Showing papers on "Organic semiconductor published in 2003"

Small molecular weight organic thin-film photodetectors and solar cells

Abstract: In this review, we discuss the physics underlying the operation of single and multiple heterojunction, vacuum-deposited organic solar cells based on small molecular weight thin films. For single heterojunction cells, we find that the need for direct contact between the deposited electrode and the active organics leads to quenching of excitons. An improved device architecture, the double heterojunction, is shown to confine excitons within the active layers, allowing substantially higher internal efficiencies to be achieved. A full optical and electrical analysis of the double heterostructure architecture leads to optimal cell design as a function of the optical properties and exciton diffusion lengths of the photoactive materials. Combining the double heterostructure with novel light trapping schemes, devices with external efficiencies approaching their internal efficiency are obtained. When applied to an organic photovoltaic cell with a power conversion efficiency of 1.0%±0.1% under 1 sun AM1.5 illuminati...

High-performance thin-film transistors using semiconductor nanowires and nanoribbons

Abstract: Thin-film transistors (TFTs) are the fundamental building blocks for the rapidly growing field of macroelectronics. The use of plastic substrates is also increasing in importance owing to their light weight, flexibility, shock resistance and low cost. Current polycrystalline-Si TFT technology is difficult to implement on plastics because of the high process temperatures required. Amorphous-Si and organic semiconductor TFTs, which can be processed at lower temperatures, but are limited by poor carrier mobility. As a result, applications that require even modest computation, control or communication functions on plastics cannot be addressed by existing TFT technology. Alternative semiconductor materials that could form TFTs with performance comparable to or better than polycrystalline or single-crystal Si, and which can be processed at low temperatures over large-area plastic substrates, should not only improve the existing technologies, but also enable new applications in flexible, wearable and disposable electronics. Here we report the fabrication of TFTs using oriented Si nanowire thin films or CdS nanoribbons as semiconducting channels. We show that high-performance TFTs can be produced on various substrates, including plastics, using a low-temperature assembly process. Our approach is general to a broad range of materials including high-mobility materials (such as InAs or InP).

Pentacene thin film transistors on inorganic dielectrics: Morphology, structural properties, and electronic transport

Abstract: The structural and transport properties of evaporated pentacene organic thin film transistors (TFTs) are reported, and they show the influence of the deposition conditions with different inorganic dielectrics. Dielectrics compatible with large area fabrication were explored to facilitate low cost electronics on glass or flexible plastic substrates. X-ray diffraction and atomic force microscopy show a clear correlation between the morphology and the structure of the highly polycrystalline films for all dielectrics investigated. The roughness of the dielectric has a distinct influence on the morphology and the structural properties, whereas the films on smooth thermal oxide are in general highly ordered and independent of the deposition conditions. The ordered films exhibit a “thin film” and a bulk phase, and the bulk phase volume fraction increases with the deposition temperature and the film thickness. Careful control of the deposition conditions gives virtually identical films on thermal oxide and silico...

Solution-processed ambipolar organic field-effect transistors and inverters.

Abstract: There is ample evidence that organic field-effect transistors have reached a stage where they can be industrialized, analogous to standard metal oxide semiconductor (MOS) transistors. Monocrystalline silicon technology is largely based on complementary MOS (CMOS) structures that use both n-type and p-type transistor channels. This complementary technology has enabled the construction of digital circuits, which operate with a high robustness, low power dissipation and a good noise margin. For the design of efficient organic integrated circuits, there is an urgent need for complementary technology, where both n-type and p-type transistor operation is realized in a single layer, while maintaining the attractiveness of easy solution processing. We demonstrate, by using solution-processed field-effect transistors, that hole transport and electron transport are both generic properties of organic semiconductors. This ambipolar transport is observed in polymers based on interpenetrating networks as well as in narrow bandgap organic semiconductors. We combine the organic ambipolar transistors into functional CMOS-like inverters.

Low-k Insulators as the Choice of Dielectrics in Organic Field-Effect Transistors

Abstract: In this paper, we present a new effect influencing the operation of organic field-effect transistors resulting from the choice of gate insulator material. In a series of studies it was found that the interaction between the insulator and the semiconductor materials plays an important role in carrier transport. The insulator is not only capable of affecting the morphology of the semiconductor layer, but can also change the density of states by local polarization effects. Carrier localization is enhanced by insulators with large permittivities, due to the random dipole field present at the interface. We have investigated this effect on a number of disordered organic semiconductor materials, and show here that significant benefits are achievable by the use of low-k dielectrics as opposed to the existing trend of increasing the permittivity for low operational voltage. We also discuss fundamental differences in the case of field-effect transistors with band-like semiconductors.

Conjugated organic molecules on metal versus polymer electrodes: Demonstration of a key energy level alignment mechanism

Abstract: Ultraviolet photoemission spectroscopy is used to determine the energy level alignment at interfaces between three electroactive conjugated organic molecular materials, i.e., N,N′-bis-(1-naphthyl)-N,N′-diphenyl1-1,1-biphenyl1-4,4′-diamine; para-sexiphenyl; pentacene, and two high work function electrode materials, i.e., gold and poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate). Although both electrode surfaces have a similar work function (∼5 eV), the hole injection barrier and the interfacial dipole barrier are found to be significantly smaller for all the interfaces formed on the polymer as compared to the metal. This important and very general result is linked to one of the basic mechanisms that control molecular level alignment at interfaces with metals, i.e., the reduction of the electronic surface dipole contribution to the metal work function by adsorbed molecules.

Comparing organic to inorganic photovoltaic cells: Theory, experiment, and simulation

Abstract: Charge carriers are photogenerated with very different spatial distributions in conventional inorganic photovoltaic (IPV) cells and in organic photovoltaic (OPV or excitonic) cells. This leads to a fundamental, and often overlooked, mechanistic difference between them. Carriers are generated primarily at the exciton-dissociating heterointerface in OPV cells, resulting in the production of electrons in one phase and holes in the other—the two carrier types are thus already separated across the interface upon photogeneration in OPV cells, giving rise to a powerful chemical potential energy gradient ∇μhv that promotes the photovoltaic effect. This occurs also in high-surface-area OPV cells, although their description is more complex. In contrast, both carrier types are photogenerated together throughout the bulk in IPV cells: ∇μhv then drives both electrons and holes in the same direction through the same phase; efficient carrier separation therefore requires a built-in equilibrium electrical potential energ...

Field-effect transistors on rubrene single crystals with parylene gate insulator

Abstract: We report on the fabrication and characterization of the organic field-effect transistors (OFETs) on the surface of single crystals of rubrene. The parylene polymer film has been used as the gate insulator. At room temperature, these OFETs exhibit the p-type conductivity with the field-effect mobility 0.1–1 cm2/V s and the on/off ratio⩾104. The temperature dependence of the mobility is discussed.

Exciton binding energies in organic semiconductors

Abstract: The exciton binding energy is one of the key parameters that govern the physics of many opto-electronic organic devices. It is shown that the previously reported values for the exciton binding energies in many organic semiconductors, which differ by more than an order of magnitude, can be consistently rationalized within the framework of the charging energy of the molecular units, with a simple dependence of the exciton binding energy on the length of these units. The implications of this result are discussed.

Increased mobility from regioregular poly(3-hexylthiophene) field-effect transistors

Abstract: Relatively high mobilities, μ=0.2 cm2 V−1 s−1 in the accumulation mode and μ=0.17 cm2 V−1 s−1 in the depletion mode, are reported for regioregular poly(3-hexylthiophene) (RR-P3HT) in field-effect transistors (FETs). Significantly higher mobility is obtained from FETs in which the RR-P3HT film is applied by dip-coating to a thickness of only 20−40 A. These observations suggest that structural order of the semiconducting polymer at the interface between the semiconducting polymer and the SiO2 gate insulator is of paramount importance for achieving high carrier mobility. Heat treatment under nitrogen at 160 °C for 3 min increases the on/off ratio of the FET.

Nonvolatile electrical bistability of organic/metal-nanocluster/organic system

Abstract: Two-terminal electrical bistable devices have been fabricated using a sandwich structure of organic/metal/organic as the active medium, sandwiched between two external electrodes. The nonvolatile electrical bistability of these devices can be controlled using a positive and a negative electrical bias alternatively. A forward bias may switch the device to a high-conductance state, while a reverse bias is required to restore it to a low-conductance state. In this letter, a model to explain this electrical bistability is proposed. It is found that the bistability is very sensitive to the nanostructure of the middle metal layer. For obtaining the devices with well-controlled bistability, the middle metal layer is incorporated with metal nanoclusters separated by thin oxide layers. These nanoclusters behave as the charge storage elements, which enable the nonvolatile electrical bistability when biased to a sufficiently high voltage. This mechanism is supported by the experimental data obtained from UV–visible ...

Metal–organic interface and charge injection in organic electronic devices

Abstract: Charge injection at the interface between metallic electrodes and organic semiconductors plays a crucial role in the performance of organic (opto-)electronic devices. This article discusses the current understanding of the formation of the metal–organic contact and the parameters which control the injection current. Organic semiconductors differ significantly from their inorganic counterparts, primarily because they are amorphous van der Waals solids. As a result the electronic states are highly localized, and charge transport is by site-to-site hopping. Organics can also form clean interfaces with many metals, free of interface states in the gap. Nevertheless, there is generally found to be a significant vacuum level offset, the origins of which are not yet fully understood. Organic semiconductors are frequently free of donor and acceptor dopants, and as a result the depletion depth is larger than the organic layer thickness. Thus the Fermi level in the organic and the charge injection barriers depend mo...

Pentacene organic transistors and ring oscillators on glass and on flexible polymeric substrates

Abstract: We have fabricated organic thin film transistors, inverters, and ring oscillators on glass and on flexible polyethylene naphthalate, using the small-molecule hydrocarbon pentacene as the semiconductor and solution-processed polyvinylphenol as the gate dielectric. Depending on the choice of substrate, the transistors have a carrier mobility between 0.3 and 0.7 cm2/V s, an on/off current ratio between 105 and 106, and a subthreshold swing between 0.9 and 1.6 V/decade. To account for the positive switch-on voltage of the transistors, circuits were designed to operate with integrated level shifting. Depending on the type of substrate, ring oscillators have a signal propagation delay as low as 15 μs per stage.

Nondispersive hole transport in amorphous films of methoxy-spirofluorene-arylamine organic compound.

Abstract: Hole transport in the organic small molecule material 2,2′,7,7′-tetrakis-(N,N-di-4- methoxyphenylamino)-9,9′-spirobifluorene (methoxy-spiro) is studied by three different experimental techniques: Time of flight (TOF), dark-injection space-charge-limited current transients and steady-state current–voltage characteristics, and over a wide range of electric fields, sample thicknesses and temperatures. It is shown that room-temperature mobilities measured by all three methods agree well over a range of studied thicknesses from 4 μm down to 135 nm. Temperature-dependent TOF measurements are analyzed within the Gaussian disorder model and the parameters of charge transport in this material are extracted. Comparison with the data reported for other spirolinked compounds indicate a significant increase of the mobility prefactor in the methoxy-spiro, which is attributed to the increased spread of the highest occupied molecular orbital wave function in this material.

Contact resistance in organic transistors that use source and drain electrodes formed by soft contact lamination

Abstract: Soft contact lamination of source/drain electrodes supported by gold-coated high-resolution rubber stamps against organic semiconductor films can yield high-performance organic transistors. This article presents a detailed study of the electrical properties of these devices, with an emphasis on the nature of the laminated contacts with the p- and n-type semiconductors pentacene and copper hexadecafluorophthalocyanine, respectively. The analysis uses models developed for characterizing amorphous silicon transistors. The results demonstrate that the parasitic resistances related to the laminated contacts and their coupling to the transistor channel are considerably lower than those associated with conventional contacts formed by evaporation of gold electrodes directly on top of the organic semiconductors. These and other attractive features of transistors built by soft contact lamination suggest that they may be important for basic and applied studies in plastic electronics and nanoelectronic systems based ...

New n-Type Organic Semiconductors: Synthesis, Single Crystal Structures, Cyclic Voltammetry, Photophysics, Electron Transport, and Electroluminescence of a Series of Diphenylanthrazolines

Abstract: The synthesis, properties, and electroluminescent device applications of a series of five new diphenylanthrazoline molecules 1a−1e are reported. Compounds 1b, 1c, and 1d crystallized in the monoclinic system with the space groups P21/c, C2/c, and P21/c, respectively, revealing highly planar molecules. Diphenylanthrazolines 1a−1e have a formal reduction potential in the range −1.39 to −1.58 V (versus SCE) and estimated electron affinities (LUMO levels) of 2.90−3.10 eV. Compounds 1a−1e emit blue light with fluorescence quantum yields of 58−76% in dilute solution, whereas they emit yellow-green light as thin films. The diphenylanthrazoline molecules as the emissive layers in light-emitting diodes gave yellow light with a maximum brightness of 133 cd/m2 and an external quantum efficiency of up to 0.07% in ambient air. Bilayer light-emitting diodes using compounds 1a−1e as the electron-transport layer and poly(2-methoxy-5-(2‘-ethyl-hexyloxy)-1,4-phenylene vinylene) as the emissive layer had a maximum external ...

Field-effect transistors on tetracene single crystals

Abstract: We report on the fabrication and electrical characterization of field-effect transistors at the surface of tetracene single crystals. We find that the mobility of these transistors reaches the room-temperature value of 0.4?cm2/V?s. The nonmonotonous temperature dependence of the mobility, its weak gate voltage dependence, as well as the sharpness of the subthreshold slope, confirm the high quality of single-crystal devices. This is due to the fabrication process that does not substantially affect the crystal quality.

Temperature dependent characteristics of poly(3 hexylthiophene)-fullerene based heterojunction organic solar cells

Abstract: Electrical and optical properties of poly(3-hexylthiophene-2.5diyl) (P3HT) used as the main component in a polymer/fullerene solar cell were studied. From the study of space-charge limited current behavior of indium-tin-oxide (ITO)/P3HT/Au hole-only devices, the hole mobility and density were estimated to range from 1.4×10−6 cm2/V s and 5.3×1014 cm−3 at 150 K to 8.5×10−5 cm2/V s and 1.1×1015 cm−3 at 250 K, respectively. The highest occupied to lowest occupied molecular orbital energetic difference was estimated from absorption spectrometry to be about 2.14 eV. Strong quenching of photoluminescence when the polymer was mixed with [6,6]-phenyl-C61 butyric acid methyl ester (PCBM), provided evidence of photoinduced charge transfer from P3HT to PCBM. Characterization of ITO/PEDOT:PSS/P3HT:PCBM/Al solar cells was done by analyzing the dependence of current density–voltage characteristics on temperature and illumination intensity. The main solar cell characteristics recorded at 300 K under 100 mW/cm2 white-light intensity were: Open-circuit voltage 0.48 V, current density 1.28 mA/cm2, with an efficiency of 0.2%, and fill factor of 30.6%. Open-circuit voltage decreased almost linearly with increasing temperature, while short circuit current density increased with temperature, saturating at around 320 K, and decreased thereafter. Power conversion efficiency and fill factor were maximum around 3 mW/cm2 due to the poor bulk transport properties of the active layer.

High Performance Organic Thin Film Transistors

Abstract: We report here methods of surface modification and device construction which consistently result in lab-scale pentacene-based TFTs with mobilities at or above 5 cm2/Vs. Surface modifications include polymeric ultrathin films presenting a passivated interface on which the semiconductor can grow. High performance TFTs have been fabricated on a variety of dielectric materials, both organic and inorganic, and are currently being implemented in manufacturable constructions. Our surface modifications have also proven useful for substituted pentacene materials and for a variety of other organic semiconductors. In addition, we report an all organic active layer, rf-powered integrated circuit. Further experiments and statistical analyses are underway to explain the elevated mobility in our samples, and efforts have been made to confirm these results through collaboration.

H2O effect on the stability of organic thin-film field-effect transistors

Abstract: Degradation of organic thin-film field-effect transistors (OTFTs) with pentacene as the active material has been studied. It was found that the field-effect mobility of the device decreased by 30% and the on/off current ratio decreased to one fifth after the OTFTs had been stored in atmosphere for 500 h. Through surface morphology analysis by atomic force microscopy and absorption analysis by infrared spectroscopy, it was found that the adsorption of H2O on the pentacene layer was the main reason for the degradation. Remarkable improvement in the device performance was achieved by device encapsulation with UV curable resin.

Uniaxial alignment of the columnar super-structure of a hexa (alkyl) hexa-peri-hexabenzocoronene on untreated glass by simple solution processing.

Abstract: Uniaxially aligned, thin films of a discotic columnar thermotropic liquid crystalline material can be prepared by a simple solution zone-casting method, without the need for modified surfaces or traditional alignment techniques. Atomic force microscopy together with X-ray diffraction reveals single-crystalline-like order over several square centimeters, far exceeding the requirements for application of such films in organic molecular electronic devices such as field-effect transistors.

Morphology and electronic transport of polycrystalline pentacene thin-film transistors

Abstract: Temperature-dependent measurements of thin-film transistors were performed to gain insight in the electronic transport of polycrystalline pentacene. Devices were fabricated with plasma-enhanced chemical vapor deposited silicon nitride gate dielectrics. The influence of the dielectric roughness and the deposition temperature of the thermally evaporated pentacene films were studied. Although films on rougher gate dielectrics and films prepared at low deposition temperatures exhibit similar grain size, the electronic properties are different. Increasing the dielectric roughness reduces the free carrier mobility, while low substrate temperature leads to more and deeper hole traps.

Light-induced bias stress reversal in polyfluorene thin-film transistors

Abstract: Gate bias-stress effects in the high-performance semiconducting polymer poly-9,9′ dioctyl-fluorene-co-bithiophene (F8T2) were studied. The bias stress in F8T2 was characterized in devices having various gate dielectric materials—different types of SiO2 and a polymer—and a variety of chemically modified dielectric/semiconductor interfaces. A bias-stress effect was reversed by illuminating the transistor structure with band gap radiation. The recovery rate was directly related to the absorption characteristics of F8T2. We conclude that bias stress in F8T2 is due to hole charge trapping inside the polymer, close to the dielectric interface and not to a structural change in the polymer, or to charge in the dielectric.

Nucleation of organic semiconductors on inert substrates

Abstract: We have adapted the microscopic theory of nucleation for the epitaxial growth of inorganic materials to the nucleation of organic small molecules on an inert substrate like the gate dielectric of an organic thin-film transistor. The parameters required to explore the model were calculated with the standard MM3 force field and also include experimentally determined vapor pressure data, as well as film growth data. Sufficient agreement is found between the experimentally determined equilibrium crystal shape and heats of sublimation on the one hand and the calculated parameters on the other hand. The growth of pentacene, tetracene, and perylene on inert substrates has been studied in terms of this theory, especially focusing on the two-dimensional (2D) to 3D nucleation transition. It is demonstrated that 3D nucleation leads to ill-connected grains, while 2D nucleated grains form continuous films suitable for charge transport. The analysis of this transition allows for the experimental determination of the molecule-substrate interactions for a given molecule on a given surface. It was found that the deposition conditions for 2D growth shift to less favorable substrate temperatures and deposition rates as the difference between interlayer interactions and molecule-substrate interactions increase and the intralayer interactions decrease. Moreover, those interactions affect the nucleation rate and therefore the ultimate 2D grain size that can be obtained.

Dopant density determination in disordered organic field-effect transistors

Abstract: We demonstrate that, by using a concentric device geometry, the dopant density and the bulk charge-carrier mobility can simultaneously be estimated from the transfer characteristics of a single disordered organic transistor. The technique has been applied to determine the relation between the mobility and the charge density in solution-processed poly(2,5-thienylene vinylene) and poly(3-hexyl thiophene) thin-film field-effect transistors. The observation that doping due to air exposure takes place already in the dark, demonstrates that photoinduced oxygen doping is not the complete picture.