Interfaces for polymer light emitting diodes
Creators: name: Susac, Darija
Publisher: name: University of British Columbia
Year: 2009

description: Interfaces in polymer light emitting diodes (PLED) made from poly(l,4 phenylene vinylene) (PPV) on indium tin oxide (ITO) substrate with evaporated Mg as a negative electrode, have been studied by X-ray photoelectron spectroscopy (XPS). Thin films of PPV on LTO were prepared by thermal conversion of undialyzed and dialyzed sulfonium precursor polymer, and Mg was evaporated on top under high vacuum conditions. Characterization of PPV by XPS indicated that two different polymer surfaces are obtained by using undialyzed and dialyzed precursor polymer. Without dialysis, more oxygen impurities are present on the polymer surface, and this has been shown to lead to differences in observations associated with subsequent processing steps. For example, upon metal deposition under high vacuum conditions the polymer surface appears more reactive to oxygen and the formation of Mg carbide is noticed. Much purer PPV can be synthesized from the dialyzed precursor, but still oxidation of the polymer/metal interface occurs when the Mg source is used under high vacuum conditions, although no evidence was found for Mg carbide formation. The relative amounts of the different carbon components detected by XPS after metal evaporation on the polymer/metal interface have been shown to depend strongly on the details of the PPV preparation process (especially on whether undialyzed or dialyzed precursor is used). A new two-layer polymer diode, using an ion exchange resin Nafion® with incorporated tris (2,2' - bipyridine) ruthenium (II) complex {Ru(bpy)₃²⁺} placed between PPV and the metal electrode, was fabricated and its electroluminescence properties tested. Interfaces in such a diode were studied by XPS. Studies of the interaction between Nafion and the Ru²⁺ complex indicated that a substantial surface rearranging occurs upon exposing the Nafion to the aqueous solution of Ru²⁺ complex. The hydrophilic sulfonic groups, which exist in a form of clusters among the hydrophobic fluorocarbon chains in the Nafion, become more oriented to the surface due to the hydrophilic interaction. A further driving force for this rearrangement is believed to originate from the electrostatic interactions between the Ru²⁺ complex and SO₃ groups on Nafion, as well as SO₄²⁻ ions formed by oxidation. XPS indicates that the amount of S-groups oriented toward the Nafion surface is a function of the thickness of the Nafion + Ru²⁺ complex film, and in particular the thinner the film the greater the structural rearrangement. XPS characterization of an interface formed between thin film of Nafion + Ru(bpy)₃²⁺ on PPV and Mg evaporated under high vacuum, indicated that the Mg interacts directly with the fluorocarbon part of the Nafion polymer, although oxygen is also picked up at this interface for the conditions used. Electroluminescence testing of the fabricated Mg/Nafion + Ru(bpy)₃²⁺/PPV/ITO diode showed yellowish light emission in spark form for values of applied current in the range 23-57 mA, while red light was emitted for 10 sec at the higher current of 63 mA. The I-V characteristic curve measured for this device followed expectations for diode behavior.