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Formation cross-sections of singlet and triplet excitons in π-conjugated polymers

Nature volume 409, pages 494497 (25 January 2001) | Download Citation

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  • A Corrigendum to this article was published on 31 May 2001

Abstract

Electroluminescence in organic light-emitting diodes arises from a charge-transfer reaction between the injected positive and negative charges by which they combine to form singlet excitons that subsequently decay radiatively. The quantum yield of this process (the number of photons generated per electron or hole injected) is often thought1 to have a statistical upper limit of 25 per cent. This is based on the assumption that the formation cross-section of singlet excitons, σS, is approximately the same as that of any one of the three equivalent non-radiative triplet exciton states, σT; that is, σS T ≈ 1. However, recent experimental2 and theoretical3 work suggests that σST may be greater than 1. Here we report direct measurements of σ ST for a large number of π-conjugated polymers and oligomers. We have found that there exists a strong systematic, but not monotonic, dependence of σST on the optical gap of the organic materials. We present a detailed physical picture of the charge-transfer reaction for correlated π-electrons, and quantify this process using exact valence bond calculations. The calculated σ ST reproduces the experimentally observed trend. The calculations also show that the strong dependence of σS T on the optical gap is a signature of the discrete excitonic energy spectrum, in which higher energy excitonic levels participate in the charge recombination process.

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Acknowledgements

We thank E.J.W. List for supplying the mLPPP polymer, and X.M. Jiang and E.J.W. List for their support in performing the experiments. The work at the University of Utah was supported in part by the DOE and the NSF. Work in Arizona was partially supported by the ONR through the MURI centre (CAMP) at the University of Arizona. Work in Bangalor was funded by CSIR, India.

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Author notes

    • M. Wohlgenannt
    •  & Kunj Tandon

    These authors contributed equally to this work

Affiliations

  1. *Department of Physics, University of Utah, Salt Lake City, Utah 84112, USA

    • M. Wohlgenannt
    •  & Z. V. Vardeny
  2. ‡Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012 , India

    • Kunj Tandon
    •  & S. Ramasesha
  3. §Department of Physics, University of Arizona, Tucson, Arizona 85721, USA

    • S. Mazumdar

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Correspondence to Z. V. Vardeny.

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https://doi.org/10.1038/35054025

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