Direct observation of ultrafast long-range charge separation at polymer-fullerene heterojunctions

March 21, 2014

Carlos Silva

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 ef fect results in distinctive
signatures in the vibrational modes of the polymer.
We probe polaron photogeneration dynamics at
polymer:fullerene heterojunctions by monitoring its
time-resolved resonance-Raman spectrum following
ultrafast photoexcitation. We conclude that polarons
emerge within 300 fs. Surprisingly, fur ther structural
evolution on < 50-ps timescales is modest, indicating
that the polymer conformation hosting nascent
polarons is not significantly dif ferent from that in
equilibrium. This suggests that charges are free from
their mutual Coulomb potential, under which
vibrational dynamics would repor t 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