Latest News in Organic and Molecular Electronics
Here we report the most recent news in organic and molecular electronics while trying to do it in a simplest possible way. However, at least basic knowledge in organic chemistry, physics, and electronics is necessary in order to follow the information provided. Our column is limited mainly to the field of nanotechnology that involves organic compounds and materials. We report also scientific discoveries in the field of inorganic electronic materials if they may affect deeply on the future developments in organic electronics. We do not pretend for full coverage of outstanding news in the field, there is a vast quantity of information coming every day. We try to do our best not to miss the information of highest importance, especially that widely recognized in the public massmedia.
Table of content: History
• Recent and upcoming conferences in the field of organic electronics • Recent and upcoming conferences in the field of organic chemistry • Latest news in
organic chemistry
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The column of highly important news that may be milestone events in the field of organic electronic science and industry. We select material according to both our opinion and opinion of experts from various information resources.
05.05.07 Thermoelectric effect in molecular junctions discovered A recent publication by a group of scientists from University of California, Berkeley: P. Reddy, S.-Y. Jang, R. A. Segalman, and A. Majumdar is obviously opening a new field in organic electronics: organic (or molecular) thermoelectricity. The work is published in Science, 2007, 315, 1568, and entitled "Thermoelectricity in molecular junctions". Thermoelectric effect in bulk metals or semiconductors as well as in junctions between different metals or semiconductors has been known and used in technology for a long time. It is observed and quantified by measuring of voltage between hot and cold sides of a bar of a material. The voltage is formed due to diffusion and concentration of charges of opposite signs on the hot and cold sides of a bar. Thermoelectric effect in bulk materials is quantified by Seebeck coefficient (S), which determines maximum voltage that can be achieved in a material when a certain temperate differential is applied and equilibrium established. Junction Seebeck coefficient (Sjunction) depends also on electrochemical potential (or Fermi levels) of materials at the junction. The sign of junction Seebeck coefficient determines which carriers: electrons (negative sign) or holes (positive sign) diffuse to the hot side. The authors demonstrated for the first time prominent thermoelectric effect at molecular junctions. Rigorously controlling the distance between a gold substrate, covered by a monomolecular layer of an aromatic dithiol, and a golden tip of scanning tunneling microscope (STM), the authors obtained monomolecular junction between two gold surfaces. Monomolecular junctions with benzenedithiol (BDT), dibenzenedithiol (DBDT), and tribenzenedithiol (TBDT) (see scheme below) have been investigated. The gold substrate was heated, whereas the temperature of the STM tip was strictly maintained at room temperature. The voltage measurements at a series of temperature differentials revealed prominent junction Seebeck coefficients for BDT +8.7, DBDT +12.9, and TBDT +14.2 mV/K.  The positive sign of the Seebeck coefficients indicates hole transport in molecular junctions. Control experiments with clean gold surfaces showed no any measurable thermoelectric effect that is consistent with a low Seebeck coefficient (~1.94 mV/K) of bulk gold itself. These convincing results impose questions: how possibly the molecular junction may work? What is the mechanism of the prominent, one-side charge transfer through a symmetrical organic molecule? We can propose following mechanism of action of this molecular junction:  Subsequently, the positive charge (hole) is transferred through the conjugated system to the opposite side of the molecule (3 on the scheme above), where it can be reduced by an electron from the 'hot' gold surface. The cycle of the charge transfer is completed after recombination of short-living intermediate biradical (4) into initial state of the molecule (5). The increase of the Seebeck coefficient in the sequence: benzenedithiol → dibenzenedithiol → tribenzenedithiol can be explained by increase of the degree of stabilization and delocalization of the positive charge and, correspondingly, increasing stability of the intermediate charge-transfer complexes in this sequence. In conclusion, this truly outstanding discovery opens virtually unlimited opportunities for the creation of highly efficient thermoelectric molecular junctions through manipulation with the chemical structures. This, in turn, may lead to efficient technologies for the direct heat energy conversion to electricity with huge positive consequences for the entire field of energy development.
Latest reviews in organic and molecular electronics
We are pleased to report here that spring 2007 is plentiful on scientific reviews in the field of organic and molecular electronics. Thus, an entire theme issue of Chemical reviews, 2007, 107, 4 "Organic electronics and optoelectronics" contains 12 awesome review articles from leading scientific groups that provide up-to date information and cover various aspects of chemistry, physics, and application of organic electronic materials. Brief overview of the issue by S. R. Forrest and M. E. Thompson can be found in the introduction part, page 923. Here we present an overview of these articles in combination with relevant articles from other journals sorted by a number of topics in organic and molecular electronics: Novel charge-transporting organic materials Novel charge-transporting organic materials
The number and variety of types of organic conductors have been increasing exponentially during the last several years and now they are reaching the point when more detailed classification is necessary. Traditionally, all organic conductors have been subdivided in two large families: small molecules and polymers. First small molecule conductors were predominantly crystalline compounds (monocrystalline or polycrystalline), whereas the polymers are usually amorphous substances. There are substantial advantages and drawbacks of both crystalline and amorphous organic semiconductors that are summarized on the diagram below:
Relatively recently, a group of small molecule conductors 'stepped forward' that combines many positive features of crystalline materials (reproducibility of characteristics, high carrier mobility) and those of conducting polymers (simple fabrication of devices from a solution, good optical and luminescent properties). This group of amorphous molecular materials is distinctly separated and thoroughly characterized in a review of Y. Shirota and H. Kageyama from Fukui University of Technology and Osaka University, Japan: "Charge carrier transporting molecular materials and their applications in devices". The article is published in the theme issue of Chemical Reviews, 2007, 107, page 953.
The review covers synthesis, charge transporting and luminescent properties of only molecular materials, e.g. the materials based on small molecules with distinct structure. Although crystalline conducting materials are discussed, the main emphasize is put on amorphous compounds, and among them small dendride-type or 'starburst' molecules with regular structure (similar to m-MTDAB on the scheme to the right) and tetraphenylmethanes are discussed in detail. Starburst molecules form stable amorphous glasses due to rotation of 'star beams', parts of a molecule brunching out from the central core. This rotation greatly reduces planarity of a molecule and hence ability to crystallize. Tetraphenylmethanes are '3-dimentional stars', where all 'beams' equidistant from each other in space due-to tetrahedral sp3 hybridization of the central carbon atom. For application of tetraphenylmethaines in OPVs, please see also our previous article. The chemistry and physics of larger, macromolecular dendrimers for use in organic electronics is covered by S.-C. Lo and P. L. Burn from University of Oxford, UK in the theme issue of Chemical Reviews: 2007, 107, page 1097. The article is entitled: "Development of dendrimers: macromolecules for use in organic light-emitting diodes and solar cells". The authors describe all aspects of the synthesis, structure, and electronic properties of macromolecular dendrimers, which are sorted by major structural types: with well defined structure (symmetrical molecules), and random structure (molecules containing variations in branching). Dendrimers are also classified by saturation of a core: saturated and conjugated as well as by generation number. Synthetic strategies that include convergent and divergent routes to dendrimers are discussed. The other class of amorphous conducting molecules are spiro-linked oligomers, which are reviewed in an article of T. P. I. Saragi, T. Spehr, A. Siebert, T. Fuhrmann-Lieker, and J. Salbeck from University of Kassel, Germany; Chemical Reviews, 2007, 107, page 1011. The article is entitled: "Spiro compounds for organic optoelectronics". Spiro-linked conjugated oligomers, like spiro-4-F on the scheme above form stable amorphous glasses because the spiro-linkage between conjugated oligomers forces them to arrange perpendicular to each other thus hampering possibility of crystallization. The review summarizes all aspects of spectroscopic, electrochemical, solid-state properties, synthesis, and application of a wide variety of conducting spiro compounds in unprecedented detail.
One of the recently emerged and promising classes of organic semiconductors are graphenes (one of the simplest graphenes HBC see scheme above), whose disk like molecules easily form 2D conducting layers and also prone to columnar self-organization of discotic type with prominent 1D - conducting properties. We have already summarized a report on graphene HBC-C12H25 in one of our previous articles. Here, we wish to introduce an excellent review on graphenes by J. Wu, W. Pisula, and K. Müllen from Max-Planck Institute for Polymer Research, Mainz, Germany. The article is published in the theme issue of Chem. Rev. 2007, 107, page 718 and entitled: "Graphenes as potential material for electronics". The article covers recent progress in the not-so-simple synthesis of graphenes as well as in physics of graphene materials, and material casting methods, which are particularly important for enhancing of the material performance. Finally, we introduce a review that covers primarily crystalline conducting oligomers and small molecules for use in organic field effect transistors (OFET) . The review by A. R. Murphy and J. M. J. Fréchet from University of California, Berkeley is published in the theme issue of Chemical Reviews 2007, 107, page 1066 and entitled: "Organic semiconducting oligomers for use in thin film transitors". The article distinctly classifies and summarizes structure-OFET performance relationships of separately p-type and n-type oligomers, vacuum-deposited, single-crystal, and solution-processed. Very interesting work written in a simple and clear fashion. Mechanism and physics of charge transport in organic semiconductors
A number of recent reviews is dedicated in greater extent to physical and mathematical aspects of the charge transport in organic semiconductors. Thus, a review by J. Zaumseil and H. Sirringhaus from Cavendish Laboratory, UK "Electron and ambipolar transport in organic field-effect transistors", Chem. Rev. 2007, 107, page 1296 describes impact of all components of organic FETs, such as structure and composition of contacts, gate dielectric, and organic layer on transistor performance. The emphasis is put primarily on relatively unexplored n-channel and ambipolar OFETs and light-emitting OFETs.
The other work, "Charge transport in organic semiconductors" by an international group of scientists from USA and Belgium: V. Coropceanu, R. Silbey, J.-L. Brédas, and coworkers summarizes modern state of theory and physics of the charge transport . The review is published in Chem. Rev. 2007, 107, page 926. Strong emphasis is put on charge carrier mobility: measurements, factors influencing mobility in organic materials. Physico-mathematical models of charge transport are discussed in detail. Although the article is full of physical expressions and mathematical formulas, the content is logically and clearly presented, and it can easily be understood by scientists who are not directly specialized in the field (chemists), as well as students and beginners.
Studies of electron transfer through organic films using low-energy electron transmission (LEET) spectroscopy and low-energy photoelectron transmission (LEPET) spectroscopy have been summarized in a review by R. Naaman and L. Sanche from Israel and Canada. The review is published in Chem. Rev. 2007, 107, page 1553 and entitled: "Low-energy electron transmission through thin-film molecular and biomolecular solids". The article deals exclusively with organic material behavior under bombardment with low-energy (0-12 eV) electrons.
Organic photovoltaics
Several review articles are dedicated to the developments in organic photovoltaics. Thus a review "Conjugated polymer-based organic solar cells", Chem. Rev. 2007, 107, 1324 by S. Günes, H. Neugebauer, and N. S. Sariciftci form Johannes Kepler University of Linz, Austria, summarizes recent progress in the development of 'plastic' photovoltaics. Significant part of the review is dedicated to general aspects of polymer-based solar cells, such as operating principles, device architecture, and device efficiency. Significant attention is also paid to fullerene-based cells as the most efficient polymeric devices fabricated to date in a separate chapter of the article. Polymer/polymer; so-called "double cable" polymer (where moieties of an acceptor are chemically bound to a donor polymer backbone); and hybrid solar cells are also briefly discussed.
The other review by T. L. Benanti and D. Venkataraman from University of Massachusetts - Amherst covers history of the development of architecture and morphology of organic solar cells. The article is published in Photosynthesis Research 2006, 87, 73, and entitled: "Organic solar cells: an overview focusing on active layer morphology". The article classifies all types of junction (planar and balk-heterojunction) and discusses basic concepts of organic photovoltaics in a simple and clear manner.
A relatively new and very efficient method of fabrication of photoelectrochemical solar cells: electrophoretic deposition technique, is summarized in a review of H. Imahori from Kyoto University. The review is entitled: "Electrophoretic deposition of donor-acceptor nanostructures on electrodes for molecular photovoltaics" published in J. Mat. Chem. 2007, 17, 31. Electrophoretic technique includes deposition of charged colloidal particles of either organic or inorganic components from a polar solution onto nanostructured electrodes (SnO2, TiO2). This allows efficient and economical assembly of active layers of a device. The author stresses common high efficiency (IPCE up to 60%) of such devices.
Recent progress in the development of organic lasers is summarized in a review of I. D. W. Samuel and G. A. Turnbull from University of St Andrews, UK. The review is published in the theme issue of Chem. Rev. 2007, 107, 1272, and entitled "Organic Semiconductor Lasers". Two general types of organic lasers are known: dye lasers, whose working medium (in resonator) is a solution of dye, and semiconductor lasers, whose resonator consists of an organic semiconductor (OSLs). The former type of lasers has been developed since the first lasers appearance (~60-ies of the last century), and it is now actively used in technology. At the same time, the OSL technology based on amorphous molecular materials or polymers is a relatively new field of laser technology with a development span within the last decade. The review covers only OSLs although comparison with dye lasers is also provided. The article is extremely interesting and easy to read. The authors stress, that OSLs provide unprecedented opportunities for diversification of laser technology due to high luminescent efficiency and luminofor density of amorphous organic semiconductors that allows fabrication of various resonator structures, such as thin film, microspheres, and other microscopic features. The fabrication itself can be done using simple solution-processing techniques, such as inkjet printing that is revolutionary in the laser technology. In addition, OSLs can be directly diode-pumped by integration of LEDs and OLEDs into organic resonators, and even direct electrical pumping is possible. The authors discuss progress toward direct electrical pumping in detail. Conducting polymer-based sensors are classified and summarized in a review by S. W. Thomas III, G. D. Joly, and T. M. Swager from MIT. The article is published in the theme issue of Chem. Rev. 2007, 107, 1339, and entitled "Chemical sensors based on amplifying fluorescent conjugated polymers". The authors describe use of amplifying fluorescent polymers (AFPs) in detection of small molecules, ions, explosives, biomolecules and DNA in great detail. Sensory mechanisms are rigorously discussed. Various aspects of organic device fabrication have been very recently reviewed. Thus, effects of doping of organic layers on device performance have been summarized in an excellent review by K. Waltzer, B. Maennig, M. Pfeiffer, and K. Leo from University of technology, Dresden, Germany. The review is published in the theme issue of Chem. Rev. 2007, 107, 1233, and entitled "Highly efficient organic devices based on electrically doped transport layers". The authors discuss separately p-type and n-type doping of organic layers, the use of doping in fabrication of OLEDs, and OPVs, and the role of doping in interfaces between contacts and organic semiconductors. The other review summarizes patterning techniques for fabrication of organic circuits (please, see also our previous overview on this topic). The review is published by E. Menard, M. A. Meitl, Y. Sun, J. A. Rogers and coworkers from University of Illinois, Urbana-Champaign, and Argonne National Laboratory, USA in Chem. Rev. 2007, 107, 1117, and entitled: "Micro- and nanopatterning techniques for organic electronic and optoelectronic systems". Among numerous patterning techniques known to date, the authors separate light-based techniques for directly photopatternable organic materials, embossing, imprint lithography, capillary molding, and various printing techniques including laser printing and inkjet printing. Inkjet printing of electroluminescent displays is reviewed in grater detail in an article by J. F. Dijksman, P. C. Duineveld, M. J. J. Hack, and coworkers from Philips Research Laboratories - Eindhoven, the Netherlands: "Precision ink jet printing of polymer light emitting displays". The article is published in J. Mater. Chem. 2007, 17, 511. This work is highly valuable also for display engineering, contains large collection of details and data in the field. Y. Sun, and J. A. Rogers from University of Illinois, Urbana-Champaign, and Argonne National Laboratory, Illinois review also stretchable electronics that can be useful for a variety of fancy applications including stretchable displays. The article, entitled: "Structural forms of single crystal semiconductor nanoribbons for high-performance stretchable electronics", is published in J. Mater. Chem. 2007, 17, 832. The article describes several techniques for the preparation of stretchable semiconductors including micropatterns chemically bonded to elastomeric poly(dimethylsiloxane) (PDMS). Modern nanopatterning techniques often employ self-assembled monolayers (SAMs) that are necessary for assembling of devices and circuits (please, see our our previous articles on this topic). Surprisingly, odd or even number of atoms in carbon chains of compounds used for SAM patterning (such as aliphatic thiols, carboxylic acids, or bifunctional molecules) may result in distinct difference of SAM properties. These, so-called odd-even effects may appear even in various diapasons of lengths of molecules. The results of studies of the odd-even effects are summarized in a review of F. Tao and S. L. Bernasek from Princeton University: "Understanding odd-even effects in organic self-assembled monolayers", Chem. Rev. 2007, 107, 1408. Molecular electronics and mechanics
Molecular electronics and mechanics deal with electronic or mechanical properties of a single molecule. This field is still somewhat futuristic, however substantial efforts are being made by scientists worldwide to bring the ideas of molecular devices into reality. A recent theme issue of Advanced Functional Materials 2007, 17, # 5 "Molecular machines and switches" is dedicated entirely to this topic. Brief overview of the field may be found in an introduction part to the issue by A. Credi, and H. Tian from University of Bologna and East China University of Science and Technology: "Big challenges for tiny machines", page 679. The issue contains also a review of S. Saha, K. C.-F. Leung, T. D. Nguyen, J. F. Stoddart, and J. I. Zink from University of California, Los Angeles: "Nanovalves" (page 685). The article summarizes recent progress on molecular switches based on rotaxanes. The authors describe functioning of rotaxane-based supramolecular systems attached to silica surface and powered either by a photosensitizer or by a lifgt-harvesting molecular triad.
03.13.07 Minireview Introduction The importance of the development of solar energy becomes increasingly obvious when our civilization faces new realities, such as global warming and global energy crisis. Some aspects of the development of organic photovoltaics we have already discussed in our previous articles. Here, we wish to summarize the most recent advances in the chemistry and physics of hybrid solar cells. Although the term "hybrid solar cells" is applied sometimes with different meanings, most commonly it means the cells whose active layer is composed of both organic and inorganic materials (semiconductors). Why one would decide to mix organics with inorganics in a solar cell, what positive results could be obtained from this? The answer is simple: inorganic semiconductors are thermally and chemically stable, possess high carrier mobility, whereas the organic ones are lightweight, flexible, and potentially inexpensive. Ideally, the combination of these attractive features could afford inexpensive, flexible, and lightweight solar cells possessing high power conversion efficiency (PCE) and high environmental stability. This is the goal. Numerous examples of unusual and prominent properties of blends of organic and inorganic materials can be found in a recent review article by a group of scientists from France and Germany: C. Sanchez, B. Julian, P. Belleville, and M. Popall, published in J. Mater. Chem., 2005, 15, page 3559. The article is entitled: "Applications of hybrid organic-inorganic nanocomposites". A fabulous introduction to the review contains striking historical facts about hybrid materials, which (surprise!) have been known and used by humans for a long time. Thus, Maya blue pigment found in ancient Maya murals looks the same pristine as when it was used by Maya twelve centuries ago. The secret of the Maya blue is in its composition: it is a true hybrid nanocomposite, where organic molecules of natural indigo are encapsulated in the channels of a special inorganic clay mineral. In the harsh tropical conditions, 'free' indigo (in the form it has been most commonly used as a dye by other civilizations), would disappear without of traces in the course of a hundred years, eaten by microorganisms, destroyed by sun, oxygen, and moisture. Would you imagine organic solar cells surviving and operating for a thousand of years? Seems, it might be possible. The review summarizes also modern strategies for the preparation of hybrid materials that include several paths, which, in turn, are subdivided in a number of routes. The most important and commonly used path (path A) is so-called sol-gel chemistry when an inorganic component (usually a metal oxide) precipitates from a solution containing an organic component. The latter is trapped within inorganic particles due to weak interactions (H-bonding, van der Waals, or p-p interactions). Path B corresponds to dispersion or assembling of pre-defined inorganic nano-building blocks in an organic medium followed by chemical interactions and strong bonding. Path C employs self-assembly of both organic and inorganic layers on templates. There are also other paths that are basically variable combinations of the aforementioned. The third section of the review summarizes applications of hybrid nanocomposites that range from kitchen appliances to microoptics, electronics, fuel cells and some completely unique applications. The photovoltaic applications of sol-gel hybrids dated by 2005 are summarized on the page 3577. In general, the review is very useful to get an orientation in the entire field of hybrid nanocomposites, which is very broad. Here we should repeat, that a generally accepted mechanism of operation of solar cells containing organic component(s) in the active layer includes formation of the excited state of an organic molecule (exciton) after the 'bombardment' with a photon. Therefore, organic solar cells sometimes are also called 'excitonic solar cells'. The exciton diffuses to p-n-heterojunction (p-n-HJ) border where it dissociates into negative and positive charge carriers (polarons), which are transferred to the corresponding electrodes driven by a difference in electrochemical potential of the p- and n-components. What is exciton, and how, and why it should diffuse to the p-n-junction border? And how far it can diffuse? Exciton, actually, is a neutral (not charged) particle, although it can be 'polarized', e.g. an excited molecule may have a partial negative charge on the one side, and positive on the other side. Then, why a neutral particle should diffuse anywhere? Indeed, excitons have no any preferences to diffuse to either p-n-junction or anywhere else, and this is a problem. Just because they diffuse randomly, only small portion of them reaches p-n-junction and generates photocurrent, especially in planar-heterojunction devices. The rest of excitons 'travel' into nowhere and die without fame through luminescent decay or thermal dissipation. So, can we, at least partially, 'direct' the excitons in the 'right' direction, e.g. to the heterojunction border? Yes we can, and we can do it using two known approaches: The importance of the second approach seems just recently has been understood. An excellent example of it could be found in an elegant work of a group of scientists from France: J. Ackermann, C. Videlot, A. El Kassmi, R. Guglielmetti, and F. Fages, published in Adv. Funct. Mater., 2005, 15, page 810. The article is entitled: "Highly efficient hybrid solar cells based on an octithiophene-GaAs heterojunction". Using external quantum efficiency (EQE) measurements the authors found, that the exciton diffusion length in a planar-heterojunction solar cell fabricated from a-octithiophene (a-8T) as a p-conductor (donor) and gallium arsenide (GaAs) as an n-conductor (acceptor) can be greatly altered by changing of organic film morphology. The authors have chosen a-8T as an organic counterpart because it possesses highest hole mobility amongst thiophene oligomers: 0.02-0.3 cm2V-1s-1. a-8T forms highly ordered microcrystalline films when deposited on GaAs at 140 oC with terrace-like monomolecular layers and orientation perpendicular to the GaAs surface (see scheme below). The same compound gives nanocrystalline disordered films while deposited at lower temperature (100 oC).  The EQE measurements of the devices revealed large photocurrent collection length and, correspondingly, exciton diffusion length of 60-100 nm in the organic layer for the case of microcrystalline a-8T films. These data are significantly larger than that of nanocrystalline a-8T films as well as other organic p-transporters, whose exciton diffusion length is normally around 10 nm. After optimization of the cathode-a-8T interface by iodide-doping, the authors obtained power conversion efficiency of 4.2% which is the highest value ever reported for planar-heterojunction organic solar cells to date. Why the exciton diffusion is so efficient in the microcrystalline a-8T films compared to the nanocrystalline films of the same compound? To propose an answer, we should look at the most probable mechanism of the exciton diffusion. An excited molecule itself cannot move or diffuse in the crystal lattice with a reasonable speed. Instead, it transfers its energy to the next molecule, and the process is continued from molecule to molecule until the excited state reaches p-n-HJ border. Probably, the exciton 'diffusion' in the dense microcrystalline films of a-8T is facilitated due to compact packing of the molecules, which makes the energy transfer from molecule to molecule easy. Seems that the exciton diffusion obey much the same rules as carrier mobility in organic semiconductors, e.g. highly ordered structures facilitate both the exciton energy transfer and the charge transfer. This could be a nice 'hint' for the future material developments.  Efficient exciton transfer is not the only problem of organic photovoltaics. An excited molecule near the p-n-border (see scheme above) may or may not dissociate to a free electron and a hole. A hole that is formed is not just a 'hole', it is a conjugated cation-radical of a-8T, where the positive charge can move freely along the entire chain of the thiophene rings (green on the scheme above). The dissociation will occur, if the lowest unoccupied molecular orbital (LUMO) of a-8T possesses higher energy than the conducting band of an inorganic semiconductor (GaAs in our case). Since one electron in an excited molecule of a-8T is located on LUMO, it may prefer 'jumping' to the lower energy conducting band of GaAs thus getting energy advantage. After the dissociation is complete, the charge carriers would have to run from each other to the electrodes as fast as possible in order to produce the photocurrent we want. The carriers, however, are very little concerned about what we want, and instead of running away, they may recombine due to mutual attraction of their electric fields (see scheme below). The process of recombination recovers the exciton, which may eventually 'dip' back into the semiconductor and dissipate. In order to improve the exciton dissociation and reduce the charge recombination, the proper combination of the energy levels of organic (LUMO) and inorganic (conducting band) counterparts is necessary.  The other critical property of semiconductors is carrier mobility, which determines how fast the charge carriers will run away from each other. In our case, a-8T possesses the highest hole mobility among oligothiophenes, whereas GeAs possesses high electron mobility. The charge transfer in a-8T film occurs in the same way as exciton transfer, from molecule to molecule, but in the opposite direction (scheme above). There are minimal obstacles for the charge transfer in the planar heterojunction structures. However, the charge transfer may become a major problem for the bulk-heterojunction structures. Here we wish to introduce the other interesting work by a group of scientists from Princeton University: J. Xue, B. P. Rand, S. Uchida, and S. R. Forrest. The work is published in Adv. Mater., 2005, 17, P. 66, and in Journal of Applied Physics 2005, 98, P. 124903. The work is devoted to hybrid solar cells, however, 'hybrid' in their case means not an organic-inorganic blend in the active layer of a device, but a combination of two composing modes: planar- heterojunction, and bulk (mixed) heterojunction in a single device. The authors designated it as a planar-mixed molecular heterojunction photovoltaic cell, or PM-HJ. Bulk-heterojunction cells are advantageous in terms of enhanced capability to 'catch' excitons and effective exciton dissociation. At the same time, they lack efficiency of the carrier transport and collection due to intrinsic structure of the bulk-heterojunction. The authors analyzed possible ways of the formation of bulk-heterojunction layers (see scheme below). The first structure, where components are segregated in 'tunnels' is ideal for the carrier transport; however, it is difficult to realize on the practice.  Homogenous mixtures of nanoparticles or molecules are much easier to obtain using sol-gel methods, or other techniques. However, the charge collection is much poorer in such structures compared to planar-heterojunction structures, because the charge-carriers have to penetrate through barriers of an alien component between particles. In addition, the particles of the components can arrange either in the way favoring the carrier trapping and terminating the carrier paths (figure 2) or in the way that favors carrier penetrating (figure 3). The authors proposed a concept of 'percolating' arrangement of components that resembles the chains of air bubbles percolating through water. Such a model would allow continuous transport of the charge carriers from 'bubble' to 'bubble' thus enhancing the charge-collection efficiency. A mixture of copper phatlocyanine as a p-component (donor), and fullerene as an n-component (acceptor) possesses the required structure. According to the author's concept, the thickness of an intermediate, mixed layer should not exceed the carrier collection length, Lc in both 'percolating' components (see scheme below). In the other words, the charge carriers that are formed in the mixed layer should be able to reach the planar layers to be collected. At the same time, the thickness of the planar layers of each component should not exceed exciton diffusion lengths, LD in both components. In this case, the diffusion of the excitons that form in the planar layers to p-n-junctions should be the most effective. The authors used also additional layers (bathocuproine (BCP)) for optimization of the carrier transport to electrodes. Power conversion efficiency of (5.0 +/- 0.3) % has been achieved that is near the highest for the solid-state organic cells obtained to date.  The model of 'segregated' bulk-heterojunction (see above) has been realized in a variation of dye-sensitized solar cells by a group of scientists from University of California, Berkeley: M. Law, L. E. Greene, J. C. Johnson, R. Saykally, and P. Yang. The work is published in Nature Materials, 2005, 4, page 455, and entitled: "Nanowire dye-sensitized solar cells". The authors obtained uniform and ideally arranged for the 'utmost' carrier transport 'nanowire' of an inorganic n-conductor: zinc oxide. The other components of the device were similar to the 'classic' dye-sensitized cells with a ruthenium N719-dye and liquid I3-/ I- redox-electrolyte. Measurements of short-circuit current density (Jsc) proved "rapid collection of carriers" in the devices, although PCE was only 1.5%, primarily due to inferior semiconductor properties of ZnO; lower surface area and dye load in ZnO nanowire compared to 'classic' devices composed of TiO2-nanoparticles (PCE ~ 10-12%). The greatest achievement of this work is in the method of the preparation of uniform 'fur' of inorganic semiconductor nanowire. Similar approach, the application of 'elongated features' for the charge collection has been used by a group of scientists from the National Taiwan University, Taiwan: Y.-T. Lin, T.-W. Zeng, W.-F. Su and coworkers. The authors applied nanorods of titanium dioxide instead of nanoparticles in an organic-inorganic photovoltaic nanocomposite. The work is published in Nanotechnology, 2006, 17, page 5781 and entitled: "Efficient photoinduced charge transfer in TiO2 nanorod/cojugated polymer hybrid materials". The authors demonstrated two order of magnitude increase in short-circuit current density (Jsc) in a device composed of MEH-PPV and titanium dioxide nanorods in comparison to that of a "pristine" MEH-PPV device. Problem of miscibility
Often, organic and inorganic compounds are 'antagonistic' in their physical properties. They 'repel' each other when mixed and tend to segregate, much like gasoline and water. This is a biggest problem of organic-inorganic composites. The problem includes both preparation of a composite and its 'preservation', prevention from phase separation and degradation with time (for example, conventional paints segregate when stored for a prolonged time). While a simple shaking may restore quality of paint, such 'restoration' would not be applicable to a solar cell.
Scientists attempt to solve this problem in a number of different ways. Thus, fullerenes can be chemically modified with organic side-chains that makes them much better miscible with most of organic counterparts. Some new, original ways have also been recently proposed.
Thus, an interesting concept has been proposed by a group of scientists from Johannes Kepler University, Linz, Austria: S. Gunes, H. Heugebauer, N. S. Sariciftci, W. Heiss, and coworkers in a recent issue of Adv. Funct. Mater., 2006, 16, page 1095. The article, is entitled: "Hybrid solar cells using HgTe nanocrystals and nanoporous TiO2 electrodes".
The authors used a sophisticated 'binding' interlayer between an inorganic n-conductor: nanoporous titanium dioxide (NP-TiO2 on the scheme below), and an organic p-conductor: P3HT. The role of interlayer is in providing of connectivity between the semiconductors plus sensitizing effect. Nanoparticles of mercury telluride (HgTe) modified with special organic ligands have been used in the interlayer.
Why HgTe? Because this semiconductor possesses very narrow band gap, high conductivity, and broad absorption spectrum similar to that of the sensitizing dye N3. Therefore, it may function as a sensitizer for titanium dioxide. At the same time, HgTe may work as an n-conductor in the bulk heterojunction mode in the P3HT layer. Thus, an interconnected (stacked) combination of two types of hybrid devices: solid state dye-sensitized type and nanocrystal-hybrid blend type was fabricated. This combination has been denoted as "ASOS device".
In order to attach the HgTe-interlayer to titanium dioxide, the authors modified the surface of HgTe nanoparticles with water-soluble organic ligands (HgTe-AS [aqua-soluble] dark-blue on the scheme above). The part of the HgTe particles intended to bind to the organic layer was modified with organic-soluble ligands (HgTe-OS [organic soluble] pale-blue on the scheme above). Although the overall PCE of the device was only 0.4%, and mercury is not an element for environmentally friendly solar cells, the authors proved that the concept affords strong improvement of the device photovoltaic characteristics compared to that of reference cells with the same semiconductors but without of the interlayer.
The other approach that includes synthesis of an electroconductive polymer directly in the pores of an inorganic semiconductor has been reported by a group of scientists form National Taiwan University: Y.-J. Lin, L. Wang, and W.-Y. Chiu is published in Thin Solid Films, 2006, 511/512, page 199. The article, entitled: "Novel poly(3-methylthiophene)-TiO2 hybrid materials for photovoltaic cells" describes electropolymerization of 3-methylthiophene directly in the pores of nanoporous titania thus eliminating any problem of the preparation of a tight blend.
This elegant solution allows for the best interpenetration of polythiophene into inorganic material, which would be impossible to achieve with a separately prepared polymer because of low solubility of P3MeT, low miscibility of the solution with inorganic particles, and inability of large molecules of the polymer to penetrate into nanopores. At the same time, small molecules of the monomeric precursor: 3-methylthiophene easily diffuse inside of the pores. Despite of the low efficiency of the device fabricated from this composite (PCE = 0.03%), the general idea deserves closest attention.
Solid state dye-sensitized solar cells
'Classic' dye sensitized solar cells with liquid redox-electrolyte possess highest efficiency (PCE 10-12%) and stability among the devices with organic components in an active layer. However, commercialization of DSCs is hampered due to technological difficulties of the preservation of volatile and corrosive electrolyte. Therefore significant efforts are made to replace volatile liquid electrolyte with either nonvolatile viscous liquid or (ideally) an appropriate solid material. The use of nonvolatile organic ionic liquids we have already discussed. Here we summarize some approaches to the solid-state alternative to the redox electrolyte. Two main approaches have been studied: As an example of the first approach we introduce the work of scientists from Universite Pierre et Marie Curie, France: E. Lancelle-Beltran, P. Prene, C. Sanchez and coworkers who used regioregular poly(3-octylthiophene) as a hole conductor instead of the redox electrolyte. The work is published in Chem. Mater., 2006, 18, 6152, and entitled: Nanostructured hybrid solar cells based on self-assembled mesoporous titania thin films".
The molecules of P3OT (as we also mentioned above) are too large, and the solution of the polymer is too viscous for the efficient penetration and interaction in the commonly used colloidal TiO2 nanoparticles. Therefore, the authors prepared special mesoporous anatase with periodically aligned TiO2 nanocrystals and the size of pores large enough for the efficient 'filling' with the polymer. After sensitizing with a rutenium dye, power conversion efficiency of 0.52% has been obtained.
The second approach is used in the work of researchers from Wuhan University, China: H. Han, W. Liu, J. Zhang, and X.-Z. Zhao, who prepared solid redox electrolyte from semicrystalline poly(ethylene oxide) (PEO) mixed with poly(vinylidene fluoride) (PVDF), and 'stuffed' with conducting TiO2 nanoparticles and LiI/I2 redox-couple. The work is published in Adv. Funct. Mater., 2005, 15, 1940, and entitled: "A hybrid poly(ethylene oxide)/poly(vinylidene fluoride)/ TiO2 nanoparticle solid-state redox electrolyte for dye-sensitized nanocrystalline solar cells".
The authors stress, that the problem of known solid ionic electrolytes (which are rather very viscous liquids than true solids) is in very low conductivity that is probably due to very low diffusion speed of ions through such systems. Addition of PVDF, which may contain small fluorine ions, could result in better conductivity. Indeed, the authors found two order of magnitude of the conductivity increase in PEO-PVDF mixtures compared to 'pure' PEO-based electrolytes. The PEO-PVDF cell gave quite impressive PCE of 4.8% and incident-photon-to-current efficiency (IPCE) of about 60% over a large part of the visible region of the solar spectrum.
This column will highlight major events in organic electronics that made the history of this science.
2000 Nobel Prize in Chemistry for pioneering work on electroconductive polymers.
Awarded Alan G. MacDiarmid, Professor of Chemistry, University of Pennsylvania, (died on February 7, 2007);
Alan J. Heeger, Professor of Physics, University of California, Santa Barbara; and
Hideki Shirakawa retired Professor of Chemistry, University of Tsukuba, Japan.
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