-controlled to produce the spot-heating duration appropriate to the material’s thickness and other characteristics.
The conveyor can test as many as 20 wafers per minute—fast enough to keep up with other production processes and cheap enough to cost just “some fraction of a penny per wafer,” according to Sopori.
Images: Dennis Schroeder/National Renewable Energy Laboratory
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