Within the scope of this project we intend to develop a fundamental understanding of charge transfer and charge transport in organic solar cells. For this purpose we will employ suitable EPR-based techniques. In particular, we will develop the experimental method of transient electrically detected magnetic resonance (transient EDMR). Combining the sensitivity of EDMR and the time resolution of transient EPR will provide us with the possibility to systematically investigate the interplay between charge transfer complexes, which are generated upon photoexcitation and subsequent charge transfer between donor and acceptor, and the photocurrent in polymer: fullerene blends. In a first step we will study the magnetic properties of these intermediate complexes in material combinations incorporated in fully processed efficient organic solar cells. We will further investigate the crucial role of triplet excitons in organic solar cells and analyse whether they influence the photocurrent via loss mechanisms or whether the long lifetime of triplet excitons can be utilized to increase the yield of free charge carriers.