Orientation Of Pentacene Research Articles

Standing Orientation of Pentacene Promoted on Octadecyltrichlorosilane Self-Assembled Monolayers Revealed by pMAIRS

Standing Orientation of Pentacene Promoted on Octadecyltrichlorosilane Self-Assembled Monolayers Revealed by pMAIRS

Comparison of organic and inorganic layers for structural templating of pentacene thin films

Effective control over the molecular orientation of pentacene was achieved with copper(i) iodide and results in a change in the functional properties with increases in both visible light absorption and work function.

Orientation analysis of pentacene molecules in organic field-effect transistor devices using polarization-dependent Raman spectroscopy

Pentacene, an organic molecule, is a promising material for high-performance field effect transistors due to its high charge carrier mobility in comparison to usual semiconductors. However, the charge carrier mobility is strongly dependent on the molecular orientation of pentacene in the active layer of the device, which is hard to investigate using standard techniques in a real device. Raman scattering, on the other hand, is a high-resolution technique that is sensitive to the molecular orientation. In this work, we investigated the orientation distribution of pentacene molecules in actual transistor devices by polarization-dependent Raman spectroscopy and correlated these results with the performance of the device. This study can be utilized to understand the distribution of molecular orientation of pentacene in various electronic devices and thus would help in further improving their performances.

Impact of molecular orientation on energy level alignment at C60/pentacene interfaces

The molecular orientation and the electronic structure at molecular donor/acceptor interfaces play an important role in the performance of organic optoelectronic devices. Here, we show that graphene substrates can be used as templates for tuning the molecular orientation of pentacene (PEN), selectively driving the formation of either face-on or edge-on arrangements by controlling the temperature of the substrate during deposition. The electronic structure and morphology of the two resulting C60/PEN heterointerfaces were elucidated using ultraviolet photoelectron spectroscopy and atomic force microscopy, respectively. While the C60/PEN (edge-on) interface exhibited a vacuum level alignment, the C60/PEN (face-on) interface exhibited a vacuum level shift of 0.2 eV, which was attributed to the formation of an interface dipole that resulted from polarization at the C60/PEN boundary.

Understanding hydroscopic properties of silk fibroin and its use as a gate-dielectric in organic field-effect transistors

Abstract Silk fibroin (SF) has attracted great interest as gate dielectric in organic field-effect transistors (OFETs), owing to outstanding characteristics such as high dielectric constant, transparency, flexibility, and solution processability. In this report, we investigated the relationship between the structural properties of SF films and their performance as SF gate dielectrics in OFETs. Solvent vapor treatment with water or methanol altered the structural properties of the SF films, which adopted a β-sheet structure; accordingly, both the surface energy and areal capacitance of the SF films were reduced. Notably, atmospheric water contributed to the increased capacitance of the SF film, especially before the solvent vapor treatment. The growth characteristics of pentacene on the SF films were determined by the surface conditions of the films; in particular, Stranski-Krastanov (layer-plus-island) growth mode with mixed standing-up/lying-down orientation of pentacene on the SF film was observed before solvent vapor treatment, whereas Volmer-Weber mode with standing-up orientation dominated after solvent vapor treatment. Pentacene OFETs based on the untreated SF film exhibited a higher on-current compared with the devices based on the solvent vapor-treated SF film. The hydroscopic characteristics of the untreated SF film enhanced its capacitance, thereby inducing accumulation of hole carriers. The present results show that the structural characteristics of SF films have a marked impact on the electrical properties of OFETs based on SF gate dielectrics. In particular, the water uptake capability of SF is a key factor in the electrical properties of organic electronic devices using SF materials.

Surface Polarity and Self-Structured Nanogrooves Collaboratively Oriented Molecular Packing for High Crystallinity toward Efficient Charge Transport.

Efficient charge transport in organic semiconductors is essential for construction of high performance optoelectronic devices. Herein, for the first time, we demonstrate that poly(amic acid) (PAA), a facilely deposited and annealing-free dielectric layer, can tailor the growth of organic semiconductor films with large area and high crystallinity toward efficient charge transport and high mobility in their thin film transistors. Pentacene is used as a model system to demonstrate the concept with mobility up to 30.6 cm2 V-1 s-1, comparable to its high quality single crystal devices. The structure of PAA has corrugations with OH groups pointing out of the surface, and the presence of an amide bond further allows adjacent polymer strands to interact via hydrogen bonding, leading to a self-rippled surface perpendicular to the corrugation. On the other hand, the strong polar groups (-COOH/-CONH) of PAA could provide repulsive forces between PAA and pentacene, which results in the vertical orientation of pentacene on the dielectric surface. Indeed, in comparison with its imidized counterpart polyimide (PI), PAA dielectric significantly enhances the film crystallinity, drastically increases the domain size, and decreases the interface trap density, giving rise to superior device performance with high mobility. This concept can be extended to more organic semiconducting systems, e.g., 2,6-diphenylanthracene (DPA), tetracene, copper phthalocyanine (CuPc), and copper hexadecafluorophthalocyanine (F16CuPc), demonstrating the general applicability. The results show the importance of combining surface nanogrooves with the strong polarity in orienting the molecular arrangement for high crystallinity toward efficient charge transport in organic semiconductors.

Enhancement of grain size and crystallinity of thin layers of pentacene grown under magnetic field

Field-effect mobilities (μ) of pentacene films, prepared by a thermal deposition under a magnetic field (H-field), were largely enhanced, in comparison with that prepared without an H-field. Under a perpendicular H-field with respect to the substrate surface, the crystallinity of the edge-on pentacene orientation is enhanced, resulting in the 9-fold enhancement of μ. Furthermore, under parallel H-field with respect to the substrate surface, μ of the pentacene films were 23-fold greater than that prepared without the H-field. The surface morphology studies by atomic force microscopy of the ultra thin films of pentacene clarified that the grain size of the pentacene at the interface with the substrate is larger for films under parallel H-field than that prepared without an H-field. The simple and effective method for enhancing the semiconducting properties of the organic thin films gives high technological impact in its application to organic electronics.

Growth and Characterizations of Indium-Doped Pentacene Thinfilm Prepared by Thermal Co-Evaporation as a Novel Nanomaterial

This research is related to growth and characterizations of indium-doped pentacene thin films as a novel hybrid material. Doped films were prepared by thermal co-evaporation under high vacuum. The doping concentration was varied from 0% to 50% by controlling the different deposition rate between these two materials while the total thickness was fixed at 100 nm. The hybrid thin films were characterized by atomic force microscopy (AFM), X-ray diffraction (XRD) and UV-Visible spectroscopy to reveal the physical and optical properties. Moreover, the electrical properties of ITO/indium-doped-pentacene/Al devices i.e. charge mobility and carrier concentration were determined by considering the relationship between current-voltage and capacitance-voltage. AFM results identify that doping of indium into pentacene has an effect on surface properties of doped films i.e. the increase of surface grain size. XRD results indicate that doping of metal into pentacene has an effect on preferential orientation of pentacene’s crystalline domains. UV-Vis spectroscopy results show evolution of absorbance at photon energy higher than 2.7 eV corresponding to absorption from oxide of indium formed in the films. Electrical measurements exhibit higher conductivity in doped films resulting from increment of both charge carrier mobility and carrier concentration. Furthermore, chemical interactions taken place inside the doped films were investigated by x-ray photoelectron spectroscopy (XPS) in order to complete the remaining questions i.e. how do indium atoms interact with the neighbor molecules?, what is the origin of the absorption at E > 2.7 eV? Further results and discussions will be presented in the publication.

Characterization of molecular organization in pentacene thin films on SiO2 surface using infrared spectroscopy, spectroscopic ellipsometry, and atomic force microscopy

Thin films of pentacene of 32 and 100nm thickness obtained by organic molecular beam deposition (OMBD) in high vacuum conditions onto silicon/native silica (Si/SiO2) and fused silica substrates were examined. Alignment, anisotropic optical properties and morphology were studied in ambient conditions using infrared (IR) transmission and polarized grazing angle attenuated total reflection (GATR) techniques, variable angle spectroscopic ellipsometry (VASE), UV–VIS absorption, and atomic force microscopy (AFM). For the first time dichroic GATR IR spectra were recorded for such thin films and conclusions on pentacene orientation were deduced on the basis of dichroic ratio of the IR-active vibrations. The symmetry assignment of the vibrational transitions is also discussed. The films exhibit continuous globular texture with uniaxial alignment of pentacene molecules and strongly anisotropic optical properties evidenced in the ellipsometric measurements. The results revealed that there are some quantitative differences in the orientation and in the dielectric properties between the two pentacene films of different thickness.

Molecular Orientation-Dependent Interfacial Energetics and Built-in Voltage Tuned by a Template Graphene Monolayer

Highly transparent and conductive monolayer graphene was used as a template to tune the crystal orientation of pentacene from generic standing-up (001) to lying-down (022) in neat films. Spatially resolved Kelvin probe force microscopy (KPFM) was used to reveal the energy levels of pentacene thin films grown on substrates with and without the template graphene layer, as well as the energy level alignment in various pentacene-containing organic–organic heterojunctions. A correlation between crystal domain orientation and the work function was directly observed using KPFM. Up to 0.36 eV shifts in work function were observed in neat pentacene films over large areas (>0.5 in.2) upon orientation transition, likely due to the transition from Fermi level pinning (standing-up pentacene on ITO) to vacuum level alignment (lying-down pentacene on graphene–ITO). Morphology-induced energy level shifts versus interfacial electronic equilibration effects were disentangled using atomic force microscopy, KPFM, X-ray diffraction, and Raman data for neat pentacene films and pentacene containing heterojunctions on monolayer graphene. The data detailed herein provide a fundamental picture of the major interfacial effects active in optoelectronic devices containing a bare graphene electrode.

A morphology control layer of a pyrene dimer enhances the efficiency in small molecule organic photovoltaic cells

Improved efficiency in organic photovoltaic (OPV) devices induced by a morphology control donor layer as an anode buffer layer (ABL) was achieved. We synthesized the pyrene dimer 1,1′-dipyrene (1DPy) and applied it in vacuum-evaporated small-molecule-type OPV devices based on pentacene. N,N′-Di(naphthalen-1-yl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine (NPD) was used for comparison in the ABLs. While NPD led to a decreased fill factor, the insertion of a 1DPy layer with only 1 nm thickness improved the power conversion efficiency (PCE) 1.5-fold compared with that of the device without 1DPy. Moreover, 1 nm thick 1DPy layers were also incorporated into devices with different donor materials: copper-phthalocyanine and tetraphenyldibenzoperiflanthene. All of the devices showed enhanced short-circuit current densities, which resulted in improvements in the power conversion efficiency. The 1DPy layer prevented hole accumulation owing to its island-like grain structure. The morphologies of the donor layers on top of the 1DPy layers were characterized using atomic force microscopy images and X-ray diffraction patterns. The roughness of the pentacene layer was decreased from 4.41 nm to 2.15 nm when a 1DPy layer was used, and the pentacene layer on 1DPy showed a tilted crystal phase. The tilted pentacene orientation may facilitate perpendicular carrier transport or exciton diffusion through an increase in molecular orbital overlap. 1DPy inserted as an ABL can be useful in controlling the morphology of the donor layer, thereby resulting in an improvement in the efficiency of the device.

Pentacene Orientation on Source/Drain Electrodes and Its Effect on Charge Carrier Transport at Pentacene/Electrode Interface, Investigated Using In Situ Ultraviolet Photoemission Spectroscopy and Device Characteristics

The valence band structures and ionization energies of pentacene layers grown on poly (3,4-ethylenedioxythiophene) polymerized with poly (4-styrenesulfonate) (PEDOT:PSS) and Au electrodes were characterized using in situ ultraviolet photoemission spectroscopy. The pentacene layers grown on Au films consist of amorphous phase and round oval grains regardless of the pentacene deposition temperature, whereas the pentacene deposited on PEDOT:PSS films showed highly distinctive ordered structures depending on the deposition conditions. The pentacene layer (thickness: 30 nm) grown on PEDOT:PSS at room temperature (RT) showed a highly crystalline thin film phase (001) and typical terrace-like structure, but that grown on PEDOT:PSS at 100°C consists of a weakly oriented thin film/bulk mixed phase with a disordered grain structure. However, irrespective of the molecular ordering structure, the pentacene layers grown on PEDOT:PSS films showed nearly the same hole-injection barriers (0.40 ∼ 0.45 eV) at both interface (10 nm) and bulk regions (30 nm). On the basis of the correlation between the molecular ordering and device characteristics, we conclude that the carrier transport at pentacene/electrode interface relies more on the hole-injection barrier than on the ordering of the pentacene layer grown on the electrode.

Current transport across the pentacene/CVD-grown graphene interface for diode applications

We investigate the electronic transport properties across the pentacene/graphene interface. Current transport across the pentacene/graphene interface is found to be strikingly different from transport across pentacene/HOPG and pentacene/Cu interfaces. At low voltages, diodes using graphene as a bottom electrode display Poole–Frenkel emission, while diodes with HOPG and Cu electrodes are dominated by thermionic emission. At high voltages conduction is dominated by Poole–Frenkel emission for all three junctions. We propose that current across these interfaces can be accurately modeled by a combination of thermionic and Poole–Frenkel emission. Results presented not only suggest that graphene provides low resistive contacts to pentacene where a flat-laying orientation of pentacene and transparent metal electrodes are desired but also provides further understanding of the physics at the organic semiconductor/graphene interface.

Surface-Directed Molecular Assembly of Pentacene on Monolayer Graphene for High-Performance Organic Transistors

Organic electronic devices that use graphene electrodes have received considerable attention because graphene is regarded as an ideal candidate electrode material. Transfer and lithographic processes during fabrication of patterned graphene electrodes typically leave polymer residues on the graphene surfaces. However, the impact of these residues on the organic semiconductor growth mechanism on graphene surface has not been reported yet. Here, we demonstrate that polymer residues remaining on graphene surfaces induce a stand-up orientation of pentacene, thereby controlling pentacene growth such that the molecular assembly is optimal for charge transport. Thus, pentacene field-effect transistors (FETs) using source/drain monolayer graphene electrodes with polymer residues show a high field-effect mobility of 1.2 cm(2)/V s. In contrast, epitaxial growth of pentacene having molecular assembly of lying-down structure is facilitated by π-π interaction between pentacene and the clean graphene electrode without polymer residues, which adversely affects lateral charge transport at the interface between electrode and channel. Our studies provide that the obtained high field-effect mobility in pentacene FETs using monolayer graphene electrodes arises from the extrinsic effects of polymer residues as well as the intrinsic characteristics of the highly conductive, ultrathin two-dimensional monolayer graphene electrodes.

Planar Growth of Pentacene on the Dielectric TiO2(110) Surface

We have studied the growth of pentacene molecules on the unreconstructed and stoichiometric surface of TiO2(110). At variance with its characteristic homeotropic growth mode, pentacene is found to be physisorbed on this dielectric substrate with its long molecular axis oriented parallel to the surface and aligned along the [001] direction. Pentacene molecules couple side-by-side into long stripes running along the [11̅0] direction, where the overlayer preserves the substrate lattice periodicity (∼6.5 Å). In the opposite direction, head-to-head pentacene repulsion drives the ordering of the stripes, whose spacing simply depends on the surface coverage. By near-edge X-ray absorption, NEXAFS, we have determined the pentacene molecules to be tilted by ∼25° off the surface around their long axis. At the monolayer coverage, the pentacene orientation and spacing are very close to that of the (010) bulk planes (also called a−c planes) of pentacene crystals. We have observed that at least two additional layers can be grown on top of the monolayer following a planar configuration. Both the strong side-by-side intermolecular attraction and the full development of the bulklike electronic states, as probed by NEXAFS, suggest an optimal charge transport along the monolayer stripes of lying-down molecules.

In situstudy of pentacene interaction with archetypal hybrid contacts: Fluorinated versus alkane thiols on gold

One approach developed to improve the performance of bottom contact source/drain electrodes is to treat the contacts with thiols before deposition of the semiconductor. There is evidence indicating that improvement is due to both morphological effects and improved work function matching. Especially suggestive evidence shows that thiols that increase the effective work function of the contacts (e.g., fluorinated thiols) yield better device performance than work function decreasing thiols (e.g., alkane thiols). Here, we compare two technologically relevant thiol treatments, an alkane thiol (1-hexadecanethiol), and a fluorinated thiol (pentafluorobenzenethiol), in pentacene organic field effect transistors. Using in situ semiconductor deposition, x-ray photoemission, and x-ray absorption spectroscopy, we are able to directly observe the interaction between the semiconductor and the thiol-treated gold layers. Our spectroscopic analysis suggests that there is not a site-specific chemical reaction between the pentacene and the thiol molecules. A homogeneous standing-up pentacene orientation was observed in both treated substrates, consistent with the morphological improvement expected from thiol treatment in both samples. Our study shows that both the highest occupied molecular orbital--Fermi level offset and $\text{C}\text{ }1s$ binding energy are shifted in the two thiol systems, which can be explained by varied dipole direction within the two thiols, causing a change in surface potential. The additional improvement of the electrical performance in the pentafluorobenzenethiol case is originated by a reduced hole injection barrier that is also associated with an increase in the density of states in the lowest unoccupied molecular orbital.

Inducement of Azimuthal Molecular Orientation of Pentacene by Imprinted Periodic Groove Patterns for Organic Thin‐Film Transistors

A new strategy for control of the in-plane orientation of pentacene is introduced using nanometer-scale periodic groove patterns as an alignment layer. Compared to pentacene films that are not aligned, an enhanced field effect mobility of 1.67 cm2 V−1 s−1 has been achieved when the optimal π-orbital overlap direction is parallel to the direction of the current flow.

Effect of Molecule−Substrate Interaction on Thin-Film Structures and Molecular Orientation of Pentacene on Silver and Gold

Evaporated pentacene thin films with thicknesses from several nm to 150 nm on gold and silver substrates have been studied by ultraviolet photoelectron spectroscopy (UPS), near-edge X-ray absorption fine structure (NEXAFS), scanning tunneling microscopy (STM), and atomic force microscopy (AFM). It was found that pentacene thin-film structures, particularly their molecular orientations, are strongly influenced by the metal substrates. UPS measurements revealed a distinct change in the valence band structures of pentacene on Au compared to those on Ag, which is attributed to the different packing between adjacent molecules. Using NEXAFS, we observed 74+/-5 degrees and 46+/-5 degrees molecular tilt angles on Ag and Au, respectively, for all measured thicknesses. We propose that pentacene molecules stand up on the surface and form the "thin-film phase" structure on Ag. On Au, pentacene films grow in domains with molecules either lying flat or standing up on the substrate. Such a mixture of two crystalline phases leads to an average tilt angle of 46 degrees for the whole film and the change in valence band structures. STM and distance-voltage (z-V) spectroscopy studies confirm the existence of two crystalline phases on Au with different conducting properties. z-V spectra on the low conducting phase clearly indicate its nature as "thin-film phase".

Mapping molecular orientation of pentacene on patterned Au surface

With a photoemission electron microscope, we investigated the orientation of pentacene films deposited on Au surface that has been patterned with self-assembled monolayers (SAMs). Organic thiols X(CH 2) nSH with X = COOH or CH 3 and an aromatic thiol; served as alignment layers to control the orientation of pentacene molecules thermally deposited on top of the SAM. According to the PEEM images and near-edge X-ray absorption fine structure spectra, a SAM exposing a non-polar methyl group, a terphenyl group or a polar carboxyl group all induced perpendicular alignment of the pentacene moiety, in contrast with parallel alignment on a bare gold surface.

Orientation and electronic properties of pentacene molecules on SiO2 and GeS(0001) studied using x-ray absorption spectroscopy

We present an investigation on the morphology of pentacene molecules, a candidate for organic semiconductors in organic electronic devices. In this case the molecules are deposited on the technologically relevant substrate SiO2. In order to observe possible changes in the morphology of a monolayer of pentacene compared to a much thicker pentacene layer, we have performed x-ray absorption experiments on both films. We did not find significant differences in the orientation of the pentacene molecules as a function of film thickness. The same is true for the orientation of pentacene on the model substrate GeS(0001), however, the molecular orientation is opposite to that of pentacene on SiO2. Whereas the pentacene molecules on GeS(0001) lie flat on the substrate surface, on SiO2 they are standing. These results have important consequences for our understanding of the functionality of organic devices based on pentacene.