Organic semiconductors have emerged as an alternative to conventional silicon-based electronics in a number of large-area applications including lighting, displays, and sensors. Their ease of processing and compatibility with solution-based deposition techniques makes them attractive for low-cost, roll-to-roll production processes. However, despite significant progress in recent years, the electronic performance of these materials remains modest relative to silicon, limiting their potential applications. In order for organic semiconductors to truly flourish in industry, electronic figures of merit such as charge carrier mobility must be improved further. The material microstructure is one of the key determinants of charge carrier mobility in organic semiconductors. While many new microstructure characterization tools have been developed and our understanding of the relationship between microstructure and electronic properties has greatly improved, significant questions remain, limiting our ability to rationally design and process new materials with improved performance. The focus of this dissertation has been in attempting to improve this understanding. In this dissertation, I discuss several ways in which the microstructure of semiconducting polymers affects their electronic properties. First, I present a procedure for determining the precise thin-film structure of a semiconducting polymer using two-dimensional grazing incidence X-ray diffraction. These packing structures can then be used in a variety of electronic structure calculations. Next, I discuss the role of molecular weight distribution as well as the impact of film confinement on the microstructure and electronic properties of two semicrystalline polythiophenes. I show how disorder, crystallinity, and chain orientation are strongly influenced by these factors and quantify their impact on charge carrier mobility. Finally, I describe our latest understanding of the factors governing charge transport in state-of-the-art materials. I suggest that disorder is an inextricable feature of semiconducting polymers that need not be highly detrimental to charge transport if it is embraced and planned for by designing materials which are resilient to this disorder.
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.
Organic electrochemical transistors (OECTs), in recent years, have emerged as promis- ing devices for fabricating biosensors using semiconducting polymers. Although in- organic materials have long dominated the semiconductor market, organic semicon- ductors have been found to be much better candidates for interfacing with biological systems due to their high chemical variability, low elastic moduli and ability to per- form both electronic and ionic transport. Because ionic species can penetrate highly porous polymer films leading to large interfacial areas, OECT devices typically ex- hibit extremely large capacitances and display among the highest transconductance values in published literature. Despite great technological advancements in device fabrication and designs over the last decade, there still lacks a thorough understanding of electronic transport, molec- ular doping, and device physics in these systems. My doctoral research focused on developing a more complete picture of these fundamental processes in OECTs. The first few chapters of this thesis will be dedicated to our work in characterizing poly- mer crystal structures, film formation and microstructures, and charge percolation in semiconducting polymer thin films. Subsequently, I will discuss how we can control- lably dope polymer thin films, and the physical and chemical properties that affect the doping process. In the last parts, I will present our electrical model for predicting OECT device responses and how we can extract useful device and biological properties in sensing experiments. Our findings provide important, fundamental insights into physical and electronic processes in semiconducting polymers, and are indispensable for designing better materials and biosensors.
A critical step to understanding charge transport in complex systems is being able to characterize them accurately and extensively. In particular, the microstucture of conjugated polymers exhibits a coexistence of ordered and amorphous regions, with the size of the ordered regions being smaller than the length of individual polymer chains. In order to study the ordered regions we use advanced X-ray diffraction analysis in combination with computational modeling and measurements of optical and electrical properties. It was possible to uncover fundamental relationships between short-range order in pi-aggregates, aggregate connectivity and macroscopic charge transport in semiconducting polymers. An unusually high and materials-independent amount of paracrystalline disorder was found in all high-performing polymers. Computer simulations and analytical models made the connection between fluctuations in molecular arrangement and electronic traps. Charge transport studies elucidated the predominant role of paracrystallites in semicrystalline and strongly disordered polymer films. The other component of the microstructure -- the amorphous regions -- deserves our attention as well since aggregate connectivity depends on it. A model for charge transport in strongly disordered polymers was developed for this reason. The morphology of individual polymer chains can be determined by well-known statistical models. Likewise, the electronic coupling between units along a polymer chain and on different molecules can be determined by Marcus theory. Combining knowledge from both areas into an analytical and computational model that incorporates the structural and electronic properties of polymers, it is possible to explain observations that previously relied on phenomenological models. The multi-scale behavior of charges in these materials (high mobility at short scales, low mobility at long scales) is naturally described with this framework. Additionally, the dependence of mobility with electric field and temperature is explained in terms of conformational fluctuations and correlations. Bringing all these concepts together it is possible to provide a more complete description of the way in which charges move in conjugated polymers, a set of materials that occupies an intermediate region between ordered and disordered systems, with a great amount of complexity at various length scales. Doing so will facilitate the feedback cycle between molecular design, microstructure optimization, and device performance.
Semiconducting polymers are of great interest for applications in electroluminescent devices, solar cells, batteries and diodes. In recent years vast advances have been made in the area of controlled synthesis of semiconducting polymers, specifically polythiophenes. The book is separated into two main sections, the first will introduce the advances made in polymer synthesis, and the second will focus on the microstructure and property analysis that has been enabled because of the recent advances in synthetic strategies. Edited by one of the leaders in the area of polythiophene synthesis, this new book will bring the field up to date with more recent models for understanding semiconducting polymers. The book will be applicable to materials and polymers chemists in industry and academia from postgraduate level upwards.
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.
Semiconducting polymers are a promising class of organic electronic materials, with the potential to have a large impact in the field of macroelectronics. In this thesis, we focus on understanding the relationship between microstructure and charge transport in semicrystalline polythiophenes. A method is presented for the measurement of complete pole figures of polymer thin films using an area detector, allowing for the first time quantitative characterization of crystalline texture and degree of crystallinity. Thin film transistors are used to measure electrical characteristics, and charge transport behavior is modeled according to the Mobility Edge (ME) model. These characterization methods are first used to investigate the effect of substrate surface treatment and thermal annealing on the microstructure of polythiophene thin films, and the effect of microstructural details on charge transport. Next, we investigate the semicrystalline microstructure in confined polythiophene films. Pole figures are used to quantify a decrease in the degree of crystallinity of films with decreasing thickness, accompanied by an improvement in crystalline texture. Next, we investigate the influence of the degree of regioregularity, molecular weight and the processing solvent on microstructure (degree of crystallinity and texture) and charge transport in high mobility P3HT thin films. Surprisingly, when processing conditions are optimized, even a polymer with moderate regioregularity can form a highly textured film with high charge carrier mobility. Finally, we use films of P3HT with engineered, anisotropic in-plane microstructure to understand the importance and mechanism of transport across grain boundaries in these semicrystalline films. Results from this study provide the first experimental evidence for the application of a percolation model for charge transport in high molecular weight semicrystalline polymer semiconductors. Understanding how characteristics of the polymer as well as details of the processing conditions can affect the film microstructure and device performance is important for future materials design and device fabrication.
The desire for more commercially feasible flexible electronic plastics has led to the development of increasingly complex conjugated polymer architectures and device geometries. Through these efforts, tremendous advances have been made in the design and performance of electronic devices fabricated with solution-processable semiconducting polymers. However, none of these materials have yet reached commercial maturity, so the opportunity for their further exploration from both a fundamental science and an application-driven point of view motivates this dissertation. Chapter 1 presents a background introduction to many of the concepts, ideas, and existing research necessary to set the context of this dissertation's work. The first component of this work (Chapters 2-6) investigates thiol-ene cross-linked conjugated polymer networks. By installing vinyl functionalities in poly(fluorene) molecules at chain ends (Chapters 2 and 3) and along the polymer backbone (Chapters 4 and 5), the polymers can be rapidly and efficiently cross-linked by photo-reaction with thiol cross-linkers into highly tunable semiconducting polymer networks. It is shown that the thiol-ene cross-linking reaction allows for a new molecular handle on modifying interchain electronic communication via network density, which is visualized using characteristic and unambiguous photoemission from low-energy fluorenone species. Light emitting diodes fabricated using these networks as an emissive layer show enhanced color stability compared to as-spun counterparts, and the robustness of the networks allows for solution processing of multiple stratified emissive layers for controlling color emission. Furthermore, the highly efficient thiol-ene cross-linking reaction is shown to work as an effective means for grafting poly(fluorene)s onto functionalized surfaces. This work's second component in Chapter 7 details the direct visualization of charge carrier density in polymeric thin film transistors using Modulation-Amplified Reflectance Spectroscopy (MARS) measurements. Owing to the unique changes in optical behavior following the formation of a charged state within the conjugated polymer film, optical spectroscopy coupled to a CCD camera offers a powerful visualization tool for observing and mapping charges across large areas as they interact with electrodes, defects, and film morphologies in active devices. The MARS technique illuminates the spatial distribution of carriers in electronic polymer films, allowing direct spatial visualization of charge density and film defects. Finally, a brief concluding comment on this work and an outlook for the field in general is presented in Chapter 8.
Flexible displays are currently one of the most researched topics within the flat panel display community. They promise to change our display-centric world by replacing bulky rigid devices with those that are paper-thin and can be rolled away or folded up when not in use. The field of flexible flat panel displays is truly unique in the sense that it is interdisciplinary to the display community, combining basic principles from nearly all engineering and science disciplines. Organized to bring the reader from the component level, through display system and assembly, to the possible manufacturing routes Flexible Flat Panel Displays: * outlines the underlying scientific theory required to develop flexible display applications; * addresses the critical issues relating to the convergence of technologies including substrates, conducting layers, electro-optic materials and thin-film transistors; * provides guidance on flexible display manufacturing; and * presents market information and a chapter dedicated to future market trends of flexible flat panel displays. Flexible Flat Panel Displays is an essential tool for scientists, engineers, designers and business and marketing professionals working at all levels of the display industry. Graduate students entering the field of display technology will also find this book an excellent reference. The Society for Information Display (SID) is an international society, which has the aim of encouraging the development of all aspects of the field of information display. Complementary to the aims of the society, the Wiley-SID series is intended to explain the latest developments in information display technology at a professional level. The broad scope of the series addresses all facets of information displays from technical aspects through systems and prototypes to standards and ergonomics
