Photolithographically patternable electroluminescent liquid crystalline materials for full-colour organic light emitting displays

Displays based on polymer light emitting diodes are attractive due to their emissive nature, their wide viewing angles and the ability of electroluminescent conjugated polymers to be solution processable at room temperature and pressure. It is difficult, however, to deposit separate red, green and blue (RGB) pixels and to maximize performance by making the devices multi-layered. Here we present recent results on a semiconducting conjugated reactive-mesogen OLED material which is solution processable, can be potentially cured and patterned by photolithography and used in multi-layer devices. This material consists of a conjugated pentathiophene core with reactive endgroups. Spectroscopy, calorimetry and microscopy show that it forms crystalline, aggregate, liquid-crystalline and isotropic phases at a range of different temperatures. The material is deposited by spincoating from solution. Low density doping with a cationic photointiator and exposure to a specific UV wavelength to avoid damage to the conjugated core leads to cross-linking into an insoluble network. Current-voltage-luminousity and spectral measurements in standard OLED device structures show the effect of cross-linking on the transport and injection properties of the material. Quenching of fluorescence and electroluminescence is discussed. Insertion of lower-energy gap, fluorescent small molecules can potentially be used to tune the emission to any desired colour but material limitations to this technique due to dopant removal during the washing procedure were observed.