Proceedings Article | 20 April 2006
KEYWORDS: Organic light emitting diodes, Polymers, Molecules, Indium, Coating, Oxides, Metals, External quantum efficiency, Doping, Optical properties
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.
