Iron related precipitates in multicrystalline silicon by conductive atomic force microscopy

Multicrystalline silicon (mc-Si) is a widely used material for photovoltaic applications. The presence of metallic contaminated grain boundaries strongly affects the crystal electronic properties enhancing electron-hole recombination, thus reducing the solar cell performance. The present study aims to investigate the electrical activity of metallic contaminated grain boundaries in mc-Si. Two sets of mc-Si wafers, contaminated with iron and aluminium, respectively, were analyzed. The wafers presented grain boundaries whose density and character were characterized by Electron Backscatter Diffraction (EBSD), while their electrical activity was analyzed using Conductive Atomic Force Microscopy (c-AFM). The grain boundary density decreases along the ingot height and the most common coherent grain boundaries have the character Σ3n. The grain boundary electrical activity is mostly due to metallic precipitates located at the grain boundaries. In particular, iron precipitates enhance the current contrast at the grain boundaries. Both fixed voltage maps and current-voltage characteristics at the grain boundaries were measured to understand and clarify the transport phenomena at grain boundaries decorated with metallic impurities. The current profiles measured by c-AFM across a grain boundary were modelled by assuming the contribution of a Coulombic potential introduced by the positively charged precipitate. Quantitative parameters regarding the segregated iron-related precipitates are estimated from the model. The results of this study, based on local electrical characterization and appropriate modelling, will contribute to improving the understanding of the recombination at iron precipitates at grain boundaries in mc-Si.