Interstitial Fe-pairs in silicon

Iron is a common deleterious transition metal contaminant in multicrystalline silicon solar cell material. Although most attention to-date has focused upon mobile interstitial Fe, we show the formation of pairs is energetically favourable. Of particular note are the magnetic order predicted for pairs as a function of internuclear distance, and that the third-neighbour pair is the most favourable form, but is predicted to exist in antiferromagnetic form, bound relative to isolated interstitials by 0.3 eV. We also note that the migration barrier for Fe-pairs is lower than individual interstitials, suggesting that pairs may be a key contributor to Fe diffusion and precipitation.