Structure Property Relationships In Semiconducting Polymers And Small Molecules Probed By Synchrotron X Ray Methods

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.

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.

This work is focused on understanding how molecular-level structural control can improve charge carrier properties in -conjugated polymers. Conjugated polymers are characterized by extended conjugation along their backbone, making them intrinsically semiconducting materials that are of interest for a wide variety of flexible, thin-film electronic applications. Polymeric semiconductors possess advantages over inorganic materials such as being lightweight, low-cost and solution processable. However, due the disordered nature of conjugated polymers and their anisotropic transport, charge carrier dynamics can be highly sensitive to structural effects. The first chapter of this dissertation gives an introduction to conjugated polymers and their relevant applications as well as how tuning morphology and doping level can influence their charge carrier properties. The second introduces a technique, known as sequential processing (SqP), that affords control over polymer domain orientation when preparing polymer films as the active layer in optoelectronic devices. We show that conventional processing methods lead to disordered, isotropic polymer networks. By contrast, SqP can be used to preserve the preferred face-on chain orientation seen with some polymer materials, yielding advantages for photovoltaics and other devices via increased vertical hole mobility. Chapter 3 turns to molecular doping of conjugated polymers and studies the effects of a bulky boron cluster dopant used to modify the charge transport properties of conjugated polymers. The design of the dopant is such that it sterically protects core-localized electron density, resulting in shielding of the electron from holes produced on the polymer. This allows the charge carriers to be highly delocalized, as confirmed both spectroscopically and by AC-Hall effect measurements. The dopants allow for high carrier mobilities to be achieved even for non-crystalline polymers. The implication is that the counterion distance is the most important factor needed to produce high carrier mobility in conjugated polymers. In the last chapter, we study a series of boron cluster dopants in which the redox potential is tuned over a large range but the anion distance is fixed. In the last chapter, we study a series of boron cluster dopants in which the redox potential is tuned over a large range but the anion distance is fixed. This allows us to disentangle the effects of energetic offset in doping on the production of free carriers. We find that the redox potential not only affects the generation of free carriers, but also the infiltration of dopants into the polymer films.

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.

Conjugated polymer blend morphology dictates performance of many organic electronic devices, including electrochemical transistors, light-emitting diodes, and solar cells. In organic photovoltaics (OPVs), electronically active layer morphology of polymer and oligomer bulk-heterojunction influences charge transport and exciton dissociation properties and governs device performance. Yet a faithful representation of the blend interface and local morphology is lacking. In principle, molecular dynamics simulation can represent these blends. However, semiconducting polymers with aromatic rings are large, stiff, and slowly relaxing, which makes equilibration challenging. We develop a new coarse-graining (CG) method, which improves simulation efficiency ten-fold by representing aromatic rings as rigidly bonded moieties, in which we represent several atoms as virtual sites. P3HT simulations with virtual site coarse graining show that the polymer persistence length and the melt density agrees with experimental results. An agreement between scattering extracted from P3HT simulations and wide-angle X-ray scattering experiment validates the simulation local morphology. In the amorphous phase, the scattering results in two wide peaks: the low q peak originates from interchain backbone correlations, and the high q peak originates from interchain side group correlations. We use the virtual site method to characterize the morphology of a typical OPV blend: P3HT (donor) and O-IDTBR (acceptor) and their pure phases. The blend morphology shows that moieties with solubilizing side-groups have fewer electronic contacts because of steric hindrance. On slow cooling, the fast simulation method enables us to observe crystallization, which occurs more readily in pure P3HT than in the blend. Simulations of a low molecular weight P3HT with O-IDTBR represent the local structures of small mixed regions. To describe a de-mixed blend interface, we need the Flory-Huggins [chi] parameter. We develop a "push-pull" technique to measure [chi], which applies robustly to polymer blends of any architecture. The method applies equal and opposite potentials to polymers in a blend to induce a concentration gradient, which is more pronounced for polymers with repulsive interactions ([chi]>0). Chain flexibility plays an important role as stiffer polymers require more energy to induce concentration gradient. We validate the method by blends of bead-spring chains with varying flexibility and PE/PEO blend. The [chi] evaluated from "push-pull'' methods are comparable to the results from previously developed "morphing'' method. We obtain a comprehensive view of the OPV blend morphology by combining local structures from our CG representation and the [chi] parameter from the "push-pull" technique. The [chi] calculated for a blend of P3HT and O-IDTBR shows that the blend follows an upper critical solution temperature behavior and predicts the critical molecular weight of P3HT for phase separation. An amorphous blend of P3HT and O-IDTBR forms an interface of a few nanometers. In contrast, the presence of a crystal acceptor crystallizes the donor polymer on its surface, forming a sharp interface. Crystallization reduces overall contact between donor and acceptor but increases face-on contact, which is important for exciton dissociation. O-IDTBR solubilized in P3HT may also aid in exciton dissociation; however, the polarons formed can not percolate to the acceptor rich region with only 15% solubility and may result in recombination losses. Much higher solubility is required for charge percolation to occur. However, increasing the acceptor solubility in the donor phase may cause crystal structure disruption. A polaron formed by exciton dissociation hops from one chain to another, and the polaron hopping rate depends on the electronic coupling between neighboring molecules governed by their local structures. Electronic coupling of a few thousand P3HT monomer pairs from an amorphous melt shows that strong contacts with high electronic coupling are rare. Feature selection in machine learning helps identify the most important feature for strong contact. The key geometric features closely relate to coherent overlap between HOMO wavefunctions on nearby moieties for hole transport. We develop a machine learning model to evaluate electronic coupling distribution with morphological changes. Slow cooling induces crystallization in P3HT and increases the number of strong contacts. Furthermore, we provide a future direction to understand the high performing organic photovoltaic blend morphology and relate the morphology to their electronic properties. The structure-property relationship will aid in developing rational design of conjugated polymers for efficient organic photovoltaic application.

The field of semiconducting polymers has attracted many researchers from a diversity of disciplines. Printed circuitry, flexible electronics and displays are already migrating from laboratory successes to commercial applications, but even now fundamental knowledge is deficient concerning some of the basic phenomena that so markedly influence a device's usefulness and competitiveness. This two-volume handbook describes the various approaches to doped and undoped semiconducting polymers taken with the aim to provide vital understanding of how to control the properties of these fascinating organic materials. Prominent researchers from the fields of synthetic chemistry, physical chemistry, engineering, computational chemistry, theoretical physics, and applied physics cover all aspects from compounds to devices. Since the first edition was published in 2000, significant findings and successes have been achieved in the field, and especially handheld electronic gadgets have become billion-dollar markets that promise a fertile application ground for flexible, lighter and disposable alternatives to classic silicon circuitry. The second edition brings readers up-to-date on cutting edge research in this field.

X-ray Absorption Spectroscopy (XAS) is a powerful technique with which to probe the properties of matter, equally applicable to the solid, liquid and gas phases. Semiconductors are arguably our most technologically-relevant group of materials given they form the basis of the electronic and photonic devices that now so widely permeate almost every aspect of our society. The most effective utilisation of these materials today and tomorrow necessitates a detailed knowledge of their structural and vibrational properties. Through a series of comprehensive reviews, this book demonstrates the versatility of XAS for semiconductor materials analysis and presents important research activities in this ever growing field. A short introduction of the technique, aimed primarily at XAS newcomers, is followed by twenty independent chapters dedicated to distinct groups of materials. Topics span dopants in crystalline semiconductors and disorder in amorphous semiconductors to alloys and nanometric material as well as in-situ measurements of the effects of temperature and pressure. Summarizing research in their respective fields, the authors highlight important experimental findings and demonstrate the capabilities and applications of the XAS technique. This book provides a comprehensive review and valuable reference guide for both XAS newcomers and experts involved in semiconductor materials research.

Scientists and engineers have long relied on the power of imaging techniques to help see objects invisible to the naked eye, and thus, to advance scientific knowledge. These experts are constantly pushing the limits of technology in pursuit of chemical imagingâ€"the ability to visualize molecular structures and chemical composition in time and space as actual events unfoldâ€"from the smallest dimension of a biological system to the widest expanse of a distant galaxy. Chemical imaging has a variety of applications for almost every facet of our daily lives, ranging from medical diagnosis and treatment to the study and design of material properties in new products. In addition to highlighting advances in chemical imaging that could have the greatest impact on critical problems in science and technology, Visualizing Chemistry reviews the current state of chemical imaging technology, identifies promising future developments and their applications, and suggests a research and educational agenda to enable breakthrough improvements.

This book provides an introduction to this exciting and relativelynew subject with chapters covering natural and synthetic polymers,colloids, surfactants and liquid crystals highlighting the many andvaried applications of these materials. Written by an expert in thefield, this book will be an essential reference for people workingin both industry and academia and will aid in understanding of thisincreasingly popular topic. Contains a new chapter on biological soft matter Newly edited and updated chapters including updated coverageof recent aspects of polymer science. Contain problems at the end of each chapter to facilitateunderstanding