Variations in Hole Injection due to Fast and Slow Interfacial Traps in Polymer Light-Emitting Diodes with Interlayers

Authors

  • M. James Harding,

    1. Department of Physics and the Center for Plastic Electronics Imperial College London Blackett Laboratory South Kensington Campus London, SW7 2AZ (UK)
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  • Dmitry Poplavskyy,

    1. OSRAM Opto Semiconductors Inc. 3870 North First Street, San Jose, CA 95134 (USA)
    2. Present address: Innovalight, Inc., 965 East Arques Avenue, Sunnyvale CA 94085 (USA)
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  • Vi-En Choong,

    1. OSRAM Opto Semiconductors Inc. 3870 North First Street, San Jose, CA 95134 (USA)
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  • Franky So,

    1. OSRAM Opto Semiconductors Inc. 3870 North First Street, San Jose, CA 95134 (USA)
    2. Present address: Department of Materials Science and Engineering, University of Florida, Gainesville, FL 33611-6400 (USA)
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  • Alasdair J. Campbell

    Corresponding author
    1. Department of Physics and the Center for Plastic Electronics Imperial College London Blackett Laboratory South Kensington Campus London, SW7 2AZ (UK)
    • Department of Physics and the Center for Plastic Electronics Imperial College London Blackett Laboratory South Kensington Campus London, SW7 2AZ (UK).
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Abstract

Detailed studies on the effect of placing a thin (10 nm) solution-processable interlayer between a light-emitting polymer (LEP) layer and a poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonic)-acid-coated indium tin oxide anode is reported; particular attention is directed at the effects on the hole injection into three different LEPs. All three different interlayer polymers have low ionization potentials, which are similar to those of the LEPs, so the observed changes in hole injection are not due to variations in injection barrier height. It is instead shown that changes are due to variations in hole trapping at the injecting interface, which is responsible for varying the hole current by up to two orders of magnitude. Transient measurements show the presence of very fast interfacial traps, which fill the moment charge is injected from the anode. These can be considered as injection pathway dead-ends, effectively reducing the active contact surface area. This is followed by slower interfacial traps, which fill on timescales longer than the carrier transit time across the device, further reducing the total current. The interlayers may increase or decrease the trap densities depending on the particular LEP involved, indicating the dominant role of interfacial chain morphology in injection. Penetration of the interlayer into the LEP layer can also occur, resulting in additional changes in the bulk LEP transport properties.

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