Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

  • Letter
  • Published:

Spin-dependent exciton formation in π-conjugated compounds

Nature volume 413, pages 828–831 (2001)Cite this article

Abstract

The efficiency of light-emitting diodes (LEDs) made from organic semiconductors is determined by the fraction of injected electrons and holes that recombine to form emissive spin-singlet states rather than non-emissive spin-triplet states. If the process by which these states form is spin-independent, the maximum efficiency of organic LEDs will be limited to 25 per cent1. But recent reports have indicated fractions of emissive singlet states ranging from 22 to 63 per cent2,3,4,5, and the reason for this variation remains unclear. Here we determine the absolute fraction of singlet states generated in a platinum-containing conjugated polymer and its corresponding monomer. The spin-orbit coupling introduced by the platinum atom allows triplet-state emission, so optically and electrically generated luminescence from both singlet and triplet states can be compared directly. We find an average singlet generation fraction of 22 ± 1 per cent for the monomer, but 57 ± 4 per cent for the polymer. This suggests that recombination is spin-independent for the monomer, but that a spin-dependent process, favouring singlet formation, is effective in the polymer. We suggest that this process is a consequence of the exchange interaction, which will operate on overlapping electron and hole wavefunctions on the same polymer chain at their capture radius.

This is a preview of subscription content, access via your institution

Access options

Buy this article

  • Purchase on Springer Link
  • Instant access to full article PDF
Buy now

Prices may be subject to local taxes which are calculated during checkout

Figure 1: The chemical structures of the platinum-containing polymer and monomer investigated.
Figure 2: Comparison of the photoluminescence and electroluminescence spectra of light-emitting diodes of the platinum-containing polymer and monomer at 290 K.
Figure 3: A simple model for the emission spectra in photoluminescence (a) and electroluminescence (b).
Figure 4: The singlet generation fraction χS for the platinum-containing polymer and monomer under different device conditions.

Similar content being viewed by others

References

  1. Friend, R. H. et al. Electroluminescence in conjugated polymers. Nature 397, 121–128 (1999).

    Article  ADS  CAS  Google Scholar 

  2. Baldo, M. A., O'Brien, D. F., Thompson, M. E. & Forrest, S. R. Excitonic singlet-triplet ratio in a semiconducting organic thin film. Phys. Rev. B 60, 14422–14428 (1999).

    Article  ADS  CAS  Google Scholar 

  3. Ho, P. K. H. et al. Molecular-scale interface engineering for polymer light-emitting diodes. Nature 404, 481–484 (2000).

    Article  ADS  CAS  Google Scholar 

  4. Cao, Y., Parker, I. D., Yu, G., Zhang, C. & Heeger, A. J. Improved quantum efficiency for electroluminescence in semiconducting polymers. Nature 397, 414–417 (1999).

    Article  ADS  CAS  Google Scholar 

  5. Wohlgenannt, M., Tandon, K., Mazumdar, S., Ramasesha, S. & Vardeny, Z. V. Formation cross-sections of singlet and triplet excitons in π-conjugated polymers. Nature 409, 494–497 (2001).

    Article  ADS  CAS  Google Scholar 

  6. Turro, N. J. Modern Molecular Photochemistry 1–195 (University Science, California, 1991).

    Google Scholar 

  7. Beljonne, D. et al. Spatial extent of the singlet and triplet excitons in transition metal-containing poly-ynes. J. Chem. Phys. 105, 3868–3877 (1996).

    Article  ADS  CAS  Google Scholar 

  8. Wittmann, H. F., Friend, R. H., Khan, M. S. & Lewis, J. Optical spectroscopy of platinum and palladium containing poly-ynes. J. Chem. Phys. 101, 2693–2698 (1994).

    Article  ADS  CAS  Google Scholar 

  9. Wilson, J. S. et al. Triplet states in a series of Pt-containing ethynylenes. J. Chem. Phys. 113, 7627–7634 (2000).

    Article  ADS  CAS  Google Scholar 

  10. Chawdhury, N. et al. Evolution of lowest singlet and triplet excited states with number of thienyl rings in platinum poly-ynes. J. Chem. Phys. 110, 4963–4970 (1999).

    Article  ADS  CAS  Google Scholar 

  11. Chawdhury, N. et al. Synthesis and electronic structure of platinum-containing poly-ynes with aromatic and heteroaromatic rings. Macromolecules 31, 722–727 (1998).

    Article  ADS  CAS  Google Scholar 

  12. Romanovskii, Y. V. et al. Phosphorescence of π-conjugated oligomers and polymers. Phys. Rev. Lett. 84, 1027–1030 (2000).

    Article  ADS  CAS  Google Scholar 

  13. Hertel, D. et al. Phosphorescence in conjugated poly(para-phenylene)-derivatives. Adv. Mater. 13, 65–70 (2001).

    Article  CAS  Google Scholar 

  14. Beljonne, D., Shuai, Z., Friend, R. H. & Brédas, J. L. Theoretical investigation of the lowest singlet and triplet states in poly(paraphenylene vinylene) oligomers. J. Chem. Phys. 102, 2042–2049 (1995).

    Article  ADS  CAS  Google Scholar 

  15. Wilson, J. S. et al. The energy gap law for triplet states in Pt-containing conjugated polymers and monomers. J. Am. Chem. Soc. 123, 9412–9417 (2001).

    Article  CAS  Google Scholar 

  16. Becker, H., Burns, S. E. & Friend, R. H. Effect of metal films on the photoluminescence and electroluminescence of conjugated polymers. Phys. Rev. B 56, 1893–1905 (1997).

    Article  ADS  CAS  Google Scholar 

  17. Kim, J. S., Ho, P. K. H., Greenham, N. C. & Friend, R. H. Electroluminescence emission pattern of organic light-emitting diodes: Implications for device efficiency calculations. J. Appl. Phys. 88, 1073–1081 (2000).

    Article  ADS  CAS  Google Scholar 

  18. Kobrak, M. N. & Bittner, E. R. Quantum molecular dynamics study of polaron recombination in conjugated polymers. Phys. Rev. B 62, 11473–11486 (2000).

    Article  ADS  CAS  Google Scholar 

  19. Blom, P. W., de Jong, M. J. M. & Breedijk, S. Temperature dependent electron-hole recombination in polymer light emitting diodes. Appl. Phys. Lett. 71, 930–932 (1997).

    Article  ADS  CAS  Google Scholar 

  20. Pope, M. & Swenberg, C. E. Electronic Processes in Organic Crystals and Polymers 2nd edn, 958–960 (Oxford Science, Oxford, 1999).

    Google Scholar 

  21. Shuai, Z., Beljonne, D., Silbey, R. J. & Brédas, J. L. Singlet and triplet exciton formation rates in conjugated polymer light-emitting diodes. Phys. Rev. Lett. 84, 131–134 (2000).

    Article  ADS  CAS  Google Scholar 

  22. Hong, T. & Meng, H. Spin-dependent recombination and electroluminescence quantum yield in conjugated polymers. Phys. Rev. B 63, 1–5 (2001).

    Google Scholar 

  23. Burin, A. L. & Ratner, M. A. Spin effects on the luminescence yield of organic light emitting diodes. J. Chem. Phys. 109, 6092–6102 (1998).

    Article  ADS  CAS  Google Scholar 

Download references

Acknowledgements

We thank N. C. Greenham for discussions and M. R. A. Al-Mandhary for the synthesis of the polymer and monomer. A.K. thanks Peterhouse, University of Cambridge, UK, for a Fellowship and the Royal Society for a University Research Fellowship. M.S.K. thanks Sultan Qaboos University, Oman, for a research grant and research leave, and the EPSRC (UK) for a Visiting Fellowship. This work was funded by the EPSRC.

Author information

Authors and Affiliations

  1. Cavendish Laboratory, Madingley Road, Cambridge, CB3 0HE, UK

    J. S. Wilson, A. S. Dhoot, A. J. A. B. Seeley, A. Köhler & R. H. Friend

  2. Department of Chemistry, Sultan Qaboos University, Al-Khod, 123, Oman

    M. S. Khan

Authors
  1. J. S. Wilson
    View author publications

    You can also search for this author in PubMed Google Scholar

  2. A. S. Dhoot
    View author publications

    You can also search for this author in PubMed Google Scholar

  3. A. J. A. B. Seeley
    View author publications

    You can also search for this author in PubMed Google Scholar

  4. M. S. Khan
    View author publications

    You can also search for this author in PubMed Google Scholar

  5. A. Köhler
    View author publications

    You can also search for this author in PubMed Google Scholar

  6. R. H. Friend
    View author publications

    You can also search for this author in PubMed Google Scholar

Corresponding author

Correspondence to A. Köhler.

Rights and permissions

Reprints and permissions

About this article

Cite this article

Wilson, J., Dhoot, A., Seeley, A. et al. Spin-dependent exciton formation in π-conjugated compounds. Nature 413, 828–831 (2001). https://doi.org/10.1038/35101565

Download citation

  • Received:

  • Accepted:

  • Issue Date:

  • DOI: https://doi.org/10.1038/35101565

This article is cited by

Comments

By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.

Search

Advanced 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