Showing papers on "Pentacene published in 2001"

Functionalized Pentacene: Improved Electronic Properties from Control of Solid-State Order

Abstract: Molecular order has proven to be a significant factor in the performance of devices based on organic semiconductors. Recent studies involving solubilized versus unsubstituted thiophene oligomers have demonstrated that modifications which increase orbital overlap in the solid state can improve device performance by more than an order of magnitude. 1 Similar studies on pentacene, a compound which has already demonstrated remarkable potential for device applications, 2 have also focused on maximizing orbital overlap by inducing order in films. 3 However, these pentacene studies have thus far relied on substrate modification, rather than on pentacene functionalization, 4 to achieve the desired goals. We report here the preparation of two functionalized pentacene derivatives, and the effect of this functionalization on both the solid-state ordering and the electronic properties of the resulting crystals. Our goal for a functionalized pentacene was two-fold: First, the substituents should impart solubility to the acene, to simplify purification and processing. Second, the substituents should induce some capability for self-assembly of the aromatic moieties into ﷿-stacked arrays to enhance intermolecular orbital overlap. We anticipated that both of these goals could be accomplished by exploiting a rigid spacer to hold the necessarily bulky solubilizing groups well away from the aromatic core, allowing the closest possible contact between the aromatic rings. 5 Our initial targets were the bis(triisopropylsilylethynyl)pentacenes 1 and 2. Both of these compounds are easily prepared in near quantitative yield in a one-pot reaction from 6,13-pentacenequinone and 5,14pentacenequinone, respectively. 6

Growth dynamics of pentacene thin films

Abstract: The recent demonstration of single-crystal organic optoelectronic devices has received widespread attention1,2,3,4. But practical applications of such devices require the use of inexpensive organic films deposited on a wide variety of substrates. Unfortunately, the physical properties of these organic thin films do not compare favourably to those of single-crystal materials. Moreover, the basic physical principles governing organic thin-film growth and crystallization are not well understood. Here we report an in situ study of the evolution of pentacene thin films, utilizing the real-time imaging capabilities of photoelectron emission microscopy. By a combination of careful substrate preparation and surface energy control, we succeed in growing thin films with single-crystal grain sizes approaching 0.1 millimetre (a factor of 20–100 larger than previously achieved), which are large enough to fully contain a complete device. We find that organic thin-film growth closely mimics epitaxial growth of inorganic materials, and we expect that strategies and concepts developed for these inorganic systems will provide guidance for the further development and optimization of molecular thin-film devices.

Contact resistance in organic thin film transistors

Abstract: We present an analysis of the electrical characteristics of pentacene OTFTs fabricated on flexible polyethylene naphthalate (PEN) film. Nickel, silicon dioxide, and palladium were deposited by ion-beam sputtering and patterned by photolithography and lift-off to form the gate electrodes, the gate dielectric layer, and the source and drain contacts, respectively. An octadecyltrichlorosilane vapor prime was used to prepare the SiO/sub 2/ gate dielectric surface for the deposition of the pentacene layer, which was deposited by thermal evaporation and patterned using a water-soluble, photo-patterned polyvinyl alcohol layer.

High-performance bottom electrode organic thin-film transistors

Abstract: Pentacene-based organic field effect transistors (FETs) exhibit enormous potential as active elements in a number of applications. One significant obstacle to commercial application remains: no completely lithographic process exists for forming high-performance devices. Processing constraints prevent electrodes from being lithographically patterned once the semiconductor is deposited, but depositing the electrodes before the semiconductor leads to low-performance transistors. By using self-assembled monolayers (SAMs) to change the surface energy of the metal electrodes and morphology of the pentacene subsequently grown on the electrodes, high-performance transistors may be formed using a process compatible with lithographic definition of the source and drain electrodes.

Pentacene TFT with improved linear region characteristics using chemically modified source and drain electrodes

Abstract: We have fabricated pentacene active layer organic thin film transistors (OTFTs) using chemically-modified source and drain contacts with improved contact and linear region characteristics. OTFTs fabricated on heavily doped, thermally oxidized single-crystal silicon substrates have linear field-effect mobility greater than 0.5 cm/sup 2//V-s at a drain-source voltage of -0.1 V, on/off current ratio greater than 10/sup 7/, and subthreshold slope as low as 0.7 V/decade.

Morphological and Transistor Studies of Organic Molecular Semiconductors with Anisotropic Electrical Characteristics

Abstract: Oriented thin films of organic semiconducting small molecules were prepared by crystallization on rubbed alignment layers. Polarized absorption spectra showed that the long axis of the conjugated backbones was highly oriented along the rubbing direction and parallel to the substrates. Transmission electron microscopy and diffraction confirmed that the molecules and in many cases the resulting crystals are aligned. Using the above aligned films as semiconducting layers, we fabricated field-effect transistors having anisotropic mobilities with ratios greater than 15. Several common organic semiconductors have been investigated, and the results indicate that this growth method is generally successful for achieving macroscopic alignment of these semiconducting molecules (and frequently their crystals, as well).

Orientation of pentacene films using surface alignment layers and its influence on thin-film transistor characteristics

Abstract: We have investigated the effect of surface order on the orientation and mobility of pentacene. The surface order was created using monolayers and polymers that are normally used to align liquid crystals. Rubbed polyvinylalcohol layers were found to align approximately 27% of the pentacene grains within a 30° range. When introduced in a thin-film transistor, they were found to enhance the saturation current by a factor of 2.5. A mechanism for this enhancement is proposed.

Valence one-electron and shake-up ionization bands of polycyclic aromatic hydrocarbons. I. Benzene, naphthalene, anthracene, naphthacene, and pentacene

Abstract: The valence ionization bands of benzene and of polyacenes ranging from naphthalene to pentacene have been entirely assigned by means of one-particle Green’s function calculations, performed using the third-order algebraic-diagrammatic construction [ADC(3)] scheme and series of basis sets of improving quality. For the sake of consistency, the computations are based on correlated (DFT/B3LYP) rather than uncorrelated geometries. Ionization bands pertaining to π-orbitals are subject to a severe shake-up contamination at already quite low binding energies (e.g., down to 8.0 eV in the case of pentacene). In sharp contrast, the orbital picture of ionization holds to a much greater extent within the σ-band system (e.g., for pentacene, up to binding energies of 14.6 eV). Despite the intricacy of ionization bands, and, possibly, vibrational complications, ADC(3) spectra consistently match photoionization measurements up to the inner-valence region, where the orbital picture completely breaks down.

Hole-Burning Studies of the Splitting in the Ground and Excited Vibronic States of Tetracene in Helium Droplets

Abstract: The laser-induced fluorescence spectra of single tetracene (C18H12) and pentacene (C24H14) molecules embedded in liquid HeN droplets (N ≃ 104) show sharp zero phonon lines (ZPL) (δν ≤ 0.2 cm-1), accompanied by weaker phonon wings (PW) on the blue side. The ZPL of tetracene is anomalously split into a doublet with a separation of 1.1 cm-1, whereas for pentacene, the ZPL is not split. Hole-burning measurements with two pulsed dye lasers and lifetime measurements indicate that inside He droplets the ground and excited states of tetracene are each split into two levels. The splitting is attributed either to the occupation of two nearly equivalent sites by localized helium atoms or to a tunneling of one or two localized helium atoms through the barrier in the double-well potential on the surface of the tetracene molecule.

Hydrostatic-pressure dependence of the photoconductivity of single-crystal pentacene and tetracene

Abstract: Pentacene and tetracene show readily observable photoconductivity when illuminated with light in the blue part of the visible spectrum. We measured the change of photoconductivity with hydrostatic pressure in single-crystal samples of both materials. Possible mechanisms for the observed increase in photoconductivity with pressure are discussed. We conclude that a carrier-mobility increase under pressure is most likely to cause the increase in photoconductivity in the case of pentacene. For tetracene, changes in the absorption spectrum in the range of the excitation wavelengths may also be significant. We also observe a phase transition near 0.3 GPa in tetracene, in agreement with previous results.

High Performance Organic Field-Effect Transistors and Integrated Inverters

Abstract: Integrated plastic circuits (IPCs) will become an integral component of future low cost electronics. For low cost processes IPCs have to be made of all-polymer Transistors. We present our recent results on fabrication of Organic Field-Effect Transistors (OFETs) and integrated inverters. Top-gate transistors were fabricated using polymer semiconductors and insulators. The source-drain structures were defined by standard lithography of Au on a flexible plastic film, and on top of these electrodes, poly(3-alkylthiophene) (P3AT) as semiconductor, and poly(4-hydroxystyrene) (PHS) as insulator were homogeneously deposited by spin-coating. The gate electrodes consist of metal contacts. With this simple set-up, the transistors exhibit excellent electric performance with a high source-drain current at source - drain and gate voltages below 30V. The characteristics show very good saturation behaviour for low biases and are comparable to results published for precursor pentacene. With this setup we obtain a mobility of 0.2cm2/Vs for P3AT. Furthermore, we discuss organic integrated inverters exhibiting logic capability. All devices show shelf-lives of several months without encapsulation.

Screen printed organic thin film transistors (OTFTs) on a flexible substrate

Abstract: OTFTs have recently received much attention because organic materials are more compatible with low temperature and low cost on flexible substrates than a-Si TFTs 1 . In addition, printing processes are more compatible with OTFT fabrication than a-Si TFTs 2 . This paper reports a materials compatibility study of various organic gate dielectrics with screen-printed electrodes for pentacene based TFTs. Standards were prepared using the polymer dielectrics and gold drain/source electrodes on a glass substrate. Poly(vinylphenol) (PVP) and benzocyclobutene (BCB) were spin coated and parylene was vapor deposited as dielectric materials. Pentacene was vapor deposited onto the substrates. Test devices were fabricated by painting drain/source electrodes onto the 3 dielectrics. The pentacene/PVP/gold devices had the best performance with mobilities near 0.3 cm 2 /V-sec, on/off current ratios between 10 3 -10 4 . When screen printable electrodes were introduced to the PVP system, device performance was significantly degraded. The BCB device performance was nearly the same for each device material; the parylene device performance was superior with the screen printable conductive inks relative to the gold devices. Using these data, a potentially low cost device was fabricated on a Mylar substrate with an ITO gate electrode, parylene dielectric, and screen-printed silver ink for drain/source electrodes. On/off current ratios were between 10 3 and 10 4 . Mobilities ranged from 0.02 - 0.08 cm 2 /V-sec.

Improved organic thin film transistor performance using chemically-modified gate dielectrics

Abstract: We report on the use of silicon dioxide gate dielectric chemically-modified with vapor-deposited octadecyltrichlorosilane (OTS) monolayers for improved organic thin film transistor (OTFT) performance. To date, silicon dioxide gate dielectric chemically-modified with OTS monolayers deposited from solvent solution have demonstrated the highest reported OTFT performance using the small-molecule organic semiconductor pentacene as the active layer. Vapor treatment is an attractive alternative, especially for polymeric substrates that may be degraded by solvent exposure. Using our OTS vapor treatment we have fabricated photolithographically defined pentacene OTFTs on flexible polymeric substrates with field-effect mobility greater than 1.5 cm2/V-s. We find the performance of pentacene as well as several other small-molecule organic active layer materials can be significantly improved using silicon dioxide gate dielectric chemically-modified with vacuum vapor prime OTS. Pentacene, naphthacene, Cu-phthalocyanine, and alpha-sexithienyl OTFTs fabricated on thermally oxidized silicon substrates with photolithographically defined bottom contacts typically show a factor of 2 to 5 improvement in field-effect mobility and reduced subthreshold slope when using silicon dioxide gate dielectric vacuum vapor treated with OTS compared to OTFTs on untreated gate dielectric.

Excitonic photoluminescence in pentacene single crystal

Abstract: Photoluminescence of pentacene single crystals is studied in the temperature range of 7 K to 200 K under excitation with He-Ne laser light. Photoluminescence spectra consist of four broad bands, L1 to L4. The highest energy band, L1, located close to the lowest exciton absorption band mainly appears for //b-polarization. The intensity of the second one, L2, with Stokes-shift of about 1500 cm-1, decreases as temperature rises above 30 K and disappears at 100 K. The bands, L3 and L4, which are located at lower energy, are observed at higher temperatures up to 200 K. Based on their energy positions, the band L2 is assigned to a shallow self-trapped exciton luminescence band, and the bands L3 and L4 to deep self-trapped exciton luminescence bands. By comparing this result with reported result on tetracene crystals, self-trapped excitons are considered to be more stable in pentacene.

Hole transport in pentacene single crystals

Abstract: Hole transport in high-quality pentacene single crystals is investigated using space-charge-limited current spectroscopy in a temperature range from 2 to 500 K. The temperature and electric-field dependence of the charge-carrier mobility below room temperature indicates a bandlike charge transport. The effective electronic bandwidth at low temperatures is estimated to be on the order of 400 meV. However, due to electron-phonon interaction the bandwidth narrows significantly, leading to a localization of the charge carriers and the formation of a lattice polaron above approximately 400 K. As a result, the transport mechanism crosses over from coherent bandlike motion to incoherent hopping.

Organic metal–semiconductor field-effect phototransistors

Abstract: We have prepared phototransistors based on bromine-doped pentacene. Such devices reveal light detection over a wide energy range with amplification. The illumination modifies the charge characteristics of the Schottky gate of an enhancement-mode the metal–semiconductor field-effect transistor leading to transistor amplification of the photodiode-like response. A gain of 8 is obtained for illumination at 632 nm. These phototransistors combine the excellent properties of pentacene field-effect transistors and photovoltaic devices.

The Effect of Surface Preparation on the Structure and Electrical Transport in an Organic Semiconductor

Abstract: Self-assembled monolayers (SAMs) with a variety of structures and terminal groups were evaluated as underlayers for pentacene deposition. It was found that the most critical factor in the formation of highly oriented thin film pentacene with large grain size was the geometric structure of the monolayer. Monolayers with terminal bonds parallel to the surface produce large pentacene grains with an angular rather than the dendridic structure normally observed on octadecyltrichlorosilane (OTS) coated substrates. The grain size, X-ray scattering, carrier mobility, and current on/off ratios are all improved with monolayers of the appropriate geometry.

Fast organic electronic circuits based on ambipolar pentacene field-effect transistors

Abstract: Inverter circuits and ring oscillators have been prepared based on the organic semiconductor pentacene. High-purity pentacene single crystals and thin films exhibit ambipolar transport properties, i.e., n- and p-channel transistor activity. This feature is very attractive for complementary logic electronic circuits, since only one organic material has to be used. Switching frequencies up to 11 MHz have been achieved for single crystal circuits. Thin film devices prepared on flexible polyimide substrates revealed delays of 0.1 μs per stage resulting in oscillation frequencies up to 900 kHz.

On the stability of organic field-effect transistor materials

Abstract: Stability and degradation of transistor performance of devices based on pentacene and α-sexithiophene are investigated. In order to distinguish between effects at grain boundaries and material issues, macroscopic bicrystals were used, where transistors were prepared on a single grain as well as across a single grain boundary. The main reason for performance instabilities is the formation of oxygen-related trapping states at the grain boundary upon exposure to air. However, especially in the case of pentacene, stable hole transport properties are observed.

Influence of the gate dielectric on the morphological and electronic structure of pentacene films for transistor applications

Abstract: The structural and transport properties of pentacene thin film transistors are reported, showing the influence of the deposition temperature, the deposition rate and the substrate on the structural and transport properties of oTFTs. The structure and morphology of pentacene films on thermal oxide and plasma CVD silicon nitride were compared by x-ray diffraction measurements and atomic force microscopy. There is a clear correlation between the morphology and the structural properties of the highly polycrystalline films on the two dielectrics. In the case of silicon nitride the roughness of the film has a distinct influence on the morphology and the structural properties, whereas the films on thermal oxide are in general highly ordered independent of the deposition conditions. The ordered films exhibit a thin film and a crystalline bulk phase, and the crystalline bulk phase fraction increases with the deposition temperature and the film thickness. We find that careful control of the deposition conditions give virtually identical films on the oxide and nitride dielectrics. To study the electronic properties we have realized inverted staggered transistors. The mobility of the TFTs is correlated with the morphology and the structural properties of the films, and increases with the size of the crystals. The TFTs exhibit very similar mobilities of ~0.4 cm2/Vs and on/off ratios >108 on thermal oxide and flat silicon nitride. The impact of the dielectric on the device parameters of mobility, threshold voltage and sub threshold voltage slope are discussed. Bias stress experiments are performed to investigate the stability of the TFTs, and to gain understanding of the transport mechanisms of thermally evaporated pentacene TFTs.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Electron-Phonon Coupling Spectrum in Photodoped Pentacene Crystals

Abstract: The coupling between conduction charges and the vibrational modes of the molecular lattice plays a defining role in the transport characteristics of organic semiconductors. Using electron tunneling spectroscopy, we obtain the electron--optical-phonon coupling spectrum in photodoped pentacene crystals at energies $l30\phantom{\rule{0ex}{0ex}}\mathrm{meV}$. Comparison of the tunneling spectrum to infrared absorption data on the optical phonon density of states yields the energy dependence of the electron-phonon scattering matrix element. The integrated spectral weight of the electron-phonon coupling shows that superconductivity in pentacene is likely of electron-phonon origin.

Photo-responsive retention behavior of azobenzene-modified cyclodextrin stationary phase in micro-HPLC.

Abstract: An azobenzene-modified γ-cyclodextrin stationary phase (Azγ-CDSP) was prepared and its photo- and temperature-responses for the retention of perylene and pentacene were investigated using a mixture of methanol and water as the mobile phase in micro-HPLC. The retention of perylene slightly increased, whereas that of pentacene significantly decreased by UV light irradiation to Azγ-CDSP. These retentions recovered upon irradiation with visible light. Both retentions decreased upon an increase in the column temperature. It was presumed that the trans-azobenzene moiety acts as a preventive cap for perylene and a spacing for pentacene in filling the CD cavity. An azobenzene-modified stationary phase changed its retention behavior with the column temperature and the light irradiation. An improvement in the micro-HPLC system and the optimization of the molecular structure of the photo-responsive stationary phase would provide selective retention control by the irradiation of light in micro-separation systems.