Semiconducting polymers are a promising class of materials for many organic electronic applications because of their structural tunability, low cost, and solution processability, which allows for easy scale-up. However, semiconducting polymers have intrinsically poor conductivity which limits their performance in all device applications. Polymer conductivity can be improved either by adding mobile carriers to the system or by manipulating the system to make the polymer chains more ordered on a local and global scale. This thesis studies both of these methods with a goal of improving polymer conductivity, while simultaneously seeking to understand how changes in morphology affect both local and global polymer properties. We used a variety of X-ray and neutron scattering techniques to characterize polymer structure, coupled with electronic and spectroscopic experiments to gain a full picture of polymer structure-function relationship. The first half of this thesis studies the structural changes that result from introducing a molecular dopant and additional charge carriers into the polymer network, and how those change control the resulting electronic and optical properties. We start by studying a novel class of large, redox-tunable dodecaborane-based dopants. From these studies we are able to determine how redox potential controls both dopant infiltration into polymer films and the resulting film structure, providing insight into the relationship between structure and conductivity for doped conjugated polymer systems. Using traditional small-molecule dopants, we also studied various doping methods to assess scalability and application to thick polymer films. The second half of this thesis presents studies on various methods to manipulate the local morphology of polymer chains to increase their overall order. We first used an aqueous amphiphilic self-assembly system where we developed structural design rules for order assembly and demonstrate that they can be used to create polymer system that show straightened chains when self-assembled. Next, we explored a set of block-copolymers whose co-crystallization properties could be changed using the polymer molecular weight; here we show that crystallization behavior directly affects conductivity. Lastly, we studied a host-guest system of polymers aligned in straight silica mesoporous, with a goal of using confinement to understand the interplay between polymer microstructure and aggregation.

The major body of this work investigates how the chemical structure of conjugated polymers relates to the fundamental operating mechanism of organic photovoltaic devices. New conjugated polymers were characterized and their optical and electronic properties tested and correlated with their power conversion efficiencies as the active layer in polymer solar cells. From these experiments general structure/function relationships are drawn with an eye toward developing universal guidelines for conjugated polymer design and synthesis. Starting with light absorption, three major steps in the photovoltaic mechanism are examined. First, photogeneration of excited states and the migration of these states through the active layer are correlated to the polymeric backbone chemistry and the resulting device performance. Next, separation of these excited states at an interface between electron donors and electron acceptors is examined as a function of donor-acceptor distance and active layer dielectric constant. These two variables were tuned by chemical modification of polythiophene side groups. Third, charge carrier conduction is related to both polymer electronic states and to solid-state packing morphology. Design principles for effective conduction of both holes and electrons are outlined and the ambipolar nature of conjugated organic materials is discussed. In the final chapters, the solid-state polymer morphology in a solution processed thin film is examined. The impact that this morphology has on all steps in the photovoltaic mechanism is highlighted. How chemical modification of the polymer can influence this packing structure is examined as well as how new fabrication procedures can be used to pre-form nanostructured materials in solution before thin film deposition.

Improving knowledge of structure-function relationships in semiconducting polymers will help design new materials that unlock new applications. This work harnesses recent advances in transmission electron microscopy of soft materials to study length scales of microstructure in these materials that have previously been difficult to probe. Further, it combines electron microscopy with structural and charge transport simulations to study the effects of mesoscale defects on charge transport in highly ordered semicrystalline polymers. Spatially resolved nanodiffraction (4D-STEM) is used to create maps of chain direction and local order in conjugated polymers. Simulations are then built upon this experimental map, first by generating molecular geometries consistent with diffraction data, then by tracking the paths of test charges across the region. A case study in this combined method is conducted using the polymer PBTTT. Short-range charge transport is shown to be more chaotic than is often pictured, with the drift velocity accounting for a small portion of overall charge motion. Local transport is sensitive to the alignment and geometry of polymer chains. At longer length scales, the curves of this PBTTT microstructure funnel charges to specific regions, creating inhomogeneous charge distributions. While alignment generally improves mobility, these funneling effects limit the overall efficiency of charge transport. The structure is modified \textit{in silico} to explore possible design rules, showing chain stiffness and alignment to be beneficial while local homogeneity has no positive effect. These observations provide direct guidance for improving mesoscale structure for future materials.

Morphology is the origin of life and function. Defining and designing morphology, understanding the relationship between morphology and function, is an essential theme in a number of research areas. In conjugated polymer research, the major obstacles to achieving these goals are the heterogeneity and complexity of conjugated polymer films. In the study presented in this dissertation, various single molecule spectroscopy techniques were used as an approach to minimize the complexity of these problems. By using excitation polarization spectroscopy, it was discovered that single chains of poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) assume a highly ordered rod conformation despite the fact that the morphology of bulk films is known to be amorphous. The comparison of results from experiments and a coarse grained bead-on-a-chain simulation suggested that single chains have the ability to use a thermally induced defect to maximize [pi]-[pi] stacking and adopt a rod conformation as a stable conformation. Bias-induced centroid spectroscopy (BIC) on highly ordered single chains demonstrated that the energy transfer scale could be an order of magnitude larger than the value typically measured for bulk films. It was further demonstrated that such an extraordinary long energy transfer was not a unique property of single chains but was also observed in aggregates as long as the morphology was ordered. These studies were extended to another model compound poly(3-hexylthiophene) (P3HT) to generalize the mechanism of morphology formation and the structure-property relationship. For P3HT, it was shown that side-chains were a very important factor in determining single chain conformation, while the conformation of MEH-PPV was not affected by side-chains. By controlling the side-chains, both ordered and disordered P3HT chains were obtained. The comparison of results from experiments and an energy transfer model simulation quantified that energy transfer was at least twice as efficient in ordered chains as in disordered chains. In aggregates, the difference between the energy transfer efficiency of ordered and disordered morphology was even larger than that in the case of single chains. These results could suggest that there is a very fast energy transfer mechanism that occurs through interchain interactions when chains are packed in ordered fashion.

Organic semiconductors are an exciting class of materials that have potential to produce low-cost, printable, and flexible electronic devices. Moving to the next generation of organic semiconductors that will result in greater efficiency requires advancements in the areas of materials chemistry, molecular assembly, predictive modelling, and device optimization. Here, we focus on morphology and demonstrate how it is linked to each of these areas. Understanding the connections among chemistry, thin film microstructure, and charge transport remains a major challenge in the field. We examined materials systems relevant to organic solar cells, memory devices, and transistors, with a focus on synchrotron-based X-ray techniques. For a blend of a polymer and small molecule, applicable to solar cells, control of molecular orientation in the small molecule is especially important for non-fullerene based molecules that exhibit anisotropic charge transport. In ferroelectric-semiconductor polymer blends used in organic memory, improved control over phase separation length scales is achieved by altering the chemistry of the semiconducting polymer to tune polymer-polymer interactions. Complementary simulations can facilitate characterization of organic semiconductors. First-principles predictions of X-ray absorption spectroscopy are applied to semiconducting polymers, and prove critical for understanding complex experimental data related to molecular orientation and electronic structure in general. Overall, these studies provide insights into key factors that should be considered in the development of new organic semiconductors.

Organic (opto)electronic materials have received considerable attention due to their applications in perovskite and flexible electronics, OPVs and OLEDs and many others. Reflecting the rapid growth in research and development of organic (opto)electronic materials over the last few decades, this book provides a comprehensive coverage of the state of the art in an accessible format. It presents the most widely recognized fundamentals, principles, and mechanisms along with representative examples, key experimental data, and over 200 illustrative figures.