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Boron–oxygen-related degradation in multicrystalline silicon wafers

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By Rune Søndenå, Institute for Energy Technology, Kjeller, Norway

Extended crystal defects, such as grain boundaries and dislocations, have long been considered the main factors limiting the performance of multicrystalline (mc-Si) silicon solar cells. However, because the detrimental effects of these crystal defects are reduced as a result of improvements in the solidification process as well as in the feedstock and crucible quality, the degradation caused by boron–oxygen complexes is expected to be of increasing importance. Light-induced degradation (LID) occurs in both p- and n-type crystalline silicon solar cells that contain both boron and oxygen. Because of the fundamental differences in the solidification processes, mc-Si silicon contains less oxygen than Czochralski silicon; nevertheless, the oxygen content in mc-Si silicon is still sufficient to cause degradation, although to a lesser extent than in the case of Czochralski silicon. Whereas B–O-related degradation of 0.5 to 1% abs. can be found in Czochralski cells, the degradation in conventional mc-Si cells is limited to around 0.1 to 0.2% abs.

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In the past few issues of Photovoltaics International we have tracked in detail plans being implemented by the leading module manufacturers to expand production capacity. That process began tentatively last year as end-market demand began to catch up with the chronic overcapacity that had built up in the preceding years, prompting industry-wide upheaval. Our latest capacity expansion report (p.11), a unique resource in the industry, reveals that while that activity was maintained throughout much of 2014, spiking in a strong final quarter of 2014, announcements of new capacity slowed slightly in the opening quarter of this year. Nevertheless, all the signs point to the pace picking up again later this year as manufacturers look to take advantage of the surge in activity expected in the US at the back end of this year and into 2016, in anticipation of the cutting back of the solar investment tax credit at the end of that year.

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