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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.
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
To aid the design of organic semiconductors, we study the charge transport properties of organic liquid crystals and single crystals. The aim is to find structure-property relationships linking the chemical structure as well as the morphology with the bulk charge carrier mobility of the compounds. To this end, molecular dynamics (MD) simulations are performed yielding realistic equilibrated morphologies. Partial charges and molecular orbitals are calculated using quantum chemical methods. The molecular orbitals are then mapped onto the molecular positions and orientations, which allows calculation of the transfer integrals between nearest neighbors using the molecular orbital overlap method. Thus realistic transfer integral distributions and their autocorrelations are obtained. In case of organic crystals two descriptions of charge transport, namely semi-classical dynamics (SCD) and kinetic Monte Carlo (KMC) based on Marcus rates, are studied. In KMC one assumes that the wave function is localized on one molecule, while in SCD it is spread over a limited number of neighboring molecules. The results are compared amongst each other and, where available, with experimental data.
To aid the design of organic semiconductors, we study the charge transport properties of organic liquid crystals, i.e. hexabenzocoronene and carbazole macrocycle, and single crystals, i.e. rubrene, indolocarbazole and benzothiophene derivatives (BTBT, BBBT). The aim is to find structure-property relationships linking the chemical structure as well as the morphology with the bulk charge carrier mobility of the compounds. To this end, molecular dynamics (MD) simulations are performed yielding realistic equilibrated morphologies. Partial charges and molecular orbitals are calculated based on single molecules in vacuum using quantum chemical methods. The molecular orbitals are then mapped onto the molecular positions and orientations, which allows calculation of the transfer integrals between nearest neighbors using the molecular orbital overlap method. Thus we obtain realistic transfer integral distributions and their autocorrelations. In case of organic crystals the differences between two descriptions of charge transport, namely semi-classical dynamics (SCD) in the small polaron limit and kinetic Monte Carlo (KMC) based on Marcus rates, are studied. The liquid crystals are investigated solely in the hopping limit. To simulate the charge dynamics using KMC, the centers of mass of the molecules are mapped onto lattice sites and the transfer integrals are used to compute the hopping rates. In the small polaron limit, where the electronic wave function is spread over a limited number of neighboring molecules, the Schroedinger equation is solved numerically using a semi-classical approach. The results are compared for the different compounds and methods and, where available, with experimental data. The carbazole macrocycles form columnar structures arranged on a hexagonal lattice with side chains facing inwards, so columns can closely approach each other allowing inter-columnar and thus three-dimensional transport. When taking only intra-columnar transport into account, t.
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.
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.
The first reference of its kind in the rapidly emerging field of computational approachs to materials research, this is a compendium of perspective-providing and topical articles written to inform students and non-specialists of the current status and capabilities of modelling and simulation. From the standpoint of methodology, the development follows a multiscale approach with emphasis on electronic-structure, atomistic, and mesoscale methods, as well as mathematical analysis and rate processes. Basic models are treated across traditional disciplines, not only in the discussion of methods but also in chapters on crystal defects, microstructure, fluids, polymers and soft matter. Written by authors who are actively participating in the current development, this collection of 150 articles has the breadth and depth to be a major contributor toward defining the field of computational materials. In addition, there are 40 commentaries by highly respected researchers, presenting various views that should interest the future generations of the community. Subject Editors: Martin Bazant, MIT; Bruce Boghosian, Tufts University; Richard Catlow, Royal Institution; Long-Qing Chen, Pennsylvania State University; William Curtin, Brown University; Tomas Diaz de la Rubia, Lawrence Livermore National Laboratory; Nicolas Hadjiconstantinou, MIT; Mark F. Horstemeyer, Mississippi State University; Efthimios Kaxiras, Harvard University; L. Mahadevan, Harvard University; Dimitrios Maroudas, University of Massachusetts; Nicola Marzari, MIT; Horia Metiu, University of California Santa Barbara; Gregory C. Rutledge, MIT; David J. Srolovitz, Princeton University; Bernhardt L. Trout, MIT; Dieter Wolf, Argonne National Laboratory.
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.
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.