Polymer Electronic Devices: The role of oxidative defects

Electronic and optoelectronic components based on organic semiconductors (conjugated polymers in the present case) have been intensively studied over the past decades. The broad commercialization of some applications such as light emitting devices or simple circuits fabricated by (inkjet-)printing or soft lithography seems to be close at hand. A technologically relevant issue for industrial applications is the instability of the materials versus atmospheric influences (such as oxygen and water). Providing inert gas conditions during the fabrication process and using encapsulation schemes can solve some of the occurring problems. Still, contaminations and resulting (oxidative) defect states in the materials cannot be totally excluded (especially since some defects can already be incorporated during synthesis). The present project addresses the influence of such defects on the electronic structure of conjugated polymers. As far as fundamental research is concerned, it will be investigated under which circumstances oxidative defects lead to additional (unwanted) emission bands. The impact of the defect states on charge carrier generation and transport will be highlighted. The information gained in this part of the project will be used to assess to what extend literature data on charge carrier generation and transport are influenced by these defects. In a more technological part of the project, we will develop new strategies to either inhibit the formation or restore the already degraded materials to a more "pristine" state. To this aim e.g. oxygen getter materials will be blended into the active layer, which will serve to stabilize the devices. Defects incurred during synthesis will be "healed" using various (photo-)chemical approaches.

Organic semiconductors find applications in a large variety of devices including organic solar cells, displays (e.g., on MP3 players or digital cameras), TV sets, solid state lighting applications, sensors and electric circuits like RFID tags. Many of these applications are already commercially available or will come to the market in the next few years. They hold the promise of higher efficiencies (resulting in a reduced power consumption), increased brightness and low cost fabrication combined with unique properties like mechanical flexibility. One of the major challenges that one is, however, still facing for their application (especially when working with polymeric materials) is the long-time stability of the devices. Te help overcoming this limitation, the current project deals with an in depth study on the nature of the defects that limit the device lifetimes and how they can be avoided with the goal of paving the way to the development of novel, more stable materials. Therefore, we have extensively tested the stability of materials suitable for both, the fabrication of electronic circuits (poly(thiophene)) and the realization of light-emitting devices (poly(fluorene)) and performed experiments to better understand their oxidative degradation processes. In this context, we, e.g., investigated a method to a posteriori "heal" a defect containing organic semiconductor. In the later phase of the project, we refocused part of the efforts towards understanding the role played by the interfaces between the active organic semiconducting materials and the metal electrodes and dielectrics. These interfaces have recently been shown to play the dominant role for many electronic devices (in particular for field effect transistors). The generated knowledge then allowed us to fabricate field-effect transistors with unprecedented low contact resistances. This is a crucial pre-requisite for the development of fast organic circuits as needed, for example, in RFID tags. As a "side-product" of our investigations, we also discovered a new, highly promising concept for chemical sensing with organic devices: Using ultra-thin intermediate layers between the gate dielectric and the semiconducting material in organic field-effect transistors, we managed to use local doping and subsequently dedoping by an analyte (e.g., NH 3 ) to shift the turn on voltages of the devices by more than 70 V.