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. © 2014 Macmillan Publishers Limited. All rights reserved.
Bibliographical noteKAUST Repository Item: Exported on 2020-10-01
Acknowledgements: C.S. acknowledges funding from the Natural Sciences and Engineering Research Council of Canada, the Canada Research Chair in Organic Semiconductor Materials, the Royal Society and the Leverhulme Trust. The research leading to these results has received funding from LASERLAB-EUROPE (grant agreement no. 284464, EC's Seventh Framework Programme). N.S. and C.H. acknowledge funding from the UK Engineering and Physical Sciences Research Council EP/G060738/1 grant, the European Research Council (ERC) Starting Independent Research Fellowship under the grant agreement No. 279587 and King Abdullah University of Science and Technology (KAUST CRG). We acknowledge the assistance of A. Scaccabarozzi for film deposition and are grateful to Serge Beaupre and Mario Leclerc for providing the PCDTBT. S.C.H. and C.S. are grateful for illuminating discussions with David McCamant and Philip Reid.
This publication acknowledges KAUST support, but has no KAUST affiliated authors.