This book focuses on organic semiconductors with particular attention paid to their use as photovoltaic devices. It addresses a fundamental and hitherto overlooked concept in the field of organic optoelectronics, namely the role that sub-gap states play in the performance of organic semiconducting devices. From a technological point of view, organic semiconductor-based devices are of significant interest due to their lightweight, ease of processability, conformal flexibility, and potentially low cost and low embodied energy production. Motivated by these rather unique selling points, the performance of organic semiconductors has been a subject of multidisciplinary study for more than 60 years with steady progress in applications such as solar cells, transistors, light emitting diodes, and various sensors. The book begins with a review of the main electro-optical phenomena in organic solar cells and presents a new method for measuring exciton diffusion lengths based on a low-quencher-content device structure. Furthermore, the book reveals how mid-gap trap states are a universal feature in organic semiconductor donor–acceptor blends, unexpectedly contributing to charge generation and recombination, and having profound impact on the thermodynamic limit of organic photovoltaic devices. Featuring cutting-edge experimental observations supported with robust and novel theoretical arguments, this book delivers important new insight as to the underlying dynamics of exciton generation and diffusion, charge transfer state dissociation, and indeed the ultimate fate of photogenerated free carriers.