Vision shines in solar inspection
The PV solar market is still an emerging market, and its test procedures haven't yet been set in stone.
By Ann Thryft, Contributing Technical Editor -- Test & Measurement World, 8/1/2010 12:00:00 AM
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Makers of crystalline-silicon PV (photovoltaic) solar wafers and thin-film cells are under continued pressure to improve efficiency and lower prices. Kamalina Srikant, vision product manager for National Instruments, described several vision techniques that manufacturers can employ to inspect and characterize solar cells and wafers, both in-line and offline.
Q: How does the inspection of solar wafers and cells differ from that of regular semiconductors?
A: Semiconductor inspection requires wafer sorting and alignment, which is mostly done in-line. But for crystalline-silicon PV wafers and cells, there's a range of offline and in-line inspection tasks that can be done with small systems.
For example, there are a lot of checks to see if the cell has short circuits and is properly conducting current, and these tests are done both in-line and offline. When developing solar cells, you need to ensure performance and power, and those inspections, such as I-V [current-voltage] characterization, are often done in conjunction with vision tests, all offline.
Q: What types of compact hardware are used to inspect in solar wafers and cells?
A: For most crystalline-silicon PV wafer applications, we normally see PC-based systems in use, depending on the types of cameras and sensors needed. For simpler applications, some smart cameras may come into play. The small machine-vision systems, such as our Compact and Embedded Vision System products, can either be placed on the line with the product on a conveyor belt, such as in wafer sorting, or they can be used offline.
Q: How can vision help with solar-cell characterization?
A: In an efficient solar panel, the individual wafers and cells must all share the same electrical characteristics. One technology for characterizing cells is vision.
First, you connect a source-measure unit, like our NI PXI-4130, to each cell and drive current through it to get the cell's current-voltage curve. Based on what that curve shows, you can determine the cell's performance characteristics. You can also tell whether there are any significant shunts in a cell that indicate defects.
But this electrical performance testing only gives you numbers that tell you whether the cell is good or bad. To determine the nature of the defect and see where it's located, you need vision.
Near-infrared vision can reveal whether the wafer wasn't cut properly and can locate spots that lose power. You can also use vision on groups of cells in larger panels to identify the odd cells with different power characteristics, which may be due to shunts.
Sometimes you can cut around the shunts from the center of the cell to isolate them, instead of being required to scrap the entire cell, thus saving some material costs.
Q: What is the future of vision technology in the inspection of PV solar wafers and cells?
A: The PV solar market is still an emerging market, and its test procedures haven't yet been set in stone. So, there's a lot of room to put in additional types of testing, like vision, that can help guarantee better-quality products.
As this industry matures, solar manufacturers that do more testing will see better returns, and that will help them become bigger players.
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