  In polymeric semiconductors, charge carriers are polarons, which means that
the excess charge deforms the molecular structure of the polymer chain that
hosts it. This effect results in distinctive signatures in the vibrational
modes of the polymer. We probe polaron photo- generation dynamics at
polymer:fullerene heterojunctions by monitoring its time-resolved
resonance-Raman spectrum following ultrafast photoexcitation. We conclude that
polarons emerge within 200 fs, which is nearly two orders of magnitude faster
than exciton localisation in the neat polymer film. Surprisingly, further
vibrational evolution on <50-ps timescales is modest, indicating that the
polymer conformation hosting nascent polarons is not signif- icantly different
from that in equilibrium. This suggests that charges are free from their mutual
Coulomb potential, under which vibrational dynamics would report charge-pair
relaxation. Our work addresses current debates on the photocarrier generation
mechanism at organic semiconductor heterojunctions, and is, to our knowledge,
the first direct probe of molecular conformation dynamics during this
fundamentally important process in these materials.
