Increasing the efficiency and reducing the production costs are the two main goals of the optimization of crystalline silicon solar cells. This thesis addresses the impact of efficiency- limiting defects in crystalline silicon from cheaper production routes, namely upgraded metallurgical grade silicon and multicrystalline silicon, and presents an approach for the fabrication of high efficiency solar cells on low-cost multicrystalline silicon substrate.Upgraded metallurgical grade silicon typically features significant amounts of both types of dopants, acceptors and donors, which leads to a so-called charge carrier compensation. The impact of compensation on charge carrier mobility is investigated and a unified model for charge carrier mobilities in uncompensated and compensated silicon is developed.Multicrystalline silicon features structural crystal defects, such as grain boundaries and dislocations, as well as a large amount of metal impurities incorporated from the crucible system into the silicon during crystallization. In this thesis, efficiency losses due to impurities from the crucible system are quantified, the role of the doping type is evaluated, and the efficiency limit for solar cells based on high performance multicrystalline silicon substrate is assessed.