Defect analysis in directionally solidified multicrystalline silicon

This project studies how the microstructure and metallic impurities affect the electrical properties of mc-Si wafers, to improve the efficiency and the production yield of photovoltaic solar cells. Dislocations and impurities in silicon are recombination centres that reduce free carrier lifetime and thus efficiency of solar cells. The quality of the material can be improved by finding optimal growth conditions and a threshold value for the contamination that does not compromise the device efficiency. Two sets of p-type mc-Si wafers located at different heights and lateral positions of two directionally solidified ingots, one contaminated with iron and one with aluminum, were analysed with several characterization techniques. The two ingots show similar microstructure, but the top of the iron contaminated ingot has a significantly lower lifetime, as it contains more dislocation clusters decorated with segregated iron. Aluminum is less detrimental at this low concentration level and it is more homogeneously distributed along the ingot height. A Mott-Schottky analysis after evaporation of aluminum contacts confirmed the p-type nature of the samples and estimated the free charge carrier concentration. Current profiles and local I-V curves measured with Conductive Atomic Force Microscopy show that decorated grain boundaries are a preferential path for electrical conduction compared to the grain regions and iron precipitates affect more heavily the electrical properties of the wafer compared to aluminum precipitates. The shape of the current profile at the boundary was justified with a theoretical model that assumes a redistribution of charge density due to a Coulombic potential introduced by a spherical and positively charged precipitate, that can be identified with b-FeSi2. The results from this characterization show that metallic contamination at grain boundaries in Si is responsible for enhanced free carrier recombination and thus efficiency reduction in mc-Si cells.