NIR Trends: Photoluminescence Inspection of Photovoltaics with SWIR Imaging

Contactless machine-vision inspection using photoluminescence (PL) imaging with shortwave infrared (SWIR) cameras can help solar cell producers improve both efficiency and quality of their photovoltaic products. Inspection of silicon bulk ingots, sliced wafers, processed layers, and complete photovoltaic cells is possible with SWIR imaging. The PL emissions occur at the wavelengths associated with the semiconductor bandgap, which are visible to high-sensitivity, uncooled, indium gallium arsenide (InGaAs) cameras, even while imaging at video frame rates. This inspection technique illuminates the objects of interest with high optical power at one wavelength, and photons are absorbed in the bulk material. Some energy is lost as heat in the interaction with the molecular structure and the remaining energy causes a photon to be re-emitted at a longer wavelength. The uniformity and intensity of the resulting glow is sensitive to many parameters of the material, and of subsequent processing steps. In many cases, properties of the material, such as minority carrier lifetime, can be mapped from the PL image, and these maps will directly relate to how well the final product will perform as a solar cell.

SWIR photoluminescence imaging has several major advantages for machine vision inspection over visible and SWIR imaging of electroluminescence (EL).

  • Contactless – PL inspection is performed without contacting the material being inspected.
  • No or minimal sample preparation – 100 percent, on-line inspection can take place, at room temperature, at various point in the fabrication process, without disrupting the production flow.
  • SWIR PL imaging works with raw, partially, and full-finished wafers – Alternatively, EL requires contact electrodes and is primarily used for finished-product quality control, by manufacturer and for in-coming inspection by system integrators.
  • SWIR PL works with backside metalized wafers – The metal blocks SWIR imaging through the wafer, but SWIR PL inspection detects those defects with hurt minority carrier lifetime, without contact. When electrical contact is possible, SWIR EL actively reveals other defects that hurt efficiency, including cracks.
  • Video frame rate acquisition up to 30 fps – SWIR cameras have high quantum efficiency (QE) at the prime silicon emission wavelengths, permitting imaging at 30 VGS frames per second or 60 ¼ BFA fps, unlike silicon cameras used for PL, which require frame exposure times of 1 to 30 seconds per image at similar resolutions.

The image gallery below demonstrates the use of PL to find no-uniformities, and defects in the silicon wafers.

The PL images were 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 Figures 1 and 3 of the L images shown, 60 W for Figure 4. 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 (the cut-on wavelength was 1000 nm); one filter was used with the InGaAs camera, there were stacked when used with the silicon camera. The test station, silicon camera images and wafers were provided by the National Renewable Energy Labs (NREL) at Golden, CO.

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.

Figure 1a: 

SWIR PL image of a finished cell acquired with on lnGaAs area photodiode array using a 16-ms exposure time, at the rate of 60 frames/second. The InGaAs camera resolution is 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

Figure 1b: 

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. Note that the SWIR PL image (left) with its lower resolution and much shorter exposure time has revealed the same detailed map of defects and non-­ uniformities as the CCD camera.
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.

Figure 2a: 

lnGaAs-SWIR camera EL image of the same cell as above, but with an electrical contact bus bar in contact with the cell bus bar (note the thicker black bars across the wafer compared to the PL images). Applied current to the cell electrodes was 1.5 A, and camera exposure time was 11 ms with 60fps rate.
EL image of the same cell taken by the CCD using a 1 second exposure time.

Figure 2b: 

EL image of the some cell taken by the silicon CCD camera image using a 1-second exposure time. The SWIR image provides similar detail and reveals more variation in the bulk effecting efficiency /note grey patches in top and center sections that are not seen in the CCD image). The SWIR EL image also sees the misalignment of the back bus contact line through the wafer (thin line above the lower bus bar).
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.

Figure 3a: 

SWIR PL image of a wafer, before addition of electrode structures; token with the lnGaAs camera using 16ms exposure and combined laser diode illumination of 30 W.
PL image of a wafer taken with CCD using 1 second integration time.

Figure 3b: 

PL image of the some wafer token with a CCD camera using a 1-second integration time. The noise in this CCD image is eliminated by using 10-second integration, but that is 600 times longer than the SWIR camera (left)  used to get similar clarity.
PL image of wafer sliced from silicon boule, acquired with ~60 W driving the lasers and 16 ms exposure time for Goodrich camera. Outer ring is strong specular reflection of laser light from rounded edge of wafer slice penetrating single RG-1000 LP filter.

Figure 4a:

PL image of lapped wafer sliced from silicon boule, acquired with ~60 W driving the lasers and a 16-ms exposure time for the Goodrich SWIR camera. The interior rings result from variations in crystal growth; the outer ring is strong specular reflection of the 810 nm laser light from the rounded edge of the wafer slice. This light penetrated through the single RG-1000 LP filter and was still detected by the standard lnGaAs camera, despite its very low QE at that wavelength.
PL image of same boule slice, acquired with CCD using 1 second integration time.

Figure 4b: 

PL image of same boule slice with the same laser illumination, acquired with a CCD camera using a 1-second integration time. Strong silicon CCD QE at laser wavelength of 810 nm permits specular reflection to pass through 3 of the RG-1000 LP filters, causing the 4 bright spots on the edge. Again, note the acquisition time advantage of the SWIR camera.

Both Sensors Unlimited linescan and area cameras can be used for photoluminescence inspection of photovoltaic solar cells. For more information on this subject click here.

As seen in the 11/18/09 edition of the Photonics Online (www.photonicsonline.com) newsletter.

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