Pentacene Surface Research Articles

Organic/inorganic hybrid passivation layers for organic thin-film transistors

We report on the fabrication of OTFTs with organic/inorganic passivation layers to enhance the long-term stability of devices. In order to obtain a flat surface on which an inorganic thin film could be deposited, an organic smoothing layer was employed. The planarization of the rough pentacene surface would induce a very smooth and dense surface for the inorganic layer as well. When the SnO2/PVA film was introduced as a component of the encapsulation, greatly enhanced barrier properties were observed. Our work demonstrated that organic/inorganic hybrid passivation layers are a promising technique for a long-term protection of OTFTs.

Molecular dynamics simulation of the interaction of low-energy Ar ions with polyethylene and pentacene surfaces

The normal bombardment of bulk polyethylene and pentacene crystals with 100-eV Ar ions has been investigated by molecular dynamics simulation using the many-body potential for covalent bonds. Polyethylene targets with and without chain folds on the surface have been used. The sputtering yield and changes in the atomic structure of targets are discussed.

Molecular-scale structural distortion near vacancies in pentacene

Molecular vacancies form in both of the crystallographic basis sites of thin pentacene crystals. Features in scanning tunneling microscopy images of these crystals correspond to the exposed terminal atoms of molecules. The (001) and (001¯) surfaces of pentacene are distinguishable, which allows for the identification of the absolute orientation of crystals and for the unambiguous assignment of the position of molecules relative to each vacancy. For vacancies in each molecular basis site of the pentacene (001) surface, the image feature associated with one molecular nearest neighbor is displaced by significantly more than other molecules.

Dielectric layers for organic field effect transistors as gate dielectric and surface passivation

AbstractOrganic field effect transistors with the organic semiconductor pentacene, using silicon substrates, were successfully built on conventional inorganic dielectrics like silicon dioxide or silicon nitride. They can drive drain currents up to 15 mA. Beyond that, polymer films were investigated as gate dielectrics in order to achieve transistors on plastic films. On polyester substrates with an inorganic–organic gate dielectric, devices reach drain currents comparable to transistors on silicon dioxide and charge carrier mobilities of up to 0.35 cm2/V s. Analysis of the pentacene surface by atomic force microscopy showed pentacene crystallites achieving dimensions of more than 1.5 µm. The unprotected organic devices suffer from degradation due to water and oxygen incorporation. Therefore, the application of a hydrophobic polytetrafluoroethylene layer as capping layer is studied. Because of the reduced influence of water, a shift to positive threshold voltages is caused. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Surface Roughness of Organic Semiconductor Superlattice Using Pentacene as Semiconductor

An organic semiconductor superlattice using pentacene as a semiconductor has been investigated. In general, stacked pentacene shows a dendritic structure. However, in a thin pentacene layer as two monolayers, the roughness of the pentacene surface is smooth. Therefore, a smoother surface will be obtained by stacking a thin pentacene/insulator structure. From this, an insulating material of alumina is selected and the multilayer of pentacene [2 monolayer (ML)]/insulator is investigated. As a result, a relatively smoother surface roughness of 2.0 nm was observed for the structure of [pentacene (2 ML)/Al2O3 (5 Å)] × 13/pentacene (2 ML). On the other hand, in the case of the silicon dioxide layer, a nonuniform layer with a smaller grain was observed. This stacked organic layer structure will be useful for a multichannel organic layer for application in organic field-effect transistors.

Nanoimprinted large area heterojunction pentacene-C60 photovoltaic device

A pentacene-C60 bilayer heterojunction organic photovoltaic device with interpenetrating donor-acceptor interface was fabricated by nanoimprinting the pentacene layer prior to C60 deposition. An amorphous silicon substrate nanostructured using an excimer laser was imprinted onto the pentacene layer at high temperature and pressure, using a nanoimprinting lithography system to form a textured pentacene surface. A fivefold improvement in power conversion efficiency was observed due to increased exciton dissociation at the large area heterojunction. Using the proposed technique, highly efficient bilayer large area heterojunction photovoltaics based on small molecule organic materials can be envisaged.

Odd characteristics of Au film on pentacene

Anomalies of Au film formed on the pentacene surface are investigated as a counterpart of pentacene/Au structure. The Au film is found to contain pentacene derivatives originated from the pentacene layers, and it is composed of grains of various sizes formed as the Au thickness increases. The authors suggest that it is this abundance of peculiarities of the Au film that accounts for the attenuated density of states in the valence band. Deformation of Au grains is accompanied by the lift of the pentacene layers, which, in turn, brings about the device failure.

Performance Improvement of Organic Thin-Film Transistors by Electrode/Pentacene Interface Treatment Using a Hydrogen Plasma

Pentacene-based organic thin-film transistors (TFTs) have been fabricated with and without treatment by hydrogen (H2) plasma at the interface between the electrode and the pentacene surface. Processing with a H2 plasma was carried out in a plasma-enhanced chemical vapor deposition with varying treatment times. The interface treatment by H2 plasma resulted in improved device electrical properties. Removal of oxygen from the pentacene surface occurs due to slight etching by the plasma. Sufficient flux density of the H2 plasma leads to full pentacene-surface coverage by rearranging the hydrogen with carbon instead of oxygen. Atomic force microscopy profiles reveal the morphology changes of the pentacene surface after treatment, and secondary ion mass spectrometry shows the change via compositional depth profiles indicating H2 , O2, and C-H binding. Hydrogen treatment, therefore, appears to modify the interface by removing the surface oxygen layer, resulting in better performance of the organic TFT

Self organization of pentacene grown on Cu(1 1 9)

We have investigated the structural and electronic properties of a pentacene film (∼2 nm) grown on the Cu(1 1 9) substrate using scanning tunneling microscopy (STM) and synchrotron based angle-resolved UV photoelectron spectroscopy at room temperature. STM images indicate an ordered superstructure of pentacene molecules lying almost flat with respect to the substrate. From angular-resolved photoemission spectra, we have observed an energy-band dispersion of about 100 meV for the highest occupied molecular orbital in the direction perpendicular to the pentacene surface.

Structure of a pentacene monolayer deposited on SiO2: Role of trapped interfacial water

In situ synchrotron x-ray reflectivity is used to probe the early stages of pentacene growth in real time, under conditions relevant to the fabrication of organic thin film transistors. The results reveal that there is an interfacial water layer initially present on the SiO2 substrate and that this water layer is still present at the interface after the deposition of a pentacene thin film. The thickness of the trapped interfacial water layer does not significantly change subsequent to film deposition, even after exposure to atmospheric pressure or during vacuum annealing at 70°C. However, a water layer is observed to form on the free surface of pentacene after sufficient exposure to water vapor, and the thickness of this layer can be reduced by subsequent vacuum annealing. These observations are correlated with organic thin film transistor mobilities measured at atmospheric pressure versus under vacuum.

Transistor performance of top rough surface of pentacene measured by laminated double insulated-gate supported on a poly(dimethylsiloxanes) base structure

We report the fabrication and electrical characterization of pentacene field-effect transistors with a laminated double insulated-gate using poly(dimethylsiloxanes) (PDMS) as their supporting structure. The ability of PDMS to conform to surfaces enables us to directly evaluate the device performance of the top rough surface of the pentacene active layer (the pentacene-air interface). The mobility measured for the top surface was only about 20% slightly lower than that of the bottom surface. Device stability under ambient conditions is evaluated. This device structure is useful for the characterization of electrical transport in both the top and bottom surface of a thin film simultaneously.

Organic thin-film transistors using pentacene and SiOC film

Pentacene for organic thin-film transistors (OTFTs) was deposited on the SiOC film by thermal evaporation. The transfer characteristic of the pentacene channel as the active layer is dependent on the chemical properties of a surface-on-gate insulator. Hybrid-type SiOC film can have all chemical properties from organic to inorganic properties according to the deposition condition. Pentacene on SiOC film shows the gradient or normal growth because of the C=C bond in SiOC film. Normal growth of pentacene molecules increased the grain size of the surface of pentacene on SiOC film, and the mobility of OTFTs on SiOC films prepared with an O/sub 2//(BTMSM+O/sub 2/) flow rate ratio of 80% is 2.19 (cm/sup 2//V/sub s/).

Energy level alignment at interfaces with pentacene: metals versus conducting polymers

Ultraviolet photoemission spectroscopy was used to determine the work function ϕ and the hole injection barriers Δ h of interfaces formed between pentacene and two fundamentally different electrode materials: (i) conductive polymers comprising poly(3,4-ethylenedioxythiophene) and sulfonate moieties, and (ii) Au with chemisorbed Cl. ϕ of these substrates before pentacene deposition covers a wide range of 4.25–5.15 eV for the polymers, and extends to 5.8 eV for Au–Cl. Despite this huge variation in ϕ, Δ h after pentacene deposition is found to be almost identical for all cases (≈0.35 eV). The mechanism responsible for Δ h being independent of initial surface ϕ is proposed to be an interfacial charge transfer reaction leading to cationic pentacene species. These findings are compared to interfaces between pentacene and clean metal surfaces, where Δ h was found to decrease with increasing substrate ϕ.

Exploring the polymorphism of crystalline pentacene

Starting from each of the five complete X-ray structures published so far for crystalline pentacene, we have computed the structures of minimum potential energy, and obtained two local minima of the potential energy, i.e., two different “inherent structures” of mechanical equilibrium. This behavior, which has been found to be independent of the details of the potential model, indicates that there are at least two different single crystal polymorphs of pentacene. One of the two polymorphs corresponds to the structure originally determined by Campbell et al. [Acta Cryst. 14 (1961) 705]. The other polymorph corresponds to the structure found in all more recent measurements. The calculations predict significant differences between the corresponding Raman spectra of the lattice phonons, which we have checked experimentally, confirming the existence of two polymorphs. The correct identity of the samples, initially assigned only by matching experimental and calculated spectra, has been verified directly with X-ray diffraction measurements. Finally, we have obtained theoretical information on the global stability of the minima by systematically sampling the potential surface of crystalline pentacene, and found that the two polymorphs correspond to the two deepest minima. Further deep minima with layered structures, which might correspond to the thin film polymorphs found to grow on substrates, are also predicted.

Exploring the polymorphism of crystalline pentacene

Starting from each of the five complete X-ray structures published so far for crystalline pentacene, we have computed the structures of minimum potential energy, and obtained two local minima of the potential energy, i.e., two different “inherent structures” of mechanical equilibrium. This behavior, which has been found to be independent of the details of the potential model, indicates that there are at least two different single crystal polymorphs of pentacene. One of the two polymorphs corresponds to the structure originally determined by Campbell et al. [Acta Cryst. 14 (1961) 705]. The other polymorph corresponds to the structure found in all more recent measurements. The calculations predict significant differences between the corresponding Raman spectra of the lattice phonons, which we have checked experimentally, confirming the existence of two polymorphs. The correct identity of the samples, initially assigned only by matching experimental and calculated spectra, has been verified directly with X-ray diffraction measurements. Finally, we have obtained theoretical information on the global stability of the minima by systematically sampling the potential surface of crystalline pentacene, and found that the two polymorphs correspond to the two deepest minima. Further deep minima with layered structures, which might correspond to the thin film polymorphs found to grow on substrates, are also predicted.

Inherent structures of crystalline pentacene

Using a quasi-Monte Carlo scheme, we search the potential energy surface of crystalline pentacene to sample its local minima, which represent the “inherent” structures, i.e., the possible configurations of mechanical equilibrium. The system is described in terms of rigid molecules interacting through a standard atom–atom potential model. Several hundreds of distinct minima are encountered, with a surprising variety of structural arrangements. We find that deep minima are easily accessible because they exhibit a favorable energy distribution and their attraction basins tend to be wide. Thanks to these features of the potential surface, the localization the global minimum becomes entirely feasible, allowing reliable a priori predictions of the crystallographic structures. The results for pentacene are very satisfactory. In fact, the two deepest minima correspond to the structures of the two known experimental polymorphs, which are described correctly. Further polymorphs are also likely to exist.