
Read our article on NIR Trends: Maximizing Solar Cell Yield and Efficiency with Machine Vision.
The image gallery below demonstrates the use of EL for solar cell inspection to find non-uniformities, cracks, defects, mismatched cell efficiencies, and the limited usefulness of thermal imaging for these applications.


Video of electroluminescence from polycrystalline solar cell induced by 1 Hz squarewave. Recorded in room light by SU320KTSX at 60 Hz.

Left: Commercial 36 cell solar panel imaged with a Sensors Unlimited SWIR camera while forward biased with 18 V. Right: Close-up view of two cells with defects visible in the image on the left, revealing a semi-circular crack in the lower cell, dead sections, and a scatter of defects in the upper cell.
All types of photovoltaic solar cells, such as mono and poly-crystalline, thin-film ribbon, CIS and CIGS, can be tested for uniformity and defects by simply forward biasing the cell until it glows as a result of electroluminescence, as seen in the figures below.
The samples were purchased off the open market for components, and show considerable variation, both within a single device and between several samples of a device type. The Sensors Unlimited SU640KTSX camera and SUI Image Analysis software was used to acquire the videos at 30 fps.

This 30 fps video shows the non-uniformity in electroluminescence emissions from three poly-crystalline solar cells that were grown with the ribbon method as the bias voltage is ramped from 2 to 3.8 V. The brighter areas are near the connection wires to the cells, which were wired in series. Note that the back-side contacts (small rectangles along the busbars) are clearly visible through the silicon.
The following images illustrate how non-uniformity can be found with SWIR solar cell inspection, both within an individual cell (note the non-uniformity across individual cells) and among several cells assembled into a panel.

The left-hand image was taken with the Sensors Unlimited SU320KTS camera, which collects the photoemission from within the thickness of the cell. The middle image taken in the visible shows color differences due to differences in the top layer thickness, but it can be seen that those variations have little to do with the photodiode efficiency or uniformity. The right-hand image was taken with a thermal microbolometer, which exposes little or no detail.
InGaAs cameras are ideal for use by the solar industry for cell inspection due to their high Quantum Efficiency for the wavelengths that the silicon and other solar cell material luminesce as illustrated by the graph below:

Both Sensors Unlimited 1D and 2D cameras can be used for electroluminescence inspection of photovoltaic solar cells. The area cameras provide convenient still images while the digital high-speed, 2048 pixel line cameras are ideal for providing higher resolution at lower cost when used with continuous production flow or with moving inspection stages.
To read about how you can maximize your solar cell yield and efficiency with SWIR, click here: NIR Trends: Maximizing Solar Cell Yield and Efficiency with Machine Vision.
For additional information on SWIR solar cell inspection, please visit our pages on:
Photoluminescence for PV Cells
Electroluminescence of Multi-Junction PV Cells
SWIR...Solar...Synergy
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2048L Linescan Camera
