Contactless machine vision inspection of silicon ingots, raw and processed wafers and of complete photovoltaic cells is possible with imaging of photoluminescence (PL) at the silicon bandgap wavelength using high-sensitivity uncooled InGaAs cameras, complementing other methods of photovoltaic testing In this technique, the objects of interest are illuminated with high optical power at one wavelength, and photons absorbed in the bulk material. Some energy is lost as heat in the interaction with the chemical structure and the remaining energy causes a photon to be re-emitted at a longer wavelength. The uniformity and intensity of the resulting glow from steady-state carrier concentrations is sensitive to many parameters of the material, and when applicable, of subsequent processing. In many cases, properties of the material, such as minority carrier lifetime can be mapped from the PL image, and directly related to how well the final product will perform as a solar cell. Thus, this is a valuable photovoltaic testing technique.
SWIR Photoluminescence imaging has several major advantages for machine vision inspection over visible and imaging of Electroluminescence (EL):
- Contactless – PL inspection is performed without contacting the material being inspected
- No sample preparation – 100% On-line inspection can take place, at room temperature, at various points in the fabrication process without disrupting the flow
- Works with raw, partially and fully finished wafers – EL requires finished electrodes and primarily used for finished product quality control and in-coming inspection for system integrators
- Works with backside metalized wafers – which would block transmission methods
- Video frame rate acquisition – At 30 VGA frames per second, unlike silicon cameras used for PL, which require frame exposure times of 1 to 30 seconds per image
The image gallery below demonstrates the use of PL to find non-uniformities, and defects in the silicon wafers. The first pair of images was acquired with illumination provided by four semiconductor diode lasers at 810 nm with overlapping diffused beams to achieve uniform illumination, one source per side. Approximately 30 W of illumination was used for most of the PL images shown. The images were taken at different times using the same 16 mm f/1.8 lens, with the 810 nm excitation filtered at the lens by RG-1000 long pass filters; one filter was used with the InGaAs camera, three used with the silicon camera. The photovoltaic testing station, the silicon camera images and the test wafers were provided by NREL at Golden CO.
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PL image of a finished cell acquired with an InGaAs photodiode array using a 16 ms exposure time, at 60 fps. Camera resolution was 320 x 240 pixels. |
PL image taken of the same cell with a cooled silicon CCD camera using a 10 second exposure time. The camera resolution was 1k x 1k pixels |
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EL image of the same cell as above, taken with the InGaAs camera. Applied current to the cell electrodes was 1.5 A, and camera exposure time was 11 ms. |
EL image of the same cell taken by the CCD using a 1 second exposure time. |
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PL image of a wafer, before addition of electrode structures; taken with the InGaAs camera using 16 ms and laser diode illumination of 30 W. |
PL image of a wafer taken with CCD using 1 second integration time. |
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PL image of wafer sliced from silicon boule, acquired with ~60 W driving the lasers and 16 ms exposure time for Goodrich camera. Interior rings result from variations in growth; outer ring is strong specular reflection of laser light from rounded edge of wafer slice penetrating single RG-1000 LP filter. |
PL image of same boule slice, acquired with CCD using 1 second integration time. |
Both Goodrich linescan and area cameras can be used for photoluminescence inspection testing of photovoltaic solar cells. The area cameras provide convenient still images while the digital high-speed, 1024 pixel line cameras are ideal for providing higher resolution at lower cost when used with continuous production flow or with moving inspection stages.
Click on the links for more examples of electroluminescence or to read a related article on the subject: NIR Trends: Maximizing Solar Cell Yield and Efficiency with Machine Vision.
Contact Goodrich today to find how easy it is to improve your product quality while improving your bottom line and helping your customers reduce their dependence on fossil fuels through the use of photovoltaic testing.
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