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

Françoise Provencher, Nicolas Bérubé, Anthony W. Parker, Gregory M. Greetham, Michael Towrie, Christoph Hellmann, Michel Côté, Natalie Stingelin, Carlos Silva () and Sophia C. Hayes ()
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Françoise Provencher: Université de Montréal, C.P. 6128, Succursale Centre-Ville
Nicolas Bérubé: Université de Montréal, C.P. 6128, Succursale Centre-Ville
Anthony W. Parker: Central Laser Facility, Research Complex at Harwell, Science and Technology Facilities Council, Rutherford Appleton Laboratory, Harwell Oxford
Gregory M. Greetham: Central Laser Facility, Research Complex at Harwell, Science and Technology Facilities Council, Rutherford Appleton Laboratory, Harwell Oxford
Michael Towrie: Central Laser Facility, Research Complex at Harwell, Science and Technology Facilities Council, Rutherford Appleton Laboratory, Harwell Oxford
Christoph Hellmann: Centre for Plastic Electronics, Imperial College London, South Kensington Campus
Michel Côté: Université de Montréal, C.P. 6128, Succursale Centre-Ville
Natalie Stingelin: Centre for Plastic Electronics, Imperial College London, South Kensington Campus
Carlos Silva: Université de Montréal, C.P. 6128, Succursale Centre-Ville
Sophia C. Hayes: University of Cyprus, P.O. Box 20537

Nature Communications, 2014, vol. 5, issue 1, 1-11

Abstract: Abstract 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 results in distinctive signatures in the vibrational modes of the polymer. Here, 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, further structural evolution on ≲50-ps timescales is modest, indicating that the polymer conformation hosting nascent polarons is not significantly different from that near equilibrium. We interpret this as suggestive that charges are free from their mutual Coulomb potential because we would expect rich vibrational dynamics associated with charge-pair relaxation. We address current debates on the photocarrier generation mechanism at molecular heterojunctions, and our work is, to our knowledge, the first direct probe of molecular conformation dynamics during this fundamentally important process in these materials.

Date: 2014
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DOI: 10.1038/ncomms5288

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