solar cell inspection based on electroluminescence

München, (PresseBox) - In electroluminescence inspection of solar cells, a current is driven through the cell causing it to emit light in the near infrared region, around 1200 nm. This is beyond the wavelength range normally visible to a silicon detector, and so expensive cooled InGaAs cameras have been used in the past to capture images. The approach is attractive because it offers a means of inspecting the cells for defects such as local poor connection of the metal fingers, other resistive shuts, micro fractures, and others. The approach of using the cell >backwards< to emit light is an elegant way of finding problem areas affecting the performance of the cell when used >forwards< to collect light.

Good SNR and resolution is essential in order to identify small defects such as micro fractures and distinguish these from the natural vein-like structures in the silicon. Existing cameras tend to fall short of the requirements for electroluminescence inspection in two ways. Firstly, they have a low quantum efficiency in the wavelength range of interest, meaning that a long integration time is needed to capture enough light for a good enough image. Secondly, they are typically very expensive.

The Photonfocus EL1 camera has high quantum efficiency in the near infrared range, up to 1000 nm. The short wavelength tail of the emission spectrum in electroluminescence imaging reaches down into this range. The signal is weak, but it is measurable with the EL1 camera. It is the fact that the measured responsivity of the EL1 camera in the wavelength range of interest is a factor of 10 times or more higher than standard products that gives it a decisive advantage in this application.

The comparatively large 8 µm pixel size of the sensor is a good fit ot NIR optimised optics that are required to get a good image in this wavelength range and supports a good overall quantum efficiency and resolution performance of the imager. The framerate currently targeted in electroluminescence imaging ist modest, nowhere near the 240 megapixels per second the A1312 sensor is capable of, so the sensor is operated in a spectral low power mode to minimize noise.

The camera sensor is manufactured in standard CMOS process with some special optimisations for image sensors. In particular, some aspects of the sensor design have already been optimised for NIR-imaging. The camera sensor is very flexible with regard to pixel control, and a special pixel operation sequence was developed to minimize noise.

The result is that the Photonfocus EL1 camera can deliver acceptable image quality for electroluminescence inspection of solar cells with a comparatively fast integration time of 400 ms and without cooling.

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