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Application: Inspection of Multi-Junction Photovoltaic Cells with SWIR Imaging

Concentrated photovoltaic (CPV) multi-junction cell developers have achieved record solar cell energy conversion efficiencies by overlaying three light absorbing layers and associated photodiode junctions.  By careful choice of the materials, thereby matching the absorbance of each layer to the sun’s spectrum, the output of each junction will contribute equal amounts of electrical current to the cell’s output. As the junctions are photodiodes connected in series, the maximum current output of the cell will be limited by output of the lowest performing junction.  It is therefore important to ensure that all three junctions will work efficiently because the current from the good layers will be limited by the lowest current generating layer.  Multi-junction cells like these are usually positioned at the focal point of light concentrators typically designed to focus the equivalent of 500 suns onto an area of 1 cm2 and then generate amps of current.  It is vital for cell fabricators, solar concentrator integrators, and solar farm installers to inspect individual cells before incurring the expense of final assembly into large concentrator structures. Fortunately, multi-spectral electroluminescence imaging can be used to separately inspect each layer’s junction uniformity and relative output.

A typical multi-junction CPV cell is small, typically measuring just 1 cm on a side.  These cells generate a lot of current, so the collection lines and busbars have to be robust, yet can’t be large enough to block the sun light from reaching the cell surface.  In the cell used for the images below, the collection lines are only 10 µm wide, and are on 100 µm centers.  Dust particles on the surface, or defects in the various layers, can seriously impact energy collection as well.  Consequently, high resolution inspection is a requirement for good quality control. Current area camera technology for SWIR area cameras provides VGA resolution of 640 columns by 512 rows, while SWIR line-scan cameras permit imaging 1024 pixels. Thus, the highest resolution imaging can be obtained by scanning the wider linear array over the small cell.  The line-scan camera is mounted on microscope optics, and the line acquisition synchronized with the motorized stage movement.  This permits ultra-high resolution imaging by taking several passes, each offset to the side to image a different portion of the cell.    

The image gallery below demonstrates the use of imaging electroluminescence (EL) with several filters in the SWIR to find non-uniformities, cracks, and defects in the cell layers.  These images are of one type of multi-junction cell, one composed of GaInP and GaInAs layers on a Ge substrate.   The first four images were acquired by ImageXpert of Nashua, NH in a single pass per image using their wafer inspection system with a Goodrich line scan camera.  The black and white image was acquired without a wavelength filter other than the camera’s natural response from 700 to 1680 nm.  The other three were acquired through filters that isolate the emissions of each junction. (The images were converted using a false color palette to help bring contrast at both low and high intensity portions of the image.) The top layer, emitting around 700 nm, shows a number of dark spots that are not visible in the other images, indicating that the defects causing them are relatively transparent to the longer wavelengths.  However, in all of the images, some of the dark spots are noticeable.  These are likely caused by dust or digs on the top surface, which will block all wavelengths from reaching the camera.

EL image of a multi-junction cell acquired with an InGaAs 1024 pixel line scan camera array without filtering. Image cropped to 880 columns x 773 rows. The horizontal line pattern is composed of 10 µm lines on 100 µm centers.     EL image of same cell, taken with 800 nm short pass filter in front of lens to capture only the 700 emission from the top GaInP layer; note number of dark spots apparent in image. The image is displayed with warm scale false color plot generated by Goodrich SUI Image Analysis program to bring out details within dynamic range of image. Note the number of dark spots this makes visible.  
Image acquired by     Image acquired by  
         

EL image of a multi-junction cell acquired with an InGaAs 1024 pixel line scan camera array without filtering. Image cropped to 880 columns x 773 rows.  The horizontal line pattern is composed of 10 µm lines on 100 µm centers.

 

    

EL image of same cell, taken with 800 nm short pass filter in front of lens to capture only the 700 emission from the top GaInP layer; note number of dark spots apparent in image.  The image is displayed with warm scale false color plot generated by Goodrich SUI Image Analysis program to bring out details within dynamic range of image.  Note the number of dark spots this makes visible.

  
         
EL image taken with a combination of 800 nm long pass and 1250 nm short pass filter to isolate EL emission at 920 nm from middle layer of GaInAs film.      EL image of the same cell taken with 1250 nm long pass filter to capture from Ge substrate its emissions from beyond 1550 nm.  
Image acquired by     Image acquired by  
         

EL image taken with a combination of 800 nm long pass and 1250 nm short pass filter to isolate EL emission at 920 nm from middle layer of GaInAs film. 

   

EL image of the same cell taken with 1250 nm long pass filter to capture from Ge substrate its emissions from beyond 1550 nm.

  
         
Color camera image of multi-junction CPV device, glowing red due to forward bias driving the top layer to emit at 700 nm. This particular cell has physical damage to the fine collector lines in the lower part of the cell.     InGaAs 2-D camera image of damaged multi-junction cell under room light illumination.  

Color camera image of multi-junction CPV device, glowing red due to forward bias driving the top layer to emit at 700 nm.  This particular cell has physical damage to the fine collector lines in the lower part of the cell.

    

InGaAs 2-D camera image of damaged multi-junction cell under room light illumination.

  
         
InGaAs 2D camera image taken with 980 nm bandpass filter with 40 nm FWHM.     Same cell and camera, but using a 1550 nm long pass filter to show emissions from Ge substrate.  

InGaAs 2D camera image taken with 980 nm bandpass filter with 40 nm FWHM.

    

Same cell and camera, but using a 1550 nm long pass filter to show emissions from Ge substrate.

  
         
     
InGaAs SU640KTSX-NIR-1.7RT camera image of cell with damaged collector lines, taken without filters. Cell voltage @ 2.64V and drawing 50 mA.     Same camera and cell, image taken with 790 nm short pass filter to show emissions from top layer (peak emission at 700 nm).  
         
     
Same cell and camera, but using a 940 nm bandpass filter to show emissions from GaInAs layer. Note that center bright area in contrast with dark area at shorter wavelength.     Same cell and camera, imaged through 1050 nm long pass filters to show emissions from the Ge substrate.  
         
         

 

Both Goodrich linescan and area cameras can be used for electroluminescence inspection 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 on other types of PV cells or photoluminescence to read a related article on the subject:   NIR Trends: Maximizing Solar Cell Yield and Efficiency with Machine Vision.


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