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Molecular-scale interface engineering for polymer light-emitting diodes

Nature volume 404, pages 481–484 (2000)Cite this article

Abstract

Achieving balanced electron–hole injection and perfect recombination of the charge carriers is central to the design of efficient polymer light-emitting diodes1,2 (LEDs). A number of approaches have focused on modification of the injection contacts, for example by incorporating an additional conducting-polymer layer at the indium-tin oxide (ITO) anode3,4. Recently, the layer-by-layer polyelectrolyte deposition route has been developed for the fabrication of ultrathin polymer layers5,6. Using this route, we previously incorporated ultrathin (<100 Å) charge-injection interfacial layers in polymer LEDs7. Here we show how molecular-scale engineering of these interlayers to form stepped and graded electronic profiles can lead to remarkably efficient single-layer polymer LEDs. These devices exhibit nearly balanced injection, near-perfect recombination, and greatly reduced pre-turn-on leakage currents. A green-emitting LED comprising a poly(p-phenylene vinylene) derivative sandwiched between a calcium cathode and the modified ITO anode yields an external forward efficiency of 6.0 per cent (estimated internal efficiency, 15–20 per cent) at a luminance of 1,600 candelas per m2 at 5 V.

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Figure 1: Schematic diagram of the hole-injection interlayer between ITO and the LEP, and chemical structure of some of the polymers.
Figure 2: Schematic electronic profile of the graded interlayer and some experimental confirmation.
Figure 3: Performance of LEDs with an 860-Å F8–F8BT film, Ca cathode and various ITO anodes.
Figure 4: Performance of LEDs with a 720-Å layer of green-emitting PPV (chemical structure in Fig. 1).

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Acknowledgements

We thank I. Grizzi, D. J. Lacey and E. P. Woo for support; J.-W. Cai for X-ray photoelectron spectroscopy; and A. Gerhard for electroabsorption measurements. P.K.H.H. is on leave from the National University of Singapore and thanks St John's College and IMRE for funding. This work was supported in part by the Engineering and Physical Sciences Research Council.

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Authors and Affiliations

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

    Peter K. H. Ho, Ji-Seon Kim, Thomas M. Brown, Franco Cacialli & Richard H. Friend

  2. Cambridge Display Technology, Greenwich House, Madingley Rise, Cambridge, CB3 0HJ, UK

    Peter K. H. Ho, Jeremy H. Burroughes & Richard H. Friend

  3. Covion Organic Semiconductors, Industrial Park Höchst G865, Frankfurt am Main, D-65926, Germany

    Heinrich Becker

  4. Department of Chemistry, National University of Singapore, and Institute of Materials Research and Engineering, Kent Ridge Crescent, S119260, Singapore

    Sam F. Y. Li

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Correspondence to Richard H. Friend.

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Ho, P., Kim, JS., Burroughes, J. et al. Molecular-scale interface engineering for polymer light-emitting diodes . Nature 404, 481–484 (2000). https://doi.org/10.1038/35006610

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