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Proceedings Paper

Highly efficient OLEDs on ITO-free polymeric substrates
Author(s): Karsten Fehse; Karsten Walzer; Gufeng He; Martin Pfeiffer; Karl Leo; Wilfried Lövenich; Andreas Elschner
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Paper Abstract

Standard organic light emitting diodes (OLEDs) are usually bottom-emitting, i.e. they emit light through a transparent and electrically conductive substrate. Usually, indium tin oxide (ITO) is used for this purpose. However, as indium is a very expensive metal, replacing it is of vital interest for cheap OLED mass production, especially when it comes to lighting applications. We suggest the use of a polymer instead of ITO, carrying out both hole transport and injection. In contrast to conventional approaches, which use a conductive polymer on top of ITO as smoothening and hole injection layer, we employ solely a highly conductive polymer in combination with an OLED comprising doped charge transport layers. This allows us to renounce the ITO layer underneath. We use a new, highly conductive formulation of PEDOT:PSS, called Baytron® PH 500, with a conductivity of typically 500 S/cm, providing a smooth and electrically well-conductive substrate for the OLED stack. The use of such a polymeric injection layer and of a doped small-molecule OLED stack results in a low operating voltage of the devices. The charge transport layers of the OLED consist of MeO-TPD (N,N,N',N'-tetrakis(4-methoxyphenyl)-benzidine) doped with a low percentage of F4-TCNQ (2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane) for the hole transport layer and of Bphen (4,7-diphenyl-1,10-phenanthroline) co-evaporated with Caesium for the electron transport layer. We demonstrate both fluorescent and phosphorescent monochromic OLEDs based on Baytron® PH 500 which achieve good efficiencies. The OLEDs made on Baytron® PH 500 are compared with devices made on an ITO anode. Although the polymer possesses a somewhat lower conductivity than ITO, efficient devices can be fabricated. For example, using the blue emitter Spiro-DPVBi (2,2',7,7'-tetrakis(2,2-diphenylvinyl)spiro-9,9'-bifluorene), we achieve an efficiency of up to 5.1 cd/A. As another example, we discuss green OLEDs based on the triplet emitter Ir(ppy)3 (fac tris(2-phenylpyridine) iridium) doped in a wide gap material. In this case, even a higher efficiency than on ITO is reached: 62 cd/A at a luminance of 100 cd/m2, corresponding to an increase in external quantum efficiency by 15% as compared to ITO.

Paper Details

Date Published: 20 April 2006
PDF: 6 pages
Proc. SPIE 6192, Organic Optoelectronics and Photonics II, 61921Z (20 April 2006); doi: 10.1117/12.662936
Show Author Affiliations
Karsten Fehse, Technische Univ. Dresden (Germany)
Karsten Walzer, Technische Univ. Dresden (Germany)
Gufeng He, Technische Univ. Dresden (Germany)
Martin Pfeiffer, Technische Univ. Dresden (Germany)
Karl Leo, Technische Univ. Dresden (Germany)
Wilfried Lövenich, H.C. Starck GmbH (Germany)
Andreas Elschner, H.C. Starck GmbH (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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