Freezing-in orientational disorder induces crossover from thermally-activated to temperature-independent transport in organic semiconductors.

The crystalline structure of organic materials dictates their physical properties, but while significant research effort is geared towards understanding structure-property relationships in such materials, the details remain unclear. Many organic crystals exhibit transitions in their electrical properties as a function of temperature. One example is the 1:1 charge-transfer complex trans--stilbene-2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane. Here we show that the mobility and resistivity of this material undergo a transition from being thermally activated at temperatures above 235 K to being temperature independent at low temperatures. On the basis of our experimental and theoretical results, we attribute this behaviour to the presence of a glass-like transition and the accompanied freezing-in of orientational disorder of the stilbene molecule.

Superconductivity in single crystals of the fullerene C70.

The observation of superconductivity in doped C60 has attracted much attention, as these materials represent an entirely new class of superconductors. A maximum transition temperature (Tc) of 40 K has been reported for electron-doped C60 crystals, while a Tc of 52 K has been seen in hole-doped crystals; only the copper oxide superconductors have higher transition temperatures. The results for C60 raise the intriguing questions of whether conventional electron-phonon coupling alone can produce such high transition temperatures, and whether even higher transition temperatures might be observed in other fullerenes. There have, however, been no confirmed reports of superconductivity in other fullerenes, though it has recently been observed in carbon nanotubes. Here we report the observation of superconductivity in single crystals of electric-field-doped C70. The maximum transition temperature of about 7 K is achieved when the sample is doped to approximately four electrons per C70 molecule, which corresponds to a half-filled conduction band. We anticipate superconductivity in smaller fullerenes at temperatures even higher than in C60 if the right charge density can be induced.

High-temperature superconductivity in lattice-expanded C60.

C60 single crystals have been intercalated with CHCl3 and CHBr3 in order to expand the lattice. High densities of electrons and holes have been induced by gate doping in a field-effect transistor geometry. At low temperatures, the material turns superconducting with a maximum transition temperature of 117 K in hole-doped C60/CHBr3. The increasing spacing between the C60 molecules follows the general trend of alkali metal-doped C60 and suggests routes to even higher transition temperatures.

High performance thermoelectric Tl9BiTe6 with an extremely low thermal conductivity.

Tl 9BiTe (6) exhibits a thermoelectric figure of merit of ZT approximately 1.2 around 500 K, which significantly exceeds the state-of-the-art materials in this temperature range. The extraordinary thermoelectric performance is mainly due to the extremely low thermal conductivity of Tl 9BiTe (6) [ 0.39 W/(m times K) at 300 K]. In fact, the minimum lifetime of the phonons has to be taken into account to describe the thermal conductivity data.

Universal crossover from band to hopping conduction in molecular organic semiconductors.

The charge transport in a variety of herringbone-stacked organic molecular semiconductors is investigated in the temperature range from 10 to 550 K. A crossover from coherent bandlike charge transport with mobilities up to several thousand cm (2)/V s at low temperature to an incoherent hopping motion at high temperatures is observed. This is attributed to the localization of the charge carrier due to increased electron-phonon interaction and, finally, the formation of a lattice polaron.

Josephson junctions with tunable weak links.

The electrical properties of organic molecular crystals, such as polyacenes or C60, can be tuned from insulating to superconducting by application of an electric field. By structuring the gate electrode of such a field-effect switch, the charge carrier density, and therefore also the superfluid density, can be modulated. Hence, weak links that behave like Josephson junctions can be fabricated between two superconducting regions. The coupling between the superconducting regions can be tuned and controlled over a wide range by the applied gate bias. Such devices might be used in superconducting circuits, and they are a useful scientific tool to study superconducting material parameters, such as the superconducting gap, as a function of carrier concentration or transition temperature.

Gate-induced superconductivity in a solution-processed organic polymer film.

The electrical and optical properties of conjugated polymers have received considerable attention in the context of potentially low-cost replacements for conventional metals and inorganic semiconductors. Charge transport in these organic materials has been characterized in both the doped-metallic and the semiconducting state, but superconductivity has not hitherto been observed in these polymers. Here we report a distinct metal-insulator transition and metallic levels of conductivity in a polymer field-effect transistor. The active material is solution-cast regioregular poly(3-hexylthiophene), which forms relatively well ordered films owing to self-organization, and which yields a high charge carrier mobility (0.05-0.1 cm2 V(-1) s(-1)) at room temperature. At temperatures below approximately 2.35 K with sheet carrier densities exceeding 2.5 x 10(14) cm(-2), the polythiophene film becomes superconducting. The appearance of superconductivity seems to be closely related to the self-assembly properties of the polymer, as the introduction of additional disorder is found to suppress superconductivity. Our findings therefore demonstrate the feasibility of tuning the electrical properties of conjugated polymers over the largest range possible-from insulating to superconducting.

Electron-phonon coupling spectrum in photodoped pentacene crystals.

The coupling between conduction charges and the vibrational modes of the molecular lattice plays a defining role in the transport characteristics of organic semiconductors. Using electron tunneling spectroscopy, we obtain the electron--optical-phonon coupling spectrum in photodoped pentacene crystals at energies <30 meV. Comparison of the tunneling spectrum to infrared absorption data on the optical phonon density of states yields the energy dependence of the electron-phonon scattering matrix element. The integrated spectral weight of the electron-phonon coupling shows that superconductivity in pentacene is likely of electron-phonon origin.

Superconductivity at 52 K in hole-doped C60.

Superconductivity in electron-doped C60 was first observed almost ten years ago. The metallic state and superconductivity result from the transfer of electrons from alkaline or alkaline-earth ions to the C60 molecule, which is known to be a strong electron acceptor. For this reason, it is very difficult to remove electrons from C60--yet one might expect to see superconductivity at higher temperatures in hole-doped than in electron-doped C60, because of the higher density of electronic states in the valence band than in the conduction band. We have used the technique of gate-induced doping in a field-effect transistor configuration to introduce significant densities of holes into C60. We observe superconductivity over an extended range of hole density, with a smoothly varying transition temperature Tc that peaks at 52 K. By comparison with the well established dependence of Tc on the lattice parameter in electron-doped C60, we anticipate that Tc values significantly in excess of 100 K should be achievable in a suitably expanded, hole-doped C60 lattice.

A light-emitting field-effect transistor.

We report here on the structure and operating characteristics of an ambipolar light-emitting field-effect transistor based on single crystals of the organic semiconductor alpha-sexithiophene. Electrons and holes are injected from the source and drain electrodes, respectively. Their concentrations are controlled by the applied gate and drain-source voltages. Excitons are generated, leading to radiative recombination. Moreover, above a remarkably low threshold current, coherent light is emitted through amplified spontaneous emission. Hence, this three-terminal device is the basis of a very promising architecture for electrically driven laser action in organic semiconductors.

Superconductivity in molecular crystals induced by charge injection.

Progress in the field of superconductivity is often linked to the discovery of new classes of materials, with the layered copper oxides being a particularly impressive example. The superconductors known today include a wide spectrum of materials, ranging in complexity from simple elemental metals, to alloys and binary compounds of metals, to multi-component compounds of metals and chalcogens or metalloids, doped fullerenes and organic charge-transfer salts. Here we present a new class of superconductors: insulating organic molecular crystals that are made metallic through charge injection. The first examples are pentacene, tetracene and anthracene, the last having the highest transition temperature, at 4 K. We anticipate that many other organic molecular crystals can also be made superconducting by this method, which will lead to surprising findings in the vast composition space of molecular crystals.

An organic solid state injection laser.

We report on electrically driven amplified spontaneous emission and lasing in tetracene single crystals using field-effect electrodes for efficient electron and hole injection. For laser action, feedback is provided by reflections at the cleaved edges of the crystal resulting in a Fabry-Perot resonator. Increasing the injected current density above a certain threshold value results in the decreasing of the spectral width of the emission from 120 millielectron volts to less than 1 millielectron volt because of gain narrowing and eventually laser action. High electron and hole mobilities as well as balanced charge carrier injection lead to improved exciton generation in these gate-controlled devices. Moreover, the effect of charge-induced absorption is substantially reduced in high-quality single crystals compared with amorphous organic materials.

Fractional quantum hall effect in organic molecular semiconductors.

High-quality crystals of the organic molecular semiconductors tetracene and pentacene were used to prepare metal-insulator-semiconductor (MIS) structures exhibiting hole and electron mobilities exceeding 10(4) square centimeters per volt per second. The carrier concentration in the channel region of these ambipolar field-effect devices was controlled by the applied gate voltage. Well-defined Shubnikov-de Haas oscillations and quantized Hall plateaus were observed for two-dimensional carrier densities in the range of 10(11) per square centimeter. Fractional quantum Hall states were observed in tetracene crystals at temperatures as high as approximately 2 kelvin.

A soluble and air-stable organic semiconductor with high electron mobility

Electronic devices based on organic semiconductors offer an attractive alternative to conventional inorganic devices due to potentially lower costs, simpler packaging and compatibility with flexible substrates. As is the case for silicon-based microelectronics, the use of complementary logic elements-requiring n- and p-type semiconductors whose majority charge carriers are electrons and holes, respectively-is expected to be crucial to achieving low-power, high-speed performance. Similarly, the electron-segregating domains of photovoltaic assemblies require both n- and p-type semiconductors. Stable organic p-type semiconductors are known, but practically useful n-type semiconductor materials have proved difficult to develop, reflecting the unfavourable electrochemical properties of known, electron-demanding polymers. Although high electron mobilities have been obtained for organic materials, these values are usually obtained for single crystals at low temperatures, whereas practically useful field-effect transistors (FETs) will have to be made of polycrystalline films that remain functional at room temperature. A few organic n-type semiconductors that can be used in FETs are known, but these suffer from low electron mobility, poor stability in air and/or demanding processing conditions. Here we report a crystallographically engineered naphthalenetetracarboxylic diimide derivative that allows us to fabricate solution-cast n-channel FETs with promising performance at ambient conditions. By integrating our n-channel FETs with solution-deposited p-channel FETs, we are able to produce a complementary inverter circuit whose active layers are deposited entirely from the liquid phase. We expect that other complementary circuit designs can be realized by this approach as well.

A superconducting field-effect switch.

We report here on a novel realization of a field-effect device that allows switching between insulating and superconducting states, which is the widest possible variation of electrical properties of a material. We chose C(60) as the active material because of its low surface state density and observed superconductivity in alkali metal-doped C(60). We induced three electrons per C(60) molecule in the topmost molecular layer of a crystal with the field-effect device, creating a superconducting switch operating up to 11 kelvin. An insulator was thereby transformed into a superconductor. This technique offers new opportunities for the study of superconductivity as a function of carrier concentration.

Efficient organic photovoltaic diodes based on doped pentacene.

Recent work on solar cells based on interpenetrating polymer networks and solid-state dye-sensitized devices shows that efficient solar-energy conversion is possible using organic materials. Further, it has been demonstrated that the performance of photovoltaic devices based on small molecules can be effectively enhanced by doping the organic material with electron-accepting molecules. But as inorganic solar cells show much higher efficiencies, well above 15 per cent, the practical utility of organic-based cells will require their fabrication by lower-cost techniques, ideally on flexible substrates. Here we demonstrate efficiency enhancement by molecular doping in Schottky-type photovoltaic diodes based on pentacene--an organic semiconductor that has received much attention as a promising material for organic thin-film transistors, but relatively little attention for use in photovoltaic devices. The incorporation of the dopant improves the internal quantum efficiency by more than five orders of magnitude and yields an external energy conversion efficiency as high as 2.4 per cent for a standard solar spectrum. Thin-film devices based on doped pentacene therefore appear promising for the production of efficient 'plastic' solar cells.

Ambipolar pentacene field-effect transistors and inverters.

Organic field-effect transistors based on pentacene single crystals, prepared with an amorphous aluminum oxide gate insulator, are capable of ambipolar operation and can be used for the preparation of complementary inverter circuits. The field-effect mobilities of carriers in these transistors increase from 2.7 and 1.7 square centimeters per volt per second at room temperature up to 1200 and 320 square centimeters per volt per second at low temperatures for hole and electron transport, respectively, following a power-law dependence. The possible simplification of the fabrication process of complementary logic circuits with these transistors, together with the high carrier mobilities, may be seen as another step toward applications of plastic electronics.

Fractional quantum Hall effect in organic molecular semiconductors.

High-quality crystals of the organic molecular semiconductors tetracene and pentacene were used to prepare metal-insulator-semiconductor (MIS) structures exhibiting hole and electron mobilities exceeding 10(4) square centimeters per volt per second. The carrier concentration in the channel region of these ambipolar field-effect devices was controlled by the applied gate voltage. Well-defined Shubnikov-de Haas oscillations and quantized Hall plateaus were observed for two-dimensional carrier densities in the range of 10(11) per square centimeter. Fractional quantum Hall states were observed in tetracene crystals at temperatures as high as approximately 2 kelvin.