Exciton Diffusion\nto Low Energy Sites of the Acceptor\nDrives Charge Photogeneration in D18:Y6 Solar Cells

We have investigated charge generation pathways in efficient\norganic\nphotovoltaic blends of the polymer donor D18 and the small-molecule\nacceptor Y6 using transient absorption and time-resolved fluorescence\nspectroscopies. We find that energy transfer from D18 to Y6 outcompetes\nelectron transfer and is followed by exciton diffusion from regions\nof the disordered Y6 phase to Y6 aggregates before hole transfer to\nD18. Aggregation of Y6 molecules increases their ionization energy\nby ∼0.3 eV and provides a driving force for hole transfer from\nY6 excitons and spontaneously generated charge pairs to D18. We observed\nultrafast depolarization of the Y6 ground-state bleaching in <200\nfs, which indicates delocalization of primary excitons in Y6 aggregates.\nThis delocalization can explain the spontaneous generation of charge\npairs in neat Y6 films and Y6-rich blends. Our results show that subtle\naggregation control of the low-energy absorber can be used for balancing\nphotocurrent generation with low voltage loss in photovoltaic blends.