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Spin-conserving carrier recombination in conjugated polymers

Nature Materials volume 4, pages 340–346 (2005)Cite this article

Abstract

The ultimate efficiency of polymer light-emitting diodes is limited by the fraction of charges recombining in the molecular singlet manifold. We address the question of whether this fraction can principally exceed the fundamental limit set down by spin statistics, which requires the possibility of spin changes during exciton formation. Sensitized phosphorescence at 4–300 K enables a direct quantification of spin conversion in coulombically bound electron–hole pairs, the precursors to exciton formation. These are stabilized in external electric fields over times relevant to carrier transport, capture and recombination in devices. No interconversion of exciton intermediates between singlet and triplet configurations is observed. Static magnetic fields are equally unable to induce spin mixing in electroluminescence. Our observations imply substantial exchange splitting at all times during carrier capture. Prior statements regarding increased singlet yields above 25% merely on the basis of higher singlet than triplet formation rates should therefore be re-examined.

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Figure 1: Comparison of EL and PL properties of the hydrocarbon conjugated polymer PhLPPP.
Figure 2: Transient EL intensity of a PhLPPP LED.
Figure 3: Storage of singlet excitons in external reverse-bias electric fields.
Figure 4: Field modulated recombination of optically generated singlet and triplet charge carrier pairs.
Figure 5: Effect of strong magnetic fields on fluorescence and phosphorescence under electrical excitation at 100 K.

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Acknowledgements

We are indebted to Satish Patil for the synthesis of PhLPPP and would like to thank A. Kadashchuk for making a preprint of ref. 8 available as well as Peter Gilch for insightful discussions. Technical support by W. Stadler and A. Helfrich is gratefully acknowledged, as is financial assistance by the Bundesministerium für Bildung und Forschung through the organic laser diode collaboration, OLAS, the Deutsche Forschungsgemeinschaft through a Gottfried Wilhelm Leibniz award, and the EU through the Physics of Hybrid Organic–Inorganic Heterostructures for Photonics and Telecommunications research training network HYTEC.

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Authors and Affiliations

  1. Physics Department and CeNS, Photonics and Optoelectronics Group, Ludwig-Maximilians-Universität, Amalienstr. 54, Munich, 80799, Germany

    Martin Reufer, Manfred J. Walter, Pavlos G. Lagoudakis & John M. Lupton

  2. Physikalisches Institut, Johann-Wolfgang-Goethe-Universität, Max-von-Laue-Str. 1, Frankfurt am Main, 60438, Germany

    Anne Beate Hummel, Johanna S. Kolb & Hartmut G. Roskos

  3. FB Chemie, Universität Wuppertal, Gauss-Str. 20, Wuppertal, 42097, Germany

    Ullrich Scherf

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Reufer, M., Walter, M., Lagoudakis, P. et al. Spin-conserving carrier recombination in conjugated polymers. Nature Mater 4, 340–346 (2005). https://doi.org/10.1038/nmat1354

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