SOLAR CELL INSPECTION
Sun rises on quality control
As solar power becomes an increasingly important energy source, Andrew Williams looks at how infrared imaging is used to assess solar panels
I
n recent years, infrared thermography has become a popular solar cell inspection technique that enables
manufacturers to detect hot spots and other anomalies that cannot be found by visual inspection. Tere are a number of ways it’s used. Near
infrared and shortwave infrared cameras, for example, are able to see cracks and other defects on the inside and opposite side of a silicon wafer or ingot. Longwave infrared imaging can also be used to inspect and monitor the integrity of cells configured into solar panels once they have been installed, commonly by employing a thermal imager to detect hot spots or dead cells. Mike Grodzki, product manager
for scanning products at Teledyne, noted that infrared cameras can also be used to test photovoltaic cells by techniques called photoluminescence and electroluminescence, both of which can highlight flawed regions in the solar cell. Photoluminescence uses powerful illumination to induce luminescence, while electroluminescence requires an electric current to be applied to the cell or module and the emitted light measured. Dr Jonas Kornelius Haunschild, head of the inline-wafer, process analytics and production control group at the Fraunhofer Institute for Solar Energy Systems ISE, explained that solar cells are inspected inline at two major stages in the production process. Te first is an outgoing inspection of wafers or an incoming inspection of cells. At this stage, the raw silicon wafer is
inspected to verify wafer specifications and discard poor quality material. Te second stage is during the current- voltage (I-V) tests of the finished solar
cells before module production begins. ‘Depending on different companies, inspection is also done during processing, but these two are the most important ones,’ said Haunschild.
Transparent to infrared For inspection of incoming cells, the team at Fraunhofer ISE uses three different infrared systems: infrared transmission, infrared reflection and photoluminescence imaging. Haunschild described the first system, infrared transmission, as a simple method that uses a camera to detect 1,400nm infrared light focused on each side of the wafer. ‘Te wafer should be almost transparent
16 IMAGING AND MACHINE VISION EUROPE OCTOBER/NOVEMBER 2021
to this wavelength – and so, in the images, defects, particles and cracks can be seen. Simple image processing algorithms catch the defects and the inspection system can sort them out,’ he said. Haunschild described infrared imaging
as a kind of X-ray vision, because silicon is transparent to infrared light and therefore it can be used to see inside wafers and solar cells to detect defects. ‘With normal optical inspection, these
[defects] are not visible, but they are still there, limiting the performance of the solar cells and indicating where there is potential for improvement in the production chain,’ he said.
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