Latest News in Organic and Molecular Electronics for Winter 2005-2006
02.28.06 C&EN Analysis of Printable Electronics Perspectives
Chemical & Engineering News has recently published popularized report on the current market situation and perspectives for the development of organic electronics. The article by A. H. Tullo entitled "Breaking Through" 2006, February 13, page 45 gives brief description of the situation in major companies engaged in the field along with interviewing some of their top executives. In an introduction part, advantages of plastic electronics in potential for lower cost, larger area, flexible, and disposable applications in comparison to conventional silicon-based semiconductors are formulated. According to P. Harrop, chairman of IDTechEx the sales of organic electronics may reach $250 billion by 2025 and overrun the silicon chips market. S. M. Evans, CEO of Plastic Logic, estimates that construction of a large area OLED display manufacturing plant could be order of magnitude cheaper than similar capacity LC display factory. The article gives also some ideas for the classes of organic compounds employed in the field. Thus, S. P. Williams, vice president of Plextronics underlines importance of polythiophenes for various applications including OLED, photovoltaic, RFID (radio frequency identification) tags. Other companies, such as Degussa and Cabot work on implementation of hybrid organic-inorganic materials and nanostructures. Of course, chemical and electronic giants such as Merck KGaA, Bayer, BASF, Xerox, Dow and DuPont are also in hurry to catch the moment and occupy their own positions in this rapidly growing and very promising sector of the chemical market. The article is very useful to get an orientation in general trends and world business situation in the field. Organic Compounds for Blue OLEDs Recently, two companies: UDC and Mitsubishi Chemical announced the creation of efficient and stable phosphorescent blue OLEDs. One may ask, what is so important with the blue OLEDs, why the companies hurry to compete in this area? The answer is, as we also mentioned before, that efficient and stable (possessing long operational lifetime) blue OLED materials are sparse, in contrast to the materials emitting light of two other main colors: green and red. What is wrong with the blue? Due to their intrinsic nature, the fluorescent materials emitting blue light have to possess a wide band gap e.g. big difference in energy between Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO). Therefore, an electron on the HOMO in a basic state of the molecule should receive a good punch of energy to jump on the LUMO and form an exciton. Recombination of this high-energy excited state gives higher energy (shorter wavelength) irradiation shifted to the blue region of visible light spectrum. These high energy interactions are intrinsically damaging to any of organic molecules, therefore the blue emitters tend to degrade rapidly. Of course, the companies would probably never reveal the secrets of their blue OLED materials, however we can get an idea what the structure of a blue organic fluorophore can be from the latest scientific literature. There is significant progress in the development of both small molecule (smOLED) and polymeric (pOLED) blue-emitting devices. Among smOLED materials, the various derivatives of anthracene possessing extended conjugated side chains are very popular. Thus an efficient OLEDs fabricated of 2-tert-butyl-9,10-di-2-naphtylanthracene (TBADN) (1 on the scheme below) doped with anthanthrene derivative (2) have been described in a Chem. Mater. paper 2006, 18, page 603 by B. K. Shah and D. C. Neckers from Bowling Green State University, Ohio. The paper is entitled "Anthanthrene Derivatives as Blue Emitting Materials for Organic Light-Emitting Diode Applications". The authors tested an impressive series of different derivatives of anthanthrene as dopants for (1), and the 2 was found to be one of the most efficient blue emitters. The device fabricated had Commission Internationale de l'Eclairage (CIE) color coordinates of (x,y) = (0.15,0.14). The other article "High-Purity-Blue and High-Efficiency Electroluminescent Devices Based on Anthracene" published in Advanced Functional Materials, 2005, 15, page 1799 by Y.-H. Kim, S.-K. Kwon and colleagues from Korea and USA proposes two compounds: 9,10-bis(1',2'-diphenylstyryl)anthracene (BDSA) (3 on the scheme below) and 9,10-bis(4'-triphenylsilylphenyl)anthracene (BTSA) (4). The authors stress deep-blue flourescence of non-doped smOLEDs fabricated of each compound with (CIE) coordinates of (x, y) = (0.14, 0.10) and (x, y) = (0.15, 0.09) respectively. The efficiency of pure blue fluorescence is attributed to non-coplanarity of the molecules 3 and 4.  The efficiency of industrial applications may depend not only on the performance of materials used but also on cost efficiency of production of the materials themselves. Thus, R. C. Chiechi, F. Wudl and colleagues from University of California Los-Angeles in their article "Efficient Blue-Light-Emitting Electroluminescent Devices with a Robust Fluorophore: 7,8,10-Triphenylfluoranthene" propose a blue emitting compound, a derivative of fluoranthene: TPF (5 on the scheme below). The article is published in Adv. Mater., 2006, 18, page 325. The compound exhibited good luminescent properties in thoroughly optimized smOLED with (CIE) coordinates (x, y) = (0.177, 0.24). The authors mention, that in spite of the device data were not far superior, the synthetic availability, low cost, and high thermal stability of 5 are highly attractive for further development. Interesting development on polymeric blue emitting OLEDs (pOLEDs) was reported by J. Lu, Y. Tao and colleagues from institutes of National Research Council of Canada in J. Mater. Chem., 2006, 16, page 593. The paper is entitled: "High-Efficiency Multilayer Polymeric Blue Light-Emitting Diodes Using Boronate esters as Cross-Linking Linkages". The authors fabricated a series of pOLEDs, where emitting and electron transport layer consists of fluorene-oxadiazole copolymer OxF3 (6) (or its analog). The color efficiency of one of the devices shown on the scheme below, but the main achievement of the research is in the development of a new method for the film formation. A long-standing problem in the fabrication of multilayered pOLEDs is due to use of solution processing, when every next layer is deposited on the previous from the solution (in contrast to vacuum deposition method used for smOLEDs). The solvent from the next layer usually damages the previous layer, and the method may be applied only to "very limited polymer pairs". The authors employed a special compound, a branched boronic acid that forms cross-links between the chains of the hole-transport layer thus making it resistant to damage in further solution processing. This afforded great reproducibility in the device fabrication and impressive device characteristics of a maximum luminance of 1430 cd/m2 at 18.5 V, and maximum luminous efficiency of 0.69 cd/A at 500 cd/m2.  Thiolate Self-Assembled Monolayers (SAMs) for Patterning of Metallic Microcontacts
Recently, new uses of thiolate SAMs for the growth of microcrystals of organic semiconductor and for micropatterning of metallic contacts on the surface of the SAM have been reported. All aspects of the thiolate SAMs chemistry and applications have also been summarized in a very comprehensive Chemical Review paper.
In the year 2006, new results for the use of the thiolate SAMs in the patterning of microcontacts have been published in J. Am. Chem. Soc. 2006, 128, page 392. This time these SAMs were used for the construction of the metallic microcontacts not on the surface of the SAM (see our article before), but under the bottom of the SAM. The scientists R. E. Ducker and G. J. Leggett from University of Sheffield, United Kingdom, developed a new method for the gold microcontacts patterning with an aid of the thiolate SAMs, which they call scanning near-field photolitography (SNP). In this method, the SAMs are used as photoresist materials that either prevent or allow etch in a gold substrate. Thus on the scheme below the dodecanethiolate SAM 1 is oxidized to dodecanesulfonate SAM in the spots subjected to the action of light and oxygen. The sulfonate SAMs are easily washed out with a diluted solution of ammonia to form patterned 'holes', in the SAM. Subsequent treatment of the exposed areas with a special organic compound, mercaptoethylamine 2 dissolves the gold itself to form a micropattern in the metal. The authors stress next advantages of their method: 1. The structures obtained are "sharply defined with good edge definition"; 2. The method is applicable on length scales "from tens of microns to a few nanometers". The micron scale micropatterns were obtained using known dip-pen nanolithography method (DPN, three articles quoted). And for the nanometer scale patterns, the authors employed their SNP method. In both, very good edge definition has been achieved, what the authors attribute to a very mild etch property of the new etch compound - mercaptoethylamine 2. The action of 2 on the gold as a bidentate ligand that coordinates to a gold atom through both thiol and amine moieties of the molecule has been suggested.
Year 2005 'Chemical Reviews' for Organic Electronics and Nanotechnology
During the year 2005 Chemical Reviews has published a number of highly interesting and comprehensive review articles closely related to the field of organic electronics. Issue #4, entitled "Functional Nanostructures" is entirely dedicated to the field. All of its very high quality articles are extremely interesting and valuable. Here, we would like to briefly summarize some of them.
A big and remarkable work of a group of scientists from Eindhoven University of Technology, the Netherlands: F. J. M. Hoeben, P. Jonkheijm, E. W. Meijer, and A. P. H. J. Schenning; entitled "About Supramolecular Assemblies of p-Conjugated Systems", page 1491, summarizes information on importance of supramolecular organization of materials in the field of organic and molecular electronics. This research classifies all kinds of supramolecular organization, self-assembly, and aided-assembly of organic oligomers, polymers, liquid crystals, and multicomponent systems used in electronic and optoelectronic devices. Classification is based upon thorough analysis of different noncovalent forces (such as p-p-interaction, hydrogen bond, hydrophobic effects, different ionic interactions, and others) acting between different types of molecules and components. A very interesting chapter of the review is dedicated to mimicry of the supramolecular organization from the Nature, especially in the field of Solar Energy implication. The authors also introduce a new concept of "supramolecular electronics", where a single electronic device is composed of a single supramolecular element such as nanotube, or nanofiber, or nanocrystal etc. the size of around 100 nm. It is a sort of 'medium-size' devices between one molecule size devices (Angstrom - size) reserved for the molecular electronics and modern organic electronic devices the size of micrometers. The review is also written in a very clear and easy-reading fashion. We would highly recommend it to the professionals as well as to the beginners in organic electronics.
Another highly interesting article by B. D. Gates, C. G. Willson, G. M. Whitesides, and coworkers form Harvard University and The University of Texas at Austin is entitled "New Approaches to Nanofabrication: Molding, Printing, and Other Techniques", page 1171. This article is just priceless for the technologists working in the field of organic electronics. It introduces essentials and comparisons for both 'conventional' fabrication procedures, such as photolithography, scanning beam lithography; and some new and very new processes, such as molding, embossing, printing, scanning probe lithography, edge lithography, self-assembly. The article is extremely useful also for the beginners. A very well organized article "Self-Assembled Monolayers of Thiolates on Metals as a Form of Nanotehnology", page 1103 by J. C. Love, R. G. Nuzzo, G. M. Whitesides and coworkers from Harvard University and University of Illinois - Urbana Champaign summarizes all aspects of chemistry and technology for the thiolate SAMs (for some examples of this kind of SAMs see our article before). While just mentioning (and quoting) different methods of the SAMs formation, such as Langmuir-Blodgett techniques, surfactants, and siloxane SAMs on silica surfaces, the authors focus on the thiolate SAMs on metals in great detail. Preparation, characterization, removing, patterning, application, and opportunities for new applications, a lot of highly useful information.
Other very interesting articles in the #4 are: "Synthesis and Properties of Molecular Rods. 2. Zig-Zag Rods", page 1197, by P. F. H. Schwab, J. R. Smith, and J. Michl from Germany and USA. Describes all aspects of chemistry and potential use in the nanotechnology of zig-zag molecular rods. "Supramolecular Nanotube Architectures Based on Amphiphilic Molecules", page 1401, by T. Shimizu, M. Masuda, and H. Minamikawa from Japan summarizes chemistry, formation, control, and potential application of lipid and polymer (not carbon) nanotubes. For a very recent progress in this area, please, see also a communication article "Narrow Pore-Diameter Polypyrrole Nanotubes" by X. Zhang and S. K. Manohar (USA) published in a recent issue of JACS, 2005, 127, page 14156. The article gives a recipe for the preparation of "highly electroactive polypyrrole nanoparticles". "Artificial Molecular Rotors", page 1281, by G. S. Kottas, J. Michl and coworkers (USA) describes 'topic in fashion' for molecular electronics and mechanics in detail. Another review, "Toward Intelligent Molecular Machines: Directed Motions of Biological and Artificial Molecules and Assemblies", page 1377, is dedicated to a similar molecular mechanisms but predominantly in the field of molecular biology. The article is presented by Japanese scientists: K. Kinbara, and T. Aida. The other article "Nanostructures in Biodiagnostics", page 1547, by N. L. Rosi and C. A. Mirkin (USA), summarizes the use of nanotechnology for biological applications such as molecular diagnostics. Some latest issues of Chemical Reviews also contain interesting papers related to organic electronics. Thus, a review of M. Kertesz, C. H. Choi, and S. Yang from USA and South Korea "Conjugated Polymers and Aromaticity", page 3448, summarizes physical and mathematical aspects of various aromatic systems with a focus on conjugated electroconductive materials. Theoretical calculations of band gaps in correlation with structural features and geometry of the molecules and experimental data allow for better understanding of electronic properties. The information presented may be very useful for the design of new organic electroconductive materials. Very interesting review on "Ionic Liquid Crystals", page 4148, by K. Binnemans from Katholieke Universiteit Leuven, Belgium, introduces details for types, structures, and application of ionic liquid crystals (liquid crystals that contain charged, ionic moiety). Very well organized work useful for both the specialists and the beginners in the field of liquid crystalline materials.
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