Introduction to Liquid Crystalline Phases
This chapter deals principally with structural aspects of liquid crystalline phases – the necessary asymmetry of the molecular structure, the way in which molecules assemble into layers or columns, and the larger-scale structures visible in the optical mi
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Optical Properties of Light-Emitting Liquid Crystals Mary O’Neill and Stephen M. Kelly
6.1 Introduction Over many years the liquid crystal display (LCD) industry has stimulated an enormous research effort in the development of new liquid crystals, mostly required to be transparent, highly insulating as well as easily reoriented in an electric field. A wide range of chemically, photochemically and electrochemically stable liquid crystals and additives were developed to meet these criteria over the last four decades. However, the continued dominance of LCDs as a flat panel display technology is now being challenged by rapid improvements in Organic LightEmitting Displays (OLEDs), which represent an emissive and, potentially, more efficient display device technology [1]. Unsurprisingly, OLEDs have a different spectrum of material requirements, e.g., some must be highly conjugated for lightemission and their molecular energies must be engineered for electronic injection and transport. The organic materials should not move in electric fields, so that an extremely high viscosity is required for stable OLEDs leading to the formation of glassy liquid crystalline phases. These commercially important developments in OLEDs are occurring concurrently with rapid progress in organic photovoltaics [2–4], and solid-state lasers [5], leading to a renaissance in the study of lightemitting organic materials. Although many of these new materials are not liquid crystalline, the self-assembling properties of liquid crystals can be used to enhance the performance of light-emitting organic devices. This chapter discusses the optical properties of light-emitting liquid crystalline materials and systems using them [6].
M. O’Neill () Department of Physics and Mathematics, University of Hull, Hull HU6 7RX, UK e-mail: [email protected] S.M. Kelly Department of Chemistry, University of Hull, Hull HU6 7RX, UK e-mail: [email protected] R.J. Bushby et al. (eds.), Liquid Crystalline Semiconductors, Springer Series in Materials Science 169, DOI 10.1007/978-90-481-2873-0 6, © Springer ScienceCBusiness Media Dordrecht 2013
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M. O’Neill and S.M. Kelly
In Sect. 6.2 we briefly describe the mechanism of electroluminescence for OLEDs, in order to identify the main parameters which influence performance. In Sects. 6.3 and 6.4, we discuss the luminescence of liquid crystalline semiconductors and the out-coupling of light from thin films with particular reference to the parameters identified in Sect. 6.2. The extended length of the aromatic core of liquid crystalline semiconductors results in a very high birefringence, as outlined in Sect. 6.5, which also discusses possible applications of birefringence for thin film lasers and photonic band-gap structures. Progress in chiral liquid crystal lasers is reviewed in Sect. 6.6 followed by a brief discussion and conclusion in Sect. 6.7.
6.2 OLEDs OLEDs based on conjugated molecular or polymeric thin film devices were first pioneered by groups in Rochester and Cambridge [7, 8]. They are increasing
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