  Grain boundaries (GBs) are defects originating in multi-crystalline silicon
during crystal growth for device Si solar cell fabrication. The presence of GBs
changes the coordination of Si, making it advantageous for charge carriers to
recombine, which brings a significant reduction of carrier lifetimes.
Therefore, GBs can be highly detrimental for device performances. Furthermore,
GBs easily form vacancies with deep defect electronic states and are also
preferential segregation sites for various impurity species, such as C, N, and
O. We studied from first principles the correlation between structural,
energetics, and electronic properties of the $\Sigma$3{111} Si GB with and
without vacancies, and the segregation of C, N, and O atoms. C and O atoms
strongly increase their ability to segregate when vacancies are present.
However, the electronic properties of the $\Sigma$3{111} Si GB are not affected
by the presence of O, while they can strongly change in the case of C. For N
atoms, it is not possible to find a clear trend in the energetics and
electronic properties both with and without vacancies in the GB. In fact, as N
is not isovalent with Si, as C and O, it is more flexible in finding new
chemical arrangements in the GB structure. This implies a stronger difficulty
in controlling the properties of the material in the presence of N impurity
atoms compared to C and O impurities.
