Efficient charge generation by relaxed charge-transfer states at organic interfaces

Abstract

Interfaces between organic electron-donating (D) and electron-accepting (A) materials have the ability to generate charge carriers on illumination. Efficient organic solar cells require a high yield for this process, combined with a minimum of energy losses. Here, we investigate the role of the lowest energy emissive interfacial charge-transfer state (CT1) in the charge generation process. We measure the quantum yield and the electric field dependence of charge generation on excitation of the charge-transfer (CT) state manifold via weakly allowed, low-energy optical transitions. For a wide range of photovoltaic devices based on polymer:fullerene, small-molecule:C60 and polymer:polymer blends, our study reveals that the internal quantum efficiency (IQE) is essentially independent of whether or not D, A or CT states with an energy higher than that of CT1 are excited. The best materials systems show an IQE higher than 90% without the need for excess electronic or vibrational energy.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.

from$8.99

All prices are NET prices.

Figure 1: Energetics of the relevant states at a D/A interface.
Figure 2: Current density and relative number of photogenerated charge carriers as a function of applied voltage.
Figure 3: Determination of IQE(E) in the spectral region of CT emission for polymer:fullerene photovoltaic devices.
Figure 4: Determination of IQE(E) in the spectral region of CT emission for small-molecule:C60 and polymer:polymer photovoltaic devices.

References

  1. 1

    Deibel, C. & Dyakonov, V. Polymer–fullerene bulk heterojunction solar cells. Rep. Prog. Phys. 73, 096401 (2010).

    Article  Google Scholar 

  2. 2

    Riede, M., Mueller, T., Tress, W., Schueppel, R. & Leo, K. Small-molecule solar cells—status and perspectives. Nanotechnology 19, 424001 (2008).

    CAS  Article  Google Scholar 

  3. 3

    Li, G., Zhu, R. & Yang, Y. Polymer solar cells. Nature Photon. 6, 153–161 (2012).

    CAS  Article  Google Scholar 

  4. 4

    Arkhipov, V. I., Emelianova, E. V. & Bässler, H. Hot exciton dissociation in a conjugated polymer. Phys. Rev. Lett. 82, 1321–1324 (1999).

    CAS  Article  Google Scholar 

  5. 5

    Tong, M., Coates, N. E., Moses, D., Heeger, A. J., Beaupré, S. & Leclerc, M. Charge carrier photogeneration and decay dynamics in the poly (2, 7-carbazole) copolymer PCDTBT and in bulk heterojunction composites with PC70BM. Phys. Rev. B 81, 125210 (2010).

    Article  Google Scholar 

  6. 6

    Vandewal, K. et al. Quantification of quantum efficiency and energy losses in low bandgap polymer: Fullerene solar cells with high open-circuit voltage. Adv. Funct. Mater. 22, 3480–3490 (2012).

    CAS  Article  Google Scholar 

  7. 7

    Hou, J., Chen, H.-Y., Zhang, S., Chen, R. I., Yang, Y., Wu, Y. & Li, G. Synthesis of a low band gap polymer and its application in highly efficient polymer solar cells. J. Am. Chem. Soc. 131, 15586 (2009).

    CAS  Article  Google Scholar 

  8. 8

    Liang, Y. et al. For the bright future-bulk heterojunction polymer solar cells with power conversion efficiency of 7.4%. Adv. Mater. 22, E135–E138 (2010).

    CAS  Article  Google Scholar 

  9. 9

    Park, S. H. et al. Bulk heterojunction solar cells with internal quantum efficiency approaching 100%. Nature Photon. 3, 297–302 (2009).

    CAS  Article  Google Scholar 

  10. 10

    Lin, L-Y. et al. A donor–acceptor–acceptor molecule for vacuum-processed organic solar cells with a power conversion efficiency of 6.4%. Chem. Commun. 48, 1857–1859 (2012).

    Article  Google Scholar 

  11. 11

    Bartelt, J. A. et al. The importance of fullerene percolation in the mixed regions of polymer–fullerene bulk heterojunction solar cells. Adv. Energy Mater. 3, 364–374 (2013).

    CAS  Article  Google Scholar 

  12. 12

    Brédas, J. L., Norton, J. E., Cornil, J. & Coropceanu, V. Molecular understanding of organic solar cells: The challenges. Acc. Chem. Res. 42, 1691–1699 (2009).

    Article  Google Scholar 

  13. 13

    Benson-Smith, J. J. et al. Formation of a ground-state charge-transfer complex in polyfluorene/[6, 6]-phenyl-c61 butyric acid methyl ester (PCBM) blend films and its role in the function of polymer/PCBM solar cells. Adv. Funct. Mater. 17, 451–457 (2007).

    CAS  Article  Google Scholar 

  14. 14

    Loi, M. A. et al. Charge transfer excitons in bulk heterojunctions of a polyfluorene copolymer and a fullerene derivative. Adv. Funct. Mater. 17, 2111–2116 (2007).

    CAS  Article  Google Scholar 

  15. 15

    Vandewal, K., Tvingstedt, K., Gadisa, A., Inganäs, O. & Manca, J. V. On the origin of the open-circuit voltage of polymer–fullerene solar cells. Nature Mater. 8, 904–909 (2009).

    CAS  Article  Google Scholar 

  16. 16

    Ohkita, H. et al. Charge carrier formation in polythiophene/fullerene blend films studied by transient absorption spectroscopy. J. Am. Chem. Soc. 130, 3030–3042 (2008).

    CAS  Article  Google Scholar 

  17. 17

    Bakulin, A. A. et al. The role of driving energy and delocalized states for charge separation in organic semiconductors. Science 335, 1340–1344 (2012).

    CAS  Article  Google Scholar 

  18. 18

    Grancini, G. et al. Hot exciton dissociation in polymer solar cells. Nature Mater. 12, 29–33 (2013).

    CAS  Article  Google Scholar 

  19. 19

    Kniepert, J., Schubert, M., Blakesley, J. C. & Neher, D. Photogeneration and recombination in P3HT/PCBM solar cells probed by time-delayed collection field experiments. J. Phys. Chem. Lett. 2, 700–705 (2011).

    CAS  Article  Google Scholar 

  20. 20

    Albrecht, S. et al. On the field dependence of free charge carrier generation and recombination in blends of PCPDTBT/PC70BM: Influence of solvent additives. J. Phys. Chem. Lett. 3, 640–645 (2012).

    CAS  Article  Google Scholar 

  21. 21

    Mihailetchi, V. D., Koster, L. J. A., Hummelen, J. C. & Blom, P. W. M. Photocurrent generation in polymer–fullerene bulk heterojunctions. Phys. Rev. Lett. 93, 216601 (2004).

    CAS  Article  Google Scholar 

  22. 22

    Mingebach, M., Walter, S., Dyakonov, V. & Deibel, C. Direct and charge transfer state mediated photogeneration in polymer–fullerene bulk heterojunction solar cells. Appl. Phys. Lett. 100, 193302 (2012).

    Article  Google Scholar 

  23. 23

    Parkinson, P., Lloyd-Hughes, J., Johnston, M. B. & Herz, L. M. Efficient generation of charges via below-gap photoexcitation of polymer–fullerene blend films investigated by terahertz spectroscopy. Phys. Rev. B 78, 115321 (2008).

    Article  Google Scholar 

  24. 24

    Lee, J. et al. Charge transfer state versus hot exciton dissociation in polymer–fullerene blended solar cells. J. Am. Chem. Soc. 132, 11878–11880 (2010).

    CAS  Article  Google Scholar 

  25. 25

    Van der Hofstad, T. G. J., Di Nuzzo, D., van den Berg, M., Janssen, R. A. J. & Meskers, S. C. J. Influence of photon excess energy on charge carrier dynamics in a polymer–fullerene solar cell. Adv. Energy Mater. 2, 1095–1099 (2012).

    CAS  Article  Google Scholar 

  26. 26

    Würfel, P. The chemical potential of radiation. J. Phys. C 15, 3967–3985 (1982).

    Article  Google Scholar 

  27. 27

    Würfel, P. The Physics of Solar Cells (Wiley, 2007).

    Google Scholar 

  28. 28

    Kasha, M. Characterization of electronic transitions in complex molecules. Discuss. Faraday Soc. 9, 14–19 (1950).

    Article  Google Scholar 

  29. 29

    Lupton, J. M. Frequency up-conversion as a temperature probe of organic opto-electronic devices. Appl. Phys. Lett. 80, 186–188 (2002).

    CAS  Article  Google Scholar 

  30. 30

    Gresser, R., Hummert, M., Hartmann, H., Leo, K. & Riede, M. Synthesis and characterization of near-infrared absorbing benzannulated Aza-BODIPY dyes. Chem. Eur. J. 17, 2939–2947 (2011).

    CAS  Article  Google Scholar 

  31. 31

    Meiss, J., Holzmueller, F., Gresser, R., Leo, K. & Riede, M. Near-infrared absorbing semitransparent organic solar cells. Appl. Phys. Lett. 99, 193307 (2011).

    Article  Google Scholar 

  32. 32

    Yin, C. et al. Tuning of the excited-state properties and photovoltaic performance in PPV-based polymer blends. J. Phys. Chem. C 112, 14607–14617 (2008).

    CAS  Article  Google Scholar 

  33. 33

    Jailaubekov, A. E. 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).

    CAS  Article  Google Scholar 

Download references

Acknowledgements

This publication was supported by the Center for Advanced Molecular Photovoltaics (Award No KUS-C1-015-21) and the Department of Energy, Laboratory Directed Research and Development funding, under contract DE-AC02-76SF00515. The PCDTBT used in this work was provided by St-Jean Photochemicals. M.K.R. acknowledges financial support by the BMBF through project 03IP602 and J.W. acknowledges support from the Heinrich-Böll-Stiftung. S.A. and M.S. acknowledge financial support by the BMBF within PVcomB (FKZ 03IS2151D) and the DFG (SPP 1355). D.N. thanks the DFG for financially supporting a travel grant. K.R.G. and A.A. acknowledge SABIC for a post-doctoral fellowship. The authors thank J. Kurpiers for technical assistance with the TDCF set-up.

Author information

Affiliations

Authors

Contributions

K.V., D.N., S.A. and A. Salleo designed the experiments. S.A. prepared devices for TDCF experiments and performed the TDCF experiments. K.V., W.R.M., E.T.H., K.R.G., J.T.B., M.S., J.W. and M.K.R. prepared photovoltaic devices and optimized their processing parameters for photovoltaic performance. E.T.H. and J.T.B. adjusted the EQE and electroluminescence measurement set-ups for the detection of weak signals, crucial for this work. K.V., E.T.H. and K.R.G. measured the EQE and electroluminescence spectra. K.V. measured the PDS spectra. J.D.D. synthesized PBDTTPD. A. Sellinger, J.M.J.F., A.A., M.K.R. and M.D.M. supervised their team members involved in the project. D.N. and A. Salleo supervised the overall project. All authors contributed to analysis and writing.

Corresponding authors

Correspondence to Koen Vandewal or Dieter Neher or Alberto Salleo.

Ethics declarations

Competing interests

The authors declare no competing financial interests.

Supplementary information

Supplementary Information

Supplementary Information (PDF 1308 kb)

Rights and permissions

Reprints and Permissions

About this article

Cite this article

Vandewal, K., Albrecht, S., Hoke, E. et al. Efficient charge generation by relaxed charge-transfer states at organic interfaces. Nature Mater 13, 63–68 (2014). https://doi.org/10.1038/nmat3807

Download citation

Further reading

Search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing