Transiently Delocalised Hybrid Quantum States are the Gateways for Efficient Exciton Dissociation at Organic Donor-Acceptor Interfaces

The field of organic photovoltaics research has witnessed a renaissance in recent years owing to the development of non-fullerene acceptor materials reaching record power conversion efficiencies of >20%. New regimes of photophysics are reached in these materials that are currently not well understood suggesting that new computational models are urgently needed to rationalise, explain and further build on these advances. Here we report on a novel implementation of eXcitonic state-based Surface Hopping (X-SH), a powerful non-adiabatic molecular dynamics method for the simulation of photo-induced charge generation in truly nanoscale donor-acceptor interfaces. We observe a transition from an inefficient "cold" to an efficient "hot" exciton dissociation mechanism as the electronic coupling between the molecules within the donor and acceptor phase is increased. In the latter mechanism, Frenkel excitons are observed to convert to transiently delocalised hybrid exciton-charge transfer states that subsequently form separated charge carriers. This way the kinetically trapped interfacial charge transfer states, which are prone to non-radiative recombination, are avoided. Similar observations are made when the dielectric constant of the donor and acceptor materials is increased instead of the electronic coupling. Both modifications result in a better energetic alignment of excitonic and charge transfer states that leads to the emergence of transiently delocalised hybrid exciton-charge transfer states as gateways for hot exciton dissociation. Our results highlight the importance of electronic couplings, an often overlooked property, for opto-electronic charge generation. Three design rules for efficient hot exciton dissociation mechanism in heterojunction organic photovoltaics are discussed.