  Organic solar cells (OSCs) based on non-fullerene acceptors can show high
charge generation yields despite near-zero donor-acceptor energy offsets to
drive charge separation and overcome the mutual Coulomb attraction between
electron and hole. Here we use time-resolved optical spectroscopy to show that
free charges in these systems are generated by thermally activated dissociation
of interfacial charge-transfer excitons (CTEs) that occurs over hundreds of
picoseconds at room temperature, three orders of magnitude slower than
comparable fullerene-based systems. Upon free electron-hole encounters at later
times, CTEs and emissive excitons are regenerated, thus setting up an
equilibrium between excitons, CTEs and free charges. This endothermic charge
separation process enables these systems to operate close to
quasi-thermodynamic equilibrium conditions with no requirement for energy
offsets to drive charge separation and achieve greatly suppressed non-radiative
recombination.
