Polymeric light-emitting diodes with highly polarized blue emission

Polymer light-emitting diodes (LED) with aligned conjugated molecules and polymers as the active layer can emit polarized light as shown recently in several reports. We have synthesized several liquid-crystalline polyfluorenes with different side chain patterns. The thin polymer films were aligned at elevated temperatures on a rubbed polyimide layer. For the most suitable substitution, a dichroic ratio of twelve was observed in absorption. This polymer was used to construct light-emitting devices with ITO and Ca electrodes. Suitable hole transport molecules were added to the polyimide in order to obtain layers with good alignment properties and large hole mobility. Polyimide and poly-fluorene layer thicknesses were varied to optimize the device performance. For those optimized devices, blue light with a dichroic ratio in emission of fifteen was measured and the luminance was approx. 100 Cd/m(2) at 18 V. Even though these values are still below the requirements in application, the high dichroic ratio combined with a reasonable brightness represents an important step towards the use of organic LEDs as illumination sources in LCD displays.