Residual strain and electrical activity of defects in multicrystalline silicon solar cells

Abstract

The growth process by casting methods of multi-crystalline Si results in a crystalline material with, among other defects, a high density of dislocations and grain boundaries. Impurity incorporation and their gathering around grain boundaries and dislocations seem to be the main factor determining the electrical activity of those defects, which limit the minority carrier lifetime. In this work, we analyze multi-crystalline Si samples by combining etching processes to reveal the defects, Raman spectroscopy for strain measurements, and light beam induced current measurements for the localization of electrically active defects. In particular, we have explored the etching routes capable to reveal the main defects (grain boundaries and dislocation lines), while their electrical activity is studied by the light beam induced current technique. We further analyze the strain levels around these defects by Raman micro-spectroscopy, aiming to obtain a more general picture of the correlation between residual stress and electrical activity of the extended defects. The higher stress levels are observed around intra-grain defects associated with dislocation lines, rather than around the grain boundaries. On the other hand, the intra-grain defects are also observed to give dark light beam induced current contrast associated with a higher electrical activity of these defects as compared to the grain boundaries