The conversion of photoexcitations into charge carriers in organic solar cells is facilitated by the dissociation of excitons at the donor/acceptor interface. The ultrafast timescale of charge separation demands theoretical models that go beyond equilibrium theories like Marcus theory and furthermore raises questions about coherence and the role of non-equilibrium nuclear configuration. We turned to Two-dimensional electronic spectroscopy (2D ES) to provide incisive information about charge separation dynamics that helps to shape and test theoretical models. 2D ES on poly(3-hexylthiophene)/PCBM (P3HT/PCBM) blends shows the pathways of charge transfer that clearly expose the significance of hot electron transfer. During this ultrafast electron transfer, vibrational coherence is directly transferred from the P3HTexciton to the P3HT hole polaron in the crystalline domain. This result reveals that the exciton converts to a hole with a similar spatial extent on a timescale far exceeding other photophysical dynamics including vibrational relaxation.
Interest in singlet fission has recently been surged due to the recognition that this process represents a potential means to circumvent the thermodynamic limitations of conventional single-junction photovoltaic cells. Specifically, the energy of a singlet exciton in excess of the optical gap that would otherwise be thermalized prior to charge separation is retained by splitting the photogenerated singlet exciton into a pair of triplet excitons. Transient absorption studies on a series of functionalized pentacene nanoparticles demonstrated versatility of pentacene core as a singlet fission chromophores. However, timescale of triplet pair formation is in qualitative disagreement with the predictions of theoretical models of singlet fission based on individual molecular pairs. We use 2D ES in order to gain insight on the mechanisms of singlet fission which aids testing and development of theoretical models.
Pensack,R. D.; Tilley, A. J.; Parkin, S. R.; Lee, T. S.; Payne, M. M.; Gao, D; Jahnke, A. A.; Oblinsky , D. G.; Li, P-F; Anthony, J. E.; Seferos, D. S.; Scholes, G. D. Exciton Delocalization Drives Rapid Singlet Fission in Nanoparticles of Acene Derivatives. J. Am. Chem. Res. 2015 (in press).
Song, Y.; Hellmann, C.; Stingelin, N.; Scholes, G. D. The separation of vibrational coherence from ground- and excited-electronic states in P3HT film. J. Chem. Phys. 2015, 142, 212410
Song, Y.; Clafton, S. N.; Pensack, R. D.; Kee, T. W.; Scholes, G. D. Vibrational coherence probes the mechanism of ultrafast electron transfer in polymer-fullerene blends. Nature. Commun. 2014, 5:4933.
Pensack, R. D.; Song, Y.; McCormick, T. M.; Jahnke, A. A.; Hollinger, J.; Seferos, D. S.; Scholes, G. D. Evidence for the rapid conversion of primary photoexcitations to triplet states in seleno- and telluro- analogues of poly(3-hexythiophene). J. Phys. Chem. B. 2014, 118 (9), 2589–2597.
Jumper, C. C.; Anna, J. M.; Stradomska, A.; Schins, J.; Myahkostupov, M.; Prusakova,V.; Oblinsky, D. G.; Castellano, F. N.; Knoester, J.; Scholes, G. D. Intramolecular radiationless transitions dominate exciton relaxation dynamics. Chem. Phys. Lett. 2014, 559, 23-33.
Hwang, I.; Selig, U.; Chen, S. S. Y.; Shaw, P. E.; Brixner, T.; Burn, P. L.; Scholes G. D. Photophysics of delocalized excitons in carbazole dendrimers. J. Phys. Chem. A. 2013, 117 (29), 6270–6278.
Hwang, I.; Beaupré, S.; Leclerc, M.; Scholes, G. D. Ultrafast relaxation of charge-transfer excitons in low-bandgap conjugated copolymers. Chem. Sci. 2012, 3, 2270-2277.