Solar cells made from multicrystalline silicon material may suffer from defects affecting the power conversion. Such detrimental effects manifest themselves in current–voltage characteristics of the cells showing deviations from the ideal case. In this book a large variety of defects and the corresponding mechanisms, which influence the solar cell performance, are discussed. Several defects affecting mainly the forward current–voltage characteristic are reviewed. The main focus is on the breakdown behavior of solar cells, i.e. on the reverse current of solar cells. Spatially resolved lock-in thermography methods are presented for the quantitative analysis of the physical parameters of breakdown sites in solar cells, such as temperature coefficient and reverse current. The physical mechanisms behind the different breakdown types, namely early breakdown, defect-induced breakdown, and avalanche breakdown, are described in detail. The book is addressed to scientists and engineers working in the field of solar cells and solar cell materials.