  Organic semiconductors have the remarkable property that their optical
excitation not only generates charge-neutral electron-hole pairs (excitons) but
also charge-separated polaron pairs with high yield. The microscopic mechanisms
underlying this charge separation have been debated for many years. Here we use
ultrafast two-dimensional electronic spectroscopy to study the dynamics of
polaron pair formation in a prototypical polymer thin film on a sub-20-fs time
scale. We observe multi-period peak oscillations persisting for up to about 1
ps as distinct signatures of vibronic quantum coherence at room temperature.
The measured two-dimensional spectra show pronounced peak splittings revealing
that the elementary optical excitations of this polymer are hybridized
exciton-polaron-pairs, strongly coupled to a dominant underdamped vibrational
mode. Coherent vibronic coupling induces ultrafast polaron pair formation,
accelerates the charge separation dynamics and makes it insensitive to
disorder. These findings open up new perspectives for tailoring
light-to-current conversion in organic materials.
