Pentacene Surface Research Articles

Stress-driven recrystallization of pentacene films via diffusion-induced ingress of MOF nanocrystals into the grain boundaries

Metal-organic frameworks (MOFs) have been identified as versatile candidates for producing functional materials for various applications. However, the growth of MOF thin film becomes challenging due to non-availability of a proper solvent that restricts the use of the materials in electronic sensor fabrication. The receptor molecules in sensors are often used to functionalize the semiconducting channel. However, integrating such molecules degrade the crystallinity of channel materials, leading to poor sensor performance. In this work, we report the growth of a MOF (CPO-27-Ni) thin film and the recrystallization of semiconducting pentacene thin films due to diffusion of the crystallites into the grain boundaries. MOF films were successfully spin coated at various revolutions per minute (rpm) on pentacene surfaces. The growth of MOF films was studied using statistical analysis of rough surfaces by utilizing scaling theory to capture the evolution of surface morphologies. The x-ray reflectivity study unveiled the diffusion-induced ingress of the MOF through the grain boundaries of pentacene films that imparts compressive stress within the grains leading to subsequent rearrangement of pentacene molecules in the thin-film phase and recrystallizes the bulk crystallographic phase, which in a way will enhance the device performance.

Top-Contact Pentacene-Based Organic Thin Film Transistor with a Rubrene Layer in between Pentacene-Electrode Interface

Top-contact pentacene-based organic thin-film transistor (OTFT) with a rubrene interlayer in between pentacene-electrode [Al, Au] interface is reported. A study of the interlayer behavior of rubrene shows enhanced device performance of OTFT than that of the conventional OTFTs with only metal source-drain electrodes [Al, Au]. The improved performances of the device are attributed to the smoother pentacene surface for high carrier injection and mobility and decrease in contact resistance of the device. The device with a rubrene interlayer in between pentacene/Au interface shows better field-effect mobility of 3.3 cm2 v−1 s−1, On/Off ratio of 1.22 × 107, the threshold voltage of −3.8 V, and sub-threshold-slope of 0.31 V decade−1 respectively.

High Detectivity Ammonia Gas Sensor of Pentacene Based Organic Material with OXIDE MESH

Ammonia gas sensing properties of pentacene based organic gas sensor fabricated by thermal evaporation method on amorphous oxide semiconductor (a-SiInZnO) mesh. The amorphous oxide semiconductor mesh layer was fabricated by RF sputtering and conventional photolithography. Insertion of a dense mesh layer has been found to significantly improve gas sensing properties. In the case of a conventional pentacene thin film, the SEM image analysis resulted in a uniform surface. This means that ammonia gas cannot be adsorbed well on the surface. The insertion of the mesh layer improves the surface volume ratio of the organic thin film and induces more ammonia gas to be adsorbed on the pentacene surface. As a result, it was confirmed that even when the ammonia gas concentration was 5 ppm or less, the detection rate was higher than that of a conventional pentacene thin film. It is expected to be applied to various high-sensitivity organic-based gas sensors.

Morphology, Structure, and Dynamics of Pentacene Thin Films and Their Nanocomposites with [C2 C1 im][NTf2 ] and [C2 C1 im][OTF] Ionic Liquids.

In this study, a homogeneous thin film growth of pentacene onto indium tin oxide (ITO) coated glass surfaces is explored using a high-resolution and reproducible vapor deposition methodology. Moreover, vacuum thermal evaporation of ionic liquids (ILs) ([C2 C1 im][NTf2 ] and [C2 C1 im][OTF]) onto ITO, gold/palladium (AuPd) and pentacene surfaces were performed. A greater wettability behavior of ILs is observed for surfaces containing AuPd. Sequential and simultaneous depositions of ILs and pentacene were explored. Simultaneous depositions lead to the formation of nanocomposites films, consisting of IL micro- and nanodroplets covered by pentacene layers. Plasma surface treatment was used to induce the ILs droplets coalescence and explore the dynamics and phase separation of the nanocomposites. The [C2 C1 im][OTF] droplets were found to be completely covered with pentacene, which suggests a great affinity between cation-anion pairs and the aromatic moiety. Pentacene films and their nanocomposites with ILs exhibit a typical optical band gap of Egap =1.77 eV, indicating that the nanocomposite phase domains are large enough to behavior as the bulk.

Exciton Dissociation and Electron Transfer at a Well-Defined Organic Interface of an Epitaxial C60 Layer on a Pentacene Single Crystal

The electron transfer of a well-defined single-crystalline organic P–N interface of the epitaxial layer of C60 on the surface of a single-crystalline pentacene (Pen-SC) was observed using time-reso...

A Pentacene -Based Organic Mis Structures

Organic devices based on pentacene thin film are demonstrated. Gold nanoparticles were used as the charge storage elements while pentacene has been used as the active semiconductor layer of Metal-Insulator-Semiconductor (MIS) structures. Polymethylmethacrylate (PMMA) formed the gate insulator. Four devices at different evaporation rates were fabricated and characterised by the atomic force microscopy and electrical characteristics. An optimal deposition rate was identified and supporting topographical images are presented. The device performance strongly correlates with the surface morphology, and the structural properties of the organic thin films have been investigated. The Atomic Force Microscopy (AFM) investigation of very slow evaporated pentacene showed larger grain sizes. Evaporating of gold electrodes on pentacene layers proved to change the morphology of the pentacene surface. The characteristics of bottom contact structures showed much-improved pentacene structures.

Effect of a pentacene anode buffer on the performance of small-molecule organic solar cells

The effect of a thin pentacene anode buffer layer on the performance of small-molecule organic solar cells (OSCs) was experimentally investigated primarily from morphological and crystallographic viewpoints. The OSC had a structure of indium-tin oxide (ITO, anode)/pentacene (anode buffer)/copper phthalocyanine (CuPc, donor)/fullerene (acceptor)/bathocuproine (cathode buffer)/Ag (cathode). The thin pentacene layer provided an enhanced initial device performance, that is, an increase of 5–29% in the short-circuit current density, resulting in an increase of 10–43% in the power conversion efficiency ηp. Atomic force microscopy showed that the roughness of the pentacene surface was large and that this roughness was increased and transferred to the overlaid CuPc surface. X-ray diffraction and near-edge X-ray absorption fine structure spectroscopy analyses showed that the standing-up CuPc crystallite orientation became randomized as the pentacene thickness was increased. The pentacene layer did not bring about the lying-down configuration of CuPc film molecules with respect to the substrate surface. The origin of the increase in ηp was the increase in the area between the donor–acceptor interface with enhanced roughness, which brought about an increase in the number of carriers generated at the interface. The reduction of energy barrier height for hole extraction from CuPc to ITO was also a possible reason for the increase in ηp.

Reaction induced morphology changes of tetracene and pentacene surfaces.

Morphology plays a critical role in determining the properties of solid-state molecular materials, yet fluctuates wildly as these materials undergo reaction. A prototypical system, a vapor–solid Diels–Alder reaction of tetracene and pentacene thin-films, is used to observe the evolution of morphology features as the reaction transitions from surface to bulk. The initial stages of reaction display little topographical change as measured by atomic force microscopy (AFM) and scanning electron microscopy (SEM), and substrates are coated with a uniform layer of product 1–2 molecules thick, as determined by energy-dispersive X-ray (EDX) spectroscopy. The highly textured surfaces of late stage reactions are a result of aggregated products, as identified via EDX spectroscopy and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS); areas of the surface in between product aggregates resemble the initial stages. The mechanism by which products aggregate into surface asperities requires the assistance of a facilitating media – in this case condensed vapor; simple thermally assisted surface diffusion was unable to generate these morphology changes. The combined data indicate that reactions of molecular solids, could be confined to the surface in the absence of condensate of the vapor phase reactant.

Temperature Dependent Epitaxial Growth of C60 Overlayers on Single Crystal Pentacene

AbstractHeteroepitaxy of one material onto another molecular single crystal surface is one promising route for resolving questions about formation criteria of molecular heterojunction structures as well as for the development of next‐generation organic electronic devices allowing efficient intermolecular charge carrier exchange. In the present work, the in‐plane and out‐of‐plane crystallinity of an epitaxial molecular p–n heterojunction, C60 (acceptor) overlayers formed on the single crystal surface of pentacene (donor), and its evolution, depending on the growth temperature, are systematically elucidated. It is demonstrated that the crystallinity of the C60 on pentacene is dominated by the temperature during the growth rather than the postannealing of the sample. The mean crystallite size in the in‐plane directions grows from 50 to 150 nm proportionally to the growth temperature in a range of 125–370 K. The present results suggest that the formation mechanisms of the C60/pentacene heterojunction are kinetically controlled, by diffusion processes at the molecular interface, rather than by the thermal equilibrium conditions.

Accurate description of charged excitations in molecular solids from embedded many-body perturbation theory

We present a novel hybrid quantum/classical approach to the calculation of charged excitations in molecular solids based on the many-body Green's function $GW$ formalism. Molecules described at the $GW$ level are embedded into the crystalline environment modeled with an accurate classical polarizable scheme. This allows the calculation of electron addition and removal energies in the bulk and at crystal surfaces where charged excitations are probed in photoelectron experiments. By considering the paradigmatic case of pentacene and perfluoropentacene crystals, we discuss the different contributions from intermolecular interactions to electronic energy levels, distinguishing between polarization, which is accounted for combining quantum and classical polarizabilities, and crystal field effects, that can impact energy levels by up to $\ifmmode\pm\else\textpm\fi{}0.6$ eV. After introducing band dispersion, we achieve quantitative agreement (within 0.2 eV) on the ionization potential and electron affinity measured at pentacene and perfluoropentacene crystal surfaces characterized by standing molecules.

Heterojunction effect on contact resistance minimization in staggered pentacene thin-film transistors

The MoO3/pentacene heterojunction is demonstrated to be effective for reducing the contact resistance in staggered organic thin-film transistors. The heterojunction-induced doping is nondestructive and may form a top conducting channel close to the pentacene surface. Contact interface doping and channel doping both significantly reduced the contact resistance. The effect of channel doping was prominent at low gate bias values, which is ascribed to the negligible access resistance owing to the presence of the top channel. Interface doping and channel doping were combined to obtain a complete heterojunction, which exhibited minimized contact resistance for a wide range of gate bias values.

Balancing Surface Hydrophobicity and Polarizability of Fluorinated Dielectrics for Organic Field‐Effect Transistors with Excellent Gate‐Bias Stability and Mobility

It is demonstrated that treating dielectrics with fluorinated polymers, which have excellent hydrophobicity, chemical inertness, and the lowest polarizability, yields a semiconductor‐compatible surface energy and excellent charge detrapping characteristics. Fluorocopolymers, polystyrene‐random‐poly(2,3,4,5,6‐pentafluorostyrene) (PS‐r‐PPFS) copolymers with different 2,3,4,5,6‐pentafluorostyrene (PFS) loadings, are synthesized to modify a SiO2 gate dielectric using radical polymerization. Surface energy (γ) of the copolymer‐treated SiO2 dielectrics decreases from 40.7 to 24.0 mJ m−2 with increasing PFS mol% in the copolymer. Pentacene organic field‐effect transistors (OFETs) show field‐effect mobility (μFET) values ranging from 0.82 (for 0 mol% PFS) to 0.25 cm2 V−1 s−1 (for 100 mol% PFS). Enhancing the bias stress stability without affecting the μFET value is achieved via the introduction of a small mol% fluorocarbon segments onto the PS‐r‐PPFS backbone. 15 mol% PFS‐loaded fluorocopolymer‐coated SiO2 substrate yields a γ value of 35 mJ m−2, close to that (38 mJ m−2) of the lowest‐γ crystal surface of pentacene, and the corresponding OFETs have μFET values up to 0.81 cm2 V−1 s−1 and excellent gate‐bias stress stability in comparison to the rich fluorinated dielectric systems, which has degraded μFET values ranging from 0.2 to 0.4 cm2 V−1 s−1.

Epitaxial Growth of an Organic p-n Heterojunction: C60 on Single-Crystal Pentacene.

Designing molecular p-n heterojunction structures, i.e., electron donor-acceptor contacts, is one of the central challenges for further development of organic electronic devices. In the present study, a well-defined p-n heterojunction of two representative molecular semiconductors, pentacene and C60, formed on the single-crystal surface of pentacene is precisely investigated in terms of its growth behavior and crystallographic structure. C60 assembles into a (111)-oriented face-centered-cubic crystal structure with a specific epitaxial orientation on the (001) surface of the pentacene single crystal. The present experimental findings provide molecular scale insights into the formation mechanisms of the organic p-n heterojunction through an accurate structural analysis of the single-crystalline molecular contact.

Multiscale Simulation and Modeling of Multilayer Heteroepitactic Growth of C60 on Pentacene.

We apply multiscale methods to describe the strained growth of multiple layers of C60 on a thin film of pentacene. We study this growth in the presence of a monolayer pentacene step to compare our simulations to recent experimental studies by Breuer and Witte of submonolayer growth in the presence of monolayer steps. The molecular-level details of this organic semiconductor interface have ramifications on the macroscale structural and electronic behavior of this system and allow us to describe several unexplained experimental observations for this system. The growth of a C60 thin film on a pentacene surface is complicated by the differing crystal habits of the two component species, leading to heteroepitactical growth. In order to probe this growth, we use three computational methods that offer different approaches to coarse-graining the system and differing degrees of computational efficiency. We present a new, efficient reaction-diffusion continuum model for 2D systems whose results compare well with mesoscale kinetic Monte Carlo (KMC) results for submonolayer growth. KMC extends our ability to simulate multiple layers but requires a library of predefined rates for event transitions. Coarse-grained molecular dynamics (CGMD) circumvents KMC's need for predefined lattices, allowing defects and grain boundaries to provide a more realistic thin film morphology. For multilayer growth, in this particularly suitable candidate for coarse-graining, CGMD is a preferable approach to KMC. Combining the results from these three methods, we show that the lattice strain induced by heteroepitactical growth promotes 3D growth and the creation of defects in the first monolayer. The CGMD results are consistent with experimental results on the same system by Conrad et al. and by Breuer and Witte in which C60 aggregates change from a 2D structure at low temperature to 3D clusters along the pentacene step edges at higher temperatures.

Light emitting field-effect transistors with vertical heterojunctions based on pentacene and tris-(8-hydroxyquinolinato) aluminum

We report a top-contact light emitting field-effect transistor based on an asymmetric vertical heterojunction using pentacene as a field-effect layer and tris-(8-hydroxyquinolinato) aluminum (Alq3) as an electron transport and luminescent material, which is fabricated on an indium tin oxide (ITO)-coated glass substrate with poly (methyl methacrylate) (PMMA) as a gate dielectric. The Alq3 layer underneath the drain electrode roughly occupies one half of the pentacene surface forming an asymmetric heterojunction with pentacene. A hole transport layer N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) is introduced to occupy the other half of the pentacene surface underneath the source electrode to allow vertical hole transport in the device. We have realized the electrical switching functionality of a field-effect transistor (FET) and the control of electroluminescence (EL) simultaneously under ambient atmosphere. The device exhibits typical p-channel characteristics and green emission from Alq3 is observed adjacent to the drain electrode. A working principle of the device is discussed in detail. Furthermore, this device configuration enables high-spatial-resolution fluorescence imaging of device operation, which is a simple and powerful tool for studying organic luminescent materials.

Correlating optimal electrode buffer layer thickness with the surface roughness of the active layer in organic phototransistors

Inserting a C60 buffer layer between Au source/drain electrodes and pentacene active layer has been proved to improve the performances of pentacene organic phototransistors (PENT-OPTs) in our previous study. Buffer layer certainly has an optimal thickness with which the modified device can achieve the best performance. Based on the surface morphology analysis of different thickness C60 buffer layer on pentacene film, we further optimized the thickness of C60 buffer layer for best performance of PENT-OPTs and investigated its physical origins. Studies on PENT-OPTs with different pentacene surface morphology realized by different substrate temperatures indicate that the optimal thickness of C60 buffer layer directly related to the surface roughness of pentacene active layer and it is found that the optimized buffer layer thickness increases with the roughness of pentacene layer. Besides, we found that the photogenerated current of OPTs increases with the increasing of gate electric bias and then gradually reach saturation. An approximate analytical expression for gate voltage dependence of the photogenerated current was derived and used to fit the experiment data. An important parameter, saturated photoresponsivity, was introduced for better comparing the performances of OPTs.

Degradation of pentacene deposited on gold, aluminum and parylene surfaces: Impact of pentacene thickness

In this work, the impact of film thickness in thermally evaporated pentacene thin films and metal electrodes is studied. In particular, we report electrical performance and stability of un-encapsulated organic Schottky diodes. The chemical composition, surface morphology and ambient stability of the resulting films were also studied. The electrical measurements were carried out in vertical Schottky diodes and its performance and stability was evaluated as function of shelf life time. Oxygen was detected at the pentacene surface. This oxygen is the main cause of the diode degradation and attributed to diffusion of oxygen into the bulk of the pentacene films. Thicker pentacene films resulted in more stable for Schottky diodes.

A label-free, organic transistor-based biosensor by introducing electric bias during DNA immobilization

We report an improved label-free DNA sensor based on pentacene organic thin-film transistors (OTFTs). OTFT with top-contact structure was first fabricated by using pentacene as the active layer. Different electric biases were introduced between the source and gate contacts of OTFT during the single-stranded DNA (ssDNA) immobilization process in order to improve the immobilization efficiency of DNA molecules on the pentacene substrate. Atomic force microscopy (AFM) images show the application of positive bias can significantly improve the amount of the immobilized ssDNA. The potentiostatic effect of bias can induce more ssDNA molecules onto the pentacene surface, which leads to the improvement of sensor sensitivity reflected by the electric signals of OTFTs. Furthermore, an optimized immobilization time of 30min was obtained at a constant bias that exhibits the highest immobilization efficiency. With this design, the optimized process conditions (+50V bias and an immobilization of 30min) for the ssDNA immobilization on the pentacene film were also obtained. As a result, the introduction of bias during immobilizing ssDNA molecules has been proposed to improve the immobilized efficiency, which provides an effective path to enhance the sensitivity of OTFT-based DNA sensors.

Organic Ferroelectric Field-Effect Transistor Memory Using Flat Poly(vinylidene fluoride–tetrafluoroethylene) and Pentacene Thin Films

Organic ferroelectric field-effect transistor (FET) memories have been fabricated using pentacene as the semiconductor and a flat poly(vinylidene fluoride–tetrafluoroethylene) [P(VDF–TeFE)] thin film as the ferroelectric gate. The P(VDF–TeFE) film is prepared by spin coating, and it was cooled slowly with a flattening process after annealing. The polarization–electric field (P–E) hysteresis of the P(VDF–TeFE) thin film prepared by slow cooling is larger than that in the case of quick cooling. Moreover, the flattening process does not have a negative effect on ferroelectric properties. The obtained remanent polarization (Pr) of 5.2 µC/cm2 is sufficient for controlling the pentacene surface potential. Good memory characteristics are obtained in the P(VDF–TeFE) gate FET with pentacene deposited on the flat P(VDF–TeFE). The maximum drain current is about twice larger than that deposited on the rough P(VDF–TeFE) prepared by quick cooling, and the memory retention is over 1 week.

Optimizing Pentacene Growth in Low-Voltage Organic Thin-Film Transistors Prepared by Dry Fabrication Techniques

ABSTRACTWe have studied the effect of pentacene purity and evaporation rate on low-voltage organic thin-film transistors (OTFTs) prepared solely by dry fabrication techniques. The maximum field-effect mobility of 0.07 cm2/Vs was achieved for the highest pentacene evaporation rate of 0.32 Å/s and four-time purified pentacene. Four-time purified pentacene also led to the lowest threshold voltage of -1.1 V and inverse subthreshold slope of ∼100 mV/decade. In addition, pentacene surface was imaged using atomic force microscopy, and the transistor channel and contact resistances for various pentacene evaporation rates were extracted and compared to field-effect mobilities.