Studying And Controlling The Structure Of Doped Semiconducting Polymers

This thesis focuses on studying and controlling the structure of pristine and doped semiconducting polymers. Semiconducting polymers have many applications in flexible electronics due to their structural tunability, low cost and solution processability. Intrinsically, semiconducting polymers have poor conductivities due to a lack of mobile carriers. Charge transfer between a semiconducting polymer and a dopant molecule is necessary to introduce carriers into a polymer system. If an electron is fully transferred, commonly called "integer charge transfer (ICT)", this will result in a polaron and a dopant anion. On the other hand, the electron charge could be shared between the polymer and a dopant molecule to form a "charge transfer complex (CTC)". In the first part of the thesis, we explored factors that affect the charge transfer pathways in doped semiconducting polymers and were able to control the formation of CTCs. Semiconducting polymers are composed of both crystalline and amorphous parts. Compared to crystalline regions, amorphous polymer parts are disordered, thus the dopant anion is usually close to the polarons, resulting in poor carrier mobility due to Columb attraction between polarons and counterions. CTCs also tend to form in amorphous polymer regions compared to crystallites and result in less carriers due to the charge-sharing nature of CTC. In our second project, we explored ways to suppress the formation of both CTCs and localized carriers even in highly amorphous polymer films, using large boron cluster-based dopants. The electron density of these dopants is core-localized and is shielded from the holes on the polymer, resulting in increased crystallinity and higher film conductivities. In our third project, we further explored how polymer crystallite orientation influences the ease of doping and found that polymer regions with structures similar to the final doped structure could be doped more easily. In the last chapter, we designed amphiphilic semiconducting polyelectrolytes that form ordered cylindrical micelles in water. Our results demonstrate that we can achieve relatively precise control between electron donor and acceptor co-assemblies by varying the structural properties of component amphiphilic polymers and acceptors, which can provide guidelines for designing systems with controllable excited-state transfers.

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.

Semiconducting polymers show promise for use in a variety of applications such as photovoltaic cells, light emitting diodes, and thermoelectric generators. For many of these devices, the electronic properties are tuned through the introduction of chemical dopants. This dissertation is focused on understanding several key aspects of the chemical doping process. The first chapter gives an overview of semiconducting polymers, introduces doping by sequential processing methods and looks at how the chemical doping process works on a basic level. We also explore dopant transport methods, discuss the electrical and thermoelectrical characterization of these materials, and finally consider the structural morphology of conjugated polymer thin films. Chapter 2 takes an analytical approach to understanding how the underlying morphology and electrical/thermoelectrical properties of doped polymer films are affected when introducing the dopant either via the solutionphase or using vapor transport. Chapter 3 explores the fundamental charge transfer interactions that occur between polymer and dopant. We introduce a novel processing technique that enables the tunable production of dopant-polymer charge transfer complexes (CTCs), which represent a poorly understood but widely seen doping mechanism in these materials. We provide the first comprehensive picture of the forces that drive CTC formation and offer guidelines for limiting CTC occurrence in doped conjugated polymers as their electrical properties are usually undesirable. Finally, in Chapter 4 we solve a long-standing mystery in the literature of the highly variable vibrational spectra of certain dopant molecules, which should nominally show consistent and predictable frequencies. We show that the wide range of vibrational energies observed for these dopant molecules can be fully understood through the framework of the vibrational Stark effect. Our experimental evidence shows a clear and predictable shift for these modes as a function of their locally experienced electric field, which arises due to Coulomb interactions with the charge carriers on the polymer. Thus, the vibrational shifts of these dopant molecules are actually exquisite reporters on the local environment of the charge carriers in doped conjugated polymers. We use our experimentally-measured shifts to quantitatively estimate the change in polaron coherence length, the extent to which the charge carriers on the polymer spread over multiple polymer repeat units. These chapters cover a variety of themes which highlight the sometimes unexpected path from experiment to manuscript. I sincerely hope they can be of use to others who study similar systems and motivate additional works in the future.

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.

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 self-assembly of organic semiconducting molecules and polymers is critical for their electrical properties. This project addressed the design of organic semiconductors with novel synthetic building blocks for proton-dopable conducting materials and the molecular order and microstructure of high performance semiconducting polymers blended with charge transfer dopants. Novel azulene donor-acceptor materials were designed and synthesized with unique electronic effects upon protonation to generate charged species in solution. The microstructure and optical properties of these derivatives were examined to develop structure-property relationships. Studies of the microstructure of blends of charge transfer doped semiconducting polymers revealed highly ordered conductive phases in blends. The molecular packing of one blend was studied in detail using a combination of solid-state NMR and x-ray scattering revealing that dopant incorporation is unlikely to be random as assumed in transport models. Studies of the electrical properties of these highly ordered blends revealed a universal trend between the thermopower and electrical conductivity of semiconducting polymers that is independent of the doping mechanism.

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.

An A-to-Z of doping including its definition, its importance, methods of measurement, advantages and disadvantages, properties and characteristics—and role in conjugated polymers The versatility of polymer materials is expanding because of the introduction of electro-active behavior into the characteristics of some of them. The most exciting development in this area is related to the discovery of intrinsically conductive polymers or conjugated polymers, which include such examples as polyacetylene, polyaniline, polypyrrole, and polythiophene as well as their derivatives. "Synmet" or "synthetic metal" conjugated polymers, with their metallic characteristics, including conductivity, are of special interest to researchers. An area of limitless potential and application, conjugated polymers have sparked enormous interest, beginning in 2000 when the Nobel Prize for the discovery and development of electrically conducting conjugated polymers was awarded to three scientists: Alan J. Heeger, Alan G. MacDiarmid, and Hideki Shirakawa. Conjugated polymers have a combination of properties—both metallic (conductivity) and polymeric; doping gives the conjugated polymer's semiconducting a wide range of conductivity, from insulating to low conducting. The doping process is a tested effective method for producing conducting polymers as semiconducting material, providing a substitute for inorganic semiconductors. Doping in Conjugated Polymers is the first book dedicated to the subject and offers a comprehensive A-to-Z overview. It details doping interaction, dopant types, doping techniques, and the influence of the dopant on applications. It explains how the performance of doped conjugated polymers is greatly influenced by the nature of the dopants and their level of distribution within the polymer, and shows how the electrochemical, mechanical, and optical properties of the doped conjugated polymers can be tailored by controlling the size and mobility of the dopants counter ions. The book also examines doping at the nanoscale, in particular, with carbon nanotubes. Readership The book will interest a broad range of researchers including chemists, electrochemists, biochemists, experimental and theoretical physicists, electronic and electrical engineers, polymer and materials scientists. It can also be used in both graduate and upper-level undergraduate courses on conjugated polymers and polymer technology.

My thesis focuses on the development, characterization, and application of a new method for doping semiconducting polymer films. Semiconducting polymers represent a versatile class of materials used in many electronic device applications. However, due to their low intrinsic conductivity, doping is often necessary to achieve the desired electrical properties for specific applications. Adding large amounts of dopant often leads to detrimental effects that hinder both film quality and electrical properties. I developed new methodologies to overcome these specific issues present in the high doping regime as well as new insights gained general to molecular doping of polymers afforded by these methods. The first chapter gives a brief history of doping of semiconducting polymers, including characterization methods and challenges. The second chapter introduces the new method, sequential doping, specifically designed to overcome one such challenge and its comparison to a more traditional doped-film fabrication method. We demonstrate that this new method produces highly doped films of superior film quality allowing for accurate optical and electrical measurements, including Hall effect measurements, which had previously been unrealized. Chapter three focuses on how sequential doping offers the unique advantage of being able to tune the polymer film morphology prior to doping and maintain that same morphology after doping allowing for a detailed study of how polymer crystallinity effects the optical and electrical properties of the doped state. Of particular note was the discovery that for the specific polymer:dopant system studied, the dopant resides in the side chain regions between polymer chains as opposed to in the -stacks as previously claimed. The final chapter continues to use the advantages offered by sequential doping to study the interplay between polymer crystallinity and energy level offset between the polymer and the dopant by the use of statistical copolymers with independently tuned crystallinity and energy levels. It was found that the crystallinity of the polymer after being doped could vary depending on the structural composition of the undoped polymer, and that the doped state crystallinity had a more drastic effect on the resulting electrical properties than the energy level offset. Overall, sequential doping is an effective way to produce highly doped semiconducting polymer films and allows for novel studies on how physical properties of materials influence their doped counterparts.