Fabrication and properties of light-emitting diodes based on self-assembled multilayers of poly(phenylene vinylene)

Abstract

Light‐emitting diodes have been fabricated from self‐assembled multilayers of poly(p‐phenylene vinylene) (PPV) and two different polyanions; polystyrene sulfonic acid (SPS) and polymethacrylic acid (PMA). The type of polyanion used to assemble the multilayer thin films was found to dramatically influence the behavior and performance of devices fabricated with indium tin oxide and aluminum electrodes. Light‐emitting devices fabricated from PMA/PPV multilayers were found to exhibit luminance levels in the range of 20–60 cd/m², a thickness dependent turn‐on voltage and classical rectifying behavior with rectification ratios greater than 10⁵. In sharp contrast, the devices based on SPS/PPV exhibited near symmetric current–voltage curves, thickness independent turn‐on voltages and much lower luminance levels. The significant difference in device behavior observed between these two systems is primarily due to a doping effect induced either chemically or electrochemically by the sulfonic acid groups of SPS. It was also found that the performance of these devices depends on the type of layer that is in contact with the Al top electrode thereby making it possible to manipulate device efficiency at the molecular level.