Molecular understanding of the electronic and optical processes in organic solar cells: The challenges

We first provide an overview of the optical and electronic processes that take place in a solid-state organic solar cell. For each of these processes, we describe the theoretical challenges that need to be overcome in order to gain a comprehensive understanding at the molecular level. We highlight recent theoretical advances, in particular regarding the determination of the energetics and dynamics at organic-organic interfaces, and discuss the balance that needs to be found in the optimization of materials parameters.