Organic Materials for Photonic Devices
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Organic Materials for Photonic Devices
Thomas Fuhrmann and Josef Salbeck Abstract Current issues in the development of organic materials for photonic applications are reviewed. Organic light-emitting diodes, which are a main focus of industrial research at the moment, are given special emphasis. Other applications in optical communications technology, including organic solid-state lasers, optical switching devices, and data storage, are also covered. Keywords: molecular materials, optical communications, organic light-emitting diodes (OLEDs), photochromics, photonic materials, stimulated emission.
Introduction In today’s communications technologies, organic materials have already found application in liquid-crystal displays (LCDs), xerographic photoreceptors, optical fibers, and data-storage media. The current boom in organic light-emitting diode (OLED) technology and related fields, however, indicates that there is still enormous competitive potential for organic materials in the photonics market. The advantages of “plastic” (i.e., organic) materials are obvious: they are cheap, easily processable, environmentally friendly, and can be specifically tailored to obtain the desired properties. For photonic applications, the optical quality of the materials is of the utmost importance. If an organic material is to be used as an optical film, measures have to be taken to ensure that the morphology of the material does not change. Scattering due to grain boundaries is especially undesirable, so one of the primary demands for organic photonic materials—unless they are used as single crystals or fluids—is the formation of morphologically stable glassy states. The materials under consideration can be divided into two main groups, polymers and low-molecular-weight glasses. For materials with a low molecular weight, it is far more difficult to obtain glasses that are stable far above ambient temperature. On the other hand, they offer some advantages over polymers. Because of their distinct molecular weight, they can be more easily purified, and their properties are not affected by chain length distribution. Both vacuum vapor deposition and solution-based techniques such as 354
spin-coating or ink-jet printing can be used to fabricate devices using these materials. In this review, we focus on new classes of low-molecular-weight materials with high Tg, noting that the discussion of their functional properties also applies to corresponding polymers with the same functional groups. Some recent reviews are available that are specifically dedicated to polymers in the context of light-emitting diodes (LEDs)1,2 and optical waveguides.3
Organic Glasses:The Second Generation A set of common design concepts has been proven to be successful over the last decade in obtaining organic glasses with high Tg. The most important requirements are large and rigid molecules with few intramolecular degrees of freedom, leading to entanglements in the vitreous state and thereby preventing crystallization.4,5 Since crystallization was identified a
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