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Improved quantum efficiency for electroluminescence in semiconducting polymers

Nature volume 397, pages 414–417 (1999)Cite this article

Abstract

Some conjugated polymers have luminescence properties that are potentially useful for applications such as light-emitting diodes, whose performance is ultimately limited by the maximum quantum efficiency theoretically attainable for electroluminescence1, 2,. If the lowest-energy excited states are strongly bound excitons (electron–hole pairs in singlet or triplet spin states), this theoretical upper limit is only 25% of the corresponding quantum efficiency for photoluminescence: an electron in the π*-band and a hole (or missing electron) in the π-band can form a triplet with spin multiplicity of three, or a singlet with spin multiplicity of one, but only the singlet will decay radiatively3. But if the electron–hole binding energy is sufficiently weak, the ratio of the maximum quantum efficiencies for electroluminescence and photoluminescence can theoretically approach unity. Here we report a value of ∼50% for the ratio of these efficiencies (electroluminescence:photoluminescence) in polymer light-emitting diodes, attained by blending electron transport materials with the conjugated polymer to improve the injection of electrons. This value significantly exceeds the theoretical limit for strongly bound singlet and triplet excitons, assuming they comprise the lowest-energy excited states. Our results imply that the exciton binding energy is weak, or that singlet bound states are formed with higher probability than triplets.

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Figure 1: External electroluminescence quantum efficiency, QEext(EL), as a function of film thickness.
Figure 2: Temperature dependence of QEext(EL) and QEext(PL).
Figure 3: Comparison of external QEext(EL), filled circles, with QEext(PL), for devices with different luminescent polymer thickness.

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Acknowledgements

Research at UNIAX was partially supported by the Office of Naval Research.

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Authors and Affiliations

  1. UNIAX Corporation, 6780 Cortona Drive, Santa Barbara, 93117-3022, California, USA

    Yong Cao, Ian D. Parker, Gang Yu, Chi Zhang & Alan J. Heeger

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Cao, Y., Parker, I., Yu, G. et al. Improved quantum efficiency for electroluminescence in semiconducting polymers. Nature 397, 414–417 (1999). https://doi.org/10.1038/17087

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