  Organic photovoltaics (OPVs) are promising candidates for solar-energy
conversion, with device efficiencies continuing to increase. However, the
precise mechanism of how charges separate in OPVs is not well understood
because low dielectric constants produce a strong attraction between the
charges, which they must overcome to separate. Separation has been thought to
require energetic offsets at donor-acceptor interfaces, but recent materials
have enabled efficient charge generation with small offsets, or with none at
all in neat materials. Here, we extend delocalised kinetic Monte Carlo (dKMC)
to develop a three-dimensional model of charge generation that includes
disorder, delocalisation, and polaron formation in every step from
photoexcitation to charge separation. Our simulations show that delocalisation
dramatically increases charge-generation efficiency, partly by enabling
excitons to dissociate in the bulk. Therefore, charge generation can be
efficient even in devices with little to no energetic offset, including neat
materials. Our findings demonstrate that the underlying quantum-mechanical
effect that improves the charge-separation kinetics is faster and
longer-distance hops between delocalised states, mediated by hybridised states
of exciton and charge-transfer character.
