We study incoherent charge separation in a lattice model of an all-organic bilayer. Charge delocalization is taken into account by working in the basis of electron−hole pair eigenstates, and the separation is described as a series of incoherent hops between these states. We find that relatively weak energetic disorder, in combination with good charge delocalization, can account for efficient and weakly fieldand temperature-dependent separation of the strongly bound charge transfer (CT) state. The separation efficiency is determined by the competition between the recombination from the initial CT state and the escape toward intermediate CT states, from which free-charge states can be reached with certainty. The separation of donor excitons also exhibits quite high yields, less bound excitons separating more efficiently. Overall, our results support the notion that efficient charge separation can be achieved even out of strongly bound pair states without invoking coherent effects.

We present a detailed investigation of ultrafast (subpicosecond) exciton dynamics in the lattice model of a donor/acceptor heterojunction. Exciton generation by means of a photoexcitation, exciton dissociation, and further charge separation are treated on equal footing. The experimentally observed presence of space-separated charges at 100 fs after the photoexcitation is usually attributed to ultrafast transitions from excitons in the donor to charge transfer and charge separated states. Here, we show, however, that the space-separated charges appearing on 100-fs time scales are predominantly directly optically generated. Our theoretical insights into the ultrafast pump-probe spectroscopy challenge usual interpretations of pump-probe spectra in terms of ultrafast population transfer from donor excitons to space-separated charges.

The exciton dissociation and charge separation occurring on subpicosecond time scales following the photoexcitation are studied in a model donor/acceptor heterojunction using a fully quantum approach. Higher-than-LUMO acceptor orbitals which are energetically aligned with the donor LUMO orbital participate in the ultrafast interfacial dynamics by creating photon-absorbing chargebridging states in which charges are spatially separated and which can be directly photoexcited. Along with the states brought about by single-particle resonances, the two-particle (exciton) mixing gives rise to bridge states in which charges are delocalized. Bridge states open up a number of photophysical pathways that indirectly connect the initial donor states with states of spatially separated charges and compete with the efficient progressive deexcitation within the manifold of donor states. The diversity and efficiency of these photophysical pathways depend on a number of factors, such as the precise energy alignment of exciton states, the central frequency of the excitation, and the strength of carrier-phonon interaction.