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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.
Organic semiconductors have attracted significant interest in recent years for applications in low-cost and large area electronics; for example, flexible displays and solid state lighting, photovoltaics, biosensors, disposable electronics, and low cost RFID tags. Their unique properties make them compatible with high throughput roll-to-roll printing and low temperature deposition, thus allowing the utilization of inexpensive and flexible substrates. Although some commercial applications, such as organic light emitting diode displays, already exist; organic semiconductors still need further development. The success of organic semiconductors in commercial applications requires a deeper understanding of the factors limiting or degrading their performance. In particular those creating defects that lead to reduction of mobility or creation of electronic traps. Identifying those traps and linking them to their physical origin is therefore an important step forward in the evolution of organic semiconductors. Modeling electrical characteristics is an interesting technique that can be used to understand how processing parameters or other environmental factors affect material and device performance. However, attention must be paid to assess that the model fully describes measured devices in order to obtain reliable parameters estimations. In this thesis a series of models are described that allow to estimate semiconductor properties, such as mobility and trap density, from electrical measurements of thin film transistors and unipolar diodes. First, the analysis of transfer curves from polymeric transistors is used to understand the effect that regioregularity defects, degree of crystallinity, and angular distribution of crystallites have on the electrical properties of the material. Results indicate that none of them play a significant role on the total concentration of trap states. The model is then extended to study the electrical properties in unipolar diodes, in which current is space-charge limited. This particular geometry requires the model to account for diffusion current, asymmetries in the contacts, and non-homogeneities in the semiconductor; three factors that are typically ignored in the literature. A thorough error analysis allows us to estimate the energy range where the trap distribution can be estimated reliably. Finally, defects are induced in a rubrene single-crystals by means of ultra-violet ozone exposure and X-ray irradiation. The models developed in this work are used to determine how different the energetic and spatial signatures of the induced traps are. Oxygen-related states centered around 0.35 eV and spatially located near the surface of the crystal, are generated after ultra-violet ozone exposure. In addition the mobility in the same region is severely affected. X-ray irradiation, in contrast, generates a much broader distribution of traps, with no preferred energy. Surprisingly, the spatial distribution indicates that, even though X-ray are supposed to be absorbed uniformly through the crystal, the induced defects have a higher concentration near the top and bottom surfaces of the crystal.
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".
The tremendous impact of electronic devices on our lives is the result of continuous improvements of the billions of nanoelectronic components inside integrated circuits (ICs). However, ultra-scaled semiconductor devices require nanometer control of the many parameters essential for their fabrication. Through the years, this created a strong alliance between microscopy techniques and IC manufacturing. This book reviews the latest progress in IC devices, with emphasis on the impact of electrical atomic force microscopy (AFM) techniques for their development. The operation principles of many techniques are introduced, and the associated metrology challenges described. Blending the expertise of industrial specialists and academic researchers, the chapters are dedicated to various AFM methods and their impact on the development of emerging nanoelectronic devices. The goal is to introduce the major electrical AFM methods, following the journey that has seen our lives changed by the advent of ubiquitous nanoelectronics devices, and has extended our capability to sense matter on a scale previously inaccessible.
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.