Optical properties of single crystals and vacuum-deposited thin films of a substituted oligo( p-phenylene vinylene)

Abstract

The optical properties and structural organization of a 5-ring n-octyloxy-substituted oligo( p-phenylene vinylene) have been investigated in the solid state. For this study solution-grown single crystals and vacuum-deposited thin films were used. The unit cell of the solution-grown single crystals was determined to be monoclinic, space group I2 a . The as-deposited oligomer films showed a polycrystalline structure. Annealing the film at a temperature of 120°C resulted in a morphological change, which manifested itself in an increase of crystalline domain size from approximately 4 μm to 20 μm. Both single crystals and thin films exhibit a high photoluminescence quantum yield of 0.5 and 0.7, respectively. In order to determine the relative importance of oxygen and water on photoluminescence quenching, we measured the luminescence decay of a vacuum-deposited film exposed to water and air. It was found that quenching is mainly due to contamination with oxygen from air rather than water and that this contamination is reversible to a great extent. We demonstrate that under intense laser excitation it is possible to obtain stimulated emission from both single crystals and thin films. Non-optimized single-layer light-emitting diodes prepared from the oligomer showed uniform green/yellow light emission with external electroluminescence efficiencies up to 0.15% using Ca as electron-injecting electrode. The electroluminescence efficiency is drastically increased by annealing.