Project description

In the last decade, research on electronic devices based on organic materials, like organic light-emitting diodes (OLEDs), has gained much attention due to the potential for cheap and ultrathin illumination sources with high viewing angle and color range.

Charges, which are induced from the electrodes into an organic semiconducting layer, move through the layer by a hopping process along the molecules and recombine to form an exciton. The relaxation from the excited to the ground state results in the emission of light, what is called electroluminescence. In multilayer devices it is possible to improve the carrier injection efficiency from the electrodes into the light-emitting layer (EML), ensuring the emission color purity, but increasing cost and complexity. The structure may include specific layers for hole or electron injection (HIL), transport (HTL) and blocking (HBL).

As OLED material, low and high molecular weight compounds can be used. Small-molecule-based OLEDs (SMOLEDs) for applications in the lighting market are commonly fabricated via vacuum deposition techniques, which include an additional purification and allow the multi-layer preparation without serious problems. However, the sublimation technique is relatively expensive and it is limited to vaporizable and thermoresistant low molecular weight materials. Alternatively, solution processes allow continuous, low cost and large-scale production with less material usage, but there are still challenges to face, like the solubility and coatability of the materials, the formation of homogeneous thin films and the dissolution between layers concerning multilayer devices. Polymeric materials shows good processability and film forming properties, but reveal uncontrollable batch-to-batch variations, hence this work will primarily handle SMOLEDs only.

The goal of the „Cluster of excellence: Print-OLED“ is a solution processed monochrome multilayer OLED, that achieves 70% of the light emitting efficiency of the comparable OLED fabricated via vacuum deposition technique. The deposition method and the drying step during the solution process of a low-molecular OLED will be topic of the investigation.
In this work, special attention will be drawn to the drying process of the casted solutions, since this is an important step during the production. The exact knowledge of the drying process is not only required for the construction of dryers in an economical production, it has also a strong influence on the formation of the film (homogeneous material spreading, dewetting, surface structure, alignment of the molecules) and hence an influence on the electronic properties and electroluminescence of the device.