Exciton dissociation from non-adiabatic molecular dynamics: The role of initial conditions, dimensionality, and disorder

We have recently introduced excitonic state-based surface hopping, a powerful non-adiabatic molecular dynamics method capable of simulating charge photogeneration in nanoscale molecular systems. Here, we assess the impact of the initial conditions, treatment of thermal fluctuations, and model dimensionality when simulating exciton dissociation at an organic donor-acceptor interface. We find that realistic modeling of the initial photoexcitation into the partially delocalized excitonic band states is essential to capture the short-term relaxation dynamics that are often observed in experiment. Yet, the population dynamics on longer timescales remain largely insensitive to the initialization procedure. Two-dimensional systems are found to exhibit a significantly increased density of hybrid Frenkel exciton-charge transfer states at the same excitation energy when compared to one-dimensional systems, resulting in accelerated exciton decay. Moreover, the increase in the ratio of charge transfer to Frenkel exciton states alongside dimensionality entropically favors charge generation. Finally, we find that the impact of neglecting the thermal fluctuations of electronic couplings depends on the specific parameter regime of the interface. In the incoherent hopping regime, their neglect leads to only a very small decrease in the exciton decay rate. In the transient delocalization regime, the delocalization of charge transfer states away from (or close to) the interface is overestimated (or underestimated), resulting in a markedly different exciton decay profile when thermal fluctuations are neglected.