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Title: The influence of MoO{sub x} gap states on hole injection from aluminum doped zinc oxide with nanoscale MoO{sub x} surface layer anodes for organic light emitting diodes

The effective workfunction of Al doped ZnO films (AZO) increased from 4.1 eV to 5.55 eV after surface modification with nanoscale molybdenum sub-oxides (MoO{sub x}). Hole only devices with anodes consisting of 3 nm of MoO{sub x} on AZO exhibited a lower turn-on voltage (1.5 vs 1.8 V), and larger charge injection (190 vs 118 mA/cm{sup 2}) at the reference voltage, compared to indium tin oxide (ITO). AZO devices with 10 nm of MoO{sub x} exhibited the highest workfunction but performed poorly compared to devices with 3 nm of MoO{sub x}, or standard ITO. Ultraviolet photoelectron, X-ray photoelectron, and optical spectroscopies indicate that the 3 nm MoO{sub x} films are more reduced and farther away from MoO{sub 3} stoichiometry than their 10 nm equivalents. The vacancies associated with non-stoichiometry result in donor-like gap states which we assign to partially occupied Mo 4d levels. We propose that Fowler-Nordheim tunneling from these levels is responsible for the reduction in threshold voltage measured in devices with 3 nm of MoO{sub x}. A schematic band diagram is proposed. The thicker MoO{sub x} layers are more stoichiometric and resistive, and the voltage drop across these layers dominates their electrical performance, leading to an increase in threshold voltage. The results indicate that AZO with MoO{sub x}more » layers of optimal thickness may be potential candidates for anode use in organic light emitting diodes.« less
Authors:
; ; ;  [1]
  1. Department of Materials Science and Engineering, University of North Texas, Denton, Texas 76203-5017 (United States)
Publication Date:
OSTI Identifier:
22499253
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Applied Physics; Journal Volume: 118; Journal Issue: 6; Other Information: (c) 2015 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; 36 MATERIALS SCIENCE; ALUMINIUM; ANODES; DOPED MATERIALS; ELECTRIC POTENTIAL; ELECTRONIC STRUCTURE; EV RANGE; HOLES; INDIUM COMPOUNDS; LIGHT EMITTING DIODES; MOLYBDENUM OXIDES; NANOSTRUCTURES; SPECTROSCOPY; STOICHIOMETRY; THIN FILMS; TIN OXIDES; TUNNEL EFFECT; ULTRAVIOLET RADIATION; VACANCIES; VOLTAGE DROP; X RADIATION; ZINC OXIDES