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Hot charge-transfer excitons set the time limit for charge separation at donor/acceptor interfaces in organic photovoltaics

Nature Materials volume 12, pages 66–73 (2013)Cite this article

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Abstract

Photocurrent generation in organic photovoltaics (OPVs) relies on the dissociation of excitons into free electrons and holes at donor/acceptor heterointerfaces. The low dielectric constant of organic semiconductors leads to strong Coulomb interactions between electron–hole pairs that should in principle oppose the generation of free charges. The exact mechanism by which electrons and holes overcome this Coulomb trapping is still unsolved, but increasing evidence points to the critical role of hot charge-transfer (CT) excitons in assisting this process. Here we provide a real-time view of hot CT exciton formation and relaxation using femtosecond nonlinear optical spectroscopies and non-adiabatic mixed quantum mechanics/molecular mechanics simulations in the phthalocyanine–fullerene model OPV system. For initial excitation on phthalocyanine, hot CT excitons are formed in 10−13 s, followed by relaxation to lower energies and shorter electron–hole distances on a 10−12 s timescale. This hot CT exciton cooling process and collapse of charge separation sets the fundamental time limit for competitive charge separation channels that lead to efficient photocurrent generation.

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Figure 1: TR-SHG pump–probe profiles of CuPc/fullerene interfaces showing indirect and direct formation of interfacial CT excitons.
Figure 2: TR-2PPE spectra of D/A interfaces showing hot CT excitons.
Figure 3: Non-adiabatic QM/MM simulations of charge separation dynamics at H2 Pc/C60 interfaces.
Figure 4: Non-adiabatic QM/MM simulations of charge separation dynamics at H2Pc/C60 interfaces showing hot CT exciton transitions.

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Acknowledgements

The results reported here were based on work supported as part of the Understanding Charge Separation and Transfer at Interfaces in Energy Materials (EFRC:CST), an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001091. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the US Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. Computational resources were provided by TACC and NERSC.

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Author notes

  1. Loren G. Kaake & X-Y. Zhu

    Present address: Present addresses: Department of Physics, University of California, Santa Barbara, California 93106, USA (L.G.K.); Department of Chemistry, Columbia University, New York, New York 10027, USA (X-Y.Z.),

  2. Askat E. Jailaubekov and Adam P. Willard: These authors contributed equally to this work

Authors and Affiliations

  1. Energy Frontier Research Center (EFRC: CST), University of Texas, Austin, Texas 78712, USA

    Askat E. Jailaubekov, Adam P. Willard, John R. Tritsch, Wai-Lun Chan, Na Sai, Raluca Gearba, Loren G. Kaake, Kenrick J. Williams, Peter J. Rossky & X-Y. Zhu

  2. Sandia National Laboratory, Albuquerque, New Mexico 87185, USA

    Kevin Leung

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Contributions

X-Y.Z. supervised the experiments. P.J.R. supervised QM/MM simulations; A.E.J. and L.G.K. carried out the TR-SHG experiments; J.R.T. and W-L.C. carried out the TR-2PPE experiments; A.P.W. carried out the QM/MM simulations; R.G. assisted in sample preparation; K.J.W. helped with experimental set-up; N.S. and K.L. carried out time-dependent density functional theory calculations. X-Y.Z., A.E.J. and A.P.W. wrote the manuscript.

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Correspondence to Askat E. Jailaubekov, Adam P. Willard, Loren G. Kaake, Peter J. Rossky or X-Y. Zhu.

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Jailaubekov, A., Willard, A., Tritsch, J. et al. Hot charge-transfer excitons set the time limit for charge separation at donor/acceptor interfaces in organic photovoltaics. Nature Mater 12, 66–73 (2013). https://doi.org/10.1038/nmat3500

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