No abstract

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.

The unique combination of atomic-scale composition measurements, employing atom probe tomography, atomic structure determination with picometer resolution by aberration-corrected scanning transmission electron microscopy, and atomistic simulations reveals site-specific linear segregation features at grain boundary facet junctions. More specific, an asymmetric line segregation along one particular type of facet junction core, instead of a homogeneous decoration of the facet planes, is observed. Molecular-statics calculations show that this segregation pattern is a consequence of the interplay between the asymmetric core structure and its corresponding local strain state. Our results contrast with the classical view of a homogeneous decoration of the facet planes and evidence a complex segregation patterning.

No abstract

Grain boundaries in materials have substantial influences on device properties, for instance on mechanical stability or electronic minority carrier lifetime in multicrystalline silicon solar cells. This applies especially to asymmetric, less ordered or faceted interface portions. Here, we present the complex atomic interface structure of an asymmetric Σ9 tilt grain boundary in silicon, observed by high resolution scanning transmission electron microscopy (HR-STEM) and explained by atomistic modeling and computer simulation. Structural optimization of interface models for the asymmetric Σ9 and related symmetrical Σ9 and Σ3 tilt grain boundaries, by means of molecular-statics simulations with empirical silicon potentials in combination with first-principles calculations, results in a faceted asymmetric interface structure, whose grain-boundary energy is so low that it is likely to exist. The simulated local atomic structures match the observed HR-STEM images very well.