Latest News in Organic and Molecular Electronics for Summer 2005
Photoluminescence and electroluminescence: recent advances There are many p-type organic semiconductors that are used in OLED technology. However, n-type ones are sparse and urgently needed for 2-layered and 3-layered OLEDs. A group of scientists from Wuhan University and Institute of Chemistry, Chinese Academy of Science developed polyquinoline (PQ)-based series of n-type conductive electroluminescent polymers (Macromolecules, 2005, 38, page 6915). The best electroluminescent performance was determined for the structure 1 (see below) with a maximum of external quantum efficiency of 0.63%, maximum photometric efficiency 1.85 cd/A at brightness of 140 cd/m2, highest maximum luminance of 1768 cd/m2. The authors state these values to be the best reported up to date for PQs. A popular article in Chemical and Engineering News, 2005, August 8, page 17 entitled "Blue Light Special" describes serious achievement of a group of scientists from Bowling Green State University, Ohio. The original scientific article is published in Chemical Communications, 2005, page 3776. The authors, R. R. Islangulov, D. V. Kozlov, and F. N. Castellano found an efficient luminescent system for low power upconversion of green light (longer wavelength, lower energy) to blue light (shorter wavelength, higher energy). The effect is so pronounced that may be easily visualized by simple passing of green light beam from a commercial laser pointer through a solution of two compounds. All secret of extraordinary efficiency is in these two compounds. One of them is ruthenium complex [Ru(dmb)3]2+ that absorbs the laser light and transforms into an excited state (exciton). This ruthenium exciton transfers its energy to a molecule of 9,10-diphenylanthracene (DPA) (2) to form a triplet exciton of the DPA. Then, two triplets of DPA meet together and recombine to form one neutral molecule of DPA and one singlet exciton of DPA. Finally, this singlet emits higher energy photon and transforms to a neutral molecule of DPA too. This type of luminescence is known as anti-Stokes, when light of lower energy transforms to light of higher energy in contrast to far more usual, Stokes luminescence that transforms light of higher energy to light of lower energy. The choice of DPA (2) was not incidental: DPA possesses two 'protecting' phenyl rings in the positions 9 and 10 of the antracene core. That prevents unwanted chemical reactions of DPA excitons that usually happen with 'unprotected' anthracene and give rise to the luminescence decay. The higher concentration of the compound - the more unwanted reactions, the faster the decay is. However, the OLED technology requires very high concentration of the luminescent materials in a solid state. Therefore, extending of the discovery to the solid systems is the author's priority #1 goal.
Advances for thiophene-based conducting materials Polymers on the base of 3,4-ethylenedioxythiophene (EDOT) (1) firmly occupied the top position in use in conducting materials due to their high environmental and thermal stability, high conductivity and low band gap (difference in energy between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)). M. Turbiez, P. Frere, J. Roncali and coworkers from Universite d'Angers, France, investigated properties of polymers synthesized from sulfur-analog of EDOT (EDST) (2) and also mixed EDOT-EDST blended monomer (3) (Macromolecules, 2005, 38, page 6806). The authors discovered the band gap to be smaller for the 'mixed' polymer than that for two separate homopolymers.
Interesting results have been reported by S. S. Balamurugan, R. L. McCarley and coworkers from Louisiana State University in Angew. Chem. Int. Ed., 2005, 44, page 4872. The authors synthesized water-soluble conductive polymer (4), that possesses brash-like structure (A) at room temperature. Soluble form (A) reversibly transforms into insoluble globule-form (B) at 28-35 oC due-to thermal destruction-formation of hydrogen bonds. Prospective use of the discovery in bioelectronics and biosensors is suggested.
Many efforts have been made to improve charge carrier mobility of small molecule semiconductors used in thin film transistors (OTFTs) by means of increase of so-called p-p-stacking of p-conjugated molecules (see also articles below). In order to increase p-p-stacking in crystalline structure of thiophene oligomers, X. Zhang, A. P. Cote, and A. J. Matzger from University of Michigan synthesized completely fused heptathienoacene (J. Am. Chem. Soc., 2005, 127, page 10502, followed up by article in C&EN 2005, July 25, p.44). X-Ray-diffraction data of new compound suggest strong p-stacked packing of the molecules in the crystalline structure. The authors promised also the results of test of new thienoacenes for the TFT-performance coming soon.
08.15.05 Events in molecular electronics Two recent events in the field of molecular electronics attracted our attention. An entire issue #7 of Accounts of Chemical Research, 2005, 38, is dedicated to investigations of physical properties of a single molecule. It is well known, that physical behavior of a single, separate molecule may be quite different to the behavior of the compound to what this molecule belongs. That means, the techniques of measurement of dynamic, electrical, and spectroscopic properties of single molecules are different to those of chemical substances. Modern techniques for the measurement of a single molecule physical behavior are summarized in 13 review papers. Among them: "Single Molecule Spectroscopy" by P. F. Barbara; "Single-Molecule Optoelectronics" by T. H. Lee, J. I. Gonzalez, J. Zheng, and R. M. Dickson; "Fluorescence Imaging with One Nanometer Accuracy: Application to Molecular Motors" by A. Yildiz and P.R. Selvin. The issue is definitely a highly valuable and rich information resource in the field. Interesting article "Inversion of the Rectifying Effect in Diblock Molecular Diodes by Protonation" is published in J. Am. Chem. Soc., 2005, 127, page 10456 by G. M. Morales, L. Yu, and coworkers from The University of Chicago. They have described a molecular diode (shown in the center of the scheme below) that not only effectively rectifies the current but is also capable to change rectifying direction by means of protonation with a strong acid! By comparison with previously synthesized objects, the authors also stress improve of the rectifying effect through increase of dipole moment of the molecules.
Molecular self-assembly to enhance organic thin film transistors (OTFTs) performance
One of the main obstacles for the wide introduction of organic thin film transistors (OTFTs) in practice is relatively low charge carrier mobility of organic semiconductors in comparison to traditional silicone-based ones. One of the ways to improve the charge carrier mobility is to increase so called p-p-stacking of organic molecules to each other in their crystalline, or other packing structure. Two papers in the current issue of J. Am. Chem. Soc., 2005, 127 provide prospective ways to increase p-p-stacking through (1) special design of the semiconductor molecule, and (2) special technique of growing the crystals or casting the material. Thus, the paper "Structure of Zone-Cast HBC-C12H25 Films", page 11288 by an international team of scientist from Denmark, Switzerland, Germany, and Poland describes special technique called 'zone casting' for the grows of highly ordered films of Hexa-pery-benzocoronene (HBC) 1. The film was found to be of "extraordinary high crystalline order" with "well defined biaxial orientation" of the aromatic cores along with ordered disposition of the aliphatic long chains. Other paper "Nanobelt Self-Assembly from an Organic n-Type semiconductor: Propoxyethyl-PTCDI", page, 10496 by K. Balakrishnan, L. Zang, and coworkers from Southern Illinois University and University of Illinois at Urbana-Champaign reports formation of semiconducting "nanobelts". In this case the authors have chosen derivative of perylene tetracarboxylic diimide (2) that actually belongs to a sparse n-type group of organic semiconductors. The authors observed formation of nanocrystal "belts" of the compound as a result of self-assembling process during injection of the concentrated chloroform solution of the compound in methanol. Measurements of the material characteristics suggest "uniaxial crystal structure" with strong p-p-stacking of the molecules. Fabricating of TFT devices from the new material is currently underway.
Enhancing OFETs and OLEDs Performance A current issue #29 of J. Am. Chem. Soc., 2005, 127 is rich in articles describing new ways for the significant improvement of performance of OTFTs and OLEDs. Thus, a communication paper "High-Performance Organic Field-Effect Transistors Based on p-Extended Tetrathiafulvalene Derivatives", page 10142, by a group of scientists from Tokyo Institute of Technology led by Naraso and Y. Yamashita reports radical enhance of such important parameters of organic field-effect transistors (OFETs) as charge carrier mobility, on/off ratio, and stability. This is achieved through the modification of the structure of a p-transporter tetrathiafulvalene (TTF). The group has improved several parameters of the fabricated OFETs at once by introduction of fused aromatic rings in the TTF's skeleton. Fused aromatic rings reduce electron-donating properties of the TTF molecule thus improving its resistance against oxidation by atmospheric oxygen. At the same time, (amazing) it improves the carrier mobility through increase of 'intermolecular p-p interactions'. For example, the mobilities of OTFTs fabricated from of dibenzoTTF (1) and dinaphtoTTF (2) were 0.06 and 0.38 cm2/V·s correspondingly that are good values for OTFTs.
Q. Huang and T. J. Marks from Northwestern University, IL, and coworkers improved important parameters of OLEDs using different approach (page 10227). Instead of changing of p-transporter structure of conjugated bonds as in the previous case, the authors changed mechanism of hole injection through covalent binding of the p-transporter to an anode. Thus, traditional hole transporter NPB was replaced for modified one: NPB-Si2. The latter contains silicon-based side chains that bind to inorganic (Indium Tin Oxide, ITO) anode through chemical reaction to form covalent bonds with the anode atoms. This bonding has been shown to radically improve such device parameters as luminance and external quantum efficiency compared to 'conventional' NPB-based devices.
The same group of scientists reports use of similar silicon linkage agents to blend with commercial polymers: polyvinylphenol and polystyrene that affords 'crosslinked polymer blend dielectrics' (CPBs) for gate dielectric insulators in OTFTs (page 10388). Organic gate insulator materials are of great advantage to inorganic ones, because ultrathin films of inorganic oxides possess low flexibility and allow significant current 'leakage'. The authors fabricated series of devices based on different organic semiconductors with CPB-gate dielectric that demonstrated impressive OTFT's characteristics such as carrier mobilities (up to 0.1-0.3 cm2/V·s), Ion/Ioff ratio (up to 104-105) and low threshold voltages. The ability of OTFTs to operate at low voltages is especially important due to low thermal conductivity of organic semicondutors. Operating at high voltages makes them easily 'burning out'. Among other 'wonders' of the material, the authors state "robust, smooth, adherent, pinholefree, high-capacitance, low leakage, ultrathin (10-20 nm)". A previous comparable achievement in this area published in Nature, 2005, 434, 194 by a Cambridge group is also quoted.
Another JACS paper by a group of scientists from MESA+ Institute for Nanotechnology, The Netherlands, reports a convenient method for creating of flat stamps for microcontact printing (page 10344). The method is based on use of perfluorinated silane as a 'barrier' to improve of ink transfer control.
07.09.05 Reversible Hologram in Solution of Polymer A discovery relevant to recent advances in holographic image storage is reported in current issue of J. Am. Chem. Soc., 2005, 127, p. 9679. Scientists from F.O.R.T.H. Institute of Electronic Structure and Laser in Greece succeeded to create a reversible, grate-like image in a solution of polymer (polyisoprene) in hexane under the action of two overlapping beams of a red laser. The scientists report increase of concentration of the polymer in the grate 'planes' to achieve relatively rigid and stable configuration. They stress that the phenomenon is not caused only by irradiation but much more complicated and is not completely understood yet. As a basis for the discovery, the authors cite a paper of R. Sigel and coworkers who first observed the phenomena of organization of polymers in solution under the action of light. The paper is published in Science, 2002, 297, p.66.
C&EN's Analysis of Flat Display Market Trends A cover story "Thinning Flat Panels" published in C&EN 2005, June 27, p.20 analyses pros an cons of current development in the flat display market. A competition between inorganic plasma and organic liquid crystalline displays LCDs that share the market almost equally is mentioned. A special emphasize is given to a high cost of both types of flat panels that makes them still not quite competitive toward traditional cathode-ray tube sets. A none-competitive yet just emerging market of flexible displays based on OLED is also mentioned. The same issue of the journal provides more information about 3-D data storage (see our article above). An article "Data Storage in 3-D" stresses the success of InPhase Technologies, but mentions that some other companies have also been intensively working in that direction. A very complex problem is to make the materials rewritable that some companies have been trying to solve for a long time. InPhase succeeded because they initially 'cut off' the problem of reversibility. After accomplishing of one-time-use material, they are working on the material reversibility as a separate task. Other company with certain success in the holographic data storage is Aprilis, Inc.. For continuation of the story, please, go here.
06.27.05 ABIresearch: Printed and Organic Electronics
ABIresearch has recently released in-depth
report
on development and global marketing of organic electronics. This market research contains significant quantity of valuable information. The brochure can be purchased from the site (link above). As alternative, we can provide similar and highly focused information in the field for about an order of magnitude lower cost, please, visit our consulting page.
06.25.05 Molecular Logic Gate An article in C&EN 2005, June 20, p. 15 "A Miniature Logic Gate in Nanospace" with a quote to a J. Am. Chem. Soc. 2005, 127, 8920 paper reports a design of a 'two-input molecular logic gate'. That means, that the molecule (shown on the scheme below) produces a response (fluorescence) if two conditions are satisfied. The first condition: a proton should bind to an amine center (shown in green) of the molecule. The second condition: a sodium ion should bind to a crown ether center (shown in blue). No output if either of two conditions or both are missing.
Chemical Reviews' Entire Issue for Charge Transfer Complexes An entire issue of Chemical Reviews 2004, 104, number 11 dedicated to charge transfer complexes. A 'classic' organic charge transfer complex shown on the scheme below is composed of strong electronodonor molecule such as tetrathiafulvalene (TTF) and strong electronoacceptor counterpart such as tetracyanoquinodimethane (TCNQ). These complexes are prospective in use in organic semiconductors, magnetics, and even superconductors. A large quantity of similar complexes has been synthesized followed by detailed analysis of their electrophysical properties. The scientific data accumulated are summarized in 32 quality review papers.
 |
