Transport materials, small molecules

Carrier injection and transport in OLEDs has been treated in detail by, among others, Kalinowski.113 Most of the organic electroluminescent materials, small molecules and conjugated polymers are low-conductance materials. The h+ mobility in these materials is typically 10 7—10 3 cm2/(Vs), and the e mobility is typically lower by a factor of 10-100. However, it is now clear that the low mobility is due to the disorder in the amorphous or polycrystalline materials. Indeed, in high-quality single crystals of pentacene, the h+ and e mobility are 2.7 and... 

The unique molecular packing of rod-like chains in liquid crystalline polymers (LCP) closely resembles the extended chain structure of highly oriented flexible chain polymers, suggesting that these materials are good candidates for barrier applications. Thermotropic LCP s, first developed in the early 1970 s, have been the object of much interest because of their excellent mechanical properties and ease of product fabrication. Preliminary observations have shown that a commercially available wholly aromatic thermotropic copolyester has gas permeability coefficients that are lower than those of polyacrylonitrile (4.). These results raise some fundamental questions as to the nature of the mechanism for transport of small molecules through a matrix of ordered rigid rod-like chains. 

The remainder of this overview chapter provides fimdamental background information related to transport of small molecules in polymers and then describes materials design strategies to prepare polymers with excellent permeability and selectivity properties for both supercriticd gas separations and vapor separations. In addition to high permeability and selectivity, membranes must also be stable in industrial process environments, which may be chemically and/or thermally challenging. For example, due to chemical stability and thermal transition temperatures of polymers used in gas separations, these materials are typically used at or near ambient temperatures. The chapter by Bayer et al. in this book describes the use of selective crosslinking of polyimides to prepare high performance membrane materials that are stable to 300°C. 

Another important class of electron-transporting emitter is the distyryl-arylenes. These have been explored most extensively by workers at Idemitsu Kosan [31, 32], with the bulk of the published data focusing on a compound designated as DPVBi (see Table 13-2). This class of materials may be considered as small molecule analogs of the PPV polymers, with distyrylbenzene and its derivatives ]33] as prototypical examples. Because of the short conjugation length they tend to be blue emitters. 

Table 13-2. Chemical structures of representative small molecule transport materials ami luminescent dyes.

Trilayer structures offer the additional possibility of selecting the emissive material, independent of its transport properties. In the case of small molecules, the emitter is typically added as a dopant in either the HTL or the ETL, near the interface between them, and preferably on the side where recombination occurs (see Fig. 13-1 c). The dopant is selected to have an cxciton energy less than that of its host, and a high luminescent yield. Its concentration is optimized to ensure exciton capture, while minimizing concentration quenching. As before, the details of recombination and emission depend on the energetics of all the materials. The dopant may act as an electron or hole trap, or both, in its host. Titus, for example, an electron trap in the ETL will capture and hold an election until a hole is injected nearby from the HTL. In this case, the dopant is the recombination mmo.-... 

Returning to the discussion of the movement of small molecules in plastic materials, the kinetics and mechanisms of gas and vapour transport have been described in several review 31 33) and need not to be repeated here. 

In order for a material to act as a useful bulk charge-transport medium, one molecule (be it a polymer or a small molecule in a crystal) must be able to transfer... 

The structural hypothesis, which was formulated in response to observations that axonal transport rate components move as discrete waves, each with a characteristic rate and a distinctive composition, can explain the coherent transport of functionally related proteins and is consistent with the relatively small numbers of motor molecules in neurons. The only assumption is that the number of elements that can interact with transport motor complexes is limited, and this requires appropriate packaging of the transported material. Different rate components result from packaging of transported material into different, cytologically identifiable, structures. In fact, the faster rates reflect the transport of proteins preassembled as membranous organelles, including vesicles and... 

These authors were the first FGSE workers to make extensive use of the concept of free volume 42,44) and its effect on transport in polymer systems. That theory asserts that amorphous materials (liquids, polymers) above their glass transition temperature T contain unoccupied volume randomly distributed and in parcels of sufficient size to permit jumps of small molecules — and of polymer jumping segments — to take place. Since liquids have a fractional free volume fdil typically greater than that, f, of polymers, the diffusion rate both of diluent molecules and (uncrosslinked and unentangled) polymer molecules should increase with increasing diluent volume fraction vdi,. The Fujita-Doolittle expression 43) describes this effect quantitatively for the diluent diffusion ... 

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