Time-Resolve Raman Spectroscopy of Polaron Pair Formation in Poly(3-hexylthiophene) Aggregates
Link to Publication: ACS Publication
Contributing Authors: Tim Magnanelli, Art Bragg
Date Published: January 14, 2015
The ultrafast formation of bound charge pairs, or polaron pairs (PP), in mixed-order aggregates of poly-(3-hexylthiophene) was investigated using femtosecond stimulated Raman spectroscopy (FSRS). Time-resolved measurements of the carbon-carbon stretching region reveal a significant photo-induced depletion in steady-state features associated with lamellar-stacked, ordered polymer regions upon 500-nm photoexcitation followed by the appearance of red-shifted features delayed by a few hundred femtoseconds and attributable to PPs. PP features decay with concomitant recovery of the steady-state Raman depletion over a few picoseconds. The vibrational spectrum of the polaron pair obtained exhibits a modest redshift (<15 cm-1) and lower Raman activity relative to steady-state features in the C=C stretching region, but similar features in other regions. In total, this work demonstrates the potential of time-resolved Raman as a morphologically selective and structurally sensitive probe for tracking ultrafast charge separation and recombination dynamics within polymer regions of conjugated materials.