Download Free Quantitative Modeling Of Charge Transport In Organic Semiconductor Devices Book in PDF and EPUB Free Download. You can read online Quantitative Modeling Of Charge Transport In Organic Semiconductor Devices and write the review.

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 performance of organic semiconductor devices is heavily dependent on the precise molecular-level arrangement and overall morphology of the functional layers. In organic photovoltaic applications, exciton mobility, fission/fusion or dissociation, as well as charge transport and separation are some of the morphology-dependent processes that are of interest for efficient device design. In this work a combination of experimental and computational techniques are used to elucidate the behaviour of excitons in conjugated polymer and small-molecule semiconductor systems. While there is an emphasis on photovoltaic applications, many concepts are generally applicable to other organic electronic applications such organic light emitting diodes and photodetectors. In Chapter 3, a pump-push-probe transient absorption technique is used to observe high-energy "hot" excitons formed by photoexcitation of the conjugated polymer poly(3-hexylthiophene) (P3HT). The work demonstrates the ability to clearly isolate the transient signal of the hot exciton decay processes from the thermalised exciton population, where picosecond and sub-picosecond relaxation of hot excitons through torsional motion in the disordered polymer chain is observed. In addition, the push-induced dissociation of high-energy excitons into free charge carriers is able to be quantified and an upper bound on the exciton binding energy determined. Spectroscopic experiments on P3HT are accompanied by a hybrid quantum-classical exciton hopping model in Chapter 4. Coarse-grained molecular dynamics are used to obtain realistic structures of P3HT free chains and nanofibre aggregates, to which a Frenkel-Holstein exciton model and Monte Carlo hopping simulation is applied. This novel approach captures exciton transport properties of polymer systems with a monomer-level of detail unachievable with continuum or lattice style models, but at a large scale infeasible with fully quantum calculations. Reasonable quantitative agreement with experimental observables is obtained, offering insight into the morphology-dependence of exciton transport in conjugated polymers. In particular, the observed tendency for exciton migration to the core of the polymer aggregate can explain the relatively poor performance of highly crystalline or nanofibre-based polymer solar cells, as well as the unusually high fluorescence yield of aqueous P3HT nanoparticles. The effect of disorder in small molecule semiconductor films is investigated in Chapter 5 in the context of singlet exciton fission and triplet fusion under the influence of applied magnetic fields. A model is presented that extends the historical theory of molecular spin interactions in crystalline materials and corrects the current understanding in the literature regarding such disordered solid-phase systems. The possibility of using the fluorescence response to magnetic fields to probe the morphology and degree disorder in the films is demonstrated. Extending the model to solution-phase behaviour is then discussed in Chapter 6, where the potential of improving the light-harvesting ability of solar cells through a molecular triplet-triplet annihilation upconversion process is considered. Molecular dynamics simulations are used to obtain physical parameters and collision geometry of the emitter molecules in solution. The complications of applying a static model of triplet fusion to the dynamic solution-phase behaviour are elucidated and the potential of synthesising an ideal upconversion emitter molecule is discussed.
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 photovoltaics (OPVs) are a promising carbon-neutral energy conversion technology, with recent improvements pushing power conversion efficiencies over 10%. A major factor limiting OPV performance is inefficiency of charge transport in organic semiconducting materials (OSCs). Due to strong coupling with lattice degrees of freedom, the charges form polarons, localized quasi-particles comprised of charges dressed with phonons. These polarons can be conceptualized as pseudo-atoms with a greater effective mass than a bare charge. Here we propose that due to this increased mass, polarons can be modeled with Langevin molecular dynamics (LMD), a classical approach with a computational cost much lower than most quantum mechanical methods. Here we present LMD simulations of charge transfer between a pair of fullerene molecules, which commonly serve as electron acceptors in OSCs. We find transfer rates consistent with experimental measurements of charge mobility, suggesting that this method may provide quantitative predictions of efficiency when used to simulate materials on the device scale. Our approach also offers information that is not captured in the overall transfer rate or mobility: in the simulation data, we observe exactly when and why intermolecular transfer events occur. In addition, we demonstrate that these simulations can shed light on the properties of polarons in OSCs. In conclusion, much remains to be learned about these quasi-particles, and there are no widely accepted methods for calculating properties such as effective mass and friction. Our model offers a promising approach to exploring mass and friction as well as providing insight into the details of polaron transport in OSCs.
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.
The first advanced textbook to provide a useful introduction in a brief, coherent and comprehensive way, with a focus on the fundamentals. After having read this book, students will be prepared to understand any of the many multi-authored books available in this field that discuss a particular aspect in more detail, and should also benefit from any of the textbooks in photochemistry or spectroscopy that concentrate on a particular mechanism. Based on a successful and well-proven lecture course given by one of the authors for many years, the book is clearly structured into four sections: electronic structure of organic semiconductors, charged and excited states in organic semiconductors, electronic and optical properties of organic semiconductors, and fundamentals of organic semiconductor devices.
This 2-volume set provides the reader with a basic understanding of the foundational concepts pertaining to the design, synthesis, and applications of conjugated organic materials used as organic semiconductors, in areas including organic photovoltaic devices, light-emitting diodes, field-effect transistors, spintronics, actuation, bioelectronics, thermoelectrics, and nonlinear optics.While there are many monographs in these various areas, the emphasis here is both on the fundamental chemistry and physics concepts underlying the field of organic semiconductors and on how these concepts drive a broad range of applications. This makes the volumes ideal introductory textbooks in the subject. They will thus offer great value to both junior and senior scientists working in areas ranging from organic chemistry to condensed matter physics and materials science and engineering.Number of Illustrations and Tables: 168 b/w illus., 242 colour illus., 13 tables.
A review of recent advancements in colloidal nanocrystals and quantum-confined nanostructures, Nanocrystal Quantum Dots is the second edition of Semiconductor and Metal Nanocrystals: Synthesis and Electronic and Optical Properties, originally published in 2003. This new title reflects the book’s altered focus on semiconductor nanocrystals. Gathering contributions from leading researchers, this book contains new chapters on carrier multiplication (generation of multiexcitons by single photons), doping of semiconductor nanocrystals, and applications of nanocrystals in biology. Other updates include: New insights regarding the underlying mechanisms supporting colloidal nanocrystal growth A revised general overview of multiexciton phenomena, including spectral and dynamical signatures of multiexcitons in transient absorption and photoluminescence Analysis of nanocrystal-specific features of multiexciton recombination A review of the status of new field of carrier multiplication Expanded coverage of theory, covering the regime of high-charge densities New results on quantum dots of lead chalcogenides, with a focus studies of carrier multiplication and the latest results regarding Schottky junction solar cells Presents useful examples to illustrate applications of nanocrystals in biological labeling, imaging, and diagnostics The book also includes a review of recent progress made in biological applications of colloidal nanocrystals, as well as a comparative analysis of the advantages and limitations of techniques for preparing biocompatible quantum dots. The authors summarize the latest developments in the synthesis and understanding of magnetically doped semiconductor nanocrystals, and they present a detailed discussion of issues related to the synthesis, magneto-optics, and photoluminescence of doped colloidal nanocrystals as well. A valuable addition to the pantheon of literature in the field of nanoscience, this book presents pioneering research from experts whose work has led to the numerous advances of the past several years.
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 this book we investigate mechanism of charge carrier transport in organic semiconductor thin film devices (OTFDs). Numerical models for the current conduction in single layer OTFDs including both injection and bulk effect for both trap free organics as well as organics with traps exponentially distributed in energy are developed. The dependencies of the current density on the operation voltage, the thickness of the organic layer and the trap properties are numerically studied.