SOLAR CELL EFFICIENCY FEATURE
Greg Blackman on how luminescence imaging is improving the efficiency of solar cells
Heightened quality control measures might be low on the priority list of solar cell manufacturers at the moment, struggling as they are to make a profit with government cutbacks in the photovoltaic sector taking their toll, but work continues on improving QC techniques nonetheless. Luminescence inspection is deemed particularly important for crystalline solar cell production, as it uncovers defects in the silicon and gives a direct measure of the efficiency of the wafer, cell or module.
Dr Bram Hoex, director of the silicon PV cluster at the Solar Energy Research Institute of Singapore (SERIS) goes so far as to regard luminescence imaging as ‘the biggest innovation in solar cell and module analysis of the last decade’. ‘It’s a massive leap forward in terms of analysis and it’s so quick,’ he says. ‘It should be used in quality control checks, because so many performance-limiting defects can be picked up with this characterisation technique. Some of these defects cannot be detected by the current standard testing sequence.’
An edge isolation system developed
by German company Eckelmann using cameras from
Stemmer Imaging. The system incorporates a vision-guided laser to make the scribes
Luminescence imaging assesses non- uniformities in the silicon wafer or solar cell by exciting the semiconductor to emit light, either by driving a current through the cell, known as electroluminescence (EL), or bombarding the silicon with photons, called photoluminescence (PL). Exciting silicon causes it to act like an inefficient LED, the light produced from which gives diagnostic information on the material’s efficiency at generating electricity. ‘EL is already widely used in the solar industry,’ states Hoex. ‘Most solar manufacturers employ EL off-line currently, but inline EL will become more prevalent in the future.’ EL is simple to implement, but it requires electrical contacts to be made and a current to be passed through the cell, so it can’t be used to test raw silicon. PL imaging is a little more complicated,
because the luminescence signals tend to be relatively small. It involves a high-power light source, typically a laser, and the ability to filter out undesired wavelengths. However, the technique is non-contact, so it can be implemented right at the beginning of the process to test the unrefined silicon. PL is a less mature technology, but there
are some companies selling inline equipment, notably BT Imaging, based in Surry Hills, NSW, Australia, which provides equipment for sub-second PL imaging of silicon wafers. Isra Vision (Darmstadt, Germany) also supplies both inline EL and PL inspection systems as part of its Solarscan product range. The Solarscan-PL system incorporates homogenised illumination to provide an inspection field of up to 170 x 170mm. Both Isra Vision’s and BT Imaging’s systems offer inspection speeds of 3,600 wafers or cells per hour (one wafer per second), exceeding current production speeds, which, according to Hoex, run at around 2,000 wafers per hour. Hoex feels the first wide-scale industrial application of PL imaging will be applied in the sorting of raw silicon wafers. ‘After sawing of the wafers, their electronic quality can be assessed before they are processed into a solar cell,’ he says. The test means wafers can be sorted according to quality and any defective ones removed. Production lines can therefore be optimised for processing a more specific range of raw wafer quality.
The type of luminescence typically employed is bandgap luminescence, which is measured around the bandgap of silicon at 1.1eV, emitting at around 1,100nm. Silicon CCD cameras are the usual choice for measuring emissions, as they are still sensitive at this infrared wavelength and are significantly cheaper than InGaAs cameras. There have been, however, some
improvements in InGaAs cameras that make them more suitable for luminescence imaging. Belgian company Xenics provides line scan InGaAs cameras used for PL and EL inspection. Raf Vandersmissen, CEO of sInfraRed, a Xenics subsidiary based in Singapore, comments that line scan gives the advantage of relatively low cost compared to area scan InGaAs versions, combined with high resolution. Xenics’ newest line scan InGaAs camera offers 1,024 pixel resolution and the company also plans to release a 2,048 pixel version later in the year. ‘We see a lot of
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