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Showing papers on "Organic semiconductor published in 2007"


Journal ArticleDOI
TL;DR: This review gives a general introduction to the materials, production techniques, working principles, critical parameters, and stability of the organic solar cells, and discusses the alternative approaches such as polymer/polymer solar cells and organic/inorganic hybrid solar cells.
Abstract: The need to develop inexpensive renewable energy sources stimulates scientific research for efficient, low-cost photovoltaic devices.1 The organic, polymer-based photovoltaic elements have introduced at least the potential of obtaining cheap and easy methods to produce energy from light.2 The possibility of chemically manipulating the material properties of polymers (plastics) combined with a variety of easy and cheap processing techniques has made polymer-based materials present in almost every aspect of modern society.3 Organic semiconductors have several advantages: (a) lowcost synthesis, and (b) easy manufacture of thin film devices by vacuum evaporation/sublimation or solution cast or printing technologies. Furthermore, organic semiconductor thin films may show high absorption coefficients4 exceeding 105 cm-1, which makes them good chromophores for optoelectronic applications. The electronic band gap of organic semiconductors can be engineered by chemical synthesis for simple color changing of light emitting diodes (LEDs).5 Charge carrier mobilities as high as 10 cm2/V‚s6 made them competitive with amorphous silicon.7 This review is organized as follows. In the first part, we will give a general introduction to the materials, production techniques, working principles, critical parameters, and stability of the organic solar cells. In the second part, we will focus on conjugated polymer/fullerene bulk heterojunction solar cells, mainly on polyphenylenevinylene (PPV) derivatives/(1-(3-methoxycarbonyl) propyl-1-phenyl[6,6]C61) (PCBM) fullerene derivatives and poly(3-hexylthiophene) (P3HT)/PCBM systems. In the third part, we will discuss the alternative approaches such as polymer/polymer solar cells and organic/inorganic hybrid solar cells. In the fourth part, we will suggest possible routes for further improvements and finish with some conclusions. The different papers mentioned in the text have been chosen for didactical purposes and cannot reflect the chronology of the research field nor have a claim of completeness. The further interested reader is referred to the vast amount of quality papers published in this field during the past decade.

6,059 citations


Journal ArticleDOI
TL;DR: Electronic Coupling in Oligoacene Derivatives: Factors Influencing Charge Mobility, and the Energy-Splitting-in-Dimer Method 3.1.
Abstract: 2.2. Materials 929 2.3. Factors Influencing Charge Mobility 931 2.3.1. Molecular Packing 931 2.3.2. Disorder 932 2.3.3. Temperature 933 2.3.4. Electric Field 934 2.3.5. Impurities 934 2.3.6. Pressure 934 2.3.7. Charge-Carrier Density 934 2.3.8. Size/molecular Weight 935 3. The Charge-Transport Parameters 935 3.1. Electronic Coupling 936 3.1.1. The Energy-Splitting-in-Dimer Method 936 3.1.2. The Orthogonality Issue 937 3.1.3. Impact of the Site Energy 937 3.1.4. Electronic Coupling in Oligoacene Derivatives 938

3,635 citations


Journal ArticleDOI
15 Feb 2007-Nature
TL;DR: This work demonstrates an organic circuit with very low power consumption that uses a self-assembled monolayer gate dielectric and two different air-stable molecular semiconductors (pentacene and hexadecafluorocopperphthalocyanine, F16CuPc) to implement transistors, circuits, displays and sensors on arbitrary substrates.
Abstract: Organic transistors and circuits show great promise for the realization of futuristic roll-up displays, adaptive sensors for humanoid robots and ubiquitous radio-frequency identification tags. But today's organic circuits require operating voltages of 15 to 30 volts (10 to 20 batteries' worth), and they draw enough power to drain those batteries in a day. To overcome this major hurdle, Hagen Klauk et al. have developed a method of fabricating organic circuits that run on a single 1.5-volt battery for several years. The key to the method is the use of a layer of an insulating organic material just one molecule thick; although the layer is very thin, it leaks only a small amount of current, while it provides for a large capacitance. Two different types of organic semiconductors are used to fabricate transistors, logic gates and ring oscillators. A report of the development of organic electronic circuits, which require only a single 1.5V battery and last for several years. The main ingredient is the use of a single layer of an insulating organic material. Although the layer is very thin, it leaks only small amount of current, while providing for a large capacitance. The prospect of using low-temperature processable organic semiconductors to implement transistors, circuits, displays and sensors on arbitrary substrates, such as glass or plastics, offers enormous potential for a wide range of electronic products1. Of particular interest are portable devices that can be powered by small batteries or by near-field radio-frequency coupling. The main problem with existing approaches is the large power consumption of conventional organic circuits, which makes battery-powered applications problematic, if not impossible. Here we demonstrate an organic circuit with very low power consumption that uses a self-assembled monolayer gate dielectric and two different air-stable molecular semiconductors (pentacene and hexadecafluorocopperphthalocyanine, F16CuPc). The monolayer dielectric is grown on patterned metal gates at room temperature and is optimized to provide a large gate capacitance and low gate leakage currents. By combining low-voltage p-channel and n-channel organic thin-film transistors in a complementary circuit design, the static currents are reduced to below 100 pA per logic gate. We have fabricated complementary inverters, NAND gates, and ring oscillators that operate with supply voltages between 1.5 and 3 V and have a static power consumption of less than 1 nW per logic gate. These organic circuits are thus well suited for battery-powered systems such as portable display devices2 and large-surface sensor networks3 as well as for radio-frequency identification tags with extended operating range4.

1,324 citations


Journal ArticleDOI
TL;DR: Evaluations of the devices under ambient conditions showed typical p-channel FET responses with the field-effect mobility higher than 1.0 cm2 V-1 s-1 and Ion/Ioff of approximately 10(7).
Abstract: 2,7-Dialkyl[1]benzothieno[3,2-b]benzothiophenes were tested as solution-processible molecular semiconductors. Thin films of the organic semiconductors deposited on Si/SiO2 substrates by spin coating have well-ordered structures as confirmed by XRD analysis. Evaluations of the devices under ambient conditions showed typical p-channel FET responses with the field-effect mobility higher than 1.0 cm2 V-1 s-1 and Ion/Ioff of ∼107.

847 citations


Journal ArticleDOI
TL;DR: In this article, the fundamental aspects behind the structural design/realization of p- and n-channel semiconductors for organic field effect transistors (OFETs) are discussed.

804 citations


Journal ArticleDOI
TL;DR: This review will focus on synthetic strategies that have been investigated for the preparation of optoelectronically active solution-processable dendritic materials and concentrate on two different applications, namely OLEDs and solar cells, in which they have been used.
Abstract: Introduction: Branched macromolecules or dendrimers have provided a rich seam of research in terms of both innovative chemistry and applications.1-14 For example, dendrimers have been studied for use as low-dielectric materials,15 as templates for the growth of single-wall carbon nanotubes,16 as catalysts,17-19 and in biological applications,20-23 including biosensors,24 magnetic resonance imaging,25-28 and drug delivery.29-33 However, it has only been more recently that such macromolecular structures have been explored in terms of their electronic and optoelectronic properties, which is the focus of this series of reviews. For example, charge-transporting dendrimers have become an important class of organic semiconducting material34 and significant effort has focused on light harvesting and energy transfer from a peripheral dye or chromophore to an emissive dye at the center or focus of the dendrimer.35-39 Organic semiconductors have become increasingly important as the active component in applications including organic light-emitting diodes (OLEDs),40-42 transistors,43,44 photovoltaic (PV) cells,45,46 optical amplifiers,47,48 and lasers.49-51 Traditionally, organic semiconductors have fallen into two main classes, small molecules and polymers, and these materials and their applications will be covered in detail by other authors. Small molecules are generally processed by evaporation techniques and have the advantages that the structure−property relationships are relatively simple to understand, the materials are mono(disperse), and they are deposited in a pure form. On the other hand, conjugated polymers are soluble and can be deposited from solution by processes such as spin-coating and ink-jet printing, which opens up the exciting prospect of simple, fast, large-area, low-temperature device manufacturing. An additional advantage for conjugated polymers is that solution processing is potentially less wasteful of material than evaporation for devices that require patterning. However, it is often difficult to control the polydispersity, molecular weight, backbone defects, and end groups of conjugated polymers reproducibly. Branched macromolecules, known as dendrimers, also have the advantage of being solution processable but by careful design can incorporate the control over the optoelectronic properties that is reminiscent of small molecules. In addition, the dendritic architecture provides a number of other attractive properties, including the ability to independently control the processing and optoelectronic properties; providing the processing power to enable simple chromophores to be deposited as stable amorphous films; dendrimer generation as a tool for controlling the intermolecular interactions that govern device performance; and the ability in well-defined dendrimers to have high chemical purity. In this review, we will focus on synthetic strategies that have been investigated for the preparation of optoelectronically active solution-processable dendritic materials and concentrate on two different applications, namely OLEDs and solar cells, in which they have been used. In the context of OLEDs, we limit the discussion to light emission, as branched macromolecules for charge transport will be discussed in the review by Shirota. We will also briefly comment on other recent light-emitting and -absorbing branched molecular materials that have been used in OLEDs and solar cells.

707 citations


Journal ArticleDOI
TL;DR: Very high-mobility organic transistors are fabricated with purified rubrene single crystals and high-density organosilane self-assembled monolayers in this paper, where the interface with minimized surface levels allows carriers to distribute deep into the crystals by more than a few molecular layers under weak gate electric fields, and the inner channel plays a significant part in the transfer performance.
Abstract: Very high-mobility organic transistors are fabricated with purified rubrene single crystals and high-density organosilane self-assembled monolayers. The interface with minimized surface levels allows carriers to distribute deep into the crystals by more than a few molecular layers under weak gate electric fields, so that the inner channel plays a significant part in the transfer performance. With the in-crystal carriers less affected by scattering mechanisms at the interface, the maximum transistor mobility reaches 18cm2∕Vs and the contact-free intrinsic mobility turned out to be 40cm2∕Vs as the result of four-terminal measurement. These are the highest values ever reported for organic transistors.

666 citations


Journal ArticleDOI
TL;DR: In this article, a solution-processed organic thin film transistors (OTFTs) with carrier mobility > 1cm2∕Vs, current on/off ratio greater than 107, and subthreshold slope < 0.3V/decade were fabricated.
Abstract: Using the small molecule organic semiconductor 6,13-bis(triisopropyl-silylethynyl) pentacene (TIPS-pentacene), the authors have fabricated the solution-processed organic thin film transistors (OTFTs) with carrier mobility >1cm2∕Vs, current on/off ratio greater than 107, and subthreshold slope <0.3V/decade. The high mobility TIPS-pentacene solution-processed films are deposited from high boiling point solvents and show strong molecular ordering including molecular terracing. Film ordering varies substantially for different solvents and film deposition techniques and OTFT mobility correlates well with film ordering.

584 citations


Journal ArticleDOI
TL;DR: In this article, the physical processes that lead to photocurrent generation in organic solar cells, as well as the various architectures employed to optimize device performance are discussed, including donor-acceptor heterojunction for efficient exciton dissociation, the exciton blocking layer, the mixed or bulk heterjunction, and the stacked or tandem cell.
Abstract: In this review, we focus on the field of organic photovoltaic cells based on small molecular weight materials. In particular, we discuss the physical processes that lead to photocurrent generation in organic solar cells, as well as the various architectures employed to optimize device performance. These include the donor–acceptor heterojunction for efficient exciton dissociation, the exciton blocking layer, the mixed or bulk heterojunction, and the stacked or tandem cell. We show how the choice of materials with known energy levels and absorption spectra affect device performance, particularly the open-circuit voltage and short-circuit current density. We also discuss the typical materials and growth techniques used to fabricate devices, as well as the issue of device stability, all of which are critical for the commercialization of low-cost and high-performance organic solar cells. Copyright © 2007 John Wiley & Sons, Ltd.

504 citations


Proceedings Article
01 May 2007
TL;DR: In this paper, the chemistry, the physics and engineering of solar cells based on organic materials are discussed. And a tutorial is provided to provide an overview of the chemistry and physics of the solar cells.
Abstract: Solid-state organic photovoltaic technologies are emerging and maturing with reports of power conversion efficiencies close to 5%. This tutorial will provide an overview of the chemistry, the physics and engineering of solar cells based on organic materials.

476 citations


Journal ArticleDOI
TL;DR: In this paper, the effects of surface modification of titania (TiO2) in hybrid TiO2∕regioregular poly(3-hexylthiophene) (P3HT) photovoltaic cells were systematically investigated.
Abstract: We have systematically investigated the effects of surface modification of titania (TiO2) in hybrid TiO2∕regioregular poly(3-hexylthiophene) (P3HT) photovoltaic cells. By employing a series of para-substituted benzoic acids with varying dipoles and a series of multiply substituted benzene carboxylic acids, the energy offset at the TiO2∕polymer interface and thus the open-circuit voltage of devices can be tuned systematically by 0.25 V. Transient photovoltage measurements showed that the recombination kinetics was dominated by charge carrier concentration in these devices and were closely associated with the dark current. The saturated photocurrent of TiO2∕P3HT devices exhibited more than a twofold enhancement when molecular modifiers with large electron affinity were employed. The ability of modifiers to accept charge from polymers, as revealed in photoluminescence quenching measurement with blends of polymers, was shown to be correlated with the enhancement in device photocurrent. A planar geometry photo...

Journal ArticleDOI
TL;DR: The field effect transistor (FET) has served as a versatile tool for electrical characterization of many facets of organic molecular crystals as discussed by the authors, including the Hall effect and the intrinsic transport properties.

Journal ArticleDOI
TL;DR: Perylenetetracarboxyldiimide nanowires self-assembled from commercially available materials are demonstrated as the n-channel semiconductor in organic field-effect transistors (OFETs) and as a building block in high-performance complementary inverters.
Abstract: Perylenetetracarboxyldiimide (PTCDI) nanowires self-assembled from commercially available materials are demonstrated as the n-channel semiconductor in organic field-effect transistors (OFETs) and as a building block in high-performance complementary inverters. Devices based on a network of PTCDI nanowires have electron mobilities and current on/off ratios on the order of 10-2 cm2/Vs and 104, respectively. Complementary inverters based on n-channel PTCDI nanowire transistors and p-channel hexathiapentacene (HTP) nanowire OFETs achieved gains as high as 8. These results demonstrate the first example of the use of one-dimensional organic semiconductors in complementary inverters.

Journal ArticleDOI
TL;DR: In this article, the authors illustrate possible alternative strategies based on the development of organic semiconductors with higher dimensionality, capable of exhibiting isotropic electronic properties, such as organic light-emitting diodes (OLEDs), organic field effect transistors (OFETs), or solar cells.
Abstract: Organic semiconductors based on π-conjugated systems are the focus of considerable interest in the emerging area of soft or flexible photonics and electronics. Whereas in recent years the performances of devices such as organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), or solar cells have undergone considerable progress, a number of technical and fundamental problems related to the low dimensionality of organic semiconductors based on linear π-conjugated systems remain unsatisfactorily resolved. This low dimensionality results in an anisotropy of the optical and charge-transport properties, which in turn implies a control of the material organization/molecular orientation during or after device fabrication. Such a constraint evidently represents a problem when device fabrication by solution-based processes, such as printing techniques, is envisioned. The aim of this short Review is to illustrate possible alternative strategies based on the development of organic semiconductors with higher dimensionality, capable to exhibit isotropic electronic properties.

Journal ArticleDOI
TL;DR: In this paper, a semitransparent metal electrode fabricated by nano-imprint lithography (NIL) was evaluated as an anode material for OLED anodes and evaluated its potential as anodes.
Abstract: Organic light-emitting diodes (OLEDs) are promising for full-color, full-motion, flat panel display applications because they offer several advantageous features, for example, ease of fabrication, low costs, light weight, bright self-emission, a wide viewing angle, and the possibility of flexible displays. The basic OLED structure consists of a number of organic semiconductor layers sandwiched between a cathode and an anode. For efficient electron injection into the organic layers, low-work-function materials are required for the cathode. A very thin LiF layer with a thick Al capping is widely used for this purpose. For the anode, indium tin oxide (ITO) is the predominant choice because it offers transparency in the visible range of the electromagnetic spectrum as well as electrical conductivity. However, several aspects of ITO are far from optimal for high-performance OLEDs. It is known that the migration of indium and oxygen from ITO into organic semiconductors during OLED operation causes device degradation. The electrical properties of ITO greatly depend on the film preparation. The rough surface of the deposited ITO film and the work function of ITO, ca. 4.7 eV, limit the efficiency of the hole injection. The typical sheet resistance of a 100 nm thick ITO layer, 20–80 X/ , is still high, which causes a voltage drop along the addressing line, thus limiting the operation of a large-area passive matrix OLED array. Moreover, the cost of ITO has escalated in recent years because of the jump in price of the element indium. Several alternative materials, for example, TiN, Al-doped ZnO, and fluorine tin oxide, have been investigated as anode materials instead of ITO; however, none are optimal as anode in OLEDs because they have either a lower work function or a lower conductivity than ITO. Other transparent conducting oxides, such as Ga–In–Sn–O (GITO), Zn–In–Sn–O (ZITO), Ga–In–O (GIO), and Zn–In–O (ZIO), that have a higher work function and a similar electrical conductivity when compared to ITO have also been examined as OLED anode materials. However, they are potentially problematic because they also contain the element indium that i) may diffuse into the organic layer in the OLED; and ii) has a high price, making these electrodes expensive. Besides these materials several metals with a high work function, such as Au, Ni, and Pt, have been investigated as anodes for OLEDs. In these cases the metal was used to modify the surface of the ITO electrode, or as an anode for top-emitting devices. A surface-modified thin Ag film has been used as a semitransparent electrode instead of ITO, but its transparency was low. Recently, carbon nanotube films have been investigated as transparent, conductive electrodes, but they have a high sheet resistance that may limit the device performance. In this Communication, we report semitransparent metal electrodes fabricated by nanoimprint lithography (NIL), and evaluate their potential as OLED anodes. NIL, an emerging lithographic technique, is well-suited to the area of organic electronics, which requires low-cost and high-throughput fabrication at high resolution. The fabricated semitransparent metal electrode offers several advantages over ITO for OLED applications. First, several problems associated with ITO can be eliminated, such as device degradation by indium diffusion and high costs. Second, efficient hole injection into the organic semiconductor can be realized by choosing metals with a high work function, such as Au or Pt. Third, a semitransparent metal electrode is potentially suitable for topemitting devices and tandem structures. Last, but not least, the output efficiency of the OLED can be enhanced by preventing waveguiding in the ITO layer, which occurs as a result of its high refractive index and is one of the limitations to the external efficiency of OLEDs, and by forming a two-dimensional (2D) hole array with proper periodicity. We demonstrate here that a unique property of such an electrode is that its optical transparency and the electrical conductivity can be tuned separately by changing the aperture ratio and the metal thickness, thereby making it possible to tailor the structures for different applications. To our knowledge, a nanoimprinted semitransparent metal electrode has not been reported before. The semitransparent metal electrodes are in the form of a nanometer-scale periodically perforated dense metal mesh on glass. Two design considerations led to such structures: i) the line width of the metal mesh was designed to be subwavelength, to provide sufficient transparency and to minimize light scattering; and ii) the period of the mesh was chosen to be sub-micrometer to ensure the uniformity of the current injection into the organic semiconductors. Such large-area dense nanostructures can be fabricated by NIL, which is ideal for this application because of its inherently high resolution C O M M U N IC A IO N

Journal ArticleDOI
TL;DR: In this paper, multiple junction solar cells incorporating polymer:fullerene bulk heterojunctions as active layers and solution processed electron and hole transport layers are presented, and the recombination layer, deposited between the active layers, is fabricated by spin coating ZnO nanoparticles from acetone, followed by spin-coating neutral pH poly(3,4-ethylenedioxythiophene) from water and short UV illumination of the completed device.
Abstract: Multiple junction solar cells incorporating polymer:fullerene bulk heterojunctions as active layers and solution processed electron and hole transport layers are presented. The recombination layer, deposited between the active layers, is fabricated by spin coating ZnO nanoparticles from acetone, followed by spin coating neutral pH poly(3,4-ethylenedioxythiophene) from water and short UV illumination of the completed device. The key advantage of this procedure is that each step does not affect the integrity of previously deposited layers. The open-circuit voltage (Voc) for double and triple junction solar cells is close to the sum of the Voc’s of individual cells.

Journal ArticleDOI
TL;DR: Inorganic Semiconductors for Light-emitting Diodes (E Fred Schubert, Thomas Gessmann, and Dieter Neher) as mentioned in this paper have been used in the development of organic light emitting devices (OLEDs).
Abstract: 1 Inorganic Semiconductors for Light-emitting Diodes (E Fred Schubert, Thomas Gessmann, and Jong Kyu Kim) 11 Introduction 12 Optical Emission Spectra 13 Resonant-cavity-enhanced Structures 14 Current Transport in LED Structures 15 Extraction Efficiency 16 Omnidirectional Reflectors 17 Packaging 18 Conclusion References 2 Electronic Processes at Semiconductor Polymer Heterojunctions (Arne C Morteani, Richard H Friend, and Carlos Silva) 21 Introduction 22 Charge Capture at Polymer Heterojunctions 23 Exciton Dissociation at Polymer Heterojunctions 24 Morphology-dependent Exciton Retrapping at Polymer Heterojunctions 25 Summary Acknowledgments References 3 Photophysics of Luminescent Conjugated Polymers (Dirk Hertel and Heinz Bssler) 31 Introduction 32 Spectroscopy of Singlet States 33 Optically Induced Charge Carrier Generation 34 Triplet States 35 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) 41 Introduction 42 LEDs Fabricated from Semiconducting Polymers 43 Accurate Measurement of OLED/PLED Device Parameters 44 Fowler-Nordheim Tunneling in Semiconducting Polymer MIM Diodes 45 Pixilated Displays 46 Thickness Dependence of Electroluminescence Efficiency 47 Limits on the Electroluminescence Efficiency 48 White-light emission 49 Conclusion Note Acknowledgement References 5 Metal/Polymer Interface Studies for Organic Light-Emitting Devices (Man-Keung Fung, Chun-Sing Lee, and Shuit-Tong Lee) 51 Review of Organic Light-Emitting Diodes and their Fundamental Interface Studies 52 Polymer Materials, their Preparations, and Experimental Details 53 Chemistry and Electronic Properties of Metal/F8BT 54 Role of Ytterbium and Ytterbium/Cesium Fluoride on the Chemistry of F8BT 55 Highly Efficient and Substrate-Independent Ytterbium/Cesium Fluoride Cathodes 56 Conclusions Acknowledgements References 6 The Synthesis of Electroluminescent Polymers (Andrew C Grimsdale) 61 Introduction 62 Poly(arylene vinylene)s 63 Poly(arylene ethynylene)s 64 Polyarylenes 65 EL Polymers with Isolated Chromophores 66 Stability of EL Polymers 67 Conclusion References 7 Charge-transporting and Charge-blocking Amorphous Molecular Materials for Organic Light-emitting Diodes (Yasuhiko Shirota) 71 Introduction 72 Amorphous Molecular Materials 73 Requirements for Materials in OLEDs 74 Amorphous Molecular Materials for Use in OLEDs 75 Charge Transport in Amorphous Molecular Materials 76 Outlook References 8 Dendrimer Light-Emitting Diodes (John M Lupton) 81 Introduction 82 The Dendrimer Concept 83 Electroluminescent Dendritic Materials 84 Electronic Properties 85 Dendrimer Devices 86 Dendronized Polymers 87 Conclusions References 9 Crosslinkable Organic Semiconductors for Use in Organic Light-Emitting Diodes (OLEDs) (Klaus Meerholz, Christoph-David Mller, Oskar Nuyken) 91 Introduction 92 Multiple-Layer Deposition 93 Patterning 94 Conclusion and Outlook Acknowledgements References 10 Hybrid OLEDs with Semiconductor Nanocrystals (Andrey L Rogach and John M Lupton) 101 Introduction 102 LEDs in the Visible based on Composites of Semiconductor Nanocrystals and Polymers or Nanocrystals and Small Organic Molecules 103 Near-infrared LEDs based on Composites of Semiconductor Nanocrystals and Polymers or Small Organic Molecules 104 Concluding Remarks References 11 Polymer Electrophosphorescence Devices (Xiaohui Yang and Dieter Neher) 111 Introduction 112 Phosphorescent Dyes 113 Transfer Processes in Polymer Hosts Doped with Phosphorescent Dyes 114 Polymer Phosphorescence Devices based on PVK 115 Phosphorescent Devices with Other Host Polymers 116 Fully Functionalized Polymers 117 Conclusion and Outlook Acknowledgement References 12 Low-threshold Organic Semiconductor Lasers (Daniel Schneider, Uli Lemmer, Wolfgang Kowalsky, Thomas Riedl) 121 Introduction 122 Fundamentals of Organic Semiconductor Lasers 123 Low-threshold Organic Lasing 124 Comparison of Organic Laser Properties 125 Electrically Driven Organic Lasers 126 Summary and Outlook References Subject Index

Journal ArticleDOI
TL;DR: Spin polarization (P) of the tunnel current through the Alq3 layer, directly measured using superconducting Al as the spin detector, shows that minimizing formation of an interfacial dipole layer between the metal electrode and organic barrier significantly improves spin transport.
Abstract: Electron spin-polarized tunneling is observed through an ultrathin layer of the molecular organic semiconductor tris(8-hydroxyquinolinato)aluminum (Alq3). Significant tunnel magnetoresistance (TMR) was measured in a Co/Al2O3/Alq3/NiFe magnetic tunnel junction at room temperature, which increased when cooled to low temperatures. Tunneling characteristics, such as the current-voltage behavior and temperature and bias dependence of the TMR, show the good quality of the organic tunnel barrier. Spin polarization (P) of the tunnel current through the Alq3 layer, directly measured using superconducting Al as the spin detector, shows that minimizing formation of an interfacial dipole layer between the metal electrode and organic barrier significantly improves spin transport.

Journal ArticleDOI
Bin Hu1, Yue Wu1
TL;DR: This finding reveals that the magnetic-field-dependent generation of secondary charge carriers from the dissociation and charge reaction affects the injection current by forming further space charges at the organic-electrode interfaces and therefore accounts for the tunable magnetoresistance.
Abstract: Magnetic-field-dependent injection current, namely magnetoresistance, is readily observable in organic semiconductor devices. This provides a non-contact approach to tune organic optoelectronic properties by using a magnetic field. Here, we demonstrate that this magnetoresistance can be changed between positive and negative values by adjusting the dissociation and charge reaction in excited states through changing the bipolar charge injection in organic light-emitting diodes. This finding reveals that the magnetic-field-dependent generation of secondary charge carriers from the dissociation and charge reaction affects the injection current by forming further space charges at the organic–electrode interfaces and therefore accounts for the tunable magnetoresistance. Furthermore, the dissociation and charge reaction have opposite dependences on magnetic field in the generation of secondary charge carriers, consequently leading to negative and positive magnetoresistance, respectively. As a result, adjusting the dissociation and charge reaction in excited states provides a convenient pathway to tune the magnetoresistance in organic semiconductors.

Journal ArticleDOI
TL;DR: In this paper, a reliable method for fabricating field effect transistors (FETs), which involves careful control of the semiconductor/gate interface (see figure), is presented.
Abstract: The achievement of high mobilities in field-effect transistors (FETs) is one of the main challenges for the widespread application of organic conductors in devices. Good device performance of a single-crystal pentacene FET requires both removal of impurity molecules from the bulk and the manipulation of interface states. A reliable method for fabricating FETs, which involves careful control of the semiconductor/gate interface (see figure), is presented.

Book
17 Apr 2007
TL;DR: In this article, the authors describe the properties of thin-film transistors and their properties, and present a historical perspective (II-VI semiconducting channel materials for TFTs), W. Howard and Mark Brodsky amorphous silicon and polycrystalline silicon TFT, S.J. Fonash device structures, Jerzy Kanicki organic small molecule TFT and Christos D. Dimitrakopoulos organic oligomers.
Abstract: Introduction and device characteristics of thin-film transistors, Michael S. Shur historical perspective (II-VI semiconducting channel materials for TFTs), W. Howard and Mark Brodsky amorphous silicon and polycrystalline silicon TFTs, S.J. Fonash device structures of amorphous silicon, Jerzy Kanicki organic small molecule TFTs, Christos D. Dimitrakopoulos organic oligomers as semiconducting channel materials for TFTs, Francis Garnier solution deposited organic semiconductors for TFTs -molecules to polymers, Zhenan Bao and Howard Katz organic-inorganic hybrid TFTs, Cherie Kagan and David Mitzi integration and deposition of organic semiconductors for TFTs, John Rogers and Ananth Dodabalapur TFTs in active-matrix liquid crystaldisplays, Kouji Susuki and F. Libsch future applications, Richard Friend.


Journal ArticleDOI
TL;DR: The planar geometry of organic light emitting field effect transistors (OLEFETs) offers direct access to the light emission region, providing a unique experimental configuration to investigate fundamental optical and electronic properties in organic semiconductors.
Abstract: Light emitting field effect transistors based on molecular and polymeric organic semiconductors are multifunctional devices that integrate light emission with the current modulating function of a transistor. The planar geometry of organic light emitting field effect transistors (OLEFETs) offers direct access to the light emission region, providing a unique experimental configuration to investigate fundamental optical and electronic properties in organic semiconductors. OLEFETs show great potential for technological applications such as active matrix full color electroluminescent displays. In this Feature Article we review advances in OLEFETs since their first demonstration in 2003 and we highlight exciting challenges associated with their future development.

Journal ArticleDOI
TL;DR: The successive replacement of CH moieties by nitrogen atoms in oligoacenes (benzene to hexacene) has been studied computationally at the B3LYP/6-311+G(d,p)//6-31 G(d) level of theory, and the effects of different heteroatomic substitution patterns on structures, electron affinities, excitation, ionization, and reorganization energies are discussed.
Abstract: The successive replacement of CH moieties by nitrogen atoms in oligoacenes (benzene to hexacene) has been studied computationally at the B3LYP/6-311+G(d,p)//6-31G(d) level of theory, and the effects of different heteroatomic substitution patterns on structures, electron affinities, excitation, ionization, and reorganization energies are discussed. The calculated tendencies are rationalized on the basis of molecular orbital arguments. To achieve electron affinities of 3 eV, a value required to allow for efficient electron injection from common metal electrodes, at least seven nitrogen atoms have to be incorporated into tetracenes or pentacenes. The latter require rather small reorganization energies for electron transfer (<0.20 eV) making these compounds promising candidates for n-channel semiconducting materials. Particularly interesting are heptaazapentacenes 5 and 6 in which the nitrogen atoms are arranged to form self-complementary systems with a maximum number of intermolecular CH−N contacts in planar...

Journal ArticleDOI
TL;DR: In this paper, an organic near-infrared photodetector using an ester group modified polythieno[3,4-b]thiophene (PTT) was presented.
Abstract: Organic photodetectors (PDs) have been the subject of extensive research in the past decade due to several inherent advantages: large-area detection, wide selection of materials, and low-cost fabrication on flexible substrates. High external quantum efficiency (EQE), full-color, fast-response, and position-sensitive PDs have been reported in the past. However, there are few reports on organic near-infrared photodetectors (NIR-PDs) in spite of their tremendous potential in industrial and scientific applications, such as remote control, chemical/biological sensing, optical communication, and spectroscopic and medical instruments. S. Meskers and co-workers reported an infrared PD in which doped poly(2, 4-ethylenedioxythiophene)/poly(styrene sulfonic acid) (PEDOT/PSS) was used as the active material. More recently, G. Konstantatos and coworkers fabricated NIR-PDs by spin-coating colloidal quantum dots from solution onto gold interdigitated electrodes. The device showed a large photoconductive gain and high detectivity at 1.3 lm. However, 3-dB bandwidth was only about 18 Hz and the working voltage was as high as 40 V. These characteristics strongly restrict their applications in the fields of imaging and communication where high-speed and low-power PDs are desired. Thus, there is a strong need for the development of fast response and low working voltage NIR-PDs while simultaneously maintaining the benefit of low-cost solution process. Here we report an organic near-infrared photodetector using a new low band gap polymer. By utilizing an ester group modified polythieno[3,4-b]thiophene, we have successfully lowered the highest occupied molecular orbital (HOMO) energy level of the low band gap (LBG) polymer, so that it can match the energy level of (6,6)-phenyl C61-butyric acid methyl ester (PCBM), and has good solubility and easy processing ability. In this communication, we report a device which has a donoracceptor type energy structure whose operation shows excellent NIR detection capability. Reports on LBG polymers for solar energy conversion have emerged recently. The preparation of LBG, high mobility, solution-processable polymers is not trivial and requires judicious design. Among several band gap tuning strategies for conjugated polymers, polymerization of fused heterocyclic rings has been known to yield polymers with very low band gaps. Polythieno[3,4-b]thiophene (PTT) is one kind of LBG polymers in which the fused thiophene moieties can stabilize the quinoid structure of the backbone, thereby reducing the band gap of the conjugated system. Several PTTs without side chains have been reported previously, but the poor solubility makes them difficult to process and limits their use in electro-optical and electronic devices. Synthesis of alkyl chains substituted thieno[3,4-b]thiophenes monomers have been reported and the resulting polymers exhibit better solubility, but poor oxidative stability. It was found that the HOMO levels of these polymers are too high to match the energy levels of the commonly used electron acceptor, PCBM. We report a new type of ester group modified PTT polymer (Scheme 1). The introduction of an ester group at the 2-position of thieno[3,4-b]thiophene has two effects. First, the electron withdrawing ester group can stabilize the electron-rich thienothiophene and lower the HOMO energy level of the polymer to match the energy level of PCBM. Second, a long tertiary alkyl side chain from the ester group can increase the solubility of the polymer. Polymer was synthesized by Stille polycondensation reaction between the bisbrominated thieno[3,4-b]thiophene and bis-stannylated thiophene. (Scheme 1; see Supporting Information for details) The resulting polymer has good solubility in chloroform and chlorobenzene. In contrast to inorganic semiconductors, photoexcitation of organic semiconductors generates strong bound excitons rather than free charge carriers. To dissociate excitons efficiently, the donor/acceptor bulk heterojunction approach is typically used. The active layer in our PD comprises of PTT and PCBM (Fig. 1a), forming interpenetrating donor/acceptor networks. Details of the device fabrication process are given in the Experimental section. Figure 1b shows the absorption spectra of PTT and PTT: PCBM films. Pure PTT thin film abC O M M U N IC A IO N

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TL;DR: In this article, the authors review recent progress in interface engineering for the fabrication of high-performance OFETs and, in particular, engineering of the interfaces between semiconductors and insulators, and the effects of interfacial characteristics on the molecular and mesoscale structures of π-conjugated molecules and the performance of OFET devices.

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TL;DR: The crystal truncation rod X-ray scattering technique is extended to fiber structured thin films and it is found that the molecular arrangement within the unit cell is substrate dependent, which may lead to a controlled fine-tuni...
Abstract: It is widely recognized that the intrinsic charge transport properties in organic thin-film transistors (OTFTs) depend strongly on the crystal structure of the organic semiconductor layer. Pentacene, showing one of the highest charge carrier mobilities among organic semiconductors, is known to crystallize in at least four polymorphs, which can be distinguished by their layer periodicity d(001). Only two polymorphs grow as single crystals, and their detailed crystal structure has been solved. The substrate-induced 15.4 A polymorph is the most relevant for OTFT applications; however, its crystal structure has remained incomplete as it only grows as a fiber structured thin film. Here we extend the crystal truncation rod X-ray scattering technique to fiber structured thin films. We determined the complete crystal structure of this polymorph grown on technologically relevant substrates. We found that the molecular arrangement within the unit cell is substrate dependent, which may lead to a controlled fine-tuni...

Journal ArticleDOI
TL;DR: In this article, the authors report on highly efficient organic light emitting diodes (OLEDs) consisting of only two organic layers, and the key to the simplification is the direct injection of holes into the wide band gap hole transport material 4,4′,4″-tris(N-carbazolyl)-triphenyl amine (highest occupied molecular orbital is 5.9eV) through an indium tin oxide/tungsten oxide (WO3) anode.
Abstract: The authors report on highly efficient organic light emitting diodes (OLEDs) consisting of only two organic layers. The key to the simplification is the direct injection of holes into the wide band gap hole transport material 4,4′,4″-tris(N-carbazolyl)-triphenyl amine (highest occupied molecular orbital is 5.9eV) through an indium tin oxide/tungsten oxide (WO3) anode. Kelvin probe analysis has revealed an extremely high work function of 6.4eV for WO3. The efficiencies of the simplified OLEDs exceed 40lm∕W and 45cd∕A at a brightness of 100cd∕m2, unsurpassed by other comparably simple OLED devices. Therefore, our OLED architecture demonstrates highly efficient, yet easy to fabricate devices.

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TL;DR: The experimental observations strongly suggest that the primary spin relaxation mechanism in the organic is the Elliott-Yafet mode, in which the spin relaxes whenever a carrier scatters and its velocity changes.
Abstract: Organic semiconductors that are π-conjugated are emerging as an important platform for ‘spintronics’, which purports to harness the spin degree of freedom of a charge carrier to store, process and/or communicate information1. Here, we report the study of an organic nanowire spin valve device, 50 nm in diameter, consisting of a trilayer of ferromagnetic cobalt, an organic, Alq3, and ferromagnetic nickel. The measured spin relaxation time in the organic is found to be exceptionally long—between a few milliseconds and a second—and it is relatively temperature independent up to 100 K. Our experimental observations strongly suggest that the primary spin relaxation mechanism in the organic is the Elliott–Yafet mode, in which the spin relaxes whenever a carrier scatters and its velocity changes.

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TL;DR: In this article, the first flexible, air-stable n-type organic transistors with a transparent channel on conventional overhead transparency film or on ITO/glass substrates were synthesized.
Abstract: Core-cyanated naphthalene diimide organic semiconductors were synthesized for use in field-effect transistors. The unusual combination of wide band gap and high mobility in these materials enables the fabrication of the first flexible, air-stable n-type organic transistors with a transparent channel on conventional overhead transparency film or on ITO/glass substrates.