Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV16)
Publication date: 28th March 2016
Organic photovoltaics offer a cheaper alternative to the conventional crystalline Si-based solar cells. In order to improve their efficiency, it is imperative to track the fate of photogenerated excitons in the organic backbone to achieve higher charge transfer (CT) and polaron yields at the molecular interfaces. Studies on pristine conjugated polymers using transient absorption spectroscopy provided the structural correlations for direct generation of polaron pairs with optical excitation, [1] although the evolution of molecular structure in the excited state were not explored. However, transient absorption signatures of excitons and polarons are usually difficult to deconvolve due to inhomogeneity in the excited states. Therefore structure-sensitive vibrational spectroscopy is required to distinguish the complex excited state pathways in conjugated polymers. Femtosecond stimulated Raman spectroscopy (FSRS) provides both good spectral (~10 cm-1) as well as high temporal resolution (< 50 fs) for tracking the changes in the polymer structure after photoexcitation. Recent work has shown that indeed FSRS as a technique can be used to track the fate of excitons and polarons in both solution and spin-casted films.[2, 3] Here we report the exciton dynamics in Donor-Bridge-Acceptor (D-B-A) low bandgap conjugated polymers using FSRS as a spectroscopic tool. Our results suggest an ultrafast sub-picosecond localization of the exciton in solution through the thiophene bridge moiety along with a surprisingly long-lived excitonic state. The mode-dependent intensity changes reflect the nature of the exciton-phonon coupling in these D-B-A polymeric systems. We will discuss our results in the context of correlating the polaron generation yields with the molecular structure which is the key in determining the charge generation performance of the organic photovoltaic devices.
References
[1] R. Tautz, E. D. Como, T. Limmer, J. Feldmann, H. J. Egelhaaf, E. v. Hauff, V. Lemaur, D. Beljonne, S. Yilmaz, I. Dumsch, S. Allard and U. Scherf, Nat. Commun., 2012, 3; [2] T. J. Magnanelli and A. E. Bragg, J. Phys. Chem. Lett., 2014, 6, 438-445; [3] F. Provencher, N. Berube, A. W. Parker, G. M. Greetham, M. Towrie, C. Hellmann, M. Cote, N. Stingelin, C. Silva and S. C. Hayes, Nat. Commun., 2014, 5.
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