Reduction of Tungsten Oxide: A Path Towards Dual Functionality Utilization for Efficient Anode and Cathode Interfacial Layers in Organic Light-Emitting Diodes

• Published in: Advanced functional materials 2011-04, Vol.21 (8), p.1489-1497
• Main Authors: Vasilopoulou, Maria, Palilis, Leonidas C., Georgiadou, Dimitra G., Douvas, Antonios M., Argitis, Panagiotis, Kennou, Stella, Sygellou, Labrini, Papadimitropoulos, Georgios, Kostis, Ioannis, Stathopoulos, Nikos A., Davazoglou, Dimitris
• Format: Article
• Language: English
• Subjects: electron injection layer; hybrid polymer light-emitting diodes; polyfluorene; reduction; tungsten oxide
• ISSN: 1616-301X; 1616-3028
• DOI: 10.1002/adfm.201002171

Here, we report on the dual functionality of tungsten oxide for application as an efficient electron and hole injection/transport layer in organic light‐emitting diodes (OLEDs). We demonstrate hybrid polymer light‐emitting diodes (Hy‐PLEDs), based on a polyfluorene copolymer, by inserting a very thin layer of a partially reduced tungsten oxide, WO2.5, at the polymer/Al cathode interface to serve as an electron injection and transport layer. Significantly improved current densities, luminances, and luminous efficiencies were achieved, primarily as a result of improved electron injection at the interface with Al and transport to the lowest unoccupied molecular orbital (LUMO) of the polymer, with a corresponding lowering of the device driving voltage. Using a combination of optical absorption, ultraviolet spectoscopy, X‐ray photoelectron spectroscopy, and photovoltaic open circuit voltage measurements, we demonstrate that partial reduction of the WO3 to WO2.5 results in the appearance of new gap states just below the conduction band edge in the previously forbidden gap. The new gap states are proposed to act as a reservoir of donor electrons for enhanced injection and transport to the polymer LUMO and decrease the effective cathode workfunction. Moreover, when a thin tungsten oxide film in its fully oxidized state (WO3) is inserted at the ITO anode/polymer interface, further improvement in device characteristics was achieved. Since both fully oxidized and partially reduced tungsten oxide layers can be deposited in the same chamber with well controlled morphology, this work paves the way for the facile fabrication of efficient and stable Hy‐OLEDs with excellent reproducibility. The dual functionality of tungsten oxide for application as an efficient electron and hole injection/transport layer in hybrid PLEDs is demonstrated. When a thin tungsten oxide layer in its fully oxidized state (WO3) was inserted at the ITO anode/polymer interface and a thin layer of a partially reduced tungsten oxide (WO2.5) was used at the polymer/Al cathode interface, significantly improved current densities, luminances, and efficiencies were achieved.