Structure Function Relationships In Semiconducting Polymers

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.

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.

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.

Conjugated polymers are a versatile class of materials useable in a variety of organic electronic applications, but their utilization is limited by their intrinsically low conductivity. However, doping of semiconducting polymers via oxidation of their backbone can add mobile charge carriers and increase their electrical conductivity. This can be accomplished via electrochemical doping, where an applied potential oxidizes the polymer, or via chemical doping, where a molecular oxidizer is introduced to the polymer.Electrochemical doping of semiconducting polymers is of interest because of this class of material's ability to be utilized in electrochemical cells, such as lithium-ion batteries (LIBs). This is explored in the first part of this dissertation (Chapters 2 through 7), where we investigate the application of semiconducting polymers as LIB binders. Binders are typically designed to be chemically and mechanically durable during cycling. Utilizing conjugated polymers as binders increases the electrical conductivity of the electrode, leading to reduced resistive losses and faster charging. We show that dihexyl-substituted poly(3-4-propylenedioxy-thiophene) (PPrODOT-Hx2) can serve as a binder at relevant electrochemical potentials. Additionally, we show that by either creating co-polymers with oligoether side-chains or by adding conjugation break-spacer units, we can tune ionic conductivity, heat generation, swelling, and the mechanical properties of the semiconducting polymers. While electrochemical doping has the advantage of allowing the selection of an exact potential (i.e., doping level), chemical doping is advantageous because it is fairly simple to accomplish. In the second half of this dissertation (Chapters 8 through 12), we study a variety of semiconducting polymers and dopants to understand what results in the highest conductivity in chemically doped semiconducting films. We explore the energetics and the role of crystallinity, dielectric constant, and Coulomb binding in chemical doping. Throughout this dissertation, we utilize grazing incidence wide-angle X-ray scattering (GIWAXS) to see how the structure of these polymers change upon doping, and how these structural changes map onto changes in both electronic and ionic conductivity, and optical properties.

The attractive optoelectronic and mechanical properties of conjugated polymers have lead to intense research in understanding of their semiconducting properties and application in various devices. The properties of conjugated polymers are highly dependent and sensitive to intra- and inter-polymer chain contacts and new methods to manipulate and control these solution and solid state interactions and optoelectronic properties are thus continually being pursued. Conjugated polyelectrolytes (CPEs) are a class of conjugated polymers containing ionic substituents in which polymer chain interactions are known to be influence by long-range electrostatic interactions, not feasible in non-ionic conjugated polymers. Although, CPEs have exhibit device improvement when incorporated in light emitting and photovoltaic devices, the optical and particularly charge carrier transport properties are not fully understood. In this dissertation the influence of the ionic substituents on both the optical and charge transport properties of CPEs are systematically investigated in pursue of constructing concrete structure-function-property relationships that can be use as guidelines for the future design of CPEs that have tailored properties and function.

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.

In the last 10 years there have been major advances in fundamental understanding and applications and a vast portfolio of new polymer structures with unique and tailored properties was developed. Work moved from a chemical repeat unit structure to one more based on structural control, new polymerization methodologies, properties, processing, and applications. The 4th Edition takes this into account and will be completely rewritten and reorganized, focusing on spin coating, spray coating, blade/slot die coating, layer-by-layer assembly, and fiber spinning methods; property characterizations of redox, interfacial, electrical, and optical phenomena; and commercial applications.

Semiconducting polymers are of great interest for applications in electroluminescent devices, solar cells, batteries, and diodes. This volume provides a thorough introduction to the basic concepts of the photophysics of semiconducting polymers as well as a description of the principal polymerization methods for luminescent polymers. Divided into two main sections, the book first introduces the advances made in polymer synthesis and then goes on to focus on the photophysics aspects, also exploring how new advances in the area of controlled syntheses of semiconducting polymers are applied. An understanding of the photophysics process in this kind of material requires some knowledge of many different terms in this field, so a chapter on the basic concepts is included. The process that occurs in semiconducting polymers spans time scales that are unimaginably fast, sometimes less than a picosecond. To appreciate this extraordinary scale, it is necessary to learn a range of vocabularies and concepts that stretch from the basic concepts of photophysics to modern applications, such as electroluminescent devices, solar cells, batteries, and diodes. This book provides a starting point for a broadly based understanding of photophysics concepts applied in understanding semiconducting polymers, incorporating critical ideas from across the scientific spectrum.

Very thin film materials have emerged as a highly interesting and useful quasi 2D-state functionality. They have given rise to numerous applications ranging from protective and smart coatings to electronics, sensors and display technology as well as serving biological, analytical and medical purposes. The tailoring of polymer film properties and functions has become a major research field. As opposed to the traditional treatise on polymer and resin-based coatings, this one-stop reference is the first to give readers a comprehensive view of the latest macromolecular and supramolecular film-based nanotechnology. Bringing together all the important facets and state-of-the-art research, the two well-structured volumes cover film assembly and depostion, functionality and patterning, and analysis and characterization. The result is an in-depth understanding of the phenomena, ordering, scale effects, fabrication, and analysis of polymer ultrathin films. This book will be a valuable addition for Materials Scientists, Polymer Chemists, Surface Scientists, Bioengineers, Coatings Specialists, Chemical Engineers, and Scientists working in this important research field and industry.