PHOTOVOLTAICS Resolution vs noise
effective method of weeding out defective wafers or cells, as well as a means of sorting them based on their efficiency. Crystalline silicon PV modules have a broad emission peaking at 1,150nm, while luminescence from copper indium gallium di-selenide (CIGS) and copper indium di-selenide (CIS) thin film solar cells peaks at around 1,330nm. The signal from crystalline solar cells can be measured with a silicon-based detector such as a CCD. However, 1,150nm is right at the edge of a CCD’s sensitivity and the sensor will only be operating at a few per cent quantum efficiency in the near-infrared. A deep-cooled CCD has high sensitivity but
requires a long exposure time, which doesn’t lend itself to making inline quality control inspections. ‘A typical photovoltaic production line will produce six wafers per second,’ states Ludger Kemper, general manager of Allied Vision Technologies’ Osnabrück site. ‘If you want to inspect silicon wafers on the production floor at this speed then you have to use SWIR cameras because the exposure time is so short.’
Dr Communal at Raptor Photonics agrees: ‘Deep-cooled CCDs are expensive, and more of a laboratory application; I don’t see them being used on the production floor or for preventive maintenance.’
Thermal cameras can detect which cells in a solar panel are working correctly just by the surface temperature
An EMCCD is another option for luminescence inspection, where the amplification of the signal and the elimination of the readout noise give the detector a high sensitivity. The exposure time required for an amplified signal from an EMCCD will be shorter, but again an EMCCD will be limited in frame rate – a few frames per second, according to Dr Communal. EMCCDs are also expensive and restricted to 1 megapixel resolution. A third potential sensor for these applications is scientific CMOS (sCMOS), which has low readout noise, large resolution, small pixels, and is sensitive enough to detect a large electroluminescence signal. The electroluminescence signal does have to be high, however, says Dr Communal, because sCMOS pixels are small and the exposure times are short. Sensitivity therefore might be an issue. The alternative to silicon detectors is an InGaAs sensor, which, says Dr Communal, is ideal because it is sensitive where it matters, i.e. in the SWIR region and not below 900nm. ‘This is the most sensitive system and you can reach high frame rates,’ he says. The downside with SWIR cameras are that they are expensive and have low resolution, typically VGA (640 x 512 pixels). Inspecting thin-film solar cells, such as cadmium telluride or CIGS, which luminesce at 1,300-1,700nm, is restricted to InGaAs cameras because the emission is outside the sensitivity of CCD detectors. ‘Everyone wants more resolution and less noise with SWIR,’ says Dr Communal. ‘Sensitivity and resolution are increasing slowly, but the price of these cameras is still large. There are at least two orders of magnitude in price between a SWIR camera and an industrial vision silicon- based camera. ‘Those that use SWIR cameras do so because they have to,’ he adds. ‘SWIR cameras are perfect for scientific photovoltaic material characterisation because the luminescence peak matches the sensitivity readout of InGaAs detectors. Now technology needs to be improved to offer larger fields of view and higher sensitivities.’
Inspecting at the
level of single wafers means microcracks can be detected early
Wafer inspection The process of fabricating a PV module begins with sawing silicon ingots into wafers. These are treated, printed and electrically connected to form a solar panel. ‘The silicon can have inherent defects within its crystal structure and the sawing process can also introduce defects in the wafers, such as variations in surface flatness and cracks,’ explains Simon Stanley, managing director of the LED business unit at lighting specialists ProPhotonix. ProPhotonix’s infrared Cobra Slim LED line lights are used to inspect silicon wafers for microcracks. The company’s 3D Pro and InViso structured light lasers can be used to inspect for surface flatness, including for roughness and undulations on the wafer surface.
Inspecting at the level of single wafers means
microcracks can be detected early in production. Crystalline silicon is brittle and has to be handled very carefully and any microcracks can become larger as wafers pass further along production. ‘The earlier microcracks are detected in the production cycle, the more cost-effective the process will be,’ states Kemper at AVT. Isra Vision provides its Solarscan-Micro-D system specifically for microcrack detection in silicon wafers, calculating the position, shape and size of the cracks. The company also provides a photoluminescence system, the Yieldmaster-PL, which can inspect more than 3,600 wafers per hour, for defects like microcracks, inclusions, and low efficiency regions. Wafers and finished panels are also tested with a solar simulator, a light source that simulates the electromagnetic spectrum of the Sun. Solar simulators are typically based on a xenon light source, but ProPhotonix’s LEDs have also been used. ‘LEDs can be regulated more effectively to provide an accurate solar spectrum with a uniform output,’ comments Stanley. ‘An LED source gives greater opportunity to optimise the spectrum and monitor it over time. An LED system will also last longer than a xenon source.’ ProPhotonix uses chip-on-board technology in the manufacture of its LEDs, meaning it is able to produce compact LED sources with multiple wavelengths.
FEATURE
➤ PHOTONICS FOR RENEWABLE ENERGY 2013 9
Flir
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