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The electrical characteristics of organic semiconductors have been studied intensively ever since conductivity in organic materials has been discovered. The focus has been the understanding of factors that affect charge transport so that the performance of organic devices can be improved. This work focuses on the electrical characterization of organic semiconductors. We have investigated a series of problems related to capacitance voltage measurements and current voltage measurements of organic devices. The physics of organic electronic devices are often interpreted by invoking the concept of -- "unintentional doping". However, the applicability and usefulness of this controversial concept is not very clear and is under much debate, recently. In this thesis, we have reevaluated the validity of this concept through careful experiments and detailed numerical simulations. Specifically, we have used Capacitance Voltage (CV) measurements of pentacene devices as a test bed to unravel the role of injecting electrodes and unintentional doping (if any). Our results have indicated that the CV of pentacene capacitors can be solely understood in terms of properties of the contact electrodes. The unintentional doping, if present, has an inconsequential role in device performance. Our conclusions have indicated that, often, an incorrect interpretation of CV results leads to unphysical values of unintentional doping. It has obvious implications in the fundamental understanding of organic semiconductor device physics, modeling, and characterization thus resolving many ambiguities in literature by providing a consistent interpretation through a coherent conceptual framework. The impact of atmospheric exposure on pentacene devices has been explained based on the contact barrier degradation at the metal-semiconductor interface. An analytical model based on the timing analysis of the capacitance frequency measurements has been proposed in order to extract the injection barrier. It was found that on atmospheric exposure, the pentacene gold injection barrier is reduced to 0.51 eV limiting the number of carriers transporting in the devices. The extracted value is close to different values reported in various photoelectron spectroscopic studies. Mechanical flexibility is one of the key advantages of organic semiconducting films in applications such as wearable-electronics or flexible displays. We have studied the electrical characteristics of C60-based top gate organic field effect transistors (OFET). The devices were characterized by curling the substrates in a concave and convex manner, to apply varying values of compressive and tensile strain, respectively. Electron mobility was found to increase with compressive strain and decrease with tensile strain. The observed strain effect was found to be strongly anisotropic with respect to the direction of the current flow. The results are quantified using the Fishchuk/Kadashchuk model for the hopping charge transport. We suggest that the observed strain dependence of the electron transport is dominated by a change in the effective charge hopping distance over the grain boundaries in polycrystalline C60 films. Most studies on charge transport are focused around low temperature electrical measurements. We have electrically characterized pentacene based OFETs between the temperature ranges of 25 °C to 190 °C in ambient conditions. Material characterization studies such as X-ray photoelectron (XPS), X-ray diffraction (XRD) and atomic force microscopy (AFM) prove the stability of pentacene as a semiconductor in ambient conditions at elevated temperatures. The crystallinity of pentacene films is retained up to 110 °C; its phase changes around 150 °C. Charge transport studies reveal a strong dependence of mobility on the gate field and interface states. The degradation of device parameters is attributed to the deterioration of dielectric and phase transformation in pentacene at higher temperatures. At an above-room-temperature range, mobility is found to be thermally activated in the presence of traps, whereas, for a trap-free interface, it is temperature independent. These results validate the performance and stability of organic devices in practical environmental conditions. The different experimental works reported in this thesis have been wrapped under the thesis title, -- "Understanding and Optimization of Electrical Characteristics of Organic Devices".
This book focuses on the microscopic understanding of the function of organic semiconductors. By tracing the link between their morphological structure and electronic properties across multiple scales, it represents an important advance in this direction. Organic semiconductors are materials at the interface between hard and soft matter: they combine structural variability, processibility and mechanical flexibility with the ability to efficiently transport charge and energy. This unique set of properties makes them a promising class of materials for electronic devices, including organic solar cells and light-emitting diodes. Understanding their function at the microscopic scale – the goal of this work – is a prerequisite for the rational design and optimization of the underlying materials. Based on new multiscale simulation protocols, the book studies the complex interplay between molecular architecture, supramolecular organization and electronic structure in order to reveal why some materials perform well – and why others do not. In particular, by examining the long-range effects that interrelate microscopic states and mesoscopic structure in these materials, the book provides qualitative and quantitative insights into e.g. the charge-generation process, which also serve as a basis for new optimization strategies.
Think like an electron Organic electronic materials have many applications and potential in low-cost electronics such as electronic barcodes and in light emitting devices, due to their easily tailored properties. While the chemical aspects and characterization have been widely studied, characterization of the electrical properties has been neglected, and classic textbook modeling has been applied. This is most striking in the analysis of thin-film transistors (TFTs) using thick “bulk” transistor (MOS-FET) descriptions. At first glance the TFTs appear to behave as regular MOS-FETs. However, upon closer examination it is clear that TFTs are unique and merit their own model. Understanding and interpreting measurements of organic devices, which are often seen as black-box measurements, is critical to developing better devices and this, therefore, has to be done with care. Electrical Characterization of Organic Electronic Materials and Devices Gives new insights into the electronic properties and measurement techniques for low-mobility electronic devices Characterizes the thin-film transistor using its own model Links the phenomena seen in different device structures and different measurement techniques Presents clearly both how to perform electrical measurements of organic and low-mobility materials and how to extract important information from these measurements Provides a much-needed theoretical foundation for organic electronics
This book summarises the significant progress made in organic thermoelectric materials, focusing on effective routes to minimize thermal conductivity and maximize power factor.
Conducting polymers were discovered in 1970s in Japan. Since this discovery, there has been a steady flow of new ideas, new understanding, new conducing polymer (organics) structures and devices with enhanced performance. Several breakthroughs have been made in the design and fabrication technology of the organic devices. Almost all properties, mechanical, electrical, and optical, are important in organics. This book describes the recent advances in these organic materials and devices.
Organic solids exhibit a wide range of electrical and related properties. They occur as crystals, glasses, polymers and thin films; they may be insulators, semiconductors, conductors or superconductors; and they may show luminescence, nonlinear optical response, and complex dynamical behaviour. The book provides a broad survey of this area, written by international experts, one third being drawn from Eastern Europe. Electrical, optical, spectroscopic and structural aspects are all treated in a way that gives an excellent introduction to current themes in this highly interdisciplinary and practically important area. The coverage is especially strong in the areas where electrical and optical properties overlap, such as photoconductivity, electroluminescence, electroabsorption, electro-optics and photorefraction.
In the near future, organic semiconductors may be used in a variety of products, including flat-screen TVs, e-book readers, and third-generation organic photovoltaics applications, to name just a few. While organic electronics has received increased attention in scientific journals, those working in this burgeoning field require more in-depth cover
This book treats the important issues of interface control in organic devices in a wide range of applications that cover from electronics, displays, and sensors to biorelated devices. This book is composed of three parts: Part 1, Nanoscale interface; Part 2, Molecular electronics; Part 3, Polymer electronics.
Provides first-hand insights into advanced fabrication techniques for solution processable organic electronics materials and devices The field of printable organic electronics has emerged as a technology which plays a major role in materials science research and development. Printable organic electronics soon compete with, and for specific applications can even outpace, conventional semiconductor devices in terms of performance, cost, and versatility. Printing techniques allow for large-scale fabrication of organic electronic components and functional devices for use as wearable electronics, health-care sensors, Internet of Things, monitoring of environment pollution and many others, yet-to-be-conceived applications. The first part of Solution-Processable Components for Organic Electronic Devices covers the synthesis of: soluble conjugated polymers; solution-processable nanoparticles of inorganic semiconductors; high-k nanoparticles by means of controlled radical polymerization; advanced blending techniques yielding novel materials with extraordinary properties. The book also discusses photogeneration of charge carriers in nanostructured bulk heterojunctions and charge carrier transport in multicomponent materials such as composites and nanocomposites as well as photovoltaic devices modelling. The second part of the book is devoted to organic electronic devices, such as field effect transistors, light emitting diodes, photovoltaics, photodiodes and electronic memory devices which can be produced by solution-based methods, including printing and roll-to-roll manufacturing. The book provides in-depth knowledge for experienced researchers and for those entering the field. It comprises 12 chapters focused on: ? novel organic electronics components synthesis and solution-based processing techniques ? advanced analysis of mechanisms governing charge carrier generation and transport in organic semiconductors and devices ? fabrication techniques and characterization methods of organic electronic devices Providing coverage of the state of the art of organic electronics, Solution-Processable Components for Organic Electronic Devices is an excellent book for materials scientists, applied physicists, engineering scientists, and those working in the electronics industry.