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This study aims to better understand the influence of crystallographic structure and impurity decoration on the recombination activity at grain boundaries in multicrystalline silicon. A sample of the upper part of a multicrystalline silicon ingot with intentional addition of iron and copper has been investigated. Correlative electron-beam-induced current, electron backscatter diffraction, and atom probe tomography data for different types of grain boundaries are presented. For a symmetric coherent Σ3 twin boundary, with very low recombination activity, no impurities are detected. In case of a noncoherent (random) high-angle grain boundary and higher order twins with pronounced recombination activity, carbon and oxygen impurities are observed to decorate the interface. Copper contamination is detected for the boundary with the highest recombination activity in this study, a random high-angle grain boundary located in the vicinity of a triple junction. The 3D atom probe tomography study presented here is the first direct atomic scale identification and quantification of impurities decorating grain boundaries in multicrystalline silicon. The observed deviations in chemical decoration and induced current could be directly linked with different crystallographic structures of silicon grain boundaries. Hence, the current work establishes a direct correlation between grain boundary structure, atomic scale segregation information, and electrical activity. It can help to identify interface-property relationships for silicon interfaces that enable grain boundary engineering in multicrystalline silicon.

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Defects in multicrystalline silicon for photovoltaic applications and their impact on solar cell parameters have been investigated in the material research network project SolarFocus. A series of multicrystalline silicon ingots of ultrapure feedstock material were cast with intentional addition of typical transition metal impurities (Fe, Cu, Cr) and Ge as doping elements. The results of lifetime measurements, NAA and FTIR analysis, solar cell processing and microscopic investigations are presented in this study. For ingots intentionally contaminated with transition metals, the combined analysis reveals that despite the overall high impurity content, good solar cell efficiencies can be reached. A strong influence of the in-diffusion of metal impurities from the crucible as well as the back-diffusion from the top region of the ingot ca still be observed. All metals show a strong precipitation behaviour throughout the whole ingot. The solar cell efficiency is both limited by multiple recombination active defects and shunts, induced by a high metal contamination via indirect mechanisms. Solar cells with efficiencies up to 15.2% for material contaminated with 20 ppma Fe in the melt, 15.7% for 20 ppma Cu and 15.1% for 20 ppma Cr were processed. A positive effect of Cu added to the feedstock could not be found. Ge-rich ingots showed strong effects of increasing silicon carbide and silicon nitride formation with increasing Ge content larger than 0.5 wt.% thus reducing solar cell efficiency