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Here comes the sun

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Greg Blackman on how infrared and thermal imaging can maximise the efficiency of solar panels

Photovoltaics might not be the most vibrant market at the moment. However, according to the latest report from the European Photovoltaics Industry Association (EPIA) cumulative PV installations – which reached 100GW in 2012 – could grow to more than 400GW by 2017. At the moment, supply of PV modules dramatically exceeds demand, but analysts at Lux Research predict that supply and demand will balance out by 2015 – which should return manufacturers to profitability and herald a resurgence in the market.

All this is good news for companies supplying equipment for the solar market, including machine vision firms installing inspection systems for quality control. The construction of solar panels – each panel is made of a number of solar cells, which, in turn, are comprised of a number of silicon wafers, all connected in series – means that their efficiency relies on every wafer and cell working fully.

‘Manufacturers want to match and characterise each cell precisely, so all cells within a panel generate the same amount of electricity,’ explains Jean-Edouard Communal, regional sales manager at camera supplier Raptor Photonics. ‘The efficiency of the overall panel will be determined by the least efficient cell.’

Raptor Photonics provides cameras for the solar industry, including InGaAs and EMCCD variants for luminescence measurements. Machine vision is used for standard dimensional inspection and defect detection in PV production, but luminescence is more specific to solar cells and provides a method of determining the efficiency of a cell or module.

The properties of solar panels mean they can be induced to emit light when excited by a burst of light (photoluminescence) or when a current is passed through them (electroluminescence). A small fraction of photo-generated electrons will recombine with a hole to emit a photon in photoluminescence, while a forward-biased solar cell will essentially act as an inefficient LED in electroluminescence. In both cases, the fewer the defects, the higher the emission intensity from the wafer, cell, or panel. This makes luminescence measurements an 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,’ explains 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.’ However, he adds, there is often more time to inspect a complete panel – so cooled CCD cameras, which have a higher resolution, would be more typically used here. Allied Vision Technologies (AVT) provides SWIR cameras, among other imaging equipment.

Thermal cameras can detect which cells in a solar panel are working correctly, just by the surface temperature. Credit: Flir 

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.’

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 one 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 to 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 cameras,’ says Dr Communal. ‘Both 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 the detector technology needs to be improved to offer larger fields of view and higher sensitivities.’

Infrared and thermal imaging company Xenics has developed its XFPA-1.7-640-LN2 InGaAs detector, which is cooled with liquid nitrogen for low noise and low dark current. Since it is cooled and optimised for operation at 77K, it is extremely sensitive and is ideal for R&D in photovoltaics and luminescence measurements.

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.

The company’s infrared Cobra Slim LED line lights are used to inspect silicon wafers for microcracks. Its 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 as 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 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,’ says Stanley. ‘An LED source gives greater opportunity to optimise the spectrum and monitor it over time. It 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.

How efficient is your solar panel?

Once solar panels are installed on a building or other site, maintenance then becomes an issue. Sites with large-scale installations might be better equipped for ensuring their solar panels are operating at maximum efficiency, but home owners with panels on their roofs might be less aware of how well those panels are working.

ProPhotonix's infrared Cobra Slim LED line lights can be used to inspect silicon wafers for microcracks. 

‘There was a huge rush prior to various government tariff steps to finalise domestic installations where PV companies were interested in completing the installation, but not in the maintenance aspect. There are a lot of people out there with solar panels on their roofs who don’t know whether they are working correctly or not,’ say Tony Hargan at South Survey, a company based in Clitheroe, UK, providing a range of surveying, building and construction equipment.

Thermal imaging cameras are ideal for identifying faulty solar panels or panels where the efficiency of individual cells is unequal, simply by measuring the differences in surface temperature. A faulty cell will be colder than the surrounding cells, which a thermal camera will be able to detect. Hargan feels that thermal cameras will be the next thing to be used by the photovoltaic industry for domestic installations. South Survey supplies thermal imaging cameras from Flir for such inspections.

‘Solar farms are more geared up to maintenance, because they are built on a much bigger scale and they’re aware of the potential problems,’ says Hargan. ‘Some of the housing associations with large installations will also know about the maintenance side. We’re now getting enquiries from PV installers about thermal imaging cameras, whereas 12 months ago they were asking about laser measurers to measure the roofs so that they can quote for installing a panel. There’s a critical mass of PV systems installed; now companies are beginning to pay more attention to the maintenance side of things.’

The advantage of Flir cameras is that they allow the user to take a digital image and a thermographic image at the same time and store them together, says Hargan. This means the user can match images of faulty panels to a particular building.

Most domestic roofs have around eight panels, which can be inspected with thermal cameras. Property owners will lose money if their solar panels are not working at maximum efficiency with a feed-in tariff.

‘Thermal imaging is something home-owners should be asking for as part of their annual maintenance on their panels,’ states Hargan. ‘It would be very easy to give people a digital image and a thermographic image of their installation so they can see everything is working correctly.’

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