Abstract
Recently, significant insight into the mechanisms of charge generation and recombination in interpenetrating networks of electron-rich donor polymers and electron-deficient acceptors used in bulk heterojunction organic solar cells has been gained through optical spectroscopy. In this article, we focus on the trend of using ultrafast transient absorption spectroscopy to observe structure-function relationships in order to provide valuable feedback and guidance for materials design and device engineering. Despite the progress, many research questions in this field still remain open, and we highlight areas in which experimental and theoretical work disagree or are still lacking. As examples of the unique and important insights that can be gained from optical spectroscopy, we review the influence of dielectric environment on charge transfer in donor-acceptor-systems and the influence of morphology on charge generation and recombination in the prototypic photovoltaic blend system regioregular poly(3-hexylthiophene) and [6,6]-phenyl C61 butyric acid methyl ester (P3HT:PCBM).