The field of organic photovoltaics research has witnessed a renaissance in recent years.
In particular, the introduction of non-fullerene acceptors (NFA) in organic heterojunction
solar cells has alleviated the issue of large voltage losses in traditional fullerene-based junctions[1,2]
resulting in a major boost of power conversion efficiency from 11% to >19%[3] starting to rival those made
of perovskites. New regimes of photophysics are reached in these new materials that are currently not well understood
suggesting that more powerful experimental measurements and computational models
are urgently needed to rationalize, explain and further build on these advances.

Our group has contributed to this objective by developing a powerful non-adiabatic molecular dynamics simulation
tool in recent years. In our approach, the wavefunction of charge carriers (electrons or holes)[4,5] or electronic
excitations [6] is progapaged in nanoscale organic materials (10-100 nm) on the 10-100 ps timescale by solving the
time-dependent electronic Schrödinger equation coupled to intramolecular and lattice vibrations. The method has led to a paradigmatic
shift of our mechanistic understanding of charge[4,5] and exciton transport processes[6] in ordered (high mobility)
organic semiconductors, transient quantum delocalization.

Here we report on a timely extension of our methodology, termed excitonic state-based surface hopping (X-SH), that now allows us to simulate
the quantum dynamical dissociation of excitons to charge carriers in truly nanoscale organic materials interfaces.[*] We apply this new methodology
to study exciton dissociation at the interface between the donor material alpha-hexathiophene and the
non-fullerene acceptor perylene diamide. We find that for this system exciton dissociation proceeds
via the generally accepted picture: fast relaxation of the initial band-like electronic excitation to
a localized Frenkel exciton, diffusion of the Frenkel exciton to the interface via hopping followed by formation
of interfacial (``cold") charge transfer state that dissociates thermally to free carriers or recombines on long time scales[*].

Intriguingly, as we increase the electronic coupling between the molecules (or, equivalently, increase the electronic band width
or charge mobility of donor and acceptor), we increasingly observe a second, much more efficient ``hot" exciton dissociation channel[*].
Here Frenkel excitons convert to hybrid exciton-charge transfer states that directly form free carriers. Remarkably, the hot exciton
dissociation process is observed to occur at distances of up to several nanometers away from the interface owing to the delocalized nature
of the hybrid exciton-charge transfer states[*]. This way the formation of kinetically slow interfacial charge transfer
states that are prone to recombination is avoided. Similar observation are made when the dielectric constant of the donor and
acceptor materials are increased in place of the electronic coupling[*]. Both modifications result in a better energetic alignment of excitonic
and charge transfer states that leads to the emergence of hybrid exciton-charge transfer states as
gateways for efficient hot exciton dissociation.

Our study uncovers an important design principle for efficient hot exciton dissociation in organic materials interfaces. Moreover
our simulations may help rationalise contrasting experimental findings in regard with the nature of the exciton dissociation mechanism
at these interfaces (hot vs cold).

[*] F. Ivanovic, W.-T. Peng, S. Giannini, J. Blumberger, manuscript in preparation.