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Charge injection and transport in organic semiconductors are key factors controlling the device performance, and have been intensively investigated by conductive atomic force microscope (c-AFM) experiments in the space-charge-limited current (SCLC) regime. The simplified SCLC theory, despite being widely used to describe the unipolar SCLC, has limitations in explaining the current-voltage responses of c-AFM measurements due to two major reasons. First, the conventional planar model does not include the effect of current spreading commonly found beneath the conducting tip. Secondly, the theory only considers drift transport, and assumes that charge diffusion can be neglected, causing discrepancies in its predictions of transport behaviors that will be discussed thoroughly here. The focus of this thesis is on developing numerical models for hole-only devices with the full description of drift and diffusion transport mechanisms, which is called the drift-diffusion (DD-) SCLC model. The applications of the models in the analysis of c-AFM experimental data are presented. We generalize the theory which takes both drift and diffusion currents into account, leading to more realistic DD-SCLC models for several applications. We then develop numerical approaches that efficiently simulate the hole-only SCLCs for one-, two-, and three- dimensional systems. In the case of fully 3-D calculations, the DD-SCLC model is able to treat inhomogeneous systems including spatially varying trap distributions, nanoscale morphologies, and the tip-plane (c-AFM) geometry. In the theoretical studies, the device simulations elucidate a number of crucial factors that affect the charge transport in the SCLC regime, including charge diffusion, traps, as well as, nanoscale morphology. We introduce the methodology of characterizing the current-voltage responses from c-AFM measurements, which has been used in elucidating the experiments on semiconductor poly(3-hexylthiophene) (P3HT) thin films that develop fibrous morphologies after thermal annealing. We generalize the theory which takes both drift and diffusion currents into account, leading to more realistic DD-SCLC models for several applications. We then develop numerical approaches that efficiently simulate the hole-only SCLCs for one-, two-, and three- dimensional systems. In the case of fully 3-D calculations, the DD-SCLC model is able to treat inhomogeneous systems including spatially varying trap distributions, nanoscale morphologies, and the tip-plane (c-AFM) geometry. In the theoretical studies, the device simulations elucidate a number of crucial factors that affect the charge transport in the SCLC regime, including charge diffusion, traps, as well as, nanoscale morphology. We introduce the methodology of characterizing the current-voltage responses from c-AFM measurements, which has been used in elucidating the experiments on semiconductor poly(3-hexylthiophene) (P3HT) thin films that develop fibrous morphologies after thermal annealing.
In the field of organic semiconductors researchers and manufacturers are faced with a wide range of potential molecules. This work presents concepts for simulation-based predictions of material characteristics starting from chemical stuctures. The focus lies on charge transport – be it in microscopic models of amorphous morphologies, lattice models or large-scale device models. An extensive introductory review, which also includes experimental techniques, makes this work interesting for a broad readership. Contents: Organic Semiconductor Devices Experimental Techniques Charge Dynamics at Dierent Scales Computational Methods Energetics and Dispersive Transport Correlated Energetic Landscapes Microscopic, Stochastic and Device Simulations Parametrization of Lattice Models Drift–Diusion with Microscopic Link
Organic semiconductors could have wide-ranging applications in lightweight, efficient electronic circuits. However, several fundamental questions regarding organic electronic device behavior have not yet been fully addressed, including the nature of chemical charge traps, and robust models for injection and transport. Many studies focus on engineering devices through bulk transport measurements, but it is not always possible to infer the microscopic behavior leading to the observed measurements. In this thesis, we present scanning-probe microscope studies of organic semiconductor devices in an effort to connect local properties with local device behavior. First, we study the chemistry of charge trapping in pentacene transistors. Working devices are doped with known pentacene impurities and the extent of charge trap formation is mapped across the transistor channel. Trap-clearing spectroscopy is employed to measure an excitation of the pentacene charge trap species, enabling identification of the degradationrelated chemical trap in pentacene. Second, we examine transport and trapping in peryelene diimide (PDI) transistors. Local mobilities are extracted from surface potential profiles across a transistor channel, and charge injection kinetics are found to be highly sensitive to electrode cleanliness. Trap-clearing spectra generally resemble PDI absorption spectra, but one derivative yields evidence indicating variation in trap-clearing mechanisms for different surface chemistries. Trap formation rates are measured and found to be independent of surface chemistry, contradicting a proposed silanol trapping mechanism. Finally, we develop a variation of scanning Kelvin probe microscopy that enables measurement of electric fields through a position modulation. This method avoids taking a numeric derivative of potential, which can introduce high-frequency noise into the electric field signal. Preliminary data is presented, and the theoretical basis for electric field noise in both methods is examined.
The field of organic electronics has seen a steady growth over the last 15 years. At the same time, our scientific understanding of how to achieve optimum device performance has grown, and this book gives an overview of our present-day knowledge of the physics behind organic semiconductor devices. Based on the very successful first edition, the editors have invited top scientists from the US, Japan, and Europe to include the developments from recent years, covering such fundamental issues as: - growth and characterization of thin films of organic semiconductors, - charge transport and photophysical properties of the materials as well as their electronic structure at interfaces, and - analysis and modeling of devices like organic light-emitting diodes or organic lasers. The result is an overview of the field for both readers with basic knowledge and for an application-oriented audience. It thus bridges the gap between textbook knowledge largely based on crystalline molecular solids and those books focusing more on device applications.
Filling the gap in the literature currently available, this book presents an overview of our knowledge of the physics behind organic semiconductor devices. Contributions from 18 international research groups cover various aspects of this field, ranging from the growth of organic layers and crystals, their electronic properties at interfaces, their photophysics and electrical transport properties to the application of these materials in such different devices as organic field-effect transistors, photovoltaic cells and organic light-emitting diodes. From the contents: * Excitation Dynamics in Organic Semiconductors * Organic Field-Effect Transistors * Spectroscopy of Organic Semiconductors * Interfaces between Organic Semiconductors and Metals * Analysis and Modeling of Devices * Exciton Formation and Energy Transfer in Organic Light Emitting Diodes * Deposition and Characterization
The field of charge conduction in disordered materials is a rapidly evolving area owing to current and potential applications of these materials in various electronic devices This text aims to cover conduction in disordered solids from fundamental physical principles and theories, through practical material development with an emphasis on applications in all areas of electronic materials. International group of contributors Presents basic physical concepts developed in this field in recent years in a uniform manner Brings up-to-date, in a one-stop source, a key evolving area in the field of electronic materials