Materials science tools for organic electronics

Organic semiconductors and their electronic applications have attracted considerable interest over the last decades promising an alternative to conventional, inorganic electronics. The extensive research in the field has led to great advances and organic electronics is now entering the stage of commercial technology. To fully exploit the touted potential of this plastic electronics platform, however, other prerequisites need now to be fulfilled: for example, good mechanical stability, ease of processing, device efficiency and reliability. Material science provides unique tools to overcome these issues, e.g. a possible approach is the employment of multicomponent systems where a combination of materials having different nature is used to obtain complex structures with enhanced properties. This thesis investigates how structure-property relationships of polymer semiconductors can be modified upon addition of other components and how electronic devices, i.e. field-effect transistors and solar cells, are affected by the multicomponent approach. By doing so, we first consider a particular class of polymer blends, based on the combination of semiconducting and insulating polymers. An analysis of the solidification processes occurring during film formation is presented and the resulting microstructures are discussed. Having established the processing recipe that allows the production of electronic devices, we then show how the electronic properties of field-effect transistors can be tailored upon blending. In particular, this thesis investigates how the blending strategy affects the stability of transistors subjected to bias-stress, the control of low subthreshold parasitic currents and charge transport ambipolarity. The multicomponent approach is then extended to other systems, in which semiconducting polymers are functionalized to form complex hybrid systems with titanium derivatives. Standard organic semiconductors, i.e. polythiophenes, are synthetized with randomly hydroxylated side-chains, which can act as reacting sites for the inorganic material. We show how the presence of the hydroxyl groups affects the doping kinetics of the semiconductor when exposed to air and, therefore, we use these materials as a model system to investigate the doping process of polythiophenes. Page | iii Having shown that blend systems can be used in electronic applications, we then investigate the relationships between three critical aspects of solution processing: the solution concentration, the resulting number of entanglements per chain in the polymer and, hence, the intermixing of different components. Different strategies have been explored in this thesis, including the employment of semiconducting:insulating polymer blends, copolymers and hybrid systems. The results of this work highlight the interplay within structure, property and processing providing guidelines on how to further develop multicomponent systems in the area of organic electronics.