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Crystal growth and charge carrier transport in liquid crystals and other novel organic semiconductors

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2009, PHD, Kent State University, College of Arts and Sciences / Department of Physics.

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.

Brett Ellman, Dr. (Advisor)
Elizabeth Mann, Dr. (Committee Member)
John Portman, Dr. (Committee Member)
Robert Twieg, Dr. (Committee Member)
Deng-Ke Yang, Dr. (Committee Member)
Bryan Anderson, Dr. (Other)
John R. D. Stalvey, Dr. (Other)
197 p.

Recommended Citations

Citations

  • Pokhrel, C. P. (2009). Crystal growth and charge carrier transport in liquid crystals and other novel organic semiconductors [Doctoral dissertation, Kent State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=kent1254234736

    APA Style (7th edition)

  • Pokhrel, Chandra. Crystal growth and charge carrier transport in liquid crystals and other novel organic semiconductors. 2009. Kent State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=kent1254234736.

    MLA Style (8th edition)

  • Pokhrel, Chandra. "Crystal growth and charge carrier transport in liquid crystals and other novel organic semiconductors." Doctoral dissertation, Kent State University, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=kent1254234736

    Chicago Manual of Style (17th edition)