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Hot exciton dissociation in polymer solar cells

Nature Materials volume 12, pages 29–33 (2013)Cite this article

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Abstract

The standard picture of photovoltaic conversion in all-organic bulk heterojunction solar cells predicts that the initial excitation dissociates at the donor/acceptor interface after thermalization1,2,3,4,5,6,7,8,9,10,11,12. Accordingly, on above-gap excitation, the excess photon energy is quickly lost by internal dissipation2,3,11,13,14,15,16. Here we directly target the interfacial physics of an efficient low-bandgap polymer/PC60BM system. Exciton splitting occurs within the first 50 fs, creating both interfacial charge transfer states (CTSs) and polaron species. On high-energy excitation, higher-lying singlet states convert into hot interfacial CTSs that effectively contribute to free-polaron generation. We rationalize these findings in terms of a higher degree of delocalization of the hot CTSs with respect to the relaxed ones, which enhances the probability of charge dissociation in the first 200 fs. Thus, the hot CTS dissociation produces an overall increase in the charge generation yield.

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Figure 1: Absorption spectra of PCPDTBT, PC60BM and PCPDTBT/PC60BM (1:4) film, and vertical excited-state transitions computed for PCPDTBT.
Figure 2: Comparison of transient absorption maps at different excitation energies of the pristine PCPDTBT (top row) and PCPDTBT/PC60BM blend (bottom row).
Figure 3: Calculated excited-state energy levels for CPDTBT4, CPDTBT4/PCB60M and PC60BM, and computed IES DOS and sketch of the molecular dimer.
Figure 4: Ultrafast dynamics of singlets, IES and polarons on different photoexcitation energies.
Figure 5: Schematic of the ultrafast photophysical scenario and IQE versus energy for a PCPDTBT/PC60BM device.

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Acknowledgements

G.C. acknowledges financial support from the PRIN programme 2008JKBBK4. D.F. thanks M. Barbatti of the Max-Planck-Institut für Kohlenforschung for useful discussions.

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Authors and Affiliations

  1. Center for Nano Science and Technology@Polimi, Istituto Italiano di Tecnologia, via Pascoli 70/3 20133 Milano, Italy

    G. Grancini, D. Fazzi, A. Petrozza & G. Lanzani

  2. Dipartimento di Fisica, IFN-CNR, Politecnico di Milano, Piazza L. da Vinci, 32, 20133 Milano, Italy

    M. Maiuri, D. Brida, G. Cerullo & G. Lanzani

  3. Konarka Technologies GmbH, Landgrabenstrasse 94, 90443 Nürnberg, Germany

    H-J. Egelhaaf

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  1. G. Grancini
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Contributions

Data were taken by and analysed by G.G., M.M., A.P. and D.B. The experiments were conceived by G.L., G.G., G.C. and H-J.E. Quantum chemical calculations were carried out by D.F. Samples were prepared by G.G. All authors contributed to the writing of the paper.

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Correspondence to G. Lanzani.

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Grancini, G., Maiuri, M., Fazzi, D. et al. Hot exciton dissociation in polymer solar cells. Nature Mater 12, 29–33 (2013). https://doi.org/10.1038/nmat3502

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