Investigation of quinoline-based materials for organic light-emitting diode (OLED) applications

Date of Completion

January 2004

Keywords

Chemistry, Organic|Engineering, Materials Science

Degree

Ph.D.

Abstract

Blue electroluminescent materials are essential for the development of full-color displays. A blue-emitting quinoline-based material 8,8′ -dimethoxy-5,5′-bisquinoline (DMeOBQ) was synthesized and characterized. This material is sublimable and has significantly improved hydrolytic stability as well as promising electron transporting and emitting properties. A blue organic light-emitting diode (OLED) was made with this material. The device configuration is indium tin oxide/NPB/DMeOBQ/CsF/Al. N, N′-bis-(1-naphthyl)-N, N′-diphenyl-1,1 ′-biphenyl-4, 4′-diamine (NPB) was used as the hole transporting material, DMeOBQ as the electron transporting and blue emitting material, and CsF/Al as the cathode. The device showed a bright blue emission with a peak wavelength of 425nm (CIE coordinates: x = 0.155, y = 0.10) and narrow EL band (FWHM = 63 nm). The device also showed a low turn on voltage of 2.8 ev. ^ Tris-(8-hydroxyquinoline) aluminum (Alq3) has become one of the most widely used electroluminescent materials in OLEDs because of its good stability and luminescence properties. The existence of various isomers (meridianal and facial) as well as left- and right-handed enantiomers of the meridianal isomer could potentially contribute to structure randomization of Alq3, thereby qualitatively addressing the microcrystaline nature of this material. We studied exchange dynamics of the three inequivalent ligands and the enantiomers in Alq3 complexes. Liquid state NMR spectroscopy was used to quantify the interchange of the three ligands, the temperature dependence of this process and the enantiomers of Alq3 in the presence of a chiral shift reagent. The three inequivalent ligands were found to exchange following a first-order kinetic model. Activation parameters were obtained from the temperature dependence of the reaction rate constants. The activation energies for the flip of the three inequivalent ligands were found to be around 100 kJ/mol. The existence of the two enantiomers of Alq3 was detected and they were found to exchange on a time scale of about 2 S−1 at very low temperatures. The activation energy determined from the Arrhenius plot is 13.6–75.4 kJ/mol. These results are important for understanding the morphology of vapor deposited thin films of Alq3 as well as crystallization-assisted device failures. ^

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