  Multi-crystalline silicon is widely used for producing low-cost and
high-efficiency solar cells. During crystal growth and device fabrication,
silicon solar cells contain grain boundaries (GBs) which are preferential
segregation sites for atomic impurities such as oxygen atoms. GBs can induce
charge carriers recombination significantly reducing carrier lifetimes and
therefore they can be detrimental for Si device performance. We studied the
correlation between structural, energetic and electronic properties of
{\Sigma}3{111} Si GB in the presence of vacancies, strain and multiple O
segregation. The study of the structural and energetic properties of GBs in the
presence of strain and vacancies gives an accurate description of the complex
mechanisms that control the segregation of oxygen atoms. We analysed tensile
and compressive strain and we obtained that local tensile strain around O
impurities is very effective for segregation. We also studied the role of
multiple O impurities in the presence of Si vacancies finding that the
segregation is favorite for those structures which have restored tetrahedral
covalent bonds. The presence of vacancies attract atomic impurities in order to
restore the electronic stability: the interstitial impurity becomes
substitutional. This analysis was the starting point to correlate the change of
the electronic properties in {\Sigma}3{111}Si GBs with O impurities in the
presence of strain and vacancies. For each structure we analysed the density of
states and its projection on atoms and states, the band gaps, the segregation
energy and their correlation in order to characterise the nature of new energy
levels. Actually, knowing the origin of defined electronic states would allow
the optimization of materials in order to reduce non-radiative electron-hole
recombination avoiding charge and energy losses and therefore improving solar
cell efficiency.
