The nature of a low angle grain boundary in a Si bi-crystal with added Fe impurities

Abstract

Silicon solar cell performance can be severely degraded when low-angle grain boundaries and impurities are present in the material. These two factors often come hand in hand, and it is imperative to understand the underlying fundamental physics to make progress in the field of solar energy conversion. In this study, three different types of Lomer dislocations in a 5° Si grain boundary were identified by atomic resolution HAADF-STEM. DFT calculations and HAADF-STEM revealed that these grain boundaries are not perfectly flat and must exhibit additional shifts along <110>Si to account for the misorientation angle. DFT simulations of the Lomer type I dislocation core were in excellent agreement with the atomic structure characterised by HAADF-STEM.The nature of the segregation of impurity elements Fe and C at the grain boundary is established by atom probe tomography, DFT calculations and HAADF-STEM studies. DFT calculations predict that Fe can segregate to all interstitial sites surrounding the dislocation core except for one, while C would prefer three out of ten possible substitutional sites. These results are corroborated by atom probe tomography. Additional impurity elements were also discovered at the grain boundaries, illustrating the importance of a controlled manufacturing process and crucible coating.