2006 Breaking News in Organic and Molecular Electronics

06.23.06

'Smart' photovoltaics

Photovoltaics, whose current or light outputs may be controlled by current or light inputs represent attractive optoelectronic alternative to conventional logic gates built of transistors. Optoelectronic (photonic) logic gates may operate on two (electric + light), or even three (electric + light + chemical) types of inputs, whereas transistors operate only on one type of input - electrical. Hence, it is possible to achieve a complex logic function, that would normally require several transistors, in a single optoelectronic cell. Simplest logic optoelectronic devices - photodiodes have been known for a long time, however their logical output may not be superior over that of transistors.

Recently, two types of switchable dye-sensitized photovoltaic cells have been reported. Thus an international group of scientists from Italy, Denmark, and Ireland: M. Biancardo, C. Bignozzi, H. Doyle, and G. Redmond reported an electrochemical cell based on sensitizing ruthenium dye: Ru(dcbpy)₂(CN)₂ (1 on the scheme below). The work is published in Chemical Communications, 2005, 3918 and entitled: "A potential and ion switched molecular photonic logic gate".

The structure of the dye (1) differs from the famous N3 dye only by two ligands: two cyano groups in (1) are replaced for two thiocyano groups in N3. The device was fabricated with colloidal TiO₂ as an n-transporting semiconductor, similar to the standard Gratzel-type of cell. Working principle of the device is based on electrical + chemical information input and electrical + light (luminescent) output (see scheme below).

When the cell is irradiated (467 nm) and negative potential applied to the titanium dioxide electrode, excited dye molecules may not transfer an electron to the semiconductor as in a normally operating cell. Therefore, the dye exciton 'drops' excess of energy through luminescence (668 nm): state (1) on the scheme below. When the potential is switched to positive, the normal photovoltaic process occurs, along with quenching of the luminescence: state (2) on the scheme below. Finally, the luminescence may also be quenched in the negative potential state when a copper (II) salt is introduced in the system: state (3) on the scheme below.

The authors successfully reproduced NOR logic function on the device. However, a prospective drawback for this kind of logic cells should be mentioned: it is technically difficult to achieve rapid introduction and removal of Cu²⁺ ions from the system.

The other work by L. F. O. Furtado, A. D. P. Alexiou, L. Goncalves, H. E. Toma, and K. Araki from Universidade de Sao Paulo, Brazil, introduces a dye-sensitized logic gate based solely on light information input and electrical output. The work is published in Angew. Chem. Int. Ed., 2006, 45, 3143, (title: "TiO₂-based light-driven XOR/INH logic gates"), and echoed in C&EN "concentrates", 2006, May 1, p 29.

One of the biggest challenges in the construction of a light-driven logic gate was to choose a sensitized dye which would switch it's response under the action of light of different energy. The authors found that a ruthenium acetate trinuclear cluster (2 on the scheme above) forms an excited state that transfers an electron to the titanium dioxide as in the normal dye sensitized photovoltaic process (state (1) on the scheme below) when irradiated with 350 nm light.

At the same time, irradiation with the light of 420 nm gives rise to the other excited state, which reduces iodine molecules of the redox-electrolyte rather than transfers electrons to the titanium dioxide to afford the current of the opposite direction: state (3) on the scheme below. Finally, no current is observed when the cell is either in dark (state (2) on the scheme below) or irradiated with the light of both wavelengths at the same time: state (2') on the scheme below.

The authors successfully reproduced both XOR and INH logic functions on their switchable photovoltaic device.

06.09.06

Truly metallic conductivity of an emeraldine salt discovered

K. Lee, S.-H. Lee, A. J. Heeger and coworkers from Korea and USA reported recently in the Nature letter, 2006, 441, 65 for the first time discovery of truly metallic conductivity of an organic compound. The paper is entitled "Metallic transport in polyaniline". The report was also echoed in C&EN's "Concentrates", 2006, May 8, page 32.

To understand the point of the discovery, we should clarify what is the 'truly metallic conductivity'. High conductivity (1000-2000 S·cm^-1) of some organic salts, such as salts of strong acids and polyaniline (PANI), also known as emeraldine salts, has been known for a fairly long time. Although these conductivities were considered as 'metallic' in certain approximation, their electrical behavior was not consistent with truly metallic behavior.

The authors stress that metallic conductivity is not just high conductivity (typical conductivity of metals is over 40000 S·cm^-1, still much higher than that of emeraldine salts). Classic metallic conductivity is accompanied by two physical phenomena: constant (monotonic) decrease of resistivity along with decrease of temperature down to 0 K, and frequency-dependent conductivity according to Drude theory.

The authors demonstrated that both of these phenomena can be observed for a specially prepared emeraldine salt of defect-free polyaniline and camphor sulfonic acid (PANI-CSA, structure on the scheme below). High quality emeraldine base (free polyaniline) was prepared using special technique of polymerization of aniline. This technique utilizes oxidation of aniline in heterogeneous dispersion of organic solvent and water phase, where chains of polymer are grown on the surface of the solvent droplets to afford strictly linear, defects-free product.

This fundamental discovery may have deep impact on understanding and improving of the quality of organic materials with metallic-type conductivity.

06.07.06

Solution-Processed Silicon versus Organic Electronics

Picture from C&EN, April 10, 2006, p. 13

Information presented in a Nature paper: "Solution-processed silicon films and transistors", published recently by T. Shimoda, Y. Matsuki, M. Furusawa and coworkers from Japan, 2006, 440, 783, may strongly affect on the entire future of organic electronics. The work was also popularized in C&EN's "News of the week", 2006, April 10, page 13.

One of the most important points for the development of the field of organic electronics is in suitability of organic compounds for use in solution-processed, flexible, and lightweight electronic devices. The solution processing may be applied to large areas utilizing simple techniques such as spin coating and inkjet printing.

The present Nature report demonstrates efficient preparation of silicon TFTs from solution utilizing both spin-coating and inkjet printing techniques. Field-effect mobilities of the devices, not even fully optimized, were astonishing 108 cm²/V·s and 6.5 cm²/V·s respectively.

The following process has been invented: liquid cyclopentylsilane (CPS, 1 on the scheme below) was transformed to a mixture of soluble polysilanes (2) under the controlled action of UV-irradiation. The solution of polysilanes was used for cast of thin films followed by their controlled 'baking', which transformed the polysilanes to amorphous silicon (a-silicon). Mobility of the a-silicon films was low, and the researches successfully converted a-silicon to polycrystalline silicon using a special technique: treatment of the films with high energy UV irradiation from excimer laser. The polycrystalline films, thus obtained, possessed mobility similar to that of the films prepared via conventional vacuum deposition (CVD) method.

These unoptimized mobility values (even for the case of inkjet printing) are unlikely to be ever reached for organic semiconductors. So, shall we bury the entire idea of organic electronics under a big old pine?

We don't think so. First, lets take a close look at the paper. CPS (1) itself is not too cheap and available thus far. Second, it should be extremely pure, the researches redistilled it many times under the inert atmosphere before use. Third, the entire fabrication should be carried out in highly anaerobic conditions with concentration of oxygen less than 0.5 ppm. Slight failure in the controlling of the oxygen concentration gave rise to sharp decrease in mobility, whereas raise of the oxygen concentration up to 10 ppm affords insulating a-silicon instead of the semiconductor. Problem of use of high temperatures and high power irradiation should also be considered for the future "large area applications".

None of this is the case for the most of organic semiconductors and they often can be processed in ambient conditions with minimal precautions. There are also many other properties of semiconductors such as luminescent or optoelectronic properties, where silicon and other inorganic materials may have no match to organic compounds.

Therefore, we believe that the outstanding invention, reported here, will not suppress the development of organic electronics, but will be integrated with it to offer some new and useful solutions and applications.

02.03.06

Newest Developments in Plasmonics and Spintronics: Growing Threat to Conventional Microelectronics?

A series of latest articles in Science introduce new extraordinary concepts in microelectronics that persistently force involved specialists to think in a totally new way. Thus a review article by E. Ozbay from Bilkent University, Turkey, entitled "Plasmonics: Merging Photonics and Electronics at Nanoscale Dimensions" 2006, 311, page 189 summarizes prospective implication of a new branch of science - plasmonics.

Plasmonics imply a physical phenomenon: surface plasmons for a number of uses in microelectronics and optoelectronic devices. When electromagnetic field of light interacts with electrons near the surface of a metal, it forces them to regroup and move back and forth (see Figure 1 below. These moving groups of electrons (plasma of electrons) form their own electromagnetic waves, so called 'evanescent' waves. Since the electrons cannot move in the metal as fast as the photons of the triggering light, the 'evanescent' waves are much shorter than that of light. However, they possess the same high frequency and energy of the triggering light. They exponentially decay to the both sides of the surface and in combination with the moving plasma of electrons (they may not exist separately from each other) they form aforementioned phenomenon: the surface plasmons.

How the surface plasmons may help improve microelectronic technology? Several possible applications that may bring extraordinary breakthrough have already been proposed, modeled, and even experimentally confirmed:

1. The most important possible implication is in building of interconnections in superfast microelectronic integrated circuits. Conventional metallic interconnections may not longer provide sufficient operational speed to compete with ever growing microelectronic devices' operational speed, and they became "a substantial limitation to the speed of digital circuits". On the other hand, metallic interconnections may not be replaced with fast optical connectors, since the latter ones are simply too large. They may not be miniaturized just because the wavelength of light is about 1500 nm, whereas the size of microelectronic devices reaches the "order of 50 nm". The optical interconnections cannot be reduced to the size of light wavelength and smaller due to diffraction problem.

Fortunately there are the surface plasmons, who may save the situation. They combine the properties of both electric current of electrons and electromagnetic field of light. Due to shorter wavelength of the plasmon's electromagnetic field, the information can propagate on much shorter (subwavelength) distances that allows for efficient miniaturization. The problem that remains is to direct the plasmons in the 'right direction' and help them live longer. Significant progress in the construction of special plasmonic waveguides is summarized in the review. The operation of the plasmonic circuits is predicted to be 100.000! times faster than that of conventional metallic circuits.

2. Important in organic electronics, the second prospective implication of the surface plasmons is in OLED technology. It was found that significant amount of power of OLEDs may be lost "due to quenching by the surface plasmon modes". The author summarizes techniques that may help avoid this power lost and increase luminance efficiency by use of special thin film, or nanopatterned metallic contacts.

Other prospective implications of plasmonics are in laser technology, plasmonic (subwavelength) nanolithography, surface plasmon resonance (SPR) and surface plasmon immersion (SPI) microscopy.

New developments in spintronics have been published in the same issue of Science. An article "Majority Logic Gate for Magnetic Quantum-Dot Cellular Automata" by A. Imre, G. Csaba and coworkers from USA and Germany, page 205, and popularized article by R. P. Cowburn "Where Have All the Transistors Gone?", page 183 describe an approach in spintronics that uses specially arranged magnetic nanostructures. The approach, which is called 'magnetic quantum-dot cellular automata' (MQCA), is different from the previously described magnetic domain wall approach in arrangement and function of magnetic elements. While in the 'domain wall' approach the nanomagnets constantly appear and disappear in a single nanowire of special shape, the cellular automata approach uses networks of magnetic cells with a constant shape. These cells interact with each other and may change strength and direction of magnetization, but not the shape.

Though MQCA approach has been known before, an achievement of Imre et al is in construction of a universal logic gate based on this approach. Thus, a three input MAJORITY magnetic logic gate shown on the scheme above (Figure 2) "takes the majority state of its three inputs and then inverts it". This logic gate is universal because it can be easily reconfigured to perform both NAND and NOR key logic functions. Cowburn stresses possibility of "reconfigurable logic" to be extremely valuable for applications in reconfigurable hardware; as well as nonvolatility of magnetic systems (when data are retained when power is off) once again underlined.

01.21.06

Mitsubishi Chemical Announces Development of the First Efficient Blue Phosphorescent OLED Device

Mitsubishi Chemical headquarters in Tokyo

Shortly after announcement of Universal Display Corporation about the development of a blue phosphorescent OLED with practical operational lifetime (see our article before), Mitsubishi Chemical announces on December 20, 2005 the development of an efficient OLED device based on their proprietary blue phosphorescent OLED material. The news quickly spread elsewhere, and were published in Chemical & Engineering News, 2006, January 9, p 30.

In its press release, the company claims such marvelous achievement for the device as current efficiency of 30 cd/A at the intensity of 100 cd/m², and external quantum efficiency of 13%. These characteristics are the best reported to date for any fabricated OLED devices. Now it becomes apparent that recent achievements of both Mitsubishi Chemical and Universal Display Corporation in the creation of blue phosphorescent OLED materials and devices open a free way to the highly efficient ELD technology.