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In this study, we have utilized characterization methods to identify the nature of metal impurity precipitates in low performance regions of multicrystalline silicon solar cells. Specifically, we have utilized synchrotron-based x-ray fluorescence and x-ray absorption spectromicroscopy to study the elemental and chemical nature of these impurity precipitates, respectively. We have detected nanometer-scale precipitates of Fe, Cr, Ni, Cu, and Au in multicrystalline silicon materials from a variety of solar cell manufacturers. Additionally, we have obtained a direct correlation between the impurity precipitates and regions of low light-induced current, providing direct proof that metal impurities play a significant role in the performance of multicrystalline silicon solar cells. Furthermore, we have identified the chemical state of iron precipitates in the low-performance regions. These results indicate that the iron precipitates are in the form of oxide or silicate compound. These compounds are highly stable and cannot be removed with standard silicon processing, indicating remediation efforts via impurity removal need to be improved. Future improvements to multicrystalline silicon solar cell performance can be best obtained by inhibiting oxygen and metal impurity introduction as well as modifying thermal treatments during crystal growth to avoid oxide or silicate formation

Impurity and minority carrier lifetime distributions were studied in as-grown multicrystalline silicon used for terrestrial-based solar cells. Synchrotron-based x-ray fluorescence and the light beam induced current technique were used to measure impurity and lifetime distributions, respectively. The purpose of this work was to determine the spatial relation between transition metal impurities and minority carrier recombination in multicrystalline silicon solar cells. Our results reveal a direct correlation between chromium, iron, and nickel impurity precipitates with regions of high minority carrier recombination. The impurity concentration was typically 5×1016 atoms/cm2, indicating the impurity-rich regions possess nanometer-scale precipitates. These results provide the first direct evidence that transition metal agglomerates play a significant role in solar cell performance.

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Undercooling during the solidification of silicon is an essential parameter that plays a major role in grain nucleation and growth. In this study, the undercooling of the solid-liquid interface during growth of multi-crystalline silicon samples is measured for two types of silicon: pure, and containing light elements (carbon and oxygen) to assess and compare their impact on crystal growth. The solid-liquid interface undercooling is measured using in situ and real time X-ray synchrotron imaging during solidification. As a subsequent step, ex situ Electron Backscattered Diffraction (EBSD) is performed to obtain information about the crystalline structure, the grain orientation and the grain boundary character. Two main conclusions arise: i) the undercooling of the global solid-liquid front increases linearly with the growth rate which indicates uniform attachment, i.e. all atoms are equivalent, ii) the same trend is observed for pure silicon and silicon containing carbon and oxygen. Indeed, the growth law obtained is comparable in both cases, which suggests that the solutal effect is negligible as concern the undercooling in the case of a contamination with carbon (C) and oxygen (O). However, there is a clear effect of the impurity presence on the crystalline structure and grain boundary type distribution. Many grains nucleate during growth in samples containing C and O, which suggests the presence of precipitates on which grain nucleation is favored.