MANUFACTURINGMETROLOGY
Knowing more improves outcomes
With PV and solar manufacturing gaining wider acceptance the race is on for manufacturers to differentiate themselves in a crowded marketplace. The next few years will see the competition developing improvements to increase market share. Hungary based metrology company Semilab discusses how advanced metrology can assist manufacturers and what approach the company has developed.
sophisticated control tools, inspecting 100% of the wafers / cells for both optical and electrical properties. As the production techniques become more and more complicated, companies require such metrology tools that can deliver this performance reliably, in order to maintain cost effective and profitable manufacturing.
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How, in general, can metrology tools help to do this? Typically, a solar cell manufacturer does not sell by wafer or piece but by watt-peak, so there is a great incentive to produce units with greater efficiency, and save money and resources by the early detection of potential yield killers. There are a variety of these defects: starting materials can be of a low quality, critical errors may occur, or processes might start to deviate from the optimum.
For one, contaminated materials will certainly lead to low efficiencies. Using suitable techniques such as carrier lifetime measurements or carrier diffusion length measurements, sub-standard pieces can be identified prior to cell manufacturing. It has been shown that carrier
lifetime correlates extremely well to final cell performance and efficiency, and measurement
tate-of-the-art silicon-based solar cell manufacturing processes require
is simple, straightforward and painless (non- contact and non-destructive). It can be performed on silicon blocks, or on incoming as-cut wafers, and is fast enough to be integrated into the production line. The technique is well-known and even standardized: charge carriers are generated in the silicon by infrared laser excitation, and when the light is turned off, they recombine which can be monitored by microwave reflectivity measurements. Used together with non-contact resistivity measurement, the most important electrical properties can be obtained prior to processing the wafers.
More data can be harvested using sophisticated methods based on the determination of carrier diffusion length by the non- contact surface photo-voltage method, well known in the semiconductor industry for the effective quantitative measurement of iron contamination. Besides the performance of the final cell, long-term behaviour of the product can be deduced by quantitatively characterizing the boron- oxygen dimmer defects in the material, which lead to light-induced degradation. This, at least until now, cannot be measured in-line. However there is already an accelerated test method ALID (accelerated light induced degradation) from the Hungary-based metrology company Semilab, which gives results 10-100 times faster than any conventional test, and additionally includes possibility for high resolution mapping.
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www.solar-pv-management.com Issue VI 2010
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