23 January 2012
Optical vision solutions are pressing forward in production lines of solar wafers, cells, panels, thin-film modules and concentrator optics. Applications range from optical inspection of dimensions, colour, imprint and a myriad of defects to 3D optical shape measurement. Fast optical vision inspection is almost completely taking over from laborious manual quality assurance, significantly increasing production yield and reducing human error.
Fast optical vision significantly increases production yield and reduces human error
Automated vision systems improve many steps in PV production. They can inspect silicon crystals, wafers and cells on defects, identify wafers and PV cells and check anti-reflective-coatings. In combination with robots, they are very good at measuring the correct orientation and alignment of cells, screen-, front- and back prints. Further, they closely monitor laser edge isolation and can sort cells on colour.
The role of automated vision systems in module assembly is essential. They accurately carry out cell spacing, stringing alignment, connector and frame inspection, but also verify the entire panel assembly and its traceability. Manufacturers of vision systems for solar manufacturing include Isra Vision Solar, Cognex, Dalsa, PCO, Vitronic, Basler, Xiris Automation, JAI and Goodrich/Sensors Unlimited.
Robots with vision systems
Major robot manufacturers, including ABB, Fanuc, Kuka and Adept, are offering vision systems for additional control. Originally developed for automotive production, the technology is very suitable for solar manufacturing as well. In a vision system, software programmed with sophisticated, application-specific algorithms processes images captured from the production line and decides on the next movement of the robot. Picking and positioning of solar cells, stringers, frames and glass and EVA sheets are among the countless applications for vision controlled robots, but they also guarantee accuracy for soldering and welding.
ABB IRB340 FlexPicker robotic system picking and placing solar cells (image courtesy of ABB)
Key inspection based on optical techniques takes place on several locations in the production line. One of the most important inspections to take place before assembly is crack detection. Material defects such as cracks should be detected as early as possible in production, as they are later, once a panel is operational, posing a high risk of early power efficiency reduction or even cell breakdown. As cracks are usually barely observable in visible light conditions, several companies have developed electro-luminance (EL) systems. Showing the internal cracks very well, these systems are based on InGaAs short-wave infrared (SWIR) cameras, as silicon is transparent for SWIR wavelengths beyond 1200nm. Besides, this technology also reveals cavities in silicon crystal ingots before slicing them into wafers to produce solar cells and saw marks after slicing. SWIR camera systems also enable photoluminescence (PL) inspection for actively finding a further range of internal silicon wafer problems that will hurt cell or system power output. Manufacturers of EL and PL inspection system include Isra Vision Solar and Goodrich/Sensors Unlimited. With the introduction last year of a new 1.3 Megapixel imager at 15 µm pitch, Dr. John Trezza, Vice President for Force Protection, Goodrich's ISR Systems’ Princeton team, says: “This high-resolution camera with its industry-low dark current revolutionizes the SWIR imaging market and opens up many new applications where high resolution and sensitivity in the SWIR are critical.”
Image taken with a visible camera showing colour differences due to variation in the top layer thickness, not originating from photodiode efficiency or uniformity (image courtesy of Goodrich/Sensors Unlimited)
Image taken with an SWIR camera, which collects the photoemission from within the cell and enables detecting defects, hidden cracks or saw marks inside or on the opposite side of the wafer due to silicon’s transparency at SWIR wavelengths beyond 1.2 µm (image courtesy of Goodrich/Sensors Unlimited)
Inline thin-film inspection equipment adds multi-spectral light to find defects such as voids
Thin-film PV cells on flexible substrates permit applications not possible with traditional crystalline cells. However, flexible thin-film solar cells have placed difficult demands on machine vision systems. Inline thin-film inspection equipment adds multi-spectral light to find defects such as voids in anti-reflection- and passivation layers, stains, scratches, colour variance as well as structural problems such as digs, crinkles and discontinuities. Xiris Automation is offering such systems. And, taking a different approach is, BrightView Systems, which is offering a complete system to map and test the performance of thin-film panels. Dr. David Scheiner, CTO at BrightView Systems said, “Combined with innovative optical modelling that enables non-contact extraction of key optical and electrical properties at high throughput, our system allows users to squeeze more Watts per panel by seeing, for the first time, what’s actually manufactured over an area of several square meters, at a precision of several nanometers.”
When a solar panel is arriving at the end of the production line, it is ready for its final test: simulation of solar light falling on the panel and inspection of the electric power generated. Solar simulators are optical systems with a Xenon flashlamp and optical filters to match the spectrum of the generated light as closely as possible to the sunlight as it would fall on a panel under operational circumstances. The solar industry has agreed on a set of standards for this. These standards are limiting the deviation of the spectral match from the ideal percentage of the light in a defined frame of wavelength ranges. Also for the spatial shape and uniformity of the light beam definitions exist. Companies offering solar simulators include Newport/Oriel, Spire Corporation, Ocean Optics and StellarNet.
Solar simulator (image courtesy of Newport-ORIEL)
Increasing yield, improving quality
From robots with vision-based feedback for handling, to flasher systems to simulate sunlight on a finished PV panel, optical inspection increases speed and reliability over manual measurements or – if possible at all – mechanical automation. Detecting defects as early as possible in production and avoiding damage during handling and positioning are highly important for better yield and less scrap.
Written by Robert Molenaar, European Editor, Solar Novus Today
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