Doping Of Semiconducting Polymers For Electronic Applications

One branch of modern electronics requires avoiding the high processing costs associated with inorganic semiconductors in order to create novel low-cost, mechanically flexible, and low-profile devices for the next generation of consumer devices. Organic semiconductors can be doped to improve their charge mobility and carrier density towards creating better polymer-based photovoltaics, organic thin-film transistors, and organic light-emitting diodes. Dopants offer one route to improved device performance, but the specific interactions between the dopant molecule and the semiconductor must be designed for the desired function.This work explores the effects of sulfonic acid groups on the behavior of the common organic semiconductor poly-(3-hexylthiophene) (P3HT). P3HT was chosen for its ubiquitous use in photovoltaics and other organic electronic applications. The doping of P3HT by sulfonic acid-containing moieties was explored initially as a method to replace the poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS) electron blocking later at the photovoltaic transparent indium tin oxide electrode. Measurements of doped thiophene-based polymers were conducted in organic thin-film transistor geometries to measure the charge carrier densities. Additionally, spectroscopic evidence of doping complemented the transistor and photovoltaic studies. This work explores the extent to which P3HT can be doped at the highest density and how it may be used in modern organic electronics such as transistors, photovoltaics, and light-emitting diodes.

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.

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.

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.

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.

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.

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.

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.

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.