Spectroscopy and microscopy studies of conjugated polymers in light-emitting diodes

This thesis describes the characterization of {dollar}{bsol}pi{dollar}-conjugated polymers, namely poly(p-phenylene vinylene) PPV and its derivatives, in view of their application in light-emitting diodes (LEDs). The luminescence properties of the polymers were investigated by photoluminescence (PL) and electroluminescence (EL) spectroscopies and the characteristics of the surface of the polymer and the interfaces between the polymers and a metal were probed by scanning tunneling microscopy (STM) and x-ray photoemission spectroscopy (XPS). The characteristic spectral responses of various PPV derivatives show the relative stability of poly(2,3-diphenyl-p-phenylene vinylene) (DP-PPV). The thermal conversion of DP-PPV, which is the last step in the synthesis of the polymer, was monitored by PL spectroscopy. The onset of conversion was about 150{dollar}{bsol}sp{bsol}circ{dollar}C and a full conversion could be achieved at 170-250{dollar}{bsol}sp{bsol}circ{dollar}C. Air exposure and heating of the sample reduce the luminescence intensity of the converted polymer. Morphology and roughness characterization from the microscopy study gives a direct look of the surface of different polymers and the profile of an LED structure at a nanometer scale: the flatness of the polymer surface contrasts with the roughness of the electrodes. An XPS investigation of the interface formation between Ca and self-assembled alkanethiol monolayers, a model system of metal/organic interfaces with a well-defined geometry and no detectable impurities, reveals a charge transfer from the metal to the organic layer. The study of the interface formation between poly(p-phenylene {dollar}{bsol}alpha,{bsol}alpha{bsol}sp{bsol}prime{dollar}-diphenyl vinylene) (PPV-DP) and Ca, and between DP-PPV and Ag reveals the formation of new bonds as the metal is evaporated onto the polymer. The existence of O on the polymer surface has induced the formation of C-O species. Ca leads also to the formation of Ca-C species when Ag does not react with the organic layer. The characterization of the polymer by different techniques has permitted an overview of its properties, allowing us to understand the properties of the polymer and the device made of it, and to devise better ways to prepare them.