Latest News in Organic and Molecular Electronics for Autumn 2005
Recent Progress in Organic Materials for Solar Cell Applications
Before we present the information recently reported in J. Am. Chem. Soc. 2005, 127, page 14530 by a group of scientists from the Netherlands, we would like to introduce some general remarks about use of organic materials for solar energy applications. Apparently, there is no need to stress the importance of the solar radiation as an extremely valuable renewable energy source. It is, probably, the only viable source of energy that may meet all future needs of human civilization including environmental and other issues. Solar cells (photovoltaics) made of inorganic materials are used widely in solar technology, however they may not provide sufficient amount of energy at present moment for several reasons: 1. Although modern inorganic solar cells are superior in efficiency over known organic devices, a room for further efficiency improvement may be restricted by a limited number of possible inorganic semiconductor modifications and/or combinations. At the same time, a room for the improvement of organic structures, their adjustment to the entire solar spectrum including ultraviolet and infrared regions is virtually unlimited. 2. Inorganic cells are fragile and heavy. 3. They require significant energy consumption on the production stage that includes extensive mining, high temperatures, and thorough purification. Toxic emissions, disposal, (selenium, arsenic) are also a serious environmental problem. 4. It is problematic to make secondary structures of inorganic materials such as fibers, tubes, globules etc., whereas the well developed technologies already exist to produce a wide variety of structures, especially fibers, from organic materials.
5. Modern inorganic solar cells usually possess polished surface that reflects large amount of the solar energy back into atmosphere. Rough, curved, or furry surface may be easily constructed from organic materials that would minimize the light reflection and losses. One of the serious drawbacks of organic materials is in their low stability under the continuous action of the solar radiation and atmospheric oxygen. However once again, everything depends on the cost of the production. The stability of cells would be not so important if they are cheap and easily replaceble and disposable. In addition, the stability of organic compounds can be constantly improved, and 'hints' for this are known. Chlorophylls in leafs, for example, can survive strongest solar irradiation (in desert or in tropics) for quite a while, at least one summer, or longer. Finally, the organic photovoltaic materials may become so cheap, that the cost of an area or a land plot that a solar plant would occupy may play much more important role that the cost of the cells or materials. We expect this to happen, actually, in the not so distant future. Therefore, it will be highly important to harvest as much energy from an area unit as possible not taking into account the cost of the cells themselves. And here we can mimic from the Mother Nature again. We can also reproduce natural suborganization of the light-consuming biological objects. Lets take a tree. A tree is a natural, highly efficient system for the solar energy consumption made of organic materials. Why trees grow so high? Yes, to compete for the sunlight. But not only. The higher the tree, the more light it harvests from an area unit it occupies, because photons run through the atmosphere in all directions due to natural scattering and diffusion. A high tree harvests the reflected light coming up from the earth surface, reflected from a cloud, or running in parallel to the earth surface with the same efficiency. Not all leafs work with the same efficiency in every single moment, some leafs work more efficiently on the dawn, the others in the dusk, but in general, the entire structure is an ideal light-harvesting plant with the maximum economy of a space.  We can imagine the future solar plant not as a set of straight endless rows of brittle shiny panels in a desert, but as an artificial ... 'forest' made of artificial 'trees'. Of course, it is problematic to put artificial leafs made of heavy brittle inorganics on the 'trees'. The 'leafs' would be made of lightweight organic plastic photovoltaics. Of course, it would be nicely cool in a shadow of this forest on a hot summer day.
And back to the serious science. In the introduction part, the authors mention two approaches being used for the construction of all-organic solar cells: (1) "p-n double layer cells" (contain p-n junction), and (2) "bulk heterojunction cells, in which the p-type and n-type materials are blended". The latter form so-called bicontinuous electroconductive phase. Advantages and drawbacks of the latter type of cells that can be composed of MDMO-PPV polymer as a p- and fullerene as an n-semiconductor are discussed. The authors designed a structure (1) shown on the scheme below to replace expensive and not very efficient fullerene C60 as an n-type component. The model structure possesses a tetrahedral core that ensures amorphous properties of the compound. This prevents the blend from phase separation, a serious drawback for the C60 case. In addition, naphatalenediimide parts of the molecule can be further 'tuned' for their optoelectronic properties, as well as branched alkyl terminals may be further modified to ensure homogeneity of the system. The authors demonstrate "near-complete" quenching of fluorescence of MDMO-PPV-1 50/50 wt% mixture film. The same result was obtained for a blend with poly-(3-hexylthiophene), P3HT. Sharp increase of conductivity after short laser pulse excitation has been detected in both cases. The mobility of charge carriers formed has been measured to be 0.03 cm2/Vs.
In summary: blending of (1) into PPV and P3HT polymers in 1:1 wt ratio almost completely changes the polymers response to the visible light radiation. Instead of usual fluorescence, a sharp increase in conductivity is observed.
11.18.05 New Efficient Approaches in Organic Electronics Several recent articles published in J. Am. Chem. Soc. discuss some new approaches to composition of organic electronic materials and devices. Thus, a paper by A. Maliakal and coworkers from Bell Laboratories and DuPont, 2005, 127, page 14655, describes preparation of a new efficient "hybrid" organic-inorganic gate dielectric insulator. Creation of efficient organic gate dielectrics is very "hot" field of current research of material chemists (thus, for a new organic gate dielectric based on 'branched polymer' see our article before), because several significant problems in the fied still remain to be solved. The biggest problem is that organic materials, while possessing good processability, flexibility, and printability, have low dielectric constant K (typically ranged from 2 to 4). At the same time, some ceramic inorganic materials, especially titanium derivatives, titanium dioxide, anatase, possess high dielectric constants (K = 30-150), however they are hardly processible, not flexible, mechanically and thermally sensitive. Developing of high capacitance flexible dielectrics is vitally important for organic thin film transistor (OTFTs) applications since it would allow greater charge injection into the semiconductor thus "permitting the device to operate at lower voltage". Though attempts to create hybrid organic-inorganic dielectrics by a simple fusion of the polymer (polystyrene) and TiO2 nanoparticles have been made before, they were not very successful due-to porosity problem.
In the present research, the authors utilize so called core-shell approach, when a dielectric nanoparticle is build of high capacitance TiO2 dielectric "core" (see 1 scheme below) covered with a polystyrene shell. The authors assigned a formula for the new material: TiO2-PS. The material appeared to possess good dispersability, film-forming properties, transparence, and dielectric constant K = 8+/-0.2, that is three times higher than that of bulk polystyrene. A fabricated OTFT device composed of 1.25 mm-thick film of the gate dielectric material 1 and 50 nm-thick film of pentacene, as a channel semiconductor, afforded mobility of ~0.2 cm2/Vs, and On/Off ratio of 535.
 Another JACS report, this time by Z. Xie, Y. Ma and coworkers from Chinese Academy of Sciences, 2005, 127, page 14152, proposes an advanced luminescent compound that contain a structural unit of poly(p-phenylene vinylene) (PPV). The latter is widely used in OLED technology as a polymer with good luminescent properties. The new compound, however, is not a polymer, but a crystalline compound that forms unique crystalline structure under special crystallization conditions. This structure allows for strong increase in luminescence efficiency compare to conventional PPV materials. How that was achieved? As usual, an elegant result comes from a simple idea. It has been known for a long time that so called 'H-aggregation' in p-conjugated compounds strongly decreases quantum yield of luminescence "upon going from dilute blends or solutions to films". To avoid the H-aggregation, creation of a material where the long axes of adjacent molecular chains would be perpendicular to each other has been proposed. The authors of the article succeeded to achieve almost perpendicular (70o angle) alignment of the long axes of the molecular 1,4-distirylbenzene in it's crystalline state just by simple introduction of two phenyl rings in the central benzene ring (structure 2 on the scheme below). These two benzene rings tend to rotate thus preventing molecules from the parallel alignment. Perpendicular aligned molecules form 'columns', the columns form 'piles', and eventually the piles form needle like crystals. The authors have assigned abbreviation as trans-DPDSB for the new material. The next great characteristics of the material have been emphasized: excellent thermal stability (mp 251-254 oC), strong blue emission of the crystals in UV-light. Simple OLED device fabricated of the new material gave maximum luminescence of 2100 cd/m2 and luminous efficiency of 1.53 cd/A. The authors stress also interesting phenomenon, when the tip of a needlelike crystal of the compound shows stronger fluorescence than the body, what they attribute to a "natural self-wave guided" structure of the crystal. Preliminary experiments to use this property of the crystals for optically-pumped laser also gave rise to a success. 
10.21.05 Latest Advances for Tetrathiafulvalene-Based Electroconductive Materials Tetrathiafulvalene (TTF) is a 'grandfather' of a family of strong electron-donating compounds that form electroconductive charge transfer complexes with a strong electron-withdrawing counterpart. A resent entire issue of Chemical Reviews is dedicated to all aspects of the chemistry and physics of TTF-based complexes. A large quantity of new information on these materials constantly appears. Here, we wish to report very recent discoveries of some unique properties of new materials based on these remarkable compounds.
Thus, an article in Nature, 2005, 437, page 522, by a group of scientists from Japan, entitled "An Organic Thyristor", reports measurements of electrophysical properties of efficient thyristor made of organic salt:(BEDT-TTF)2CsCo(SCN)4 (structure of organic part of this salt see 1 on the scheme below). Thyristors are so-called "nonlinear electronic devices, that exhibit bistable resistance - that is, they can be switched between two different conductance states". They are "widely used in inventers ... and for smooth control of power in a variety of applications such as motors and refrigerators". The authors stress, that while conventional thyristors consist of series of diodes, the "present salt exhibits giant nonlinear resistance", comparable to conventional devices, "as a balk phenomenon". They attribute the origin of the effect to "melting of the charge order" under the action of external current.
Another article by a group of scientists from the University of Tokyo, published in J. Am. Chem. Soc., 2005, 127, page 14769, and entitled "Electroactive supramolecular self-assembled fibers comprised of doped tetrafulvalene-based gelators" describes formation of electroconductive fibers based on derivatives of TTF (one of them, 2 shown on the scheme below). The fibers are formed through the self-assembly due to hydrogen bonding of a specially designed part of the molecule - aminoacid moiety that is called 'gelator' (gel forming). Gelator is shown in red on the structure 2 below. The authors found that stable fibers are formed in liquid crystalline medium rather than in common solvents. They succeeded also to measure conductivity of the fiber after the charge transfer complex formation by doping with iodine.
Other report, again from Japan in J. Am. Chem. Soc., 2005, 127, page 14166 describes a phenomenon, when magnetic properties of TTF-based charge transfer complexes may be tuned just by slight varying of the molecular structure of an organic counterpart. This work is also related to a new branch of electronic science: "spintronics" that can possibly overturn and demolish all existing electronics and open a new era of electronic materials and devices (our upcoming article soon). The "spintronics" is a new "philosophy" of electronics that uses not only electric current or field to build a 'logic gate' as in conventional electronic devices, but both electric current and spin-based magnetism. Back to the article: two molecules shown on the scheme below: 3 (EDT-TTFVO) and 4 (EDO-TTVO) differ only in two heteroatoms, e.g. two sulfur atoms in dithiane ring of 3 are replaced for two oxygen atoms to form dioxane ring in 4. However, a drastic difference in electroconductive and ferromagnetic properties is observed for their iron containing charge transfer complexes. Thus, a 2:1 FeBr4 salt of 3 loses its metallic conductivity below 170 K and exhibits ferromagnetic ordering of Fe(III) d spins at ~1K, whereas 2:1 FeCl4 salt of 4 exhibits metallic behavior down to near 0.3 K and antiferromagnetic ordering of Fe(III) d spins at ~3K.
 Bis-calix arene 5 with a TTF bridge has been recently synthesized (B.-T. Zhao, M. Salle and coworkers in J. Org. Chem. 2005, 70, page 6254). Since calix arenes are well known for their chemical recognition properties, the authors proposed a model for study of the redox activity of the "hybrid" molecule and it's potential electrochemical response to a number of classes of chemical compounds. The work is interesting from a point of view of both synthetic challenge and potential for new applications in chemical sensors and receptors. 
10.15.05 A New Liquid Crystal Phase Discovered
A group of scientists from Japan, J. Yamamoto, I. Nishiyama, M. Inoue, and H. Yokoyama have reported a new liquid crystalline smectic 'blue phase' that is formed in a mixture of a chiral 'momomer' LC: 3B1M7 (1) and its 'twin' BMHBOP-6 (2) of a length exactly twice of the 'monomer', at a narrow temperature (120-130 oC) and concentration intervals, Nature 2005, 437, page 525. The authors assigned this phase as SmBPIso. The phase possess unique properties, such as optical isotropy, uniform blue color, and iridescence. This is quite unusual for smectic LC phases that are normally unisotoropic in their optical properties. The researches has attempted to explain this phenomena by spherical symmetry in the liquid crystalline orders that is spontaneously formed as a result of mixing a 'twin' (2) into 'monomer' (1). That is, probably, "equivalent to the introduction of connecting chains between two 'monomers', which freeze the motion of the monomers", however it is still not quite clear, how this can result in the spherical symmetry of the orders.
10.05.05 New Advances for Liquid Crystalline Materials With this article, we begin reviewing of the latest advances for liquid crystalline materials and devices. However, one of the reports, we would like to start with, is dated by spring of 2004. The article by X. Zhang, S. Mataka, and coworkers from Japan is published in J. Mater. Chem., 2004, 14, page 1901. It describes a new fluorescent liquid crystalline-compatible material that possess highly dichroic properties (structure 1 on the scheme below). This work belongs to a relatively new area of luminescent liquid crystal materials and devices that are seen to be advantageous in many respects to 'conventional' LC devices. The problem with 'conventional' LC systems is relatively high power consumption and structure complexity due to requirement of 'back light', a source of light that located behind of a liquid crystal layer. Introduction of luminescent unit into a molecule of LC-like material allows reduction of intensity or, in perspective, complete removal of a back light source. In addition, some usual 'contributors to complexity' of conventional LCDs, such as polarizing layer, are not required in the case of luminescent LC devices. Thus in the work we quote above, a diphenyl benzothiadiazole unit in the molecule (1) represents a highly dichroic fluorescent dye with practical dichroic ratio N > 8.0. The authors have fabricated simple 'host-guest' fluorescent LCD devices with this new material. Since the dye molecules are just photoluminescent, the devices still require a low intensity UV 'back light' source however. An article in a recent issue of J. Am. Chem. Soc., 2005, 127, page 11578, by K. Kanie and A. Muramatsu from Tohoku University describes 'hybrid' organic-inorganic LC materials that potentially can be used as analogs of Ferroelectric LCs (FLCs). In order to create hybrid LCs, the authors have modified the structure of regular LC molecules based on the observation that monodispersed particles of iron oxide a-Fe2O3 may be easily obtained in the presence of phosphate ions PO43- using special technique, so called gel-sol method. Thus, one of the structures synthesized (2) shown on the scheme below contains a phosphate moiety to bind with iron trioxide. After simple procedure of 'hybridization', the authors obtained stable thermotropic N and cubic LC phases. The authors also emphasize possibility of easy transformation of the 'hybridized' a-Fe2O3 into ferromagnetic Fe3O4 or g-Fe2O3 without sintering. Another JACS article, 2005, 127, page 11736, by a group of scientists from Italy describes chiral induction in liquid crystals. The authors have shown, that a single enantiomer of a chiral compound, methyl phenyl sulfoxide (3 on the scheme below) may induce both left- and right-handed chiral nematic phases depending on solvent properties. The practical implication of this fundamental observation is yet to be discovered.
Self-Assembled Monolayers in Service to Organic Microelectronics
Several interesting articles on use of self-assembled monolayers (SAMs) for improving of various characteristics of organic microelectronic devices have been published recently in J. Am. Chem. Soc.. A common idea of all these studies is implication of patterned organic monolayers (layers of one molecule-wide) of differently functionalized organic compounds as a 'foundation' for building microelectronic devices and circuitry. How that may work and why it so intriguing to study? Very simple, SAM can be ideal 'foundation' for the microelectronic circuitry just because it is evenly distributed on the surface of a substrate (one molecule wide layer, no more - no less) with reproducible, and easily tuned properties all across the surface. Thus, two papers by Z. Bao and coworkers describe use of SAMs for the accurate control of growth of oligoacene semiconductor monocrystals. It was found that monocrystals of oligoacenes such as antracene and pentacene possess much higher carrier mobility than polycrystalline thin films. Furthermore, the precise patterning of the monocrystals may constitute almost ready patterned circuit. That was achieved by a group of scientists from University of California-Los Angeles, Bell Laboratories, and Stanford University, J. Am. Chem. Soc., 2005, 127, page 12164. They examined a number of patterned SAMs deposited on a gold plate and differently modified with functional groups for ability to trigger growth of monocrystals of antracene. The best, oriented and large semiconductor monocrystals were obtained with terphenylthiol-based (1) self-assembled monolayer. While the gold surface may be efficiently modified with thiol (as 1) SAMs, silicon surface easy to modify with organosilanes. Moreover, difference in the structure of silane may strongly affect on the carrier mobility of pentacene films, as has been shown in the article "Conducting AFM and 2DGIXD Studies on Pentacene Thin Films", page 11542. Thus, the silicon surface modified by hexamethyl disilazane (HMDS 2) monolayer gives better conducting pentacene film than one modified by octadecyltrimethoxy silane (OTS 3).
Amazing, an efficient construction of metallic microcontacts on a similar SAM basis has been independently proposed in the same issue of JACS. As appears now, SAM-patterned monocrystalline transistors may be easily connected with metallic contacts, a new recipe for organic circuit? Thus, an article by C. Zhou, G. Nagy, and A. V. Walker from Washington University in St. Louis "Toward Molecular Electronic Circuitry: Selective Deposition of Metals on Patterned Self-Assembled Monolayer Surfaces" describes a simple method for the depositing of precise micropatterns of magnesium films on a surface of thiol-based SAM containing carboxylic group terminals (p. 12160). An idea seems really elegant and simple: the metal atoms should stick to the carboxylic groups and stay on the surface of the SAM (see 4: scheme below). At the same time, the metal atoms should drop down to the bottom of a methyl group-terminated SAM, and both was clearly demonstrated experimentally. Both SAMs can be patterned using conventional optical photolithography technique so that no any special new technology required. The authors also stress applicability of the new technique to any other metals such as aluminum, copper and silver. S. Kato (page 11538) uses amino group - terminated silicon SAM (5) to assemble layers of light-emitting polymer on an ITO surface in order to facilitate hole injection (similar motif that we mentioned before). The author's clever approach is based on formation of salt between SAM and a next transport polymer monolayer containing carboxylic groups. After several layers assembled, they were 'fused' by simple heating that transforms ammonium salts to amides and stick them "forever"...
 |
