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Title Crystal growth and charge carrier transport in liquid crystals and other novel organic semiconductors
URL
Publication Date
Degree PhD
Discipline/Department College of Arts and Sciences / Department of Physics
Degree Level doctoral
University/Publisher Kent State University
Abstract 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.
Subjects/Keywords Materials Science; Physics; Laser; Charge generation; Charge transport; Mobility; Trapping; Space charge; Hopping; Tunneling; Lattice vibration; Exciton; Polaron; HUMO; LUMO; Action Spectrum; Quantum efficiency; Crystal Growth; Liquid crystal; Disordered medium
Contributors Ellman, Brett (Advisor)
Language en
Rights unrestricted ; This thesis or dissertation is protected by copyright: all rights reserved. It may not be copied or redistributed beyond the terms of applicable copyright laws.
Country of Publication us
Format application/pdf
Record ID oai:etd.ohiolink.edu:kent1254234736
Repository ohiolink
Date Retrieved
Date Indexed 2021-01-29
Grantor Kent State University

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…diiodonaphthalene crystal by laser ablation………………………….88 Figure 3.9: A current trace as a function of time in 1,4-diiodonapthalene crystal after C60 coated on the surface. Figure 4.1: ……………………………………………90 The molecular structure of the calamitic liquid crystal…

…x28;a terpyridine derivative). ………………………………………………………………94 Figure 4.2: A representative photocurrent transient in calamitic liquid crystal (a terpyridine derivative) plotted on a linear /linear scale. ………………...96 ix Figure 4.3: The…

…3.5……………………………………………………………………….102 Figure 4.6: Electric field dependence of hole mobility in terpyridine liquid crystal. Line is a fit to the data (circle)…...………………………………..........105 Figure 4.7: Photocurrent transient in phenylpyridine…

…based liquid crystal………….106 Figure 4.8: Molecular structure of thiophenyl- bipyridinyl liquid crystal…………..107 Figure 4.9: Photo-generated ion traces at 90 degree celsius (SmF) at various applied voltages…………………………………………………………………110…

…109 Table 4.3: Average ion mobility and viscosity as function of temperature for thiophenyl-bipyridinyl liquid crystal. …………………………….……112 Table 5.1: Mobility values as a function of applied voltage at different temperatures for the experimental…

…distortions to form polarons (defined below). Traps: Through the use of modern techniques of purification, it is often possible to reduce the level of chemical impurities. However, all organic crystals and liquid crystals contain chemical…

trapping/de-trapping continues until the charge carrier reaches to the opposite electrode thereby increasing the total time of flight. An energy level diagram of a typical organic photoconductor with bands is shown in Figure 1.14. Absorption of light of…

…affinity is less than that of the host. An example of this is molecular anthracene dissolved in crystalline tetracene. Although the impurity does not act as carrier trapping center, it does induce lattice deformation. The presence of such impurities…

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