FEATURE SOLAR CELL EFFICIENCY ➤


interest in the infrared line scan business in Asia,’ Vandersmissen says.


‘In the past, [photovoltaic] customers were comparing Megapixel cooled CCD arrays with an InGaAs 2D array with a 320 x 256 pixel resolution. We had a big problem with infrared cameras in that the price was still higher than CCD cameras and the resolution was a lot lower,’ Vandersmissen states. ‘Now, with a line scan system, we are roughly at the same price level and the resolution is also about the same as CCD cameras.’


The advantage with InGaAs cameras is that they have a better efficiency in the 1,100-1,200nm wavelength band, so they can operate at much shorter exposure times. Therefore, for inline systems, InGaAs will be able to inspect at much higher speeds. ‘The problem why InGaAs


never broke through completely in solar cell production is that the most common defects are still visible with a silicon CCD camera,’ Vandersmissen summarises.


Hoex feels that the main performance-limiting defects that


Electroluminescence image of a solar cell; the dark areas show broken connections


monocrystalline silicon with the lower costs of the polycrystalline version. This material can look good on the surface, but under closer inspection with PL imaging, a lot of performance- limiting defects emerge. It just takes one PL image, Hoex says, with no need for further sophisticated analysis. ‘There will be a wide array of yield variation with cast-mono silicon wafers. This could result in a big push for more inline metrology


Photoluminescence has tremendous potential, but the cost-benefit analysis has to be positive in the short term


can be identified by bandgap luminescence would already be a step up from the current QC checks in terms of real-time monitoring on production lines. ‘Sorting wafers with PL would be a great improvement especially for the latest generation of silicon crystalline material,’ he says, referring to cast monocrystalline (cast-mono) silicon, a relatively new material that combines the higher efficiency associated with


14 PHOTOVOLTAICS 2012


at the wafer selection stage,’ he comments. However, Hoex concedes that the use of this inspection technology will ultimately come down to cost. He says: ‘PL has tremendous potential, but the cost-benefit analysis has to be positive in the very short term in the PV industry. This is still a difficult business case for equipment suppliers for metrology.’


Extended defects Bandgap luminescence will give a good indication of the efficiency of the solar cell, but there are other extended defects emitting at longer wavelengths which do require cooled InGaAs cameras to detect. Investigation into these types of defects is generally the realm of R&D, as they require more specialised equipment to study and are less well understood. Xenics sells high-end InGaAs cameras for these scientific photovoltaic applications. Vandersmissen comments: ‘This isn’t a market where you can sell hundreds of cameras a year, but it could still be a market for 20-50 cameras per year.’


SERIS is investigating extended defects – crystal imperfections in the silicon lattice – that emit at around 1,500nm. The researchers have shown that the photons emitted at around 1,500nm can have a preferential polarisation state. ‘By using standard polarisers, we’ve shown that the polarisation state of the photon carries some information from where it was emitted,’ explains Hoex. ‘We postulate that this could potentially be a way to get more information about the defect and, if it can be applied, a way to determine the


origin of defects over a whole silicon wafer.’ Hoex and his team are using InGaAs cameras for these studies, along with an IR polarisation filter.


Potentially this imaging technique at 1,500nm could be used to inspect raw silicon wafers and even silicon ingots. It would help in selecting wafers and determining which ones could be processed to mitigate defects (dislocations in the silicon lattice can be altered by processing at certain temperatures). ‘The advantage of this technique is that it is really looking at defect luminescence,’ Hoex states. ‘Dark areas in EL images, for example, can have various root causes, whereas this technique identifies the cause of the defect, to discriminate between different variations of flaws in the wafer.’ He suggests that among the first applications of this technique might be to serve as feedback to crystal growers for optimising the furnace recipes.


SERIS is also looking at quantifying material properties of silicon thin film solar cells using luminescence imaging, mainly with EL imaging as thin film cells are less efficient as an LED and therefore require higher inputs of energy to get a measurable signal.


Value-added production Luminescence isn’t the only inspection technique used in photovoltaic production; there’s a whole raft of processing steps to turn raw silicon into a solar module and cameras are employed throughout for dimensioning and quality control. Each step adds value to the semiconductor, which is why inspection between production stages is important to ensure defective parts are weeded out early in the process. Peter Keppler, machine vision sales manager at European vision distributor, Stemmer Imaging, which supplies imaging equipment to the


Solar Energy Research Institute of Singapore


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