Self-Assembled Nanostructures in Organic Electronics

Conjugated organic materials have the remarkable ability to absorb and emit light and transport electrical charge. Over the past century, this phenomenon has fascinated physicists, chemists, materials scientists and more as it allows the semiconducting properties of inorganic materials like silicon to be combined with the material properties of organic materials like polymers. By controlling the chemical properties of these organic materials, we can tune the wavelengths or colour of light that they emit and absorb, and we can use scalable printing technologies to fabricate ultra-thin, organic semiconducting films at low-cost and high throughput. These concepts have led to developments in a broad range of optoelectronic applications such as the organic light emitting diodes (OLEDs) we use in display technologies to the organic solar cells (OSCs) we use to power electrical devices. A common theme underpinning the performance of each of these technologies is the morphological optimisation of soft-matter systems, which self-assemble into complex, intricate structures at length scales intermediate between atomic and macroscopic scales. This leads to a rich hierarchical phase behaviour that strongly influences the performance and stability of the device.

In this thesis, I investigate the relationships between organic thin film nanostructure, optoelectronic performance and stability with the aim of developing a better understanding of the characteristics required to fabricate highly efficient, stable technologies. The majority of the research focuses on understanding these relationships in high performing OSC systems, which rely on the fabrication of a photoactive blend film of conjugated polymer semiconductors and small molecule electron acceptors. Over the past decade the power conversion efficiency (PCE) of OSCs has more than doubled with PCEs > 20% now in
reach. Such a rapid rise in efficiency is largely due to the development of a new class of electron acceptor material; non-fullerene electron acceptors (NFAs). In this work, I explore how factors such as the choice of casting solvent, solvent additive processing and NFA molecular design influence the structure-performance-stability relationships of polymer : NFA based systems. To do this, I use a broad range of structural characterisation techniques such as grazing incidence X-ray scattering, small angle neutron scattering and neutron reflectivity to probe the entire three-dimensional morphology of the film. In the final chapter, I expand these techniques to characterise the molecular self-assembly of novel chiral small molecules in thin films for chiro-optoelectronic applications.