  The high efficiency of charge generation within organic photovoltaic blends
apparently contrasts with the strong "classical" attraction between newly
formed electron-hole pairs. Several factors have been identified as possible
facilitators of charge dissociation, such as quantum mechanical coherence and
delocalization, structural and energetic disorder, built-in electric fields,
nanoscale intermixing of the donor and acceptor components of the blends. Our
mesoscale quantum-chemical model allows an unbiased assessment of their
relative importance, through excited-state calculations on systems containing
thousands of donor and acceptor sites. The results on several model
heterojunctions confirm that the classical model severely overestimates the
binding energy of the electron-hole pairs, produced by vertical excitation from
the electronic ground state. Using physically sensible parameters for the
individual materials, we find that the quantum mechanical energy difference
between the lowest interfacial charge transfer states and the fully separated
electron and hole is of the order of the thermal energy.
