  This work presents a novel theoretical description of the non-equilibrium
thermodynamics of charge separation process in organic solar cells (OSCs).
Using the theory of stochastic thermodynamics, we connect the phonon-assisted
dynamics and recombination of electron-hole pairs within a photo-excited
organic bilayer with the thermodynamic free energy. We analyze the impact of
energetic disorder and delocalization on the free energy, average energy and
entropy. For high energetic disorder, the site population is well described by
equilibrium. We observe significant deviations from equilibrium for delocalized
electron-hole pairs at small energetic disorder, representing efficient OSCs.
Our results emphasize that both a large Gibbs entropy and large initial
separation are required to achieve efficient charge separation. A decrease in
free energy barrier with increased distances between charges does not
necessarily correlate with the separation yield. Our presented framework can
further shed light on the transient thermodynamic free energy, allowing
previously inaccessible insight into the individual thermodynamic contributions
from energy and entropy on sub-ns timescales. Transient simulations reveal
large Gibbs entropy on ps-timescales for even highest disorder, which may
explain the efficient separation of "hot" CT states.
