High performance organic light emitting diodes using substoichiometric tungsten oxide as efficient hole injection layer

Vasilopoulou, Maria, Papadimitropoulos, George, Palilis, Leonidas C., Georgiadou, Dimitra G., Argitis, Panagiotis, Kennou, Stella, Kostis, Ioannis, Vourdas, Nikolaos, Stathopoulos, Nikolaos A. and Davazoglou, Dimitris (2012) High performance organic light emitting diodes using substoichiometric tungsten oxide as efficient hole injection layer. Organic Electronics, 13 (5), 796-806. (doi:10.1016/j.orgel.2012.01.008).

Abstract

In this work we demonstrate the unique hole injection and transport properties of a substoichiometric tungsten oxide with precise stoichiometry, in particular WO_2.5, obtained after the controlled hydrogen reduction during growth of tungsten oxide, using a simple hot-wire vapor deposition technique. We present clear evidence that tungsten suboxide exhibits metallic character and that an almost zero hole injection barrier exists at the anode/polymer interface due to the formation/occupation of electronic gap states near the Fermi level after oxide's reduction. These states greatly facilitate hole injection and charge generation/electron extraction enabling the demonstration of extremely efficient hole only devices. WO_2.5 films exhibit metallic-like conductivity and, thus, can also enhance charge transport at both anode and cathode interfaces. Electroluminescent devices using WO_2.5 as both, hole and electron injection layer, and poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-{2,1′,3}-thiadiazole)] (F8BT) as the emissive layer exhibited high efficiencies up to 7 cd/A and 4.5 lm/W, while, stability studies revealed that these devices were extremely stable, since they were operating without encapsulation in air for more than 700 h.

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Contributors

Author: Dimitra G. Georgiadou

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