SOLAR CELL INSPECTION g


Grodzki pointed out that higher resolution, shortwave infrared cameras can be used for higher detail – and that near infrared- sensitive CCDs can also be used in some cases. He added that higher resolution and lower


cost can be achieved with colloidal quantum dot SWIR sensors. ‘Although the quantum efficiency of these devices is somewhat lacking today, the technology is quite promising,’ he said.


Higher yields When it comes to assessing the advantages of using infrared imaging, Grodzki pointed out that, although other techniques exist that enable users to detect defects on the surface of solar cells, only infrared can equip observers with more information about the material being used and the resulting outputs. ‘Te use of infrared techniques results in


higher yields for the manufacturer,’ he said. ‘Finding the right mix of infrared detector, illumination and optics can be a challenge, but many machine vision distributors have the knowledge to assist with this. ‘Another challenge for customers is price


– higher sensitivity means a higher cost,’ he added. Meanwhile, Haunschild observed that


decisions relating to the application of these infrared imaging methods in industrial processes always entail detailed cost-benefit considerations, and depend on the relative value placed on internal quality control. ‘My personal impression is that a lot of emphasis is placed on this, especially in


Solar panel inspection


western countries, which is why infrared transmitted light or infrared reflection is widely used as part of the entrance test for microcrack control,’ he said. Despite the undoubted benefits,


Haunschild pointed out that photoluminescence imaging is quite expensive because a strong laser and a safe enclosure are needed – meaning it is mainly used in the laboratory and rarely on production lines. ‘If prices can be reduced and the


technical concepts simplified, this could also change, [and] we [Fraunhofer ISE] are actively working on this. We consider the hardware concept to be mature… for demanding scientific applications. If one is willing to compromise in favour of price, the rapid developments in camera and LED technology over the last few years open up new possibilities that will hopefully result in


Taking the heat out of solar


As Magnus Herz, senior expert, R&D at TÜV Rheinland, explained, one of the most important pieces of data captured by infrared images is the temperature difference between various parts of a solar cell module, or between modules or strings of modules. ‘Inactive areas of a module or string usually appear hotter than surrounding active areas. When a photovoltaic module or string is not functioning, the energy generated is not converted to DC power and remains as


excess heat, raising the temperature of the module or string,’ he explained. ‘A great number of


failures on photovoltaic modules can be detected using infrared imaging, from hot spots to mismatch losses or installation failures,’ he added. Broadly speaking,


there are two different types of infrared or thermographic cameras – those with cooled detectors and those with uncooled detectors. The sensors used in cooled thermographic


cameras are made from narrow band gap semiconductors. However, although such cameras possess very high sensitivity, they are not commonly used in photovoltaic applications because of the cost and complexity of the associated cooling system. ‘Commonly used thermographic cameras are based on uncooled sensors that can work at ambient temperatures. The most common sensor architecture is the microbolometer,’ Herz said.


‘Researchers are working feverishly to introduce tandem solar cells… [which are] only partially measurable with the established infrared systems’


simple and cost-effective photoluminescence measurement devices,’ he said.


Future developments Looking ahead, Grodzki is hopeful that detectors will become more responsive over time, which would reduce the requirement for costly and powerful infrared illumination – a development he views as especially relevant for shortwave infrared. ‘Automated solar panel inspection will also


be further enabled by the combination of drones and thermal imaging,’ he added. Meanwhile, Haunschild predicted


that the introduction of new solar cell concepts will inevitably bring new inspection requirements. ‘On the one hand, due to the boom of


monocrystalline wafers, we see more and more specific defects, which are known from earlier years but had disappeared in the industrial environment. In particular, thermal donors can be a problem for heterojunction solar cell concepts. However, with appropriate hardware and the right image analysis, these defects can be evaluated and bad wafers can be detected,’ he said. ‘On the other hand, researchers are


currently working feverishly to introduce tandem solar cells into the industrial environment,’ he added. ‘In principle, these are two solar cells [placed] on top of each other to capture and use even more light. Tis structure is only partially measurable with the established infrared systems and requires adjustments in the technology.’ O


18 IMAGING AND MACHINE VISION EUROPE OCTOBER/NOVEMBER 2021 @imveurope | www.imveurope.com


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