PixEL

Versatile Electroluminescence and Photoluminescence

PixEL is available as a high speed inline inspection module or as a customized research imaging platform that performs Photoluminescence, Electroluminescence and time-resolved infrared thermography.  It may be configured for cell imaging, module imaging, or both, and includes advanced image processing algorithms to extract device parameters and present them in high-resolution.  Inline applications include the detection of cracks, tool marks, substrate oxygen precipitates, metal contamination, non-uniform contact and sheet resistance and other common processing problems.  Please contact Tau with your specific requirements. 

The PixEL architecture is modeled after the system developed and used by Johnston et al at the National Renewable Energy Laboratory in Golden, Colorado. Detection capability extends from 350nm to 14mm when three cameras are installed, allowing the most extensive PV imaging capability available today.  R&D labs that need to measure challenging, weakly luminescent samples will find that this system exhibits industry-leading sensitivity and signal to noise ratio.  

PixEL systems are built with a variety of cameras so that hardware performance and price is matched to the end-user’s requirement.  Recent work by Tajima[1], Johnston[2], Tarasov[3], Oldenburg[4] and others has explored the near-infrared (NIR) detection of sub-gap, defect mediated luminescence in Silicon.  Modern InGaAs cameras with exceptionally high sensitivity are able to detect luminescence with wavelengths ~900-1650nm, and this allows a single camera to, with proper filters, separately measure Silicon Band Edge luminescence (900-1300nm) and Defect related luminescence (1300-1600nm).  When both images are measured using either EL or PL, simple models can be used to extract the defect density (Nakayashiki[5]):

Tau Science currently offers 640x512 and 320x256 resolution cooled, research grade InGaAs cameras and pairs them with an automated filter wheel- please note that these cameras are ITAR export controlled.  The software acquires a sequence of images as the filter wheel steps through the wavelengths of interest.  Integration times of 30ms to 10s are typically used, although for weakly luminescent samples the integration time may be extended beyond 10 minutes.  In each case images are automatically acquired and averaged to achieve the total integration time, and dark frames are automatically subtracted. 

InGaAs cameras have a significant sensitivity advantage over Silicon cameras from 900-1600nm, making them the preferred choice for several common PV materials:

• Silicon (both band edge and defect-mediated luminescence)
• Ge and SiGe
• CIGS (see example below), CZTS, InGaAs

However, the lower cost and higher resolution of Silicon CCD’s make them a viable alternative for both Silicon and higher bandgap PV materials:

• CdTe
• a-Si:H
• Silicon (band edge luminescence only)
• InGaP, etc.

CIGS minimodule Electroluminescence at 1-sun forward bias current. 

When Silicon cameras are used to inspect Silicon PV cells, the overlap between camera sensitivity curve and luminescence curve is not ideal.  Only a few percent of the luminescence is detected by the camera, but reasonable images are obtained when the cell is a strong emitter (well-passivated, high lifetime) or if the integration time is sufficiently long.  For as-cut silicon wafers, the combination of indirect bandgap and high surface recombination velocity sets the ratio of “laser photons in” to “PL photons out” near 10^9 - an exceptionally inefficient process.  However, cooled research grade Silicon CCD’s are used in PixEL, offering the low noise and long integration times needed for this application.  Tau’s Silicon cameras include several types:

• Conventional front illuminated CCD (lower cost, lower NIR performance) - 1MP to 9.3MP
• Back-illuminated, deep depletion CCD's (higher cos, good NIR performance) - 1MP only.
• sCMOS (modest QE, but excellent readout speed for inline use)- 2MP. 

The system may optionally be configured with a third camera type for hotspot detection: a time-resolved infrared thermography system named IRIS. Please contact Tau regarding this option.