Solvent-enhanced dye diffusion in polymer thin films for polymer light-emitting diode application

The method of solvent-enhanced dye diffusion for patterning full-color (red, green, and blue) polymer light-emitting diode displays was investigated in detail. After local dry transfer of dye onto a device polymer film, the dye remains on the surface of the polymer layer and must be diffused into the polymer for efficient emission. Exposure of the polymer to solvent vapor at room temperature increases the dye-diffusion coefficient by many orders of magnitude, allowing rapid diffusion of the dye into the film without a long, high-temperature anneal that can degrade the polymer. The increase in diffusion is due to absorption of the solvent vapor into the polymer film, which increases the polymer thickness and decreases its effective glass transition temperature T _g,eff. Measurements of the polymer in solvent vapor indicate that its thickness varies roughly linearly with pressure and inversely with temperature, with thickness increases as large as 15% often observed. A model based on Flory-Huggins theory is used to describe these results. The diffusion of the dye into the polymer was evaluated by photoluminescence and secondary-ion mass spectroscopy. This dye-diffusion increase is largest for high solvent-vapor partial pressures and, most surprisingly, is larger at lower temperatures than at higher temperatures. This anomalous temperature dependence is due to the increased solvent-vapor absorption and consequent reduction in the effective glass-transition temperature at lower temperatures.