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Reduction of tungsten oxide: a path towards dual functionality utilization for efficient anode and cathode interfacial layers in organic light-emitting diodes

Reduction of tungsten oxide: a path towards dual functionality utilization for efficient anode and cathode interfacial layers in organic light-emitting diodes
Reduction of tungsten oxide: a path towards dual functionality utilization for efficient anode and cathode interfacial layers in organic light-emitting diodes

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 (WO 3) 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.

electron injection layer, hybrid polymer light-emitting diodes, polyfluorene, reduction, tungsten oxide
1616-301X
1489-1497
Vasilopoulou, Maria
aad1381e-d091-4090-8c7c-b74bed22393d
Palilis, Leonidas C.
b09e9554-54da-4be5-aa08-bda894e2b86f
Georgiadou, Dimitra G.
84977176-3678-4fb3-a3dd-2044a49c853b
Douvas, Antonios M.
2e088659-e1b3-4723-aff9-4dfe385579b5
Argitis, Panagiotis
ab9c4ea6-3dd2-4e34-935d-81bfb360f358
Kennou, Stella
7ca6a6bd-6a05-4110-b3da-c8807f798370
Sygellou, Labrini
87e6de58-e0d8-46b1-aaa9-0d2150379562
Papadimitropoulos, Georgios
b4f1ecba-cc8a-4882-99a5-a29354a9bca7
Kostis, Ioannis
681af9a6-5162-4f35-b6b1-365ea7ce7e47
Stathopoulos, Nikos A.
83a4653f-c4c8-464a-8ed1-a93c52cf3833
Davazoglou, Dimitris
a946cf5d-287a-4734-ba55-b180ab4525ed
Vasilopoulou, Maria
aad1381e-d091-4090-8c7c-b74bed22393d
Palilis, Leonidas C.
b09e9554-54da-4be5-aa08-bda894e2b86f
Georgiadou, Dimitra G.
84977176-3678-4fb3-a3dd-2044a49c853b
Douvas, Antonios M.
2e088659-e1b3-4723-aff9-4dfe385579b5
Argitis, Panagiotis
ab9c4ea6-3dd2-4e34-935d-81bfb360f358
Kennou, Stella
7ca6a6bd-6a05-4110-b3da-c8807f798370
Sygellou, Labrini
87e6de58-e0d8-46b1-aaa9-0d2150379562
Papadimitropoulos, Georgios
b4f1ecba-cc8a-4882-99a5-a29354a9bca7
Kostis, Ioannis
681af9a6-5162-4f35-b6b1-365ea7ce7e47
Stathopoulos, Nikos A.
83a4653f-c4c8-464a-8ed1-a93c52cf3833
Davazoglou, Dimitris
a946cf5d-287a-4734-ba55-b180ab4525ed

Vasilopoulou, Maria, Palilis, Leonidas C., Georgiadou, Dimitra G., Douvas, Antonios M., Argitis, Panagiotis, Kennou, Stella, Sygellou, Labrini, Papadimitropoulos, Georgios, Kostis, Ioannis, Stathopoulos, Nikos A. and Davazoglou, Dimitris (2011) Reduction of tungsten oxide: a path towards dual functionality utilization for efficient anode and cathode interfacial layers in organic light-emitting diodes. Advanced Functional Materials, 21 (8), 1489-1497. (doi:10.1002/adfm.201002171).

Record type: Article

Abstract

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 (WO 3) 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.

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More information

Published date: 22 April 2011
Keywords: electron injection layer, hybrid polymer light-emitting diodes, polyfluorene, reduction, tungsten oxide

Identifiers

Local EPrints ID: 441310
URI: http://eprints.soton.ac.uk/id/eprint/441310
ISSN: 1616-301X
PURE UUID: f026d330-7953-4170-9f10-2b679b1a4286
ORCID for Dimitra G. Georgiadou: ORCID iD orcid.org/0000-0002-2620-3346

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Date deposited: 09 Jun 2020 16:30
Last modified: 07 Oct 2020 02:27

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Contributors

Author: Maria Vasilopoulou
Author: Leonidas C. Palilis
Author: Antonios M. Douvas
Author: Panagiotis Argitis
Author: Stella Kennou
Author: Labrini Sygellou
Author: Georgios Papadimitropoulos
Author: Ioannis Kostis
Author: Nikos A. Stathopoulos
Author: Dimitris Davazoglou

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