Growth of aromatic molecules on solid substrates for applications in organic electronics
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he growth of molecular adlayers on solid substrates is reviewed with a special emphasis on molecules of relevance for organic electronics. In particular, we will consider planar molecules with extended -systems, namely acenes (anthracene, tetracene, pentacene), perylene, coronenes, diindenoperylene, 3,4,9,10-perylene-tetracarboxylicacid-dianhydride, poly-phenylenes, oligothiophenes, and phthalocyanines. Special consideration is given to the importance of the formation of ordered molecular overlayers, which are compared with the structure of the corresponding bulk crystals. Whenever possible, aspects relevant for device fabrication (morphology of deposited films, mobilities of charge carriers) will be addressed.
I. INTRODUCTION
About 50 years after the invention of the Si-based transistor, recent years have seen the first devices where organic materials are used as active components. Optoelectronic devices and—more recently—also field effect transistors have been demonstrated and in a few cases are now available in commercial products. Within the field of organic electronics,1,2 currently two different classes of materials are investigated. Polymers, on the one hand, are long molecules that can be more easily processed and that are generally considered to be better suited for mass production (e.g., “smart” tags). Oligomers, on the other hand, are smaller molecules, the handling of which need more effort but which allow preparation of very well ordered films of high purity. Although it is not yet clear which class of organic materials will dominate future organic electronics, the devices that have so far reached the market are mainly made from oligomers.3,4 In this review, we will focus on these small molecules, because they are better suited for fundamental studies, mainly because many oligomers form well-ordered crystalline structures5 and thus allow for the preparation of high purity, highly ordered thin films by sublimation techniques. These high-quality films then can serve as model systems that will ultimately make it possible to also a)
Address all correspondence to this author. e-mail: [email protected] This author was an editor of this focus issue during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http:// www.mrs.org/publications/jmr/policy.html DOI: 10.1557/JMR.2004.0251 J. Mater. Res., Vol. 19, No. 7, Jul 2004
understand the properties of polymers, in particular the mechanisms governing charge transport. Although the detailed mechanism governing the transport of electrical charges in organic materials is currently not well understood, it has become clear that the structure of the molecular solid through which the charge carriers travel plays an important role. For a number of organic molecules, the electrical conductivity at sufficiently low temperatures can be described by a band-like transport similar to that found in conventional semiconductors. An important difference, however, is the pronounced anisotropy
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