Origin of efficient hole injection from conducting polymer anodes into organic light-emitting diodes

Paul A. Lane; Paul J. Brewer; Gary P. Kushto; Zakya H. Kafafi; John C. de Mello

doi:10.1117/12.742676

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17 September 2007 Origin of efficient hole injection from conducting polymer anodes into organic light-emitting diodes

Paul A. Lane, Paul J. Brewer, Gary P. Kushto, Zakya H. Kafafi, John C. de Mello

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Proceedings Volume 6643, Physical Chemistry of Interfaces and Nanomaterials VI; 664307 (2007) https://doi.org/10.1117/12.742676
Event: NanoScience + Engineering, 2007, San Diego, California, United States

Abstract

We studied hole injection from the conducting polymer blend poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) by optical spectroscopy and characterization of organic light-emitting diodes (OLEDs). Electroabsorption (EA) spectroscopy was used to measure the built-in potential of polyfluorene-based OLEDs with indium tin oxide (ITO) or poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) anodes. Although the work function of PEDOT:PSS is 5.1 eV, the inferred anode work function matches the ionization potential of the emitting polymer. We conclude that the Fermi level at the PEDOT:PSS/polyfluorene interface is pinned to the highest-occupied molecular orbital (HOMO) of the emitting polymer, permitting efficient hole injection. To test this hypothesis, we fabricated OLEDs using the archetypical molecular semiconductor, tris(8-hydroxyquinoline) aluminum (III) (Alq₃). Although the anticipated hole injection barrier is 0.7 eV, OLEDs with Alq₃ deposited onto PEDOT:PSS operate at a lower bias and higher power efficiency than OLEDs with a hole transport layer. The quantum efficiency of single layer Alq₃ and rubrene-doped Alq₃ devices is equal to that of multi-layer devices, showing that EL is not quenched by PEDOT:PSS.

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Paul A. Lane, Paul J. Brewer, Gary P. Kushto, Zakya H. Kafafi, and John C. de Mello "Origin of efficient hole injection from conducting polymer anodes into organic light-emitting diodes", Proc. SPIE 6643, Physical Chemistry of Interfaces and Nanomaterials VI, 664307 (17 September 2007); https://doi.org/10.1117/12.742676

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