Pentacene Films Research Articles

Quasi-Layer-by-Layer Growth of Pentacene on HOPG and Au Surfaces

High-resolution scanning tunneling microscopy (STM) is a promising method for characterizing organic semiconductors to obtain a deep understanding of organic semiconductor physics. However, organic films on conductive single-crystal substrates, which are required for STM, usually present different growth behaviors than the films on inert substrates such as SiO2. Here, we reported a simple modification method for modulating the organic semiconductor film growth on the highly oriented pyrolytic graphite (HOPG) and Au(111) substrates and investigated the detailed morphology evolution. Self-assembled monolayers (SAMs) fabricated from vacuum deposition and solution processing were introduced on these conductive substrates. Pentacene, a prototypical organic semiconductor, presented quasi-layer-by-layer growth on HOPG or Au(111) sufaces modified with solution-processed alkane monolayer. The pentacene film resembled the upright packing and terraced morphology but with larger grain size than that of thin-film phas...

A low temperature solution-processed ormosil film for low-voltage organic field-effect transistors

A high performance, low operating voltage organic field effect transistor (OFET) has been fabricated by utilizing organically modified silicate (ormosil) based film as dielectric. The ormosil film is fabricated by a sol-gel method at low temperature (180 °C) and exhibits a high dielectric (k = 33), a smooth surface (Rq = 0.29 nm) and a low leakage current density (5 × 10−9 A cm−2 at −2 V). The ormosil film contains hydrophobic methyl (CH3) functional groups derived from methyltriethoxysilane (MTES) and these groups produce a surface with hydrophobic character and low surface energy for pentacene film growth. The OFET with the ormosil dielectric exhibits excellent performs with high mobility (0.80 cm2 V−1 s−1), low operating voltage (−1.5 V), low threshold voltage (−0.25 V) and low sub-threshold swing (192 mV dec−1). The result demonstrates that the ormosil film can be used as a high performance dielectric for OFETs, and provides a promising way for low power and low cost organic electronics.

Molecular alignments of σ phase in co-evaporated pentacene and perfluoropentacne film on SiO2 studied by grazing-incidence X-ray diffraction

ABSTRACTGrazing-incidence X-ray diffraction experiment suggested that σ-phase of pentacene and perfluoropentacene co-evaporated film was composed of polymorphs. Two ordered alloy and one random alloy alignments were found. Tilt angles of pentacene and perfluoropentacene were 20° and 21° in one ordered alloy alignment. The other ordered alloy alignment and random alloy alignment resembles to each other. Tilt angles of pentacene and perfluoropentacene was between 5° and 14°. Tilt angles of pentacnene and perfluoropentacene in amorphous phase were also estimated to be 20°.

Surface Treatments of Poly(4-Vinyl Phenol) Insulator for High-Performance Pentacene Thin-Film Transistors

We herein demonstrate a method for reducing the surface energy in polymer insulators for high-performance organic thin-film transistors (OTFTs). The surface energy of the poly(4-vinyl phenol) (PVP) insulator was reduced from 45 mJ/m2 to 32 mJ/m2 by treatment with UV/Ozone and octadecyltrichlorosilane (ODTS). The pentacene growth was controlled by the surface energy. It caused the pentacene film deposited on the ODTS-treated PVP insulator to induce three-dimensional growth, leading to an increase in the field-effect mobility of approximately 109%. These results demonstrate that the method of adjusting the surface energy in polymer insulators is suitable for enhancing the performance of OTFTs.

Lowering the driving voltage and improving the luminance of blue fluorescent organic light-emitting devices by thermal annealing a hole injection layer of pentacene**Project supported by the National Natural Science Foundation of China (Grant No. 60906022), the Natural Science Foundation of Tianjin, China (Grant No. 10JCYBJC01100), the Key Science and Technology Support Program of Tianjin, China (Grant No. 14ZCZDGX00006), and the

We chose pentacene as a hole injection layer (HIL) to fabricate the high performance blue fluorescent organic lightemitting devices (OLEDs). We found that the carrier mobility of the pentacene thin films could be efficiently improved after a critical annealing at temperature 120 °C. Then we performed the tests of scanning electron microscopy, atomic force microscopy, and Kelvin probe to explore the effect of annealing on the pentacene films. The pentacene film exhibited a more crystalline form with better continuities and smoothness after annealing. The optimal device with 120 °C annealed pentacene film and n-doped electron transport layer (ETL) presents a low turn-on voltage of 2.6 V and a highest luminance of 134800 cd/m2 at 12 V, which are reduced by 26% and improved by 50% compared with those of the control device.

Study of copper-phthalocyanine and pentacene film growth on transferred graphene: The influence of polymer residues

Wet transferring of chemical vapor deposition (CVD) graphene with the assistance of poly(methyl methacrylate) (PMMA) usually leaves polymer contaminations onto graphene surface. Knowledge about the influence of PMMA residues on organic film growth properties is crucial for graphene-related organic electronic devices. In this study, we investigate growth properties of coper-phthalocyanine (CuPc) and pentacene on PMMA contaminated graphene substrate. Grain size and morphologies of CuPc and pentacene films were found to be governed by the density of PMMA residues on graphene. Nevertheless, the molecular packing structures of CuPc and pentacene respond differently with the density with PMMA residues. CuPc molecules tend to nucleated into grains with molecular π-π stacking parallel to graphene surface, which is irrespective of the PMMA residues. Whereas the packing behavior of pentacene on PMMA-transferred CVD graphene is sensitive to the density of PMMA residues. Molecular π-π stacking perpendicular to graphene is observed when pentacene was deposited onto as-transferred graphene. Parallel mode was observed only on the substrates without PMMA residues.

Pentacene crystal transition during the growth on SiO2 studied by in situ optical spectroscopy

We discuss the processes involved in the formation of a few monolayer thick pentacene films on the SiO2 surface. The evolution of the optical properties has been monitored in situ by optical spectroscopy and the interface morphology was investigated ex situ by atomic force microscopy. The sub monolayer shows the identical characteristic optical response with the intensity proportional to the coverage. From the second monolayer growth on, the optical properties are significantly changed. This experimental evidence indicates the formation of the first monolayer layer comprising the so-called “orthorhombic phase” crystallites with near-vertically oriented pentacene molecules, and the multilayer above comprises the so-called “thin-film phase” crystallites. Most importantly, the combination of optical spectroscopy and atomic force microscopy (AFM) investigations reveals that the first monolayer crystallize into the orthorhombic phase and the structure is persevered even after being buried by the multilayer. These observations suggest that the thickness-driven phase transformation from the orthorhombic phase to the thin-film phase starts from the second monolayer on rather than the first monolayer. This result provides a new insight into the structural evolutions and growth of pentacene thin films on SiO2.

Molecular packing correlated fluorescence in TIPS-pentacene films

Abstract The molecular packing and optical properties of exposed and buried layers (i.e. the layers at the top surface and near the substrate, respectively) were systematically studied in 6,13-Bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) films coated by spin-coated (SC) and droplet-pinned-crystallization (DPC) methods. Buried layers in both films exhibit intense photoluminescence (PL) resembling the behaviors of the molecules in dilute solution ascribing to weak π-π stacking, while the exposed layers show extremely weak PL due to strong crystallinity. Polarized excitation PL spectra demonstrate that molecular orientation of the buried layers is quasi-ordered in the film coated by DPC method and completely disordered in the film coated by SC method. Besides, the strong crystallinity of the exposed TIPS-pentacene is verified by grazing incident wide-angle X-ray scattering measurement. The distinct differences in optical and structural properties between the exposed and buried layers indicate that TIPS-pentacene films are inhomogeneous in vertical direction due to interfacial effect, which affects the performance of photodiode fabricated with both films. The understanding of the molecular packing correlated fluorescence in TIPS-pentacene films is vital for optimizing the film structure to achieve high performance organic electronic devices.

Effects of oxygen plasma generated in magnetron sputtering of ruthenium oxide on pentacene thin film transistors

Effects of oxygen plasma generated in a sputtering process for deposition of electrodes on pentacene thin films to configure top-contact (TC) transistors have been thoroughly investigated. Reactive oxygen species severely degraded electrical properties of pentacene films during the deposition of RuOx electrodes, leading to a failure of devices. In the off-region, the leakage current increased by about two orders of magnitude, and the subthreshold slope also increased by 6.5 times. The top surface of pentacene films was oxidized by oxygen plasma and C-O and C=O bonds awerere created. The pentacenequinone derivative was confirmed by X-ray photoelectron spectroscopy. The oxidation of pentacene films gives rise to charge traps at the pentacene/electrode interface, which produces a leakage channel between source and drain electrodes. We believe that this side effect of oxygen plasma on the fabrication of TC-devices should be considered carefully.

Transferable Organic Semiconductor Nanosheets for Application in Electronic Devices.

A method has been developed to stabilize and transfer nanofilms of functional organic semiconductors. The method is based on crosslinking of their topmost layers by low energy electron irradiation. The films can then be detached from their original substrates and subsequently deposited onto new solid or holey substrates retaining their structural integrity. Grazing incidence X-ray diffraction, X-ray specular reflectivity, and UV-Vis spectroscopy measurements reveal that the electron irradiation of ≈50 nm thick pentacene films results in crosslinking of their only topmost ≈5 nm (3-4 monolayers), whereas the deeper pentacene layers preserve their pristine crystallinity. The electronic performance of the transferred pentacene nanosheets in bottom contact field-effect devices is studied and it is found that they are fully functional and demonstrate superior charge injection properties in comparison to the pentacene films directly grown on the contact structures by vapor deposition. The new approach paves the way to integration of the organic semiconductor nanofilms on substrates unfavorable for their direct growth as well as to their implementation in hybrid devices with unusual geometries, e.g., in devices incorporating free-standing sheets.

Real time monitoring of ordering in pentacene films during growth by using in-situ infrared spectroscopy

By employing the in-situ Fourier transform infrared (FTIR) spectroscopy and curve-fitting, we studied the evolution in the ordering of vacuum-deposited pentacene films. In the FTIR spectra, the thin film and bulk phases appeared at 903.2 cm−1 and 907.2 cm−1, respectively. The misoriented phase was also observed at 904.4 cm−1. The position and the intensity of each phase varied with the film thickness as well as the growth temperature. The results indicated that most of pentacene molecules formed initially the thin film phase until the bulk and misoriented phases began to form at the increased thickness. Our study suggests that FTIR spectroscopy is an effective tool to monitor in real time the ordering of organic films for quality control.

Anisotropic thermal expansion in pentacene and perfluoropentacene: Effects of molecular packing motif and fixation at the interface

Thermal expansion coefficients of molecular solids are typically significantly larger than those of inorganic materials. Since they are furthermore highly anisotropic, the molecular arrangement and consequently the intermolecular orbital overlap strongly depend on temperature, hence also affecting the energetics of optoelectronic excitations and the efficiency of charge transfer processes. Here, we report on the precise determination of the anisotropic thermal expansion coefficients of the organic semiconductor pentacene in its solid state. We compare the thermal expansion coefficients of three different pentacene polymorphs and observe distinct differences between both pentacene bulk polymorphs and the interface-stabilized thin film phase. By comparing epitaxial films with films prepared on weakly interacting, amorphous substrates, we identify a notable influence of the substrate fixation on the thermal expansion in thin pentacene films. Furthermore, the results for pentacene are compared to the thermal expansion of perfluoropentacene, where an exceptionally large vertical thermal expansion coefficient is found in the substrate-mediated π-stacked polymorph. The present study underlines the importance of thermal expansion for the interpretation of temperature-dependent spectroscopic measurements and device characterizations since the notable changes in the unit cell geometries severely affect the intermolecular coupling and thus the excitonic energetics.

Study of carrier-mobility of organic thin film by dark-injection time-of-flight and electric-field-induced optical second-harmonic generation measurements

By using dark-injection time-of-flight (ToF) and time-resolved electric-field-induced optical second-harmonic generation (EFISHG) measurements, we studied carrier mobility μ of pentacene (Pen) thin film of ITO/Pen/Al and Au/Pen/polyimide/ITO diodes where pentacene film is ∼100nm in thickness. ToF showed that determination of transit time tr from trace of transient currents is difficult owing to large capacitive charging current. On the other hand, optical EFISHG is free from this charging current, and allows us to calculate hole and electron mobility as μh=1.8×10-4cm2/Vs and μe=7.6×10-7cm2/Vs, respectively, by using the relation tr=d/μ∫tctrE(0)dt (d: Pen thickness, E(0): electric field across Pen), instead of the conventional relationship tr=d2/μV (V: voltage across Pen). Time-resolved EFISHG measurement is useful for the determination of carrier mobility of organic thin film in organic devices.

The effect of interface-induced structural properties of the pentacene accumulation layer on the threshold voltage: Pentacene monolayer transistors

The effect of pentacene/gate dielectric interface-induced structural properties of a pentacene monolayer (ML) close to the SiO2 gate dielectric on the threshold voltage in the field effect transistor (FET) and van der Pauw was addressed using atomic force microscopy and near edge X-ray absorption fine structure spectroscopy (NEXAFS). Our study reveals that large negative threshold voltage found in FET and van der Pauw devices is not closely correlated to the molecular orientation and the grain size of pentacene molecules in direct contact with the SiO2 gate dielectric. In concluding our finding, NEXAFS results in the ultrathin pentacene film within the carrier accumulation thickness regime, characterized by Debye length, were correlated to the magnitude of the threshold voltage from field effect devices. Our work motivates finding and visualizing interface-induced structural properties that are directly correlated to the magnitude of the threshold voltage.

Delayed phase separation in growth of organic semiconductor blends with limited intermixing

Binary mixed thin films of picene (C22H14, PIC) and pentacene (C22H14, PEN) consist of crystallites with a statistical occupation of the lattice sites by either PEN or PIC and unit cell parameters continuously changing with the mixing ratio. For high PIC ratios a PIC phase forms which corresponds to a limited intermixing of the two compounds. The growth behavior of these mixtures is investigated in situ and in realtime using grazing incidence X-ray diffraction. We observe a delayed phase separation in PIC-rich blends, i.e. complete intermixing in the monolayer range and the nucleation of a pure PIC-phase in addition to the intermixed phase starting from the second monolayer.

Estimation of Charge-Injection Barriers at the Metal/Pentacene Interface through Accumulated Charge Measurement

The charge-injection barrier from metal electrodes to thin-film pentacene is investigated using accumulated charge measurements. When a gold electrode is deposited on a pentacene film, the interface forms a Schottky contact with a hole-injection barrier of 0.2 eV. However, interfacial carrier motion is reversible between charge injection and discharge. The result suggests that the reported electrical hysteresis in typical pentacene transistors is caused by carrier traps that are localized primarily in the SiO2/pentacene interface. The Ag/pentacene junction has a large barrier height of 0.5 eV. The barrier height is significantly reduced, and an Ohmic contact is realized by using molybdenum oxide (MoO3) as a buffer layer.

Spin current relaxation time in thermally evaporated pentacene films

The spin current relaxation time [tau] in thermally evaporated pentacene films was evaluated with the spin-pump-induced spin transport properties and the charge current transport properties in pentacene films. Under an assumption of a diffusive transport of the spin current in pentacene films, the zero-field mobility and the diffusion constant of holes in pentacene films were experimentally obtained to be ~8.0x10^-7 m^2/Vs and ~2.0x10^-8 m^2/s, respectively. Using those values and the previously obtained spin diffusion length in pentacene films of 42{plus-minus}10 nm, the [tau] in pentacene films was estimated to be 150{plus-minus}120 ns at room temperature. This estimated [tau] in pentacene films is long enough for practical use as a spintronic material.

Influence of orientation mismatch on charge transport across grain boundaries in tri-isopropylsilylethynyl (TIPS) pentacene thin films.

We present a multi-scale model for charge transport across grain boundaries in molecular electronic materials that incorporates packing disorder, electrostatic and polarisation effects. We choose quasi two-dimensional films of tri-isopropylsilylethynyl pentacene (TIPS-P) as a model system representative of technologically relevant crystalline organic semiconductors. We use atomistic molecular dynamics, with a force-field specific for TIPS-P, to generate and equilibrate polycrystalline two-dimensional thin films. The energy landscape is obtained by calculating contributions from electrostatic interactions and polarization. The variation in these contributions leads to energetic barriers between grains. Subsequently, charge transport is simulated using a kinetic Monte-Carlo algorithm. Two-grain systems with varied mutual orientation are studied. We find relatively little effect of long grain boundaries due to the presence of low impedance pathways. However, effects could be more pronounced for systems with limited inter-grain contact areas. Furthermore, we present a lattice model to generalize the model for small molecular systems. In the general case, depending on molecular architecture and packing, grain boundaries can result in interfacial energy barriers, traps or a combination of both with qualitatively different effects on charge transport.

Optical studies on vacuum evaporated pentacene thin films on glass substrates

In this paper, the effect of deposition rates and annealing on the optical properties of vacuum deposited pentacene (C22H14) thin films on glass substrates are reported. Thin films of pentacene were deposited on glass substrates using resistive evaporation technique. Optical properties of the pentacene thin films deposited for different deposition rates of 5Å/s, 10 Å/s and 20Å/s were studied. Results revealed that the optical band gap was more for the device with active material deposited at a lower deposition rate of 5 Å/s. Optical properties of the vacuum annealed (100 °C, 150 °C and 200 °C) pentacene thin films were also done and it was found that optical band gap slightly increased for the films annealed at a temperature of 100 °C and after that it remains same for the samples annealed at higher temperatures. This small optical band gap makes this material suitable for photovoltaic applications.

The influence of annealing temperature and time on the efficiency of pentacene: PTCDI organic solar cells

In this study, fabrication of a polycyclic aromatic hydrocarbon/Perylene Tetracarboxylic Di-Imide (PTCDI), donor/acceptor solar cells are presented using physical vapour deposition technique in a 1000 class glove box. An ITO/PEDOT:PSS/Pentacene/PTCDI/Al (ITO=Indium Tin Oxide and PEDOT:PSS=poly(3,4-ethylenedioxythiophene) polystyrene sulfonate) solar cell has been obtained and the power conversion efficiency, PCE (η) of about 0.33% has been obtained under simulated solar illumination of 300W/m2. Furthermore, the effects of annealing temperatures (at 100 and 150°C) and of annealing (at 100°C) times for 5 and 10min. on the power conversion efficiency, η of the solar cells have also been investigated. In general, it has been seen that the thermal annealing deteriorated the characteristics parameters of Pentacene/PTCDI solar cell such that both fill factor, FF and η decreased after annealing and with increase of annealing time. Atomic force microscopy (AFM) images showed that the phase segregation and grain size increased and the surface roughness of Pentacene film decreased and these effects reduced the η value. The η values of the solar cell have been determined as 0.33%, 0.12% and 0.06% for pre-annealing, annealing at 100 and 150°C, respectively.