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Large multicrystalline cast silicon ingots (>310 kg) are cost effective in the photovoltaic industry and attenuate
the feedstock shortage. The bulk lifetime τn and diffusion length Ln of minority carriers vary through the height due to the segregation of metallic impurities during the directional solidification. The native impurity
concentrations increase from the bottom to the top of the ingot, which is solidified last, while the ingot bottom,
which is solidified first, is contaminated by the contact with the crucible. It was found that τn and Ln are the smallest in the top and in the bottom of the ingot. In solar cells, the evolution is similar, however in the central
part of the ingot Ln is strongly increased due to the in-diffusion of hydrogen from the SiN-H antireflection
coating layer. The variations along the ingot height of the conversion efficiency η and of τn in raw wafers are
well correlated, that can predict the values of η, allowing an in-line sorting of the wafers, before solar cells are
made. If τn is smaller than 1 μs, as observed at the extremities of the ingot, η will be limited to 10% only; if τn is higher than 2.5 μs η achieve 15 % at least. In addition, impurity segregation phenomena around grain boundaries are observed at the extremities of the ingots, linked to the long duration of the solidification process. Reducing
the height of the ingots could suppress these phenomena and not much material must be discarded. Another problem can come from the use of upgraded metallurgical silicon feedstock in which the densities of
boron and phosphorus are very close. Due to the difference in the segregation coefficients, ingots may be entirely
or partly p or n type, suggesting that a purification step tawards the dopants is required.
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