Crystal Growth and Charge Carrier Transport in Liquid Crystals and Other Novel Organic Semiconductors
Crystal Growth and Charge Carrier Transport in Liquid Crystals and Other Novel Organic Semiconductors

Author: Chandra Prasad Pokhrel

Publisher:

Published: 2009

Total Pages: 180

ISBN-13:

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Due to the many advantages of organic semiconductors over their inorganic counterparts, there is a strong and growing interest in their development. However, the large intermolecular spacing and other factors in organics result in a band structure that is narrow and often thermally disrupted, introducing disorder in the system and adversely affecting the conduction of charge. In this dissertation, we concentrate on three factors that influence the motion of charge: disorder, ionic impurities, and molecular design (and, in particular, the presence of pyridine). We discuss charge carrier mobility measurement in different organic semiconductors ranging from relatively ordered liquid crystalline systems to a highly disordered glassy material. Several theoretical approaches are used to analyze the results. For example, in a terpyridine-based high-order smectic liquid crystal we found surprisingly small, Poole-Frenkel mobilities (log(mobility) ̃E1/2) which may naively be described by either the Scher-Montroll (non-Gaussian transport) or Bassler's Gaussian transport model. However, the transient current traces did not comply with the universality and logarithmic slope predictions of the non-Gaussian model, but do follow the predictions of Bassler's model of Gaussian conduction. This various roles of diagonal (site energy) and off-diagonal (transfer integral) disorder are discussed. In the organic glassy material, the energy disorder of the transport sites plays the central role in determining the mobility. Using the spatially correlated disorder model of Kenkre, Dunlap, and coworkers, we are able to extract reasonable materials' parameters such as the Gaussian width of the hopping site energy distribution and the molecular dipole moment. Impurities also play several essential roles in organic semiconductors. Here we concentrate on itinerant ions in liquid crystalline semiconductors. Due to the low viscosity of the liquid crystalline system, mobile ions may influence the effective charge carrier mobility, lowering the device performance and making extraction of the intrinsic mobility difficult. The effect of ions on charge transport, their temporal and spatial distribution, a technique to measure the intrinsic carrier mobility, and the corresponding theory is presented using a sample discotic liquid crystal material (HAT5), a quasi one-dimensional transport medium.

Liquid Crystalline Semiconductors
Liquid Crystalline Semiconductors

Author: Richard J. Bushby

Publisher: Springer Science & Business Media

Published: 2012-11-28

Total Pages: 282

ISBN-13: 9048128730

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This is an exciting stage in the development of organic electronics. It is no longer an area of purely academic interest as increasingly real applications are being developed, some of which are beginning to come on-stream. Areas that have already been commercially developed or which are under intensive development include organic light emitting diodes (for flat panel displays and solid state lighting), organic photovoltaic cells, organic thin film transistors (for smart tags and flat panel displays) and sensors. Within the family of organic electronic materials, liquid crystals are relative newcomers. The first electronically conducting liquid crystals were reported in 1988 but already a substantial literature has developed. The advantage of liquid crystalline semiconductors is that they have the easy processability of amorphous and polymeric semiconductors but they usually have higher charge carrier mobilities. Their mobilities do not reach the levels seen in crystalline organics but they circumvent all of the difficult issues of controlling crystal growth and morphology. Liquid crystals self-organise, they can be aligned by fields and surface forces and, because of their fluid nature, defects in liquid crystal structures readily self-heal. With these matters in mind this is an opportune moment to bring together a volume on the subject of ‘Liquid Crystalline Semiconductors’. The field is already too large to cover in a comprehensive manner so the aim has been to bring together contributions from leading researchers which cover the main areas of the chemistry (synthesis and structure/function relationships), physics (charge transport mechanisms and optical properties) and potential applications in photovoltaics, organic light emitting diodes (OLEDs) and organic field-effect transistors (OFETs). This book will provide a useful introduction to the field for those in both industry and academia and it is hoped that it will help to stimulate future developments.

Self-Organized Organic Semiconductors
Self-Organized Organic Semiconductors

Author: Quan Li

Publisher: John Wiley & Sons

Published: 2011-03-03

Total Pages: 376

ISBN-13: 1118009045

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This book focuses on the exciting topic on self-organized organic semiconductors – from materials to device applications. It offers up-to-date and accessible coverage of self-organized semiconductors for organic chemistry, polymer science, liquid crystals, materials science, material engineering, electrical engineering, chemical engineering, optics, optic-electronics, nanotechnology and semiconductors. Chapters cover chemistry, physics, processing, and characterization. The applications include photovoltaics, light-emitting diodes (LEDs), and transistors.

Liquid Crystals in Photovoltaics
Liquid Crystals in Photovoltaics

Author: Dr. Luz J Martinez-Miranda

Publisher: CRC Press

Published: 2021-06-30

Total Pages: 99

ISBN-13: 1351175769

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This book explores why the properties of liquid crystals make them ideal for use in photovoltaic applications. It achieves this by presenting a description of the properties of liquid crystals and how their electronic properties compare to that of polymers used in organic photovoltaics. It explores how the type of liquid crystal chosen can help in improving the efficiency of the photovoltaics. It compares experimental and theoretical ways in which the efficiency is directly or indirectly estimated between the organic photovoltaics and the organic photovoltaics that contain a liquid crystal. It first introduces liquid crystals and their different varieties, before reviewing their electronic transfer properties and how they can improve efficiency. It is an ideal text for graduate students and young researches considering entering the area of photovoltaics - specifically, organic photovoltaics – who do not yet have knowledge of this field. Introduces the field of liquid crystals and provides basic information to those new to the field, in a concise and visual manner Describes which characteristics of a liquid crystal are most advantageous to use in photovoltaics Provides basic knowledge of photovoltaics for those who do not have previous knowledge of how they behave electronically

Crystal Growth
Crystal Growth

Author: Brian R. Pamplin

Publisher: Elsevier

Published: 2013-09-11

Total Pages: 630

ISBN-13: 1483161463

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Crystal Growth, Second Edition deals with crystal growth methods and the relationships between them. The chemical physics of crystal growth is discussed, along with solid growth techniques such as annealing, sintering, and hot pressing; melt growth techniques such as normal freezing, cooled seed method, crystal pulling, and zone melting; solution growth methods; and vapor phase growth. This book is comprised of 15 chapters and opens with a bibliography of books and source material, highlighted by a classification of crystal growth techniques. The following chapters focus on the molecular state of a crystal when in equilibrium with respect to growth or dissolution; the fundamentals of classical and modern hydrodynamics as applied to crystal growth processes; creation, control, and measurement of the environment in which a crystal with desired properties can grow; and growth processes where transport occurs through the vapor phase. The reader is also introduced to crystal growth with molecular beam epitaxy; crystal pulling as a crystal growth method; and zone refining and its applications. This monograph will be of interest to physicists and crystallographers.

Chemistry of Discotic Liquid Crystals
Chemistry of Discotic Liquid Crystals

Author: Sandeep Kumar

Publisher: CRC Press

Published: 2016-04-19

Total Pages: 512

ISBN-13: 1439811458

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The self-contained properties of discotic liquid crystals (DLCs) render them powerful functional materials for many semiconducting device applications and models for energy and charge migration in self-organized dynamic functional soft materials. The past three decades have seen tremendous interest in this area, fueled primarily by the possibility

Measurement of Ionic Concentration of Nematic Liquid Crystals and Single Crystal Growth of Organic Semiconductors
Measurement of Ionic Concentration of Nematic Liquid Crystals and Single Crystal Growth of Organic Semiconductors

Author: Salma Begum (Physicist)

Publisher:

Published: 2018

Total Pages: 188

ISBN-13:

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Nematic liquid crystals (NLCs) may play an important role in applications of organic semiconductor devices (OFETs, OLEDs, etc.). However, ionic conduction can limit the success of most applications. To utilize NLCs as semiconductors, one must control the ion concentrations. There are many different methods to purify NLCs. However, it is important to know which method is more efficient when compared to others. We work on different purification methods and compare the results by analyzing the ionic transport behavior both numerically and experimentally. Organic semiconductors (OSCs) are composed of (largely) Van der Waals bonded molecules that lead to considerably different properties than their inorganic counterparts. The low melting point and high vapor pressures of OSCs need special care in crystal growth. To study the intrinsic properties of OSCs, polycrystalline or amorphous thin-film devices are not suitable because of high defect densities and grain boundaries. On the other hand, single crystals contain a small number of defects/boundaries and impurities. Therefore, to explore the physical properties of OSCs, single crystals are often a better choice. We have characterized practical growth conditions for several interesting OSCs.

Nematic Liquid Crystals for Nano-structured Organic Photovoltaic
Nematic Liquid Crystals for Nano-structured Organic Photovoltaic

Author: Manea S. Alkhalifah

Publisher:

Published: 2010

Total Pages:

ISBN-13:

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A range of novel liquid crystals and amorphous organic conjugated semiconductors were synthesised by the chemistry branch of the Organophotonics group at Hull University. These compounds are studied electrochemically and optically to investigate the suitability of these materials as good donor/acceptor with suitable pairs energy levels for use in organic photovoltaic devices. Liquid crystal compounds with a fluorene-thiophene structure were identified as potential electron donors in combination with perylene based compounds as electron acceptors. Time-of-flight was used to study the charge transport of organic semiconductors in this thesis. The nature of the functional groups of the molecules was found to have a significant influence on the charge carrier mobility. The incorporation of a reactive end group with spacer affected the charge carrier mobility of electron donors negatively, suggesting that the mobility depends on the intermolecular separation. Based on the need to correlate the charge transport of the donor/acceptor blends with photovoltaic devices, the electron and hole mobility were studied for blends. For all blends the hole mobility is lower than that of the pure electron donor. The electron mobility of the blends is much higher than that of the pure electron acceptor. The thermal activation of charge transport in the liquid crystals is investigated by applying the Gill model. The result shows that charge transport in the liquid crystals is thermally activated and the activation energy is field dependent. The Gaussian disorder model and correlated disorder model were used to analyze the mobility data of four liquid crystals compounds with the same conjugated core and different end groups. We show that the thin film nanoscale morphology and the phase separation of the donors/acceptors blends depend on the chemical structures of donors and acceptors, the casting solvents and the annealing temperature of the film. The functional groups of the perylene bisimide are found to influence the roughness. The surface roughness of the blended thin film is minimum and its phase separation finest when the electron donors component has short terminal aliphatic groups rather than long polymerisable chains. Chlorobenzene shows the best performance as coasting solvent. The annealing temperature is significant in controlling the nanoscale morphology and the phase separation of an intermixed network of the blends. We successfully demonstrate photovoltaic performance using blends of our novel donors and acceptors. The annealing temperature is very important to optimise the solar cell performance by optimisation of the phase separation. The perylene based liquid crystals have disappointing performance as electron acceptors. The donor with the shortest terminal end group gives the best result. The device performance fully correlates with the blend nanoscale morphology of the blends; the blend with the smallest domains gives best power conversion efficiency; the best device has a value of 1.1%.

Handbook of Crystal Growth
Handbook of Crystal Growth

Author: Peter Rudolph

Publisher: Elsevier

Published: 2014-11-04

Total Pages: 1420

ISBN-13: 0444633065

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Vol 2A: Basic TechnologiesHandbook of Crystal Growth, Second Edition Volume IIA (Basic Technologies) presents basic growth technologies and modern crystal cutting methods. Particularly, the methodical fundamentals and development of technology in the field of bulk crystallization on both industrial and research scales are explored. After an introductory chapter on the formation of minerals, ruling historically the basic crystal formation parameters, advanced basic technologies from melt, solution, and vapour being applied for research and production of the today most important materials, like silicon, semiconductor compounds and oxides are presented in detail. The interdisciplinary and general importance of crystal growth for human live are illustrated.Vol 2B: Growth Mechanisms and DynamicsHandbook of Crystal Growth, Second Edition Volume IIB (Growth Mechanisms and Dynamics) deals with characteristic mechanisms and dynamics accompanying each bulk crystal growth method discussed in Volume IIA. Before the atoms or molecules pass over from a position in the fluid medium (gas, melt or solution) to their place in the crystalline face they must be transported in the fluid over macroscopic distances by diffusion, buoyancy-driven convection, surface-tension-driven convection, and forced convection (rotation, acceleration, vibration, magnetic mixing). Further, the heat of fusion and the part carried by the species on their way to the crystal by conductive and convective transport must be dissipated in the solid phase by well-organized thermal conduction and radiation to maintain a stable propagating interface. Additionally, segregation and capillary phenomena play a decisional role for chemical composition and crystal shaping, respectively. Today, the increase of high-quality crystal yield, its size enlargement and reproducibility are imperative conditions to match the strong economy. Volume 2A Presents the status and future of Czochralski and float zone growth of dislocation-free silicon Examines directional solidification of silicon ingots for photovoltaics, vertical gradient freeze of GaAs, CdTe for HF electronics and IR imaging as well as antiferromagnetic compounds and super alloys for turbine blades Focuses on growth of dielectric and conducting oxide crystals for lasers and non-linear optics Topics on hydrothermal, flux and vapour phase growth of III-nitrides, silicon carbide and diamond are explored Volume 2B Explores capillarity control of the crystal shape at the growth from the melt Highlights modeling of heat and mass transport dynamics Discusses control of convective melt processes by magnetic fields and vibration measures Includes imperative information on the segregation phenomenon and validation of compositional homogeneity Examines crystal defect generation mechanisms and their controllability Illustrates proper automation modes for ensuring constant crystal growth process Exhibits fundamentals of solution growth, gel growth of protein crystals, growth of superconductor materials and mass crystallization for food and pharmaceutical industries

Organic Optoelectronic Materials
Organic Optoelectronic Materials

Author: Yongfang Li

Publisher: Springer

Published: 2015-05-30

Total Pages: 402

ISBN-13: 3319168622

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This volume reviews the latest trends in organic optoelectronic materials. Each comprehensive chapter allows graduate students and newcomers to the field to grasp the basics, whilst also ensuring that they have the most up-to-date overview of the latest research. Topics include: organic conductors and semiconductors; conducting polymers and conjugated polymer semiconductors, as well as their applications in organic field-effect-transistors; organic light-emitting diodes; and organic photovoltaics and transparent conducting electrodes. The molecular structures, synthesis methods, physicochemical and optoelectronic properties of the organic optoelectronic materials are also introduced and described in detail. The authors also elucidate the structures and working mechanisms of organic optoelectronic devices and outline fundamental scientific problems and future research directions. This volume is invaluable to all those interested in organic optoelectronic materials.