Short-wave infrared looks at solar wafers
This past year, we've seen more interest in improving solar-cell efficiencies.
Ann R. Thryft, Contributing Technical Editor -- Test & Measurement World, 5/1/2010 12:00:00 AM
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The drive to increase both production efficiency and product efficiency of crystalline-silicon PV (photovoltaic) wafers and cells is leading manufacturers to explore a number of different inspection techniques. Engineers are using SWIR (short-wave infrared) technology, primarily in design or failure-analysis labs, to check PV wafers for voids and cracks, as well as checking cells and panels for short circuits and incomplete current conduction, which can decrease efficiency or lead to the failure of an entire panel.
"Manufacturers of solar wafers and cells use other techniques to perform these checks, and they work pretty well to sort good from bad," said Doug Malchow, business development manager for industrial products for Sensors Unlimited Goodrich ISR Systems. "This past year, we've seen more interest in improving solar-cell efficiencies, and therefore more interest in the use of our SWIR technology and two imaging techniques, photoluminescence and electroluminescence, to find out why performance is bad."
Although light in the SWIR band of the spectrum is invisible to the human eye, it casts shadows and creates contrast in images created with linescan and array cameras based on InGaAs sensors, said Malchow. These sensors are extremely sensitive and can detect discontinuities in conditions with extremely low visible light.
At SWIR wavelengths, silicon becomes transparent. Since circuits are created on a wafer's front side, InGaAs sensors can create images of these circuit patterns on crystalline-silicon PV wafers and standard silicon wafers through their back sides, to determine whether they are aligned properly and to find cracks and voids, said Malchow. "When finished crystalline PV solar cells are connected to a power supply and voltage is forced into them, it creates an electroluminescent glow that can be imaged to find cracks, dead spots, contaminants, and weak areas," he said.
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The glow's intensity indicates the amount of collection efficiency in each cell and each panel. When a panel contains a mix of cells of different efficiencies, the more efficient cells waste huge amounts of energy pumping current through the less efficient or dead cells, bringing down overall panel output and efficiency.
"Cell manufacturers want to identify problems long before they put the electrodes on the cell," he continued. "Imaging photoluminescence from cells or even unprocessed wafers permits catching major material or processing problems before building expensive inventory that ends up being scrapped."
InGaAs linescan or area cameras can be used in conjunction with an IR microscope to inspect very fine detail in surface or buried layers of standard silicon wafers, said Malchow, as well as to give a rough measure of layer thicknesses. For those who need to inspect wafers at very high speeds, the new Goodrich SU1024LDH2 linescan camera will reach line rates of over 91,000 lines/s, reducing exposure times. "Our cameras are also used for examining structures inside MEMS devices, to ensure that nothing is left from the process to obstruct movement," he said.
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A Laser scanning system for solor cell testing was patented years ago. This is nothing new.
Michael Gauthier - 2010-14-7 11:39:12 EDT
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