Book excerpt: Covering both organic materials, where recent advances in the understanding of device physics is driving progress, and the newly emerging field of mixed halide perovskites, which are challenging the efficiencies of conventional thin film PV cells, this book provides a balanced overview of the experimental and theoretical aspects of these two classes of solar cell. The book explores both the experimental and theoretical aspects of these solar cell classes. Emphasis is placed on understanding the fundamental physics of the devices. The book also discusses modelling over many length scales, from nano to macro. The first book to cover perovskites, this is an important reference for industrialists and researchers working in energy technologies and materials.