Structure and properties of organic semiconductors: from single polymers and oligomers to interacting heterogeneous nanomaterials

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Materials made of organic conjugated polymers, oligomers and other carbon-based molecular systems are currently a subject of many intensive investigations because of their great potential in optoelectronics. Their applications have been demonstrated in devices such as solar cells, light-emitting diodes, transistors, integrated circuits, sensors, electro-chromic displays, optical switches, electro-optic modulators, batteries and others. It has been found that these devices perform best when they have multilayered and/or heterogeneous structures. For example, bulk heterojunction organic solar cells (where fullerenes, C60 or C70, are blended with the polymers or oligomers) give an improved performance due to a more balanced charge carrier transport. These experimental findings indicate that the weak intermolecular van der Waals (dispersion) forces play an important role in determining the (nanoscale) structures and, hence, properties of these heterogeneous organic semiconductors. Understanding the complex interactions between the building blocks of the organic semiconductors can lead to the design of novel materials. This understanding and the design of new materials is critically important for development of the next generation of organic optoelectronic applications. Using computational methods, the long term goal of my research program is to gain greater understanding of how the main components of the heterogeneous nano-filled organic composites interact and how these interactions eventually affect the materials’ overall electronic and optical properties. My group models the molecular systems as isolated chains, supramolecules, nanoclusters or solids. Our main theoretical/computational tool is density functional theory (DFT). We plan to use the various types of the dispersion corrected DFT to study the molecular structures of these systems. Time-dependent DFT (TD-DFT) is employed to study the excited states and the solid-state DFT is used to determine the polymer crystalline structures. We propose to study these systems in solutions as well as in vacuum (gas phase). This is because, even though the solvent is often removed in the final stage of the fabrication of a given material, there are numerous experimental indications (e.g. aggregate formations) that the type of solvent used in the fabrication process affects the performance of the device. The short-term objectives of my research involve the investigations of the industrially relevant (promising) organic heterogenous nano-complexes using the dispersion corrected DFT and TD-DFT with and without the presence of solvents. We will focus on the role of the van der Waals forces in determining the three-dimensional structures of these noncovalently bonded molecular systems and with the use of the high level ab initio DFT methodologies we will consider the effects of side-chains, end-chains, various chemical structural modifications along the chain backbones, different surrounding media (such as solvents), different supramolecular compositions on their structures and eventually properties such as energy levels, electronic band gaps and widths, conformational energies, absorption and emission spectra, charge transport and others. These computational studies are key to a greater understanding of how microscopic and nanoscale structures relate to macroscopic properties of these materials and will provide the scientific community and Canadian industry with information vital for the development of future more efficient devices made of organic semiconductors.