The behavior of light-emitting polymers: Film formation, optical properties, and degradation

The properties of light-emitting polymer thin films prepared via spin coating from different organic solvents were studied. The surface analysis as observed by atomic force microscopy, scanning microscopy, and x-ray reflectivity has shown that the structure of polymer thin-films is controlled by the properties of organic solvents. The photoluminescence (PL) spectrum of polymer thin-films and polymer solutions has shown the correlation between the chain conformation and light-emitting properties. A combination of viscosity measurements is used to illustrate the solvent effects on the optical properties of light-emitting polymers in solutions and thin films.; The solvent-induced film structure of poly(n-vinyl carbazole) (PVK) and poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenyl vinylene) (MEH-PPV) thin films on indium tin oxide coated glass was examined. The spin-coated polymer thin films are not in thermodynamic equilibrium; rather, the film properties are affected by the dynamics of the spin-coating process. The water in tetrahydrofuran induces rupturing of polymer films during the spin-coating process. Solvents with a high-evaporation rate lead to high-surface roughness due to Benard-Marangoni convection. Results show that the surface roughness and structure of the films are dominated by the dynamics of the film-formation process rather than the thermodynamic interactions between the polymer and solvents.; Two degradation phenomena are studied and discussed in this work. The first manifestation of degradation is the bubble formation on the polymer light-emitting diodes. Water can diffuse through pinholes to reach polymer layers. Heat and electrical bias can induce the reaction between water and metal contacts, which leads to the bubble formation. The second manifestation of degradation is the thermal-induced oxidation of poly[9,9-dioctylfluorene] (PFO). The oxidation of PFO can lead to an extra red-shifted emission and the crosslinking of polymer chains.