  Electron-hole pairs in organic photovoltaics dissociate efficiently despite
their Coulomb-binding energy exceeding thermal energy at room temperature. The
electronic states involved in charge separation couple to structured
vibrational environments containing multiple underdamped modes. The
non-perturbative simulations of such large, spatially extended
electronic-vibrational (vibronic) systems remains an outstanding challenge.
Current methods bypass this difficulty by considering effective one-dimensional
Coulomb potentials or unstructured environments. Here we extend and apply a
recently developed method for the non-perturbative simulation of open quantum
systems to the dynamics of charge separation in one, two and three-dimensional
donor-acceptor networks. This allows us to identify the precise conditions in
which underdamped vibrational motion induces efficient long-range charge
separation. Our analysis provides a comprehensive picture of ultrafast charge
separation by showing how different mechanisms driven either by electronic or
vibronic couplings are well differentiated for a wide range of driving forces
and how entropic effects become apparent in large vibronic systems. These
results allow us to quantify the relative importance of electronic and vibronic
contributions in organic photovoltaics and provide a toolbox for the design of
efficient charge separation pathways in artificial nanostructures.
