Chemical degradation mechanisms of organic semiconductor devices

Abstract

The currently starting technical exploitation of organic electronic devices requires a deep understanding of ageing and degradation mechanisms. In addition to extrinsically caused ageing processes, such as the penetration of oxygen and water in organic layers and subsequent (electro)chemical reactions, further degradation channels exist in such devices, which are based on intrinsic chemical reactions of the materials used in the devices. At this time, we know the degradation mechanisms of only few organic materials applied in organic light emitting devices (OLEDs). To detect specific reaction products, we introduced laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF-MS), a method which allows to distinguish between desired and undesired compounds in thin film organic devices. We use LDITOF- MS to detect the degradation products of different Iridium based emitter materials like Ir(MDQ)₂acac (red emitter) and FIrpic (light blue) in dc driven OLEDs and adapted test sample structures. Due to the dissociation behaviour of some Ir complexes and the ability of their fragments to form complexes with several hole blocking materials, the degradation mechanisms of the devices can be understood in terms of such chemical complex formation between the emitter molecules and neighbouring materials. On the other hand, the knowledge about these mechanisms can be used to select the right combination of materials for the benefit of long-living devices as we will show at the end of this work.