LBIC: Seeing solar’s potential

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To detect PV defects and efficiency variations, laser beam lights have been used on solar cells with digital imaging in a process called light (or laser) beam induced current (LBIC) mapping to determine the performance or power conversion efficiency of a solar cell.  With LBIC, there is a direct correspondence between the photovoltaic function in each pixel area of the solar cell and of the image, light is focused on each area, and then the electrical response is measured. The information contained in the LBIC image is absolute and can even provide the external quantum efficiency (EQE) in each pixel – but the serial process of the technique implies that speed is inherently slower. LBIC mapping has only been useful for imaging small square millimeter-sized single junctions and due to the serial nature of image formation, large panels and modules have been impossible to image in a fast industrial process. Until now, according to Danish startup InfinityPV.

“Today’s fast data handling – acquisition and storage – enables the data from an image as large as a solar panel with high resolution to be handled,” says Frederik Krebs, CEO of InfinityPV, a startup manufacturer of printed solar cells and test equipment for printed electronics. “The optical and electrical part have however been missing.”

InfinityPV has dedicated significant efforts toward devising optical methods that allow rapid scanning over the surface of a solar panel with rapid low noise amplification of the photovoltaic signal.

An illustration of the LBIC imaging of a large panel showing differences in performance due to ingress through the holes in the glass and point defects stemming from manufacturer.

Image: InfinityPV

“The InfinityPV principle increases the speed of LBIC by up to 10 million times, almost like magic,” says Krebs.

Providing an example, he says the achievable image speeds of a classical LBIC system with 100 micron resolution that employ lock-in amplification with an integration time of 100 milliseconds per pixel can image one square centimeter (10,000 pixels) in 16 minutes. And a standard photovoltaic panel measuring 1 square meter would be 100,000,000 pixels and take 115 days.

“Such a technique is hardly useful in an industrial setting and is also easily subject to changes in the environment surrounding the experiment – such as weather, light, and time of year,” says Krebs. “In terms of data rate, there is of course no challenge with this approach.”

In InfinityPV’s approach, the laser is scanned across the surface of the panel using a fast mirror. Following the 100 micron example, the company says the laser is typically scanned over the width or breadth of the solar panel at a rate of 1000 lines, or 10 cm per second. And the entire solar panel is now imaged in around 10 seconds.

This is where data rates come into play, as all data must be transferred during imaging. With 16-bit resolution, the image alone of a 1 sqm solar panel is 200 MB. The data rate is approximately 10-20 times higher than this.

The different commercially available LBIC systems that address different scales of solar cells from small devices to large panels and rolls of solar cells.

Image: InfinityPV

When pushing the technique, the current capacity of 10 micron resolution and 17,000 lines per second, the imaging speed for 1 sqm of solar panel is as low as 6 seconds with a resolution of 10 micron. Whereas the time to image 1 sqm of solar cell is currently limited to around two seconds for mechanical reasons (you have to physically move the panel or the camera). In terms of data, the 1 sqm image comprises 200 GB of data, and data handling rates of ~3 Tbit per second.

“Currently, this cannot be achieved with regular equipment,” says Krebs, adding that the best compromise is to employ standard industry components for a low resolution of 100 micron at near optimal speed followed by careful imaging at higher resolution for selected smaller areas that are identified as showing defects in lower resolution overview image.

“This can be achieved at high speed using readily available industrial technology and as a result InfinityPV is able to offer high-speed LBIC imaging solutions for large panels and modules at affordable cost, adds Krebs. “Systems are available that map from small cells and modules all the way up to large solar panels – all in a matter of seconds.”