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Showing papers on "Organic semiconductor published in 2006"


Journal ArticleDOI
TL;DR: New semiconducting liquid-crystalline thieno[3,2-b ]thiophene polymers are reported on, the enhancement in charge-carrier mobility achieved through highly organized morphology from processing in the mesophase, and the effects of exposure to both ambient and low-humidity air on the performance of transistor devices.
Abstract: Organic semiconductors that can be fabricated by simple processing techniques and possess excellent electrical performance, are key requirements in the progress of organic electronics. Both high semiconductor charge-carrier mobility, optimized through understanding and control of the semiconductor microstructure, and stability of the semiconductor to ambient electrochemical oxidative processes are required. We report on new semiconducting liquid-crystalline thieno[3,2-b ]thiophene polymers, the enhancement in charge-carrier mobility achieved through highly organized morphology from processing in the mesophase, and the effects of exposure to both ambient and low-humidity air on the performance of transistor devices. Relatively large crystalline domain sizes on the length scale of lithographically accessible channel lengths (∼200 nm) were exhibited in thin films, thus offering the potential for fabrication of single-crystal polymer transistors. Good transistor stability under static storage and operation in a low-humidity air environment was demonstrated, with charge-carrier field-effect mobilities of 0.2–0.6 cm2 V−1 s−1 achieved under nitrogen.

2,011 citations


Journal ArticleDOI
TL;DR: In this article, the authors combined and summarized the experimental findings on this nanomorphology-efficiency relationship and proposed a bicontinuous interpenetrating phase structures within these blend films.
Abstract: Within the different organic photovoltaic devices the conjugated polymer/fullerene bulk heterojunction approach is one of the foci of today's research interest. These devices are highly dependent on the solid state nanoscale morphology of the two components (donor/acceptor) in the photoactive layer. The need for finely phase separated polymer–fullerene blends is expressed by the limited exciton diffusion length present in organic semiconductors. Typical distances that these photo-excitations can travel within a pristine material are around 10–20 nm. In an efficient bulk heterojunction the scale of phase separation is therefore closely related to the respective exciton diffusion lengths of the two materials involved. Once the excitons reach the donor/acceptor interface, the photoinduced charge transfer results in the charge separation. After the charges have been separated they require percolated pathways to the respective charge extracting electrodes in order to supply an external direct current. Thus also an effective charge transport relies on the development of a suitable nanomorphology i.e. bicontinuous interpenetrating phase structures within these blend films. The present feature article combines and summarizes the experimental findings on this nanomorphology–efficiency relationship.

1,390 citations


Journal ArticleDOI
14 Dec 2006-Nature
TL;DR: The results suggest that the fabrication approach constitutes a promising step that might ultimately allow to utilize high-performance organic single-crystal field-effect transistors for large-area electronics applications.
Abstract: Organic flexible electronics are being developed for computer displays, radio frequency identification tags, sensors and devices that have not been dreamt of yet. Practical applications so far are few, as their electrical performance is poor compared with conventional electronics. In terms of charge carrier mobility, however, field-effect transistors made of organic single crystals have a very high performance. The obstacle to the use of single-crystal devices is that they have to be individually hand-made. The report of a method of fabricating large arrays of high performance transistor devices by direct patterning of single crystals onto clean silicon surfaces or flexible plastics may help to change that. The new method retains the high performance of field-effect transistors even after significant bending. Field-effect transistors made of organic single crystals are ideal for studying the charge transport characteristics of organic semiconductor materials1. Their outstanding device performance2,3,4,5,6,7,8, relative to that of transistors made of organic thin films, makes them also attractive candidates for electronic applications such as active matrix displays and sensor arrays. These applications require minimal cross-talk between neighbouring devices. In the case of thin film systems, simple patterning of the active semiconductor layer9,10 minimizes cross-talk. But when using organic single crystals, the only approach currently available for creating arrays of separate devices is manual selection and placing of individual crystals—a process prohibitive for producing devices at high density and with reasonable throughput. In contrast, inorganic crystals have been grown in extended arrays11,12,13, and efficient and large-area fabrication of silicon crystalline islands with high mobilities for electronic applications has been reported14,15. Here we describe a method for effectively fabricating large arrays of single crystals of a wide range of organic semiconductor materials directly onto transistor source–drain electrodes. We find that film domains of octadecyltriethoxysilane microcontact-printed onto either clean Si/SiO2 surfaces or flexible plastic provide control over the nucleation of vapour-grown organic single crystals. This allows us to fabricate large arrays of high-performance organic single-crystal field-effect transistors with mobilities as high as 2.4 cm2 V-1 s-1 and on/off ratios greater than 107, and devices on flexible substrates that retain their performance after significant bending. These results suggest that our fabrication approach constitutes a promising step that might ultimately allow us to utilize high-performance organic single-crystal field-effect transistors for large-area electronics applications.

968 citations


Journal ArticleDOI
TL;DR: Recent advances and future prospects of light-emitting field-effect transistors are explored, with particular emphasis on organic semiconductors and the role played by the material properties, device features and the active layer structure in determining the device performances.
Abstract: Field-effect transistors are emerging as useful device structures for efficient light generation from a variety of materials, including inorganic semiconductors, carbon nanotubes and organic thin films. In particular, organic light-emitting field-effect transistors are a new class of electro-optical devices that could provide a novel architecture to address open questions concerning charge-carrier recombination and light emission in organic materials. These devices have potential applications in optical communication systems, advanced display technology, solid-state lighting and electrically pumped organic lasers. Here, recent advances and future prospects of light-emitting field-effect transistors are explored, with particular emphasis on organic semiconductors and the role played by the material properties, device features and the active layer structure in determining the device performances.

858 citations


Journal ArticleDOI
TL;DR: Inverted organic photovoltaic devices based on a blend of poly(3-hexylthiophene) and a fullerene have been developed by inserting a solution-processed ZnO interlayer between the indium tin oxide (ITO) electrode and the active layer using Ag as a hole-collecting back contact as discussed by the authors.
Abstract: Inverted organic photovoltaic devices based on a blend of poly(3-hexylthiophene) and a fullerene have been developed by inserting a solution-processed ZnO interlayer between the indium tin oxide (ITO) electrode and the active layer using Ag as a hole-collecting back contact. Efficient electron extraction through the ZnO and hole extraction through the Ag, with minimal loss in open-circuit potential, is observed with a certified power conversion efficiency of 2.58%. The inverted architecture removes the need for the use of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) as an ITO modifier and for the use of a low-work-function metal as the back contact in the device.

814 citations


Journal ArticleDOI
TL;DR: It is shown that the neglect of electronic polarization leads to qualitatively incorrect values and trends for the transfer integrals computed with the energy splitting method, even in simple prototypes such as ethylene or pentacene dimers.
Abstract: Theoretical investigations of charge transport in organic materials are generally based on the "energy splitting in dimer" method and routinely assume that the transport parameters (site energies and transfer integrals) determined from monomer and dimer calculations can be reliably used to describe extended systems. Here, we demonstrate that this transferability can fail even in molecular crystals with weak van der Waals intermolecular interactions, due to the substantial (but often ignored) impact of polarization effects, particularly on the site energies. We show that the neglect of electronic polarization leads to qualitatively incorrect values and trends for the transfer integrals computed with the energy splitting method, even in simple prototypes such as ethylene or pentacene dimers. The polarization effect in these systems is largely electrostatic in nature and can change dramatically upon transition from a dimer to an extended system. For example, the difference in site energy for a prototypical "face-to-edge" one-dimensional stack of pentacene molecules is calculated to be 30% greater than that in the "face-to-edge" dimer, whereas the site energy difference in the pentacene crystal is vanishingly small. Importantly, when computed directly in the framework of localized monomer orbitals, the transfer integral values for dimer and extended systems are very similar.

722 citations


Journal ArticleDOI
TL;DR: In this article, the intrinsic, not limited by static disorder, charge transport in single-crystal OFETs and the nature of defects on surfaces of organic crystals are discussed.
Abstract: Small-molecule organic semiconductors, together with polymers, form the basis for the emerging field of organic electronics. Despite the rapid technological progress in this area, our understanding of fundamental electronic properties of these materials remains limited. Recently developed organic field-effect transistors (OFETs) based on single crystals of small-molecule organic materials are characterized by an unprecedented quality and reproducibility. These devices provide a unique tool to study the fundamentals of polaronic transport on organic surfaces and to explore the limits of OFET performance. This Colloquium focuses on the intrinsic, not limited by static disorder, charge transport in single-crystal OFETs and on the nature of defects on surfaces of organic crystals. In the conclusion, an outline of the outstanding problems that are now becoming within experimental reach owing to the development of single-crystal OFETs is presented.

522 citations


Journal ArticleDOI
TL;DR: In this article, the authors investigated the development of nanostructured oxide/conjugated polymer composite photovoltaic (PV) devices, which can take advantage of the high electron mobilities attainable in oxide semiconductors and can be fabricated using lowtemperature solution-based growth techniques.

521 citations


Journal ArticleDOI
TL;DR: This work has studied transport through organic single-crystal FETs with different gate insulators and finds that the temperature dependence of the mobility evolves from metallic-like to insulating-like with increasing dielectric constant of the insulator.
Abstract: In organic field-effect transistors (FETs), charges move near the surface of an organic semiconductor, at the interface with a dielectric. In the past, the nature of the microscopic motion of charge carriers—which determines the device performance—has been related to the quality of the organic semiconductor. Recently, it was discovered that the nearby dielectric also has an unexpectedly strong influence. The mechanisms responsible for this influence are not understood. To investigate these mechanisms, we have studied transport through organic single-crystal FETs with different gate insulators. We find that the temperature dependence of the mobility evolves from metallic-like to insulating-like with increasing dielectric constant of the insulator. The phenomenon is accounted for by a two-dimensional Frohlich polaron model that quantitatively describes our observations and shows that increasing the dielectric polarizability results in a crossover from the weak to the strong polaronic coupling regime. This represents a considerable step forward in our understanding of transport through organic transistors, and identifies a microscopic physical process with a large influence on device performance.

516 citations


Journal ArticleDOI
TL;DR: In this paper, it was shown that the electron transport in crystalline organic semiconductors at room temperature (RT) is neither polaronic nor a combination of thermally activated hopping and polaronics, as previously thought.
Abstract: We propose that the electron transport in crystalline organic semiconductors at room temperature (RT) is neither polaronic nor a combination of thermally activated hopping and polaronic transport, as previously thought. Thermal molecular motions cause large fluctuations in the intermolecular transfer integrals that, in turn, localize the charge carrier. This effect destroys the translational symmetry of the electronic Hamiltonian and makes the band description inadequate for RT organic crystals. We used a one-dimensional semiclassical model to compute the (temperature dependent) charge carrier mobility in the presence of thermal fluctuations of the electronic Hamiltonian. This transport mechanism explains several contrasting experimental observations pointing sometimes to a delocalized "bandlike" transport and sometimes to the existence of strongly localized charge carriers.

516 citations


Journal ArticleDOI
TL;DR: It is concluded that QD-LEDs could be made more efficient by further optimization of the organic semiconductors by using multiple spin-on HTLs.
Abstract: We report multilayer nanocrystal quantum dot light-emitting diodes (QD-LEDs) fabricated by spin-coating a monolayer of colloidal CdSe/CdS nanocrystals on top of thermally polymerized solvent-resistant hole-transport layers (HTLs). We obtain high-quality QD layers of controlled thickness (down to submonolayer) simply by spin-coating QD solutions directly onto the HTL. The resulting QD-LEDs exhibit narrow ( approximately 30 nm, fwhm) electroluminescence from the QDs with virtually no emission from the organic matrix at any voltage. Using multiple spin-on HTLs improves the external quantum efficiency of the QD-LEDs to approximately 0.8% at a brightness of 100 cd/m(2) (with a maximum brightness over 1,000 cd/m(2)). We conclude that QD-LEDs could be made more efficient by further optimization of the organic semiconductors.

Journal ArticleDOI
TL;DR: In this paper, the authors show that the rate of recombination depends on the sum of the mobilities of both carriers, and that the recombination rate in these blends is determined by the slowest charge carrier only, as a consequence of the confinement of both types of carriers to two different phases.
Abstract: Bimolecular recombination in organic semiconductors is known to follow the Langevin expression, i.e., the rate of recombination depends on the sum of the mobilities of both carriers. We show that this does not hold for polymer/fullerene bulk heterojunction solar cells. The voltage dependence of the photocurrent reveals that the recombination rate in these blends is determined by the slowest charge carrier only, as a consequence of the confinement of both types of carriers to two different phases.

Journal ArticleDOI
TL;DR: Thiophene-based n-type semiconductors exhibiting similar film morphologies and microstructures on various bilayer gate dielectrics therefore provide an incisive means to probe TFT performance parameters versus semiconductor-dielectric interface relationships.
Abstract: This study describes a general approach for probing semiconductor−dielectric interfacial chemistry effects on organic field-effect transistor performance parameters using bilayer gate dielectrics. Organic semiconductors exhibiting p-/n-type or ambipolar majority charge transport are grown on six different bilayer dielectric structures consisting of various spin-coated polymers/HMDS on 300 nm SiO2/p+-Si, and are characterized by AFM, SEM, and WAXRD, followed by transistor electrical characterization. In the case of air-sensitive (generally high LUMO energy) n-type semiconductors, dielectric surface modifications induce large variations in the corresponding OTFT performance parameters although the film morphologies and microstructures remain similar. In marked contrast, the device performance of air-stable n-type and p-type semiconductors is not significantly affected by the same dielectric surface modifications. Among the bilayer dielectric structures examined, nonpolar polystyrene coatings on SiO2 having ...

Journal ArticleDOI
TL;DR: In this article, parasitic contact effects in organic thin film transistors (OTFTs) fabricated with pentacene films have been investigated and the influence on the OTFT performance of the source and drain contact metal and the device design was investigated.
Abstract: We report on parasitic contact effects in organic thin film transistors (OTFTs) fabricated with pentacene films. The influence on the OTFT performance of the source and drain contact metal and the device design was investigated. Top contact (TC) and bottom contact (BC) gated transmission line model (gated-TLM) test structures were used to extract the combined parasitic contact resistance as a function of gate voltage swing and drain-source voltage for OTFTs with gold source and drain contacts. For comparison BC test structures with palladium contacts were studied. Differences in the bias dependence of the contact resistance for TC and BC OTFTs indicate that charge injection and device performance are strongly affected by the device design and processing. The results from this investigation show that TC and BC device performances may be contact limited for high mobility OTFTs with channel lengths less than 10μm.

Journal ArticleDOI
06 Apr 2006-Nature
TL;DR: The solution processing of silicon thin-film transistors (TFTs) using a silane-based liquid precursor is demonstrated, which shows mobilities greater than those achieved in solution-processed organic TFTs and they exceed those of a-Si T FTs.
Abstract: The manufacture of silicon semiconductor devices involves complicated photolithography and expensive machinery, so many researchers are seeking alternative semiconductor materials that can be handled by simple processes such as spin-coating or printing. Organic semiconductors are the most promising candidates but they still lack performance and reliability. Shimoda et al. have taken a different approach, printing a silicon transistor itself, not a substitute. They successfully fabricated polycrystalline silicon transistors by spin-coating a novel liquid precursor. This solution-based approach can also be adapted for ‘ink-jet’ printing of transistors. The development of a process whereby silicon can be prepared from a liquid allows the printing of semiconductor devices directly from solution. The use of solution processes—as opposed to conventional vacuum processes and vapour-phase deposition—for the fabrication of electronic devices has received considerable attention for a wide range of applications1,2,3,4,5,6,7, with a view to reducing processing costs. In particular, the ability to print semiconductor devices using liquid-phase materials could prove essential for some envisaged applications, such as large-area flexible displays. Recent research in this area has largely been focused on organic semiconductors8,9,10,11, some of which have mobilities comparable to that of amorphous silicon11 (a-Si); but issues of reliability remain. Solution processing of metal chalcogenide semiconductors to fabricate stable and high-performance transistors has also been reported12,13. This class of materials is being explored as a possible substitute for silicon, given the complex and expensive manufacturing processes required to fabricate devices from the latter. However, if high-quality silicon films could be prepared by a solution process, this situation might change drastically. Here we demonstrate the solution processing of silicon thin-film transistors (TFTs) using a silane-based liquid precursor. Using this precursor, we have prepared polycrystalline silicon (poly-Si) films by both spin-coating and ink-jet printing, from which we fabricate TFTs with mobilities of 108 cm2 V-1 s-1 and 6.5 cm2 V-1 s-1, respectively. Although the processing conditions have yet to be optimized, these mobilities are already greater than those that have been achieved in solution-processed organic TFTs, and they exceed those of a-Si TFTs (≤ 1 cm2 V-1 s-1).

Journal ArticleDOI
TL;DR: Vapor-deposited thin films of a newly developed sulfur-containing heteroarene, 2,7-diphenyl[1]benzothieno[3,2-b][1] Benzothiophene (DPh-BTBT) showed excellent FET characteristics in ambient conditions with mobilities of approximately 2.0 cm2 V-1 s-1 and Ion/Ioff of 107.5 cm2.
Abstract: Vapor-deposited thin films of a newly developed sulfur-containing heteroarene, 2,7-diphenyl[1]benzothieno[3,2-b][1]benzothiophene (DPh-BTBT), were used as an active layer of OFETs, which showed excellent FET characteristics in ambient conditions with mobilities of ∼2.0 cm2 V-1 s-1 and Ion/Ioff of 107.

Journal ArticleDOI
TL;DR: In this paper, the authors reported two viable organic excitonic solar cell structures where the conventional In2O3:Sn (ITO) hole-collecting electrode was replaced by a thin single-walled carbon nanotube layer.
Abstract: We report two viable organic excitonic solar cell structures where the conventional In2O3:Sn (ITO) hole-collecting electrode was replaced by a thin single-walled carbon nanotube layer. The first structure includes poly(3,4-ethylenedioxythiophene) (PEDOT) and gave a nonoptimized device efficiency of 1.5%. The second did not use PEDOT as a hole selective contact and had an efficiency of 0.47%. The strong rectifying behavior of the device shows that nanotubes are selective for holes and are not efficient recombination sites. The reported excitonic solar cell, produced without ITO and PEDOT, is an important step towards a fully printable solar cell.

Journal ArticleDOI
TL;DR: In this paper, a mixed quantum chemical and molecular dynamic methodology was used to assess the effect of nuclear dynamics on the modulation of the transfer integrals between close molecules in organic crystalline semiconductor molecular components.
Abstract: In organic crystalline semiconductor molecular components are held together by very weak interactions and the transfer integrals between neighboring molecular orbitals are extremely sensitive to small nuclear displacements. We used a mixed quantum chemical and molecular dynamic methodology to assess the effect of nuclear dynamics on the modulation of the transfer integrals between close molecules. We have found that the fluctuations of the transfer integrals are of the same order of magnitude of their average value for pentacene and anthracene. Under these conditions the usual perturbative treatment of the electron-phonon coupling is invalid, the band description of the crystal breaks down and the charge carriers become localized. Organic crystals of pentacene and anthracene, even in the absence of defects, can be regarded as disordered media with respect to their charge transport properties. These results suggest that the dynamic electronic disorder can be the factor limiting the charge mobility in crystalline organic semiconductors.

Journal ArticleDOI
TL;DR: In this article, the effect of hydroxyl groups on the electrical properties of pentacene-based organic thin film transistors (OTFTs) was investigated and it was confirmed that large hysteresis usually observed in OTFT devices was strongly related to the hydroxy bonds existing inside of polymeric dielectrics and could be reduced by substituting with cinnamoyl groups.
Abstract: Polymeric dielectrics having different ratios of hydroxyl groups were intentionally synthesized to investigate the effect of hydroxyl groups on the electrical properties of pentacene-based organic thin film transistors (OTFTs). Large hysteresis usually observed in OTFT devices was confirmed to be strongly related to the hydroxyl bonds existing inside of polymeric dielectrics and could be reduced by substituting with cinnamoyl groups. Although the hydroxyl groups deteriorate the capacitance-voltage characteristics and gate leakage current densities, exceptionally high hole mobility (5.5cm2V−1s−1) could be obtained by increasing the number of hydroxyl groups, which was not caused by the improvement of pentacene crystallinity but related to the interface characteristics.

Journal ArticleDOI
TL;DR: In this paper, a zero-gap semiconducting (ZGS) state in the quasi-two-dimensional organic conductor α-(BEDT-TTF) 2 I I 3 salt was shown under uniaxial pressure along the a -axis (the stacking axis of t...
Abstract: We show a zero-gap semiconducting (ZGS) state in the quasi-two-dimensional organic conductor α-(BEDT-TTF) 2 I 3 salt, which emerges under uniaxial pressure along the a -axis (the stacking axis of t...

Journal ArticleDOI
TL;DR: In this paper, the effects of optical interference and energy transfer to the quencher were considered and the effect of energy transfer was modeled when fullerenes are used as quenchers.
Abstract: Exciton diffusion is of great importance to the future design of high efficiency organic photovoltaics. Exciton diffusion studies require accurate experimental techniques. This paper addresses two important complications that can arise in exciton diffusion length measurements made by analyzing luminescence from thin films on quenching substrates: namely, the effects of optical interference and of energy transfer to the quencher. When there is modest contrast in the refractive indices of the quencher and organic material, as is the case for titania or C60 and most organic materials, interference effects can overwhelm the measurement, thereby making it impossible to accurately determine the diffusion length of excitons in the organic material. We show that this problem can be fully eliminated by using thin (<5nm) quencher films. The second complication that can occur is energy transfer to the quenching layer. We model the effect this has when fullerenes are used as quenchers. If energy transfer was ignored,...

01 Feb 2006
TL;DR: A review of organic light-emitting Diodes and their fundamental interface studies can be found in this article, where the authors propose a charge capture at polymer Heterojunctions.
Abstract: 1 Inorganic Semiconductors for Light-emitting Diodes (E. Fred Schubert, Thomas Gessmann, and Jong Kyu Kim). 1.1 Introduction. 1.2 Optical Emission Spectra. 1.3 Resonant-cavity-enhanced Structures. 1.4 Current Transport in LED Structures. 1.5 Extraction Efficiency. 1.6 Omnidirectional Reflectors. 1.7 Packaging. 1.8 Conclusion. References. 2 Electronic Processes at Semiconductor Polymer Heterojunctions (Arne C. Morteani, Richard H. Friend, and Carlos Silva). 2.1 Introduction. 2.2 Charge Capture at Polymer Heterojunctions. 2.3 Exciton Dissociation at Polymer Heterojunctions. 2.4 Morphology-dependent Exciton Retrapping at Polymer Heterojunctions. 2.5 Summary. Acknowledgments. References. 3 Photophysics of Luminescent Conjugated Polymers (Dirk Hertel and Heinz Bssler). 3.1 Introduction. 3.2 Spectroscopy of Singlet States. 3.3 Optically Induced Charge Carrier Generation. 3.4 Triplet States. 3.5 Resum. Acknowledgement. References. 4 Polymer-Based Light-Emitting Diodes (PLEDs) and Displays Fabricated from Arrays of PLEDs (Xiong Gong, Daniel Moses and Alan J. Heeger). 4.1 Introduction. 4.2 LEDs Fabricated from Semiconducting Polymers. 4.3 Accurate Measurement of OLED/PLED Device Parameters. 4.4 Fowler-Nordheim Tunneling in Semiconducting Polymer MIM Diodes. 4.5 Pixilated Displays. 4.6 Thickness Dependence of Electroluminescence Efficiency. 4.7 Limits on the Electroluminescence Efficiency. 4.8 White-light emission. 4.9 Conclusion. Note. Acknowledgement. References. 5 Metal/Polymer Interface Studies for Organic Light-Emitting Devices (Man-Keung Fung, Chun-Sing Lee, and Shuit-Tong Lee). 5.1 Review of Organic Light-Emitting Diodes and their Fundamental Interface Studies. 5.2 Polymer Materials, their Preparations, and Experimental Details. 5.3 Chemistry and Electronic Properties of Metal/F8BT. 5.4 Role of Ytterbium and Ytterbium/Cesium Fluoride on the Chemistry of F8BT. 5.5 Highly Efficient and Substrate-Independent Ytterbium/Cesium Fluoride Cathodes. 5.6 Conclusions. Acknowledgements. References. 6 The Synthesis of Electroluminescent Polymers (Andrew C. Grimsdale). 6.1 Introduction. 6.2 Poly(arylene vinylene)s. 6.3 Poly(arylene ethynylene)s. 6.4 Polyarylenes. 6.5 EL Polymers with Isolated Chromophores. 6.6 Stability of EL Polymers. 6.7 Conclusion. References. 7 Charge-transporting and Charge-blocking Amorphous Molecular Materials for Organic Light-emitting Diodes (Yasuhiko Shirota). 7.1 Introduction. 7.2 Amorphous Molecular Materials. 7.3 Requirements for Materials in OLEDs. 7.4 Amorphous Molecular Materials for Use in OLEDs. 7.5 Charge Transport in Amorphous Molecular Materials. 7.6 Outlook. References. 8 Dendrimer Light-Emitting Diodes (John M. Lupton). 8.1 Introduction. 8.2 The Dendrimer Concept. 8.3 Electroluminescent Dendritic Materials. 8.4 Electronic Properties. 8.5 Dendrimer Devices. 8.6 Dendronized Polymers. 8.7 Conclusions. References. 9 Crosslinkable Organic Semiconductors for Use in Organic Light-Emitting Diodes (OLEDs) (Klaus Meerholz, Christoph-David Mller, Oskar Nuyken). 9.1 Introduction. 9.2 Multiple-Layer Deposition. 9.3 Patterning. 9.4 Conclusion and Outlook. Acknowledgements. References. 10 Hybrid OLEDs with Semiconductor Nanocrystals (Andrey L. Rogach and John M. Lupton). 10.1 Introduction. 10.2 LEDs in the Visible based on Composites of Semiconductor Nanocrystals and Polymers or Nanocrystals and Small Organic Molecules. 10.3 Near-infrared LEDs based on Composites of Semiconductor Nanocrystals and Polymers or Small Organic Molecules. 10.4 Concluding Remarks. References. 11 Polymer Electrophosphorescence Devices (Xiaohui Yang and Dieter Neher). 11.1 Introduction. 11.2 Phosphorescent Dyes. 11.3 Transfer Processes in Polymer Hosts Doped with Phosphorescent Dyes. 11.4 Polymer Phosphorescence Devices based on PVK. 11.5 Phosphorescent Devices with Other Host Polymers. 11.6 Fully Functionalized Polymers. 11.7 Conclusion and Outlook. Acknowledgement. References. 12 Low-threshold Organic Semiconductor Lasers (Daniel Schneider, Uli Lemmer, Wolfgang Kowalsky, Thomas Riedl). 12.1 Introduction. 12.2 Fundamentals of Organic Semiconductor Lasers. 12.3 Low-threshold Organic Lasing. 12.4 Comparison of Organic Laser Properties. 12.5 Electrically Driven Organic Lasers. 12.6 Summary and Outlook. References. Subject Index.

Journal ArticleDOI
TL;DR: In this paper, a model for anomalous magnetoresistance and its change with doping based on the charge transport in these semiconductors being electron-hole recombination limited was proposed.

Journal ArticleDOI
TL;DR: In this paper, N,N′-ditridecyl-3,4,9,10 perylenetetetracarboxylic diimide (PTCDI-C13) thin-film transistors exhibited high field effect electron mobility of 2.1cm2∕Vs by just annealing at an adequate temperature (140°C) after the TFT fabrications.
Abstract: The authors demonstrated that N,N′-ditridecyl-3,4,9,10-perylenetetracarboxylic diimide (PTCDI-C13) thin-film transistors (TFTs) exhibited high field-effect electron mobility of 2.1cm2∕Vs by just annealing at an adequate temperature (140°C) after the TFT fabrications. While PTCDI-C13 formed c-axis oriented thin films, the thermal treatments improved crystallinity of the thin films as revealed by x-ray diffraction. The thermal treatment also affected thin-film morphologies; the morphologies changed from oval ball-like grains to flat and large tilelike grains, which had molecular height steps and whose size reached several micrometers.

Journal ArticleDOI
TL;DR: In this paper, organic n-channel field effect transistors and circuits based on C60 films grown by hot wall epitaxy were investigated and the electron mobility was found to be dependent strongly on the substrate temperature during film growth and on the type of the gate dielectric employed.
Abstract: We report on organic n-channel field-effect transistors and circuits based on C60 films grown by hot wall epitaxy. Electron mobility is found to be dependent strongly on the substrate temperature during film growth and on the type of the gate dielectric employed. Top-contact transistors employing LiF∕Al electrodes and a polymer dielectric exhibit maximum electron mobility of 6cm2∕Vs. When the same films are employed in bottom-contact transistors, using SiO2 as gate dielectric, mobility is reduced to 0.2cm2∕Vs. By integrating several transistors we are able to fabricate high performance unipolar (n-channel) ring oscillators with stage delay of 2.3μs.

Journal ArticleDOI
TL;DR: A method to enforce face-to-face stacking of the aromatic rings of organic semiconductor molecules in the solid state that employs bifunctional hydrogen-bond donors, in the form of semiconductor cocrystal formers, to align semiconductor building blocks.
Abstract: We report a method to enforce face-to-face stacking of the aromatic rings of organic semiconductor molecules in the solid state that employs bifunctional hydrogen-bond donors, in the form of semiconductor cocrystal formers, to align semiconductor building blocks.

Journal ArticleDOI
TL;DR: Fan-shaped electrodes were designed on Si∕SiO2 substrate to measure the anisotropic field effect mobility in freestanding single crystal pentacene as mentioned in this paper, and the highest mobility value was estimated to be ∼2.3cm2∕Vs at room temperature.
Abstract: Fan-shaped electrodes were designed on Si∕SiO2 substrate to measure the anisotropic field effect mobility in freestanding single crystal pentacene. Field effect transistor was fabricated by placing single crystal pentacene on the prepatterned electrodes. The contact between the electrodes and single crystal pentacene was enhanced by applying pressure. Angle dependence of field effect mobility in single crystal pentacene showed remarkably anisotropic behavior. The highest mobility value was estimated to be ∼2.3cm2∕Vs at room temperature.

Journal ArticleDOI
16 Nov 2006-Nature
TL;DR: Scanning tunnelling spectroscopy observations of an organic monolayer film on a silver substrate reveal a completely delocalized two-dimensional band state that is characterized by a metal-like parabolic dispersion with an effective mass of m* = 0.47me, where me is the bare electron mass.
Abstract: When a current flows through a molecular material, the electrons may either hop slowly from molecule to molecule, or they may move swiftly through the material at large The latter mechanism is possible only if the electronic states involved in current flow are dispersed and allow extensive electron delocalization Scanning tunnelling microscopy experiments on an archetypal organic semiconductor (PTCDA) deposited on a silver substrate have now made extensive dispersion and electron delocalization visible for the first time in a thin molecular layer The observed dispersion is far stronger than expected for the organic material alone This suggests that strong electron mobility enhancements — as often desired for electronic applications — may be achieved by tailoring the intermolecular coupling between organic molecules and their metal substrate Thin films of molecular organic semiconductors are attracting much interest for use in electronic and optoelectronic applications The electronic properties of these materials and their interfaces are therefore worth investigating intensively1,2,3, particularly the degree of electron delocalization that can be achieved2,4 If the delocalization is appreciable, it should be accompanied by an observable electronic band dispersion But so far only limited experimental data on the intermolecular dispersion of electronic states in molecular materials is available5,6,7,8, and the mechanism(s) of electron delocalization in molecular materials are also not well understood Here we report scanning tunnelling spectroscopy observations of an organic monolayer film on a silver substrate, revealing a completely delocalized two-dimensional band state that is characterized by a metal-like parabolic dispersion with an effective mass of m* = 047me, where me is the bare electron mass This dispersion is far stronger than expected for the organic film alone7, and arises as a result of strong substrate-mediated coupling between the molecules within the monolayer

Journal ArticleDOI
TL;DR: In this article, a two-component layered structure of organic light-emitting transistors (OLETs) with balanced ambipolar transport and mobility as large as 3 × 10 cm V s is presented.
Abstract: Today organic materials are routinely employed for the fabrication of light-emitting devices (OLEDs) and thin-film transistors (OTFTs), with the first technological realizations already having reached the market. Moreover, OTFTs with unipolar mobility values comparable to those of amorphous silicon (1 cm V s) have now been demonstrated. Applications impacting display technologies and those sectors where low cost is a key factor and low performance is acceptable include electronic paper and radio-frequency identification (RF-ID) products. In a recent development, OTFTs also exhibiting electroluminescence (EL) have been successfully demonstrated. Organic light-emitting transistors (OLETs) represent a significant technological advance by combining two functionalities, electrical switching and light emission, in a single device, thus significantly increasing the potential applications of organic semiconductors. In particular, if appropriate materials can be introduced, OLETs offer an ideal structure for improving the lifetime and efficiency of organic light-emitting heterostructures due to the intrinsically different driving conditions and charge-carrier balance compared to conventional OLEDs. Potential applications of OLETs include flat-panel display technologies, lighting, and, ultimately, easily fabricated organic lasers. The first OLET prototypes were unipolar transport devices, and recombination was expected to take place in close proximity to the metallic drain electrode where efficiency-depleting exciton quenching is also likely to occur. To avoid this significant device deficiency and to instead generate EL nearer the center of the channel, OLETs with ambipolar charge transport would be highly desirable. Furthermore, balanced ambipolar conduction is crucial for maximizing exciton recombination through efficient electron–hole balancing. Up to now various solutions have been proposed: single ambipolar materials and two-component coevaporated or layered structures. In coevaporated films, two materials are simultaneously sublimed to form bulk heterojunctions. However, carrier transport is unbalanced and the mobility values are below 10 cm V s. Devices employing a polymer film showing intrinsic ambipolar transport have also been reported but with mobility values for both charge carriers around 10 cm V s. In this paper we report OLETs based on two-component layered structures that have balanced ambipolar transport and mobility values as large as 3 × 10 cm V s. These devices are realized by sequentially depositing p-type (a,x-dihexyl-quaterthiophene, DH4T) and n-type films (N,N′-ditridecylperylene-3,4,9,10-tetracarboxylic diimide, PTCDIC13H27, P13). The combination with the highest mobility and most-balanced transport is obtained with DH4T grown in direct contact with the dielectric. For comparison, we have also employed pentacene in place of DH4T as the p-type material and showed that unbalanced ambipolarity is obtained. Morphological analysis of the outermost and buried layers, performed by laser scanning confocal microscopy (LSCM), allows selective imaging of materials with energetically separated photoluminescence (PL) spectra. Importantly, it is shown that ‘growth compatibility’ between the nand p-type materials is essential in forming a continuous interface and thereby controlling the resulting OLET optoelectronic-response properties. Each OLET material was first evaluated in a single layer in a top source–drain contact OTFT. As substrates we employed heavily doped silicon wafers with thermally grown oxides. Surface treatments such as octadecyltrichlorosilane or hexamethyldisilazane did not result in substantial improvement in the device performance. Parameters such as substrate temperature (Tsub) and evaporation rate were varied to optimize electrical characteristics. The optimum growth conditions were found to be: Tsub = 90 °C and rate = 0.1 A s –1 for DH4T, and Tsub = 25 °C and rate = 0.1 A s –1 for P13. In Table 1, the mobility (l) and threshold-voltage (Vth) values obtained are summarized. These are comparable to the highest values reported in the literature. The DH4T devices were stable even C O M M U N IC A TI O N S

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TL;DR: In this article, an overview of the materials' aspect, charge-transport, and device physics of organic field effect transistors (OFETs), focusing mainly on the organic semiconductor and organic dielectric materials and their mutual interface is presented.
Abstract: ▪ Abstract Organic field-effect transistors (OFETs) based on solution-processible polymeric as well as small molecular semiconductors have shown impressive improvements in their performance during recent years. These devices have been developed to realize low-cost, large-area electronic products. This review gives an overview of the materials’ aspect, charge-transport, and device physics of OFETs, focusing mainly on the organic semiconductor and organic dielectric materials and their mutual interface. Recent developments in the understanding of the relationship between microstructure and charge transport, the influence of processing techniques, and gate dielectric are reviewed. Comparative data of charge-carrier mobility of most organic semiconductors have been compiled. Ambipolar charge transport in OFETs and its applications to integrated circuits as well as ambipolar light-emitting transistors are also reviewed. Many interesting questions regarding how the molecular and electronic structures at the int...