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Proceedings Paper

Measurement and modeling of carrier transport and exciton formation in blue polymer light emitting diodes
Author(s): R. Coehoorn; S. Vulto; S. L. M. van Mensfoort; J. Billen; M. Bartyzel; H. Greiner; R. Assent
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Paper Abstract

An experimental and modelling study has been carried out of the current-voltage-luminance (J-V-L) characteristics of blue polyfluorene-based organic light emitting devices, with a PEDOT:PSS anode and a Ba/Al cathode. The polymer contains copolymerized hole transporting units that facilitate hole injection. The luminous efficacy for perpendicular emission as a function of the voltage shows a pronounced peak; for an 80 nm thick device, it is equal to 3.3 cd/A at 8 V. At the peak voltage, the external quantum efficiency is 2.2 %. We are working on a comprehensive device model that should provide a framework within which these results can be understood, and present in this paper our intermediate results. Hole and electron transport were studied using devices with a Au and Al cathode and anode, respectively. For hole-only devices a fair description of the temperature and layer thickness dependent J-V curves could be obtained by using a 'conventional' model for the mobility, involving a Poole-Frenkel factor for the field-dependence. For electron-only devices, the analysis is complicated by the presence of an approximately 0.5 eV injection barrier. We have found a parametrization scheme that provides a good description of the experimental J-V curves. A double carrier model that is based on the results of these studies of single-carrier devices provides a good description of the J-V curves of double carrier devices. We have developed a numerical model for the light outcoupling from the optical cavity. For the model parameters assumed, the calculated peak position and shape of the lumunous efficacy as a function of V are in good agreement with the experimental results at room temperature. An analysis is given of the factors that determine the peak height. We argue that a solid physical basis for the model used to describe the electron injection and mobility is still lacking, so that continued electron transport studies will be required.

Paper Details

Date Published: 26 April 2006
PDF: 12 pages
Proc. SPIE 6192, Organic Optoelectronics and Photonics II, 61920O (26 April 2006); doi: 10.1117/12.663096
Show Author Affiliations
R. Coehoorn, Philips Research Labs. (Netherlands)
Eindhoven Univ. of Technology (Netherlands)
S. Vulto, Philips Research Labs. (Netherlands)
S. L. M. van Mensfoort, Eindhoven Univ. of Technology (Netherlands)
J. Billen, Eindhoven Univ. of Technology (Netherlands)
M. Bartyzel, Philips Research Labs. (Netherlands)
H. Greiner, Philips Research Labs. (Germany)
R. Assent, Philips Research Labs. (Germany)

Published in SPIE Proceedings Vol. 6192:
Organic Optoelectronics and Photonics II
Paul L. Heremans; Michele Muccini; Eric A. Meulenkamp, Editor(s)

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