Science http://doi.org/qrx (2013)

To define optimization strategies for improving the light-conversion efficiency in organic solar cells, it is mandatory to understand how electrons and holes photogenerated in bulk heterojunctions can overcome their Coulomb attraction and separate into free charges. Simon Gélinas and colleagues now show that, in about 40 fs, charges can separate at distances larger than 4 nm, thus rapidly reducing their mutual attraction energy below the thermal energy. They studied bulk heterojunctions based on fullerene aggregates blended with small molecules and polymer donors. The authors determined the ultrafast charge motion by correlating the temporal evolution of the absorption spectrum of the blends with the electric field generated by the separating charges. The presence of aggregates in the fullerene domains is essential for creating delocalized band-like electronic states that couple with the wavefunction of the electrons generated in the donor material. This coupling allows electrons to resonantly transfer into the acceptor and undergo long-range motion soon after their photogeneration, without needing excess energy to separate efficiently from holes.