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

由有机共轭聚合物、低聚物和其他碳基分子体系制成的材料，由于其在光电子学方面的巨大潜力，目前是许多深入研究的主题。它们已经在太阳能电池、发光二极管、晶体管、集成电路、传感器、电致变色显示器、光学开关、电光调制器、电池等设备中得到了应用。已经发现，当这些器件具有多层和/或异质结构时，它们的性能最好。例如，体相异质结有机太阳能电池(其中富勒烯、C60或C70与聚合物或低聚物混合)由于更平衡的载流子传输而提供了更好的性能。这些实验结果表明，弱分子间范德华(色散)力在决定这些非均相有机半导体的(纳米尺度)结构和性质方面起着重要作用。了解有机半导体构件之间的复杂相互作用可以导致新材料的设计。这种理解和新材料的设计对于下一代有机光电子应用的发展至关重要。使用计算方法，我研究计划的长期目标是更好地了解多相纳米填充有机复合材料的主要成分如何相互作用，以及这些相互作用最终如何影响材料的整体电子和光学性能。我的团队将分子系统建模为孤立的链、超分子、纳米团簇或固体。我们的主要理论/计算工具是密度泛函理论(DFT)。我们计划使用各种类型的色散校正密度泛函来研究这些体系的分子结构。用含时密度泛函理论(TD-DFT)研究了聚合物的激发态，用固体密度泛函理论确定了聚合物的晶体结构。我们建议在溶液和真空(气相)中研究这些体系。这是因为，尽管溶剂通常在特定材料制造的最后阶段被去除，但有许多实验迹象(例如聚集体形成)表明，制造过程中使用的溶剂类型会影响器件的性能。本研究的短期目标包括在有无溶剂存在的情况下，利用色散校正密度泛函理论和密度泛函理论研究具有工业应用前景的有机多相纳米络合物。我们将重点讨论范德华力在确定这些非共价键合分子体系三维结构中的作用，并使用高水平从头算密度泛函方法，考虑侧链、端链、链主链上各种化学结构修饰、不同周围介质(如溶剂)、不同超分子组成对其结构和最终性质的影响，如能级、电子带隙和宽度、构象能、吸收和发射光谱、电荷输运等。这些计算研究是更好地理解微观和纳米级结构如何与这些材料的宏观性质相关的关键，并将为科学界和加拿大工业提供对开发未来由有机半导体制成的更高效设备至关重要的信息。

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.