Correlating Nanostructure and Electronic Properties of Organic Semiconductors by Electron Microscopy

Organic semiconductors enable fabrication and efficient processing of electronic devices with light weight, mechanical flexibility and tuneable properties. Despite significant progress in the last decades, efficiencies and long-term stabilities of these systems still need to be improved. The properties of organic semiconductors have been shown to be correlated to their morphology. In this thesis work, the nanostructures of organic semiconductors are studied using electron microscopy. Aggregation characteristics, morphology of the phases and the detailed structure of the interfaces have been studied using transmission electron microscopy with both imaging and spectroscopy. It is shown how these structural properties determine the electronic properties. Electron tomography is used to visualise the three-dimensional distribution of dopant molecules in an organic semiconductor at sub-nanometre resolution, which enables the determination of positions of individual molecules. Both individual dopants and clusters are observed. The clusters grow in size and change shape as the dopant concentration increases. This change affects the conductivity which initially increases with increasing concentration and thereafter decreases. The three-dimensional information about the dopant positions in the clusters show that the cluster morphology allow that each dopant molecule is in direct contact with the surrounding polymer. The changes in morphology of the dopant clusters can explain the decrease in electrical conductivity at the higher dopant levels. The work in this thesis provides detailed nanostructure information that is important for the understanding of fundamental mechanisms in organic semiconductors