SOLAR CELL INSPECTION


and LED technology over the last few years… will hopefully result in cost- effective photoluminescence devices’


‘Te rapid developments in camera


ISE is photoluminescence imaging, where infrared light of around 800nm is shone and absorbed by a sample. Using a method known as band-to-band recombination, luminescent radiation is then generated and detected by a silicon sensor. According to Haunschild, although this method is quite simple in principle, it is technically challenging because a very powerful laser and very good optical filters are required. ‘Te method [photoluminescence] can


be used in all steps of the manufacturing chain to find electrical defects and process errors. Finished solar cells or modules can simply be excited via the contacts as an alternative to the laser. Tis method, called electroluminescence, is now offered at very low cost and is part of solar cell inspection or module control,’ he added.


Remote monitoring According to Magnus Herz, senior expert, R&D at TÜV Rheinland, although optical methods of fault detection in photovoltaic power plants have been used in the photovoltaic industry for many years, uptake has been limited because it takes a lot of time and labour to make these measurements across a large solar plant comprehensively. As an alternative, he said the strategy of using remote aircraft equipped with infrared cameras can be a cost-effective and much less time-consuming option for fault diagnosis over these solar farms. He points out that solar module


In the second method, infrared reflection,


a shorter wavelength of infrared light is shone onto the wafer from the same side as the detecting camera. Here, as Haunschild explained, the trick is that infrared light is ‘coupled into the wafer and moves there like in an optical fibre’. At points of interference, especially microcracks, the light is decoupled and detected by the camera. ‘Microcracks are much more clearly


visible with this method [infrared reflection] than with classic infrared transmitted light. However, smaller defects such as SiN or SiC particles cannot be seen, so both methods have their justification,’ he said. Te third approach used at Fraunhofer


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inspection benefits from infrared and electroluminescence imaging techniques – both of which are widely used indoors and outdoors. ‘A combination of electroluminescence and infrared measurements was proposed to detect the most common defects in a solar module with high accuracy,’ he said. ‘Electroluminescence measurements


take advantage of the radiative inter-band recombination of excited charge carriers in solar cells,’ he explained. ‘Te solar cells are supplied with a defined external excitation current, while the camera takes an image of the emitted photons. Damaged areas of a solar module appear dark or radiate less than areas without defects. Te high resolution of the electroluminescence images enables the


resolution of some defects more precisely than in infrared images [alone].’


Image evaluation Broadly speaking, the Fraunhofer ISE team views the hardware of its infrared devices as mature, and does not anticipate any further additional technical innovations in the near future. Tat said, Haunschild said there has been great progress in the area of image evaluation, particularly in photoluminescence imaging. ‘Measurement results contain a lot of


information about the quality of the wafers and, since no one can look at all the high- resolution data in production, powerful image analysis based on machine learning and artificial intelligence is needed to extract the essential information from the images to make them usable for quality control and process development,’ he said. ‘As a concrete example, we have been


working on quality control of kerfless wafers from NexWafe in particular over the last year,’ he continued. ‘Tis is a very exciting technology with many advantages over classical silicon. ‘Te infrared reflection method could


also be considered as still relatively new,’ he added. ‘It was developed in industry and has some advantages over classical infrared transmitted light.’ Meanwhile, although unaware of any


major recent technological or operational breakthroughs in the use of infrared technologies for solar cell inspection,


g


Drones equipped with infrared cameras can be used to inspect solar farms


OCTOBER/NOVEMBER 2021 IMAGING AND MACHINE VISION EUROPE 17


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TÜV Rheinland


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