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Unconventional Strategies Toward Efficient Self-Assembly in Solution Processed Small Molecule Organic Photovoltaics


Broadly described by conductive π-conjugated cores with electronically-inert solubilizing groups, solution-processed organic semiconductors have wide applications in modern electronics such as transistor, sensor, lighting and energy-harvesting technologies. However, their optical and electronic properties in the solid-state strongly depend upon their ability to organize into ordered phases during solution deposition. Frustrated self-assembly contributes to performance variation and thermal instability. Organic photovoltaics (OPVs) offer a relevant case-in-point, as they rely on a blend of electron-donating and electron-accepting semiconductors to achieve light harvesting and photocurrent generation. Efficient charge generation requires the self-assembly of internal ordered domains and broad interfaces between these components, while charge carrier extraction simultaneously demands that each form continuous pathways. Understanding how the chemical structure and processing of these materials impacts their organizational tendencies is essential for the design of materials that readily and reliably reach this desired complex morphology.

This dissertation discusses the novel strategies for directing self-assembly in OPV materials suggested by this understanding. Chapter 1 presents our current understanding of the processes involved in the formation of the bulk heterojunction and processing methods for manipulating morphology, such as solvent additives. Chapter 2 discusses how a solid and electronically-inert additive, polystyrene, provides additional interfaces for crystalline nucleation during solution-casting, a different mechanism than solvent additives. In Chapter 3, we find that molecular design that reduces available structural conformation decreases morphological disorder during solution-deposition, with minimal impact on other molecular properties. Chapter 4 discusses how molecules designed to change topology in solution and film states can readily create ordered phases without sacrificing processibility. Chapter 5 concludes this work by discussing how these strategies can be applied toward emerging fields within organic semiconductor research, namely processing from environmentally-friendly solvents and designing of electron-accepting materials.

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