Silicon is the dominant material for production of solar cells. Research work constantly aims at improving the quality of the silicon materials to achieve higher solar-cell conversion efficiencies. It has been demonstrated that defects and impurities, often rising during the silicon ingot production and due to the presence of impurity sources throughout the silicon solar-cell value chain (e.g. silicon feedstock, crucible and coating, furnace atmosphere, etc.), have a detrimental effect on device performances. Crystallization is the first step in the solar-cell value chain. During crystallization, crystal defects, such as grain boundaries and dislocations, develop, and impurities, such as dopants and metals, distribute. By controlling the process, we can also control the defect formation and impurity segregation. In this study, we review some of the work carried out at NTNU to understand the role of defects and impurities on materials properties. Specifically, we report on some of the work done on defects, crystallization, and characterization.