Visualising Functional Nanostructures of Organic Semiconductors using Electron Microscopy

Doctoral thesis, 2024

Organic semiconducting materials have enabled the solution-processable fabrication of light-weight and flexible electronic devices. In particular, doping using molecular dopants has enabled the fabrication of high-performance organic electronics. Despite the significant progress in the last decades, organic electronics are still outperformed by their inorganic counterparts regarding device efficiencies, and an optimisation of the material properties is necessary. The properties of organic semiconductors have been shown to be correlated to their fine-scale microstructure.

In this thesis work, the nanostructure of organic semiconducting thin-films and the spatial distribution of molecular dopants are studied using electron microscopy. The aggregation characteristics, the morphology of the phases and the detailed interface structure have been studied using transmission electron microscopy. The films have been studied using two- and three-dimensional imaging and spectroscopy. The results show that the electrical properties depend on the nanostructure. Furthermore, the three-dimensional spatial distribution of individual molecular dopants in the organic semiconductors is revealed by electron tomography. The dopants are present as individual species or in clusters. The cluster size increases as the dopant concentration increases. The polar side-chain length for a semiconducting polymer is shown to affect the cluster size, where short side-chains lead to a more fine-dispersed distribution. This promotes the charge transfer from the dopants to the surrounding polymer, resulting in increased ionisation efficiency and increased electrical conductivity. The nanostructure-conductivity correlation provides important information for the understanding of the fundamental mechanisms determining the electrical conductivity in the doped organic semiconductors, which in turn enables the optimisation of the properties.