Imaging of crystal morphology and molecular simulations of surface energies in pentacene thin films

We have investigated the crystal growth of the organic semiconductor pentacene by complementing molecular simulations of surface energies with experimental images of pentacene films. Pentacene thin films having variations in thickness and grain size were produced by vacuum sublimation. Large (approximately 20 microm) faceted crystals grew on top of the underlying polycrystalline thin film. The films were characterized using optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Single crystals most commonly grew in a truncated diamond shape with the largest crystal face, (001), growing parallel to the substrate. Crystal morphologies and surface energies were calculated using force field-based molecular simulations. The (001) surface was found to have the lowest energy, at 76 mJ/m(2), which was consistent with experimental observations of crystal face size. It was demonstrated that the morphology of the large faceted crystals approached the equilibrium growth shape of pentacene. From contact angle measurements, the critical surface tension of textured pentacene thin films in air was determined to be 34 mJ/m(2).     

A photopatternable pentacene precursor for use in organic thin-film transistors

We demonstrate that modifying pentacene to incorporate an acid-labile moiety into its molecular structure leads to a new precursor that can be easily deposited, photopatterned, and processed via wet-chemical methods to produce organic semiconducting devices exhibiting good electrical characteristics. Acidic conditions produced by ultraviolet illumination of a co-deposited photoacid generator greatly accelerate the local conversion of this N-sulfinyl-tert-butylcarbamate pentacene adduct back to pentacene. Photopatterned thin-film transistors exhibit carrier mobilities in excess of 0.1 cm2 V-1 s-1, making this an attractive precursor for fabrication of large-area organic electronics via solution-phase methods.     

Synthesis and thermolysis of a Diels-Alder adduct of pentacene and thiophosgene

Pentacene has excellent semi-conducting properties but its practical use in organic thin film transistors (OTFTs) gives rise to a lot of problems caused by its sensitivity to oxygen and its very low solubility. In order to solve the problems involved in the use of pentacene, we have synthesized a Diels-Alder adduct of pentacene with thiophosgene.     

Photogeneration and thermal generation of pentacene from soluble precursors for OTFT applications

A CO adduct of pentacene with an unsymmetrical structure is synthesized; it is soluble and can be spin-coated into thin films. Pentacene is regenerated in near quantitative yield by either thermal or photoinduced elimination of CO. OTFT devices fabricated by this compound exhibit typical FET characteristics.     

High mobility of dithiophene-tetrathiafulvalene single-crystal organic field effect transistors

Single-crystal field effect transistors of the organic semiconductor dithiophene-tetrathiafulvalene (DT-TTF) were prepared by drop casting. Long, thin crystals connected two microfabricated gold electrodes, and a silicon substrate was used as a back gate. The highest hole mobility observed was 1.4 cm2/Vs, which is the highest reported for an organic semiconductor not based on pentacene. A high ON/OFF ratio of at least 7 x 105 was obtained for this device.     

Studies of tetracene- and pentacene-based organic thin-film transistors fabricated by the neutral cluster beam deposition method

The neutral cluster beam deposition (NCBD) method has been applied to produce and characterize organic thin-film transistors (OTFTs) based upon tetracene and pentacene molecules as active layers. Organic thin films were prepared by the NCBD method on hexamethyldisilazane (HMDS)-untreated and -pretreated silicon dioxide (SiO2) substrates at room temperature. The surface morphology and structures for the tetracene and pentacene thin films were examined by atomic force microscopy (AFM) and X-ray diffraction (XRD). The measurements demonstrate that the weakly bound and highly directional neutral cluster beams are efficient in producing high-quality single-crystalline thin films with uniform, smooth surfaces and that SiO2 surface treatment with HMDS enhances the crystallinity of the pentacene thin-film phase. Tetracene- and pentacene-based OTFTs with the top-contact structure showed typical source-drain current modulation behavior with different gate voltages. Device parameters such as hole carrier mobility, current on/off ratio, threshold voltage, and subthreshold slope have been derived from the current-voltage characteristics together with the effects of surface treatment with HMDS. In particular, the high field-effect room-temperature mobilities for the HMDS-untreated OTFTs are found to be comparable to the most widely reported values for the respective untreated tetracene and pentacene thin-film transistors. The device performance strongly correlates with the surface morphology, and the structural properties of the organic thin films are discussed.     

Conducting AFM and 2D GIXD studies on pentacene thin films

Among all organic semiconductors, pentacene has been shown to have the highest thin film mobility reported to date. The crystalline structure of the first few pentacene layers deposited on a dielectric substrate is strongly dependent on the dielectric surface properties, directly affecting the charge mobility of pentacene thin film OTFTs. Herein, we report that there is a direct correlation between the crystalline structure of the initial submonolayer of a pentacene film and the mobility of the corresponding 60-nm-thick films showing terrace-like structure, as confirmed by 2D grazing-incidence X-ray diffraction and atomic force microscopy. Specifically, multilayered pentacene films, grown from single crystal-like faceted islands on HMDS-treated surface, have shown much higher charge mobility (mu = 3.4 +/- 0.5 cm2/Vs) than those with polycrystalline dendritic islands (mu = 0.5 +/- 0.15 cm2/Vs) on OTS-treated ones.     

A high-resolution near-edge x-ray absorption fine structure investigation of the molecular orientation in the pentacene/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) pentacene/system

We present x-ray photoemission spectroscopy and highly resolved near-edge x-ray absorption fine structure spectroscopy measurements taken on pentacene thin films of different thicknesses deposited on a spin coated poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) substrate. Thin films of pentacene were prepared by using organic molecular beam deposition in situ using strictly controlled evaporation conditions. Our investigations show that pentacene thin films on PEDOT:PSS are characterized by upright standing molecules. Due to the strong dichroic behavior, the calculated values of the molecular orientation give a clear indication not only of the real molecular arrangement in the films but also of a high orientational order. This high degree of molecular orientation order is a characteristic already of the first layer. The films show the tendency to grow on the PEDOT:PSS substrate following an island-fashion mode, with a relatively narrow intermixing zone at the interface between the pentacene and the polymer blend. The peculiarity of the growth of pentacene on PEDOT:PSS is due to the fact that the substrate does not offer any template for the nucleated films and thus exerts a lateral order toward the crystal structure arrangement. Under these conditions, the upright orientation of the molecules in the films minimizes the energy required for the system stability.     

Thin film structure of tetraceno[2,3-b]thiophene characterized by grazing incidence X-ray scattering and near-edge X-ray absorption fine structure analysis

Understanding the structure-property relationship for organic semiconductors is crucial in rational molecular design and organic thin film process control. Charge carrier transport in organic field-effect transistors predominantly occurs in a few semiconductor layers close to the interface in contact with the dielectric layer, and the transport properties depend sensitively on the precise molecular packing. Therefore, a better understanding of the impact of molecular packing and thin film morphology in the first few monolayers above the dielectric layer on charge transport is needed to improve the transistor performance. In this Article, we show that the detailed molecular packing in thin organic semiconductor films can be solved through a combination of grazing incidence X-ray diffraction (GIXD), near-edge X-ray absorption spectra fine structure (NEXAFS) spectroscopy, energy minimization packing calculations, and structure refinement of the diffraction data. We solve the thin film structure for 2 and 20 nm thick films of tetraceno[2,3-b]thiophene and detect only a single phase for these thicknesses. The GIXD yields accurate unit cell dimensions, while the precise molecular arrangement in the unit cell was found from the energy minimization and structure refinement; the NEXAFS yields a consistent molecular tilt. For the 20 nm film, the unit cell is triclinic with a = 5.96 A, b = 7.71 A, c = 15.16 A, alpha = 97.30 degrees, beta = 95.63 degrees, gamma = 90 degrees; there are two molecules per unit cell with herringbone packing (49-59 degree angle) and tilted about 7 degrees from the substrate normal. The thin film structure is significantly different from the bulk single-crystal structure, indicating the importance of characterizing thin film to correlate with thin film device performance. The results are compared to the corresponding data for the chemically similar and widely used pentacene. Possible effects of the observed thin film structure and morphology on charge carrier mobility are discussed.     

Self-assembled submicron-height terrace structures of pentacene single crystals formed in liquid crystal cells

A flat-surface single crystal structure of pentacene organic semiconductor was formed, with a submicron-height terrace structure, in liquid crystal solvent cells; the formation mechanism is discussed. By cooling the pentacene solution in a heated cell until supersaturated, a variety of segregated crystal morphology was observed, including dendrite, lozenge and needle-like crystals. Segregation of lozenge crystals was promoted by the appropriate pentacene concentration combined with the rubbing process of polyimide alignment layers, and the crystal morphology was examined in detailed. As a result, based on terrace-structural growth, low-profile flat-surface crystal morphology was found in addition to conventional pyramidal morphology. The molecular alignment of the flat pentacene crystal was confirmed by anisotropy of the microscopic Raman scattering intensity of the polarized incident light used for excitation. The self-assembly of flat thin single crystal plates, whose maximum size reach 150 µm approximately, may be applicable to practical electronic devices such as organic transistors.     

Structural characterization of a pentacene monolayer on an amorphous SiO2 substrate with grazing incidence x-ray diffraction

Grazing incidence X-ray diffraction reveals that a pentacene monolayer, grown on an amorphous SiO2 substrate that is commonly used as a dielectric layer in organic thin film transistors (OTFTs), is crystalline. A preliminary energy-minimized model of the monolayer, based on the GIXD data, reveals that the pentacene molecules adopt a herringbone arrangement with their long axes tilted slightly from the substrate normal. Although this arrangement resembles the general packing features of the (001) layer in single crystals of bulk pentacene, the monolayer lattice parameters and crystal structure differ from those of the bulk. Because carrier transport in pentacene OTFTs is presumed to occur in the semiconductor layers near the dielectric interface, the discovery of a crystalline monolayer structure on amorphous SiO2 has important implications for transport in OTFTs.     

Molecular orientation transition of organic thin films on graphite: the effect of intermolecular electrostatic and interfacial dispersion forces

In situ low-temperature scanning tunnelling microscopy investigation reveals a molecular orientation transition of organic thin films of pentacene and p-sexiphenyl on graphite, arising from the delicate balance between the intermolecular electrostatic and interfacial dispersion forces.     

Vapor phase self-assembly of molecular gate dielectrics for thin film transistors

Organic-inorganic films grown entirely via a vapor-phase deposition process and composed of highly polarizable molecular structures are investigated as gate dielectrics in organic field-effect transistors (OFETs). Molecules 1 and 2 form self-ordered thin films via hydrogen bonding, and these organic-inorganic structures exhibit large capacitances and large pentacene OFET mobilities.     

Preparation of high-quality organic semiconductor nanoparticle films by solvent-evaporation-induced self-assembly

Organic semiconductor nanoparticles are expected to be used in organic optical and electronic devices due to their unique optical and electrical properties. However, no method has been reported for the preparation of high-quality organic nanoparticle films without remaining additives and being capable of dealing with binary nanoparticle blends. We developed a simple approach to fabricate high-quality organic semiconductor nanoparticle films from their aqueous solutions by solvent-evaporation-induced self-assembly. Only volatile solvents are employed in the nanoparticle solutions, so the self-assembled nanoparticle films are free of additives. Moreover, this method is also suitable for fabricating thin films containing binary nanoparticles. Therefore, it paves the way for potential applications of organic semiconductor nanoparticles in nanoscale optical and electronic devices.     

Tuning the ionization energy of organic semiconductor films: the role of intramolecular polar bonds

For the prototypical conjugated organic molecules pentacene and perfluoropentacene, we demonstrate that the surface termination of ordered organic thin films with intramolecular polar bonds (e.g., -H versus -F) can be used to tune the ionization energy. The collective electrostatics of these oriented bonds also explains the pronounced orientation dependence of the ionization energy. Furthermore, mixing of differently terminated molecules on a molecular length scale allows continuously tuning the ionization energy of thin organic films between the limiting values of the two pure materials. Our study shows that surface engineering of organic semiconductors via adjusting the polarity of intramolecular bonds represents a generally viable alternative to the surface modification of substrates to control the energetics at organic/(in)organic interfaces.     

Influence of SAM Quality on the Organic Semiconductor Thin Film Gas Sensors

Gas sensors based on organic semiconductors receive tremendous attentions owing to their advantages on high selectivity and room temperature operation. However, until now, most organic semiconductor based sensors still suffered from problems, such as low sensitivity, slow response/recovery speed and poor stability. In addition, a clear correlation between the sensing performance and the film property is still absent. Herein, we report the investigation on sensing performance of a series of organic films with various morphologies. By simply adjusting the quality of self-assembled monolayer(SAM) on the silicon wafer surface, we obtain organic semiconductor 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) films with varied morphologies and different charge transport abilities. The film with a small grain size and a continuous morphology presents the highest sensing performance to NO₂, with a sensitivity up to 730%/ppm(ppm=parts per million, vo-lume ratio). We thus reveal that the high sensitivity of the organic film is evident related with the charge transport ability and initial conductivity of the films, as well as the morphologies of both modification layer and the active films.     

Ultrathin organic semiconductor films — Soft matter effect

The growth of organic semiconductor thin films has been a crucial issue in organic electronics, especially the growth at the early stages. The thin-film phase has been found to be a common phenomenon in many organic semiconductor thin films, which is closely related with the weak van der Waals interaction between organic molecules, the long-range interaction between organic molecules and the substrate, as well as the soft matter characteristics of ultrathin films. The growth behavior and soft matter characteristics of the thin-film phase have great effects on thin film morphology and structure, for example, the formation and coalescence of grain boundaries, which further influences the performance of organic electronic devices. The understanding of thin-film phase and its intrinsic quality is necessary for fabricating large-size, highly ordered, continuous and defect-free ultrathin films. This review will focus on the growth behavior of organic ultrathin films, i.e., the level of the first several molecular layers, and provide an overview of the soft matter characteristics.     

Oriented growth of crystalline pentacene films with preferred a-b in-plane alignment on a rubbed crystalline polymethylene surface

We have achieved a growth of highly oriented crystalline pentacene thin films, with preferred a-b in-plane orientation with respect to the rubbing direction of a rubbed polymethylene surface. The polymethylene thin film, generated on a gold surface by gold-catalyzed decomposition of diazomethane, was annealed and gently rubbed in a fixed direction by a flannelette cloth to serve as an alignment layer during the deposition of pentacene molecules. Various surface analysis techniques, including reflection absorption IR spectroscopy (RAIRS), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, grazing incidence X-ray diffraction (GIXD), and atomic force microscopy were used to elucidate the structural details of the polymethylene and the pentacene thin films deposited on it. Two crystalline morphologies of pentacene thin film were observed: the minor one of rod-like molecular crystals having their long axes of the crystals perpendicular to the rubbing direction, and the dominant one of platelet-like and layered crystals having the molecular axes stand near vertical to the surface. Moreover, GIXD revealed that the rubbing on polymethylene indeed induced a preferential azimuthal alignment of pentacene crystallites. The deposition of pentacene at 25 degrees C led to a twin growth of crystallites with the [110] direction predominately aligned perpendicular to the rubbing direction. In contrast, the pentacene deposition at 50 degrees C produced twinned crystallites of lower twin angle and the [120] direction aligned parallel to the rubbing direction.     

Inorganic oxide core, polymer shell nanocomposite as a high K gate dielectric for flexible electronics applications

Organic/inorganic core shell nanoparticles have been synthesized using high K TiO(2) as the core nanoparticle, and polystyrene as the shell. This material is easy to process and forms transparent continuous thin films, which exhibit a dielectric constant enhancement of over 3 times that of bulk polystyrene. This new dielectric material has been incorporated into capacitors and thin film transistors (TFTs). Mobilities approaching 0.2 cm(2)/V.s have been measured for pentacene TFTs incorporating the new TiO(2) polystyrene nanostructured gate dielectric, indicating good surface properties for pentacene film growth. This novel strategy for generating high K flexible gate dielectrics will be of value in improving organic and flexible electronic device performance.     

Orientation of pentacene molecules on SiO2: from a monolayer to the bulk

Near edge x-ray absorption fine structure (NEXAFS) spectroscopy is used to study the orientation of pentacene molecules within thin films on SiO2 for thicknesses ranging from monolayers to the bulk (150 nm). The spectra exhibit a strong polarization dependence of the pi* orbitals for all films, which indicates that the pentacene molecules are highly oriented. At all film thicknesses the orientation varies with the rate at which pentacene molecules are deposited, with faster rates favoring a thin film phase with different tilt angles and slower rates leading to a more bulklike orientation. Our NEXAFS results extend previous structural observations to the monolayer regime and to lower deposition rates. The NEXAFS results match crystallographic data if a finite distribution of the molecular orientations is included. Damage to the molecules by hot electrons from soft x-ray irradiation eliminates the splitting between nonequivalent pi* orbitals, indicating a breakup of the pentacene molecule.