|M.Sc Student||Nouzman Yelizabetha|
|Subject||Interlayers at the Organic/Metal Interface in Organic|
|Department||Department of Materials Science and Engineering||Supervisor||Professor Gitti Frey|
|Full Thesis text|
The architecture of high efficiency OLED and PLED devices consists multiple layers of several functional organic materials. Cathode interlayers, for example, play an important role in facilitating electron injection from the metal cathode into the light-emitting materials, reduce driving voltage, and improve efficiency. While deposition of sequential layers is relatively straightforward in evaporation, constructing multilayers becomes more difficult in solution processing techniques used in the fabrication of PLEDs, such as spin coating, slot die coating, inkjet coating, etc. However, these processes are compatible with roll-to-roll manufacturing. Therefore, much effort is directed to find approaches to form organic multilayer by solution processing.
This work suggests a mechanism for the spontaneous formation of interlayers in PLEDs. The process is based on the migration of additives from the polymer layer to the polymer/metal interface driven by the interaction between the additive and the evaporated metal atoms. The additive/metal interactions also modify the effective electrode work function and dramatically improve the device performance.
In this work, we show that PEG molecules initially blended with the emissive polymer migrate during Al deposition to form an interlayer at the organic/Al cathode interface in PLEDs. As a result, devices with PEG molecules exhibit reduced turn on voltage, and increased luminance, current density and current efficiency compared to devices with no PEG. XPS measurements confirm that PEG migration occurs during the Al deposition and thermal treatments reveal that annealing the films prior to Al depositions suppresses PEG migration and interlayer formation. Therefore, we suggested and studied a new, simple, roll-to-roll-compatible methodology for solution processing interlayers in PLEDs that may also be useful beyond PLEDs technology.