Science and Technology at the Nanometer Scale Using Vacuum-Deposited Organic Thin Films
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Science and
Technology at the Nanometer Scale Using VacuumDeposited Organic Thin Films Stephen Forrest
The following article is an edited transcript of the talk given by MRS Medalist Stephen Forrest at the MRS Fall Meeting on November 29, 1999. Forrest received the MRS Medal for his “pioneering contributions to the growth and optoelectronic applications of organic semiconductor thin films.”
Organic Thin Films as Optoelectronic Materials Organic thin films have been studied for their properties as active optoelectronic materials for at least 50 years. Yet, until recently, they have failed to make a significant impact in the commercial world. However, display products based on organic light emission are now available commercially, indicating that a major change in the acceptance of organic optoelectronics is in the offing. I would like to discuss some of the reasons for these developments from an engineering perspective. First, let us consider some of the advantages and disadvantages of organic compounds. In terms of advantages, it is clear that they provide an enormous variety of structures and properties. More than two million organic compounds are known today, and many are commercially available. Organic materials can be deposited in an ordered structure without necessarily being lattice-matched to the substrate structure. They are relatively inexpensive, although labor, not materials, is usually
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the cost-limiting factor in producing devices. Finally, some organic compounds possess very high luminescent efficiencies. Among the disadvantages of organic materials are their low mobilities, which are about 1 cm2/Vs at room temperature for the best-ordered organic thin films, compared with somewhere between 500 cm2/Vs and 106 cm2/Vs for semiconductors (depending on the test conditions and structures used). This suggests that the conductivities of organic materials will be low, the speed of the devices will be slower, and so on. We can make these films very thin, however, and by that means obtain very high device operating bandwidths. Organic materials for thin films are very soft, so they are easily destroyed on contact and must be treated carefully. It is also difficult to control and to measure their purity because measurement techniques are generally not sensitive to better than 0.1%. Molecular organics cannot be conveniently doped to make them n- or p-type: they intrinsically have either a higher electron or hole mobility, but generally we cannot use extrinsic dopants and expect them to have the same effect as in, for example, silicon, where a vacancy or a dopant provides an extra charge carrier. Organic materials are also sensitive to water and oxygen; many are unstable, and they are difficult to pattern into devices.
Methods of Deposition Numerous methods can be used to deposit organic thin films: spin-casting or spray-on methods, vacuum and other types of vapor deposition, and self-assembly. This article will focus on vacuum deposition, which is particularly amenable to smallmolecule materials. “Small
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