Showing papers on "Pentacene published in 2005"
TL;DR: The device parameters for organic field-effect transistors fabricated from solution-deposited films of functionalized pentacene and anthradithiophenes, which showed behavior strongly dependent on the substituents, are presented.
Abstract: We present the device parameters for organic field-effect transistors fabricated from solution-deposited films of functionalized pentacene and anthradithiophenes. These materials are easily prepared in one or two steps from commercially available starting materials and are purified by simple recrystallization. For a solution-deposited film of functionalized pentacene, hole mobility of 0.17 cm2/V·s was measured. The functionalized anthradithiophenes showed behavior strongly dependent on the substituents, with hole mobilities as high as 1.0 cm2/V·s.
735 citations
TL;DR: Ultrathin insulating NaCl films have been employed to decouple individual pentacene molecules electronically from the metallic substrate, which allows the inherent electronic structure of the free molecule to be preserved and studied by means of low-temperature scanning-tunneling microscopy.
Abstract: Ultrathin insulating NaCl films have been employed to decouple individual pentacene molecules electronically from the metallic substrate. This allows the inherent electronic structure of the free molecule to be preserved and studied by means of low-temperature scanning-tunneling microscopy. Thereby direct images of the unperturbed molecular orbitals of the individual pentacene molecules are obtained. Elastic scattering quantum chemistry calculations substantiate the experimental findings.
712 citations
TL;DR: In this paper, the effects of the surface energy of gate dielectrics on pentacene morphology and the electrical properties of FETs are reported, using surface energy-controllable poly(imide-siloxane)s as gate-dielectric layers.
Abstract: The effects of the surface energy of polymer gate dielectrics on pentacene morphology and the electrical properties of pentacene field-effect transistors (FETs) are reported, using surface-energy-controllable poly(imide-siloxane)s as gate-dielectric layers. The surface energy of gate dielectrics strongly influences the pentacene film morphology and growth mode, producing Stranski–Krastanov growth with large and dendritic grains at high surface energy and three-dimensional island growth with small grains at low surface energy. In spite of the small grain size (≈ 300 nm) and decreased ordering of pentacene molecules vertical to the gate dielectric with low surface energy, the mobility of FETs with a low-surface-energy gate dielectric is larger by a factor of about five, compared to their high-surface-energy counterparts. In pentacene growth on the low-surface-energy gate dielectric, interconnection between grains is observed and gradual lateral growth of grains causes the vacant space between grains to be filled. Hence, the higher mobility of the FETs with low-surface-energy gate dielectrics can be achieved by interconnection and tight packing between pentacene grains. On the other hand, the high-surface-energy dielectric forms the first pentacene layer with some voids and then successive, incomplete layers over the first, which can limit the transport of charge carriers and cause lower carrier mobility, in spite of the formation of large grains (≈ 1.3 μm) in a thicker pentacene film.
447 citations
TL;DR: Provided a large enough alkyne substituent is used, these large aromatic rods are both stable and soluble and can be characterized spectroscopically as well as by single-crystal X-ray diffraction.
Abstract: We have extended our functionalization strategy for pentacene to the higher acenes hexacene and heptacene. Provided a large enough alkyne substituent is used, these large aromatic rods are both stable and soluble and can be characterized spectroscopically as well as by single-crystal X-ray diffraction.
391 citations
TL;DR: In this paper, the authors demonstrate bilayer source-drain (S-D) electrodes for organic thin film transistors (OTFTs), which consists of a transition metal oxide (MoO3,WO3 or V2O5) layer and a metal layer.
Abstract: We demonstrate bilayer source-drain (S-D) electrodes for organic thin film transistors (OTFT). The bilayer consists of a transition metal oxide (MoO3,WO3, or V2O5) layer and a metal layer. The metal oxide layer, directly contacting the organic semiconducting layer, serves as the charge-injection layer. The overcoated metal layer is responsible for the conduction of charge carriers. We found that the metal oxide layer coupled between pentacene and metal layers played an important role in improving the field-effect transistor characteristics of OTFTs. Devices with the bilayer S-D electrodes showed enhanced hole-injection compared to those with only metal electrode. High field-effect mobility of 0.4cm2∕Vs and on/off current ratios of 104 were obtained in the pentacene based TFTs using the bilayer S-D electrodes at a gate bias of −40V. The improvement is attributed to the reduction in the contact barrier and the prevention of metal diffusion into the organic layer and/or unfavorable chemical reaction between ...
386 citations
TL;DR: The synthesis and characterization of a fused-ring compound, dithieno[2,3-d:2',3'-d']thieno [3,2-b:4,5-b']dithiophene (pentathienoacene, PTA), demonstrating the potential of PTA for application in future organic electronics.
Abstract: We present the synthesis and characterization of a fused-ring compound, dithieno[2,3-d:2‘,3‘-d‘]thieno[3,2-b:4,5-b‘]dithiophene (pentathienoacene, PTA). In contrast to pentacene, PTA has a larger band gap than most semiconductors used in organic field-effect transistors (OFETs) and therefore is expected to be stable in air. The large π-conjugated and planar molecular structure of PTA would also form higher molecular orders that are conductive for carrier transport. X-ray diffraction and atomic force microscopy experiments on its films show that the molecules stack in layers with their long axis upright from the surface. X-ray photoelectron spectroscopy suggests that there are no chemical bonds at the PTA/Au interface. OFETs based on the PTA have been constructed, and their performances as p-type semiconductors are also presented. A high mobility of 0.045 cm2/V s and an on/off ratio of 103 for a PTA OFET have been achieved, demonstrating the potential of PTA for application in future organic electronics.
317 citations
TL;DR: The morphology, structure, and transport properties of pentacene thin film transistors (TFTs) are reported showing the influence of the gate dielectric surface roughness.
Abstract: The morphology, structure, and transport properties of pentacene thin film transistors (TFTs) are reported showing the influence of the gate dielectric surface roughness Upon roughening of the amorphous SiO2 gate dielectric prior to pentacene deposition, dramatic reductions in pentacene grain size and crystallinity were observed The TFT performance of pentacene films deposited on roughened substrates showed reduced free carrier mobility, larger transport activation energies, and larger trap distribution widths Spin coating roughened dielectrics with polystyrene produced surfaces with 2 A root-mean-square (rms) roughness The pentacene films deposited on these coated surfaces had grain sizes, crystallinities, mobilities, and trap distributions that were comparable to the range of values observed for pentacene films deposited on thermally grown SiO2 (roughness also ∼2 A rms)
314 citations
TL;DR: In this article, the field effect mobility of organic thin-film transistors (OTFTs) was measured using top-contact devices and the results indicated that the mobility saturates when 6 monolayers of pentacene are deposited on the gate dielectric, SiO2.
Abstract: The dependence of the field-effect mobility of organic thin-film transistors (OTFT) was measured using top-contact devices. OTFTs were fabricated by vacuum sublimation of pentacene on thermally oxidized silicon wafers to form films with nominal thickness between 2 and 25 monolayers. The deposition was carried out at a low rate and the substrate was heated in order to yield high-quality films. The results indicate that the mobility saturates when 6 monolayers of pentacene are deposited on the gate dielectric, SiO2.
311 citations
TL;DR: In the absence of charge storage or slow polarization in the gate dielectric, the hysteresis in the currentvoltage (I−V) characteristics of pentacene-based organic thin-film transistors (OTFTs) is dominated by trapped electrons in the semiconductor as mentioned in this paper.
Abstract: In the absence of charge storage or slow polarization in the gate dielectric, the hysteresis in the current-voltage (I−V) characteristics of pentacene-based organic thin-film transistors (OTFTs) is dominated by trapped electrons in the semiconductor. The immobile previously stored negative charge requires extra holes to balance it, resulting in the early establishment of the channel and extra drain current. Inferred from I−V characteristics, this simple electrostatic model qualitatively explains memory effects in pentacene-based OTFTs, and was verified by a time domain measurement.
285 citations
TL;DR: In this article, the first few pentacene layers on a dielectric substrate are strongly dependent on the surface properties, directly affecting the charge mobility of Pentacene thin film OTFTs.
Abstract: 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 (μ = 3.4 ± 0.5 cm2/Vs) than those with polycrystalline dendritic islands (μ = 0.5 ± 0.15 cm2/Vs) on OTS-treated ones.
281 citations
TL;DR: The bromopentacenes could be further elucidated to pentacene nitriles, further decreasing the acene's reduction potential, while the charge-carrier mobility in the fluorinated systems was shown to scale with the degree of fluorine substitution.
TL;DR: In this article, a pentacene OFET gated by a solution-deposited polymer electrolyte film was shown to achieve a sub-threshold slope of 180mV per decade of current at a source-drain bias of −1V, and the estimated dielectric layer specific capacitance was 5μF∕cm2.
Abstract: Large operating voltages are often required to switch organic field-effect transistors (OFETs) on and off because commonly used gate dielectric layers provide low capacitive coupling between the gate electrode and the semiconductor. We present here a pentacene OFET gated by a solution-deposited polymer electrolyte film in which the current was modulated over four orders of magnitude using gate voltages less than 2V. A subthreshold slope of 180mV per decade of current was observed during transistor turn on at a source-drain bias of −1V; the estimated dielectric layer specific capacitance was 5μF∕cm2. Sweep rate-dependent hysteresis may be attributed to a combination of ion migration and charge carrier trapping effects. Strategies to improve switching speeds for polymer electrolyte-gated OFETs are also discussed.
TL;DR: In this article, direct and inverse photoemission spectroscopy was used to investigate the electronic polarization induced by the pentacene anion and cation at the Au/pentacene interface.
TL;DR: In this article, the authors have fabricated very flexible pentacene field effect transistors with polyimide gate dielectric layers on plastic films with a mobility of 0.3cm2∕Vs and an on/off ratio of 105, and have measured their electrical properties under various compressive and tensile strains while changing the bending radius of the base plastic films systematically.
Abstract: We have fabricated very flexible pentacene field-effect transistors with polyimide gate dielectric layers on plastic films with a mobility of 0.3cm2∕Vs and an on/off ratio of 105, and have measured their electrical properties under various compressive and tensile strains while changing the bending radius of the base plastic films systematically. We have found that the change in source-drain current with bending radius is reproducible and reversible when the bending radius is above 4.6mm, which corresponds to strains of ∼1.4±0.1%. Furthermore, the change in source-drain current does not depend on the direction of strain versus direction of current flow.
TL;DR: This novel strategy for generating high K flexible gate dielectrics will be of value in improving organic and flexible electronic device performance.
Abstract: Organic/inorganic core shell nanoparticles have been synthesized using high K TiO2 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 cm2/V·s have been measured for pentacene TFTs incorporating the new TiO2 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.
TL;DR: In this article, a mobility model for organic thin-film transistors (OTFTs) has been considered that fully accounts for the effect of grains and grain boundaries of the organic layer.
Abstract: A mobility model for organic thin-film transistors (OTFTs) has been considered that fully accounts for the effect of grains and grain boundaries of the organic layer. The model has been applied to a top contact pentacene OTFT. Comparison between simulation results and experimental data shows a strong dependence of mobility as a function of grain size. The field-effect-extracted mobility is not linearly related to the grain size, but presents a rather abrupt reduction for a grain size smaller than 2μm.
TL;DR: In this article, the effects of surface treatment of SiO2 on the electric characteristics of organic thin-film transistors were investigated using pentacene as an active layer with chemically modified siO2 gate dielectrics.
TL;DR: In this paper, a high performance OFET with pentacene and copper phthalocyanine (CuPC) was fabricated from organic phototransistors (OPTs) based on the geometry of organic field effect transistors (OFETs); and the effect of the wavelength of the incident light source on their performance was examined.
Abstract: Organic phototransistors (OPTs) were fabricated from pentacene and copper phthalocyanine (CuPC) based on the geometry of organic field-effect transistors (OFETs); and the effect of the wavelength of the incident light source on their performance was examined High performance OFETs with pentacene and CuPC were fabricated and the characteristics of the OPTs were examined under UV and visible-light irradiations with top illumination The CuPC and pentacene OPTs show a high responsivities of 05–2 and 10–50A∕W and maximum IPh∕IDark of 3000 and 13×105, respectively, under 365nm UV light However, under visible light, at a wavelength of 650nm, the pentacene OPTs had 100 times less responsivity, 015–045A∕W, and a IPh∕IDark of 1000, even though an absorption coefficient three times larger was observed at this wavelength than at 365nm A strong correlation was found between the performance of the OPTs and the incident photon to current conversion efficiency spectra of an organic semiconductor The strong depen
TL;DR: In this article, the photovoltaic response at a given illumination level is parameterized and modeled using the equivalent circuit model developed for inorganic pn-junction solar cells.
Abstract: We present studies of the current–voltage characteristics of organic solar cells based on heterojunctions of pentacene and C60 as a function of illumination intensity. The photovoltaic response at a given illumination level is parameterized and modeled using the equivalent circuit model developed for inorganic pn-junction solar cells. Reduction in shunt resistance and increase in diode reverse saturation current density are observed upon increase of the light intensity. We demonstrate that this effect can be modeled by a refined equivalent circuit model that contains an additional shunt resistance and an additional diode the properties of which are functions of the light intensity. The effects of these additional components on the overall photovoltaic performance are discussed.
TL;DR: The results clarify the importance of purification of semiconductive materials for measurements of intrinsic mobility and optimal device performance.
Abstract: At moderate temperatures in flowing gas, pentacene undergoes a disproportionation reaction to produce 6,13-dihydropentacene (DHP) and a series of polycondensed aromatic hydrocarbons, including the previously unknown peripentacene (PP). The process requires activation by heating to 320 degrees C and is possibly catalyzed by impurities such as DHP, 6,13-pentacenequinone (PQ), Al, or Fe found in the starting materials. These impurities also result in a decrease in the intrinsic field-effect mobility (FEM) of pentacene crystals. Subsequent purifications remove such impurities, thus inhibiting the formation of the disproportionation products and increasing the FEM of pentacene (2.2 cm(2)/Vs). These results clarify the importance of purification of semiconductive materials for measurements of intrinsic mobility and optimal device performance.
TL;DR: Finite regions of high conductivity were observed in both n- and p-channel organic thin film transistors based on polycrystalline organic semiconductor films and a solution-processed, solid polymer electrolyte gate dielectric.
Abstract: Finite regions of high conductivity were observed in both n- and p-channel organic thin film transistors based on polycrystalline organic semiconductor films and a solution-processed, solid polymer electrolyte gate dielectric The transition from a highly conductive state to a more insulating state with increasing gate bias may be attributed to the realization of carrier densities greater than 1014 charges/cm2 in the semiconductor film
TL;DR: In this article, a nanotransfer printing process for high-resolution transfer of all components for organic electronic devices on plastic substrates has been developed and demonstrated for pentacene (Pn), poly (3-hexylthiophene) and carbon nanotube (CNT) thin-film transistors (TFTs).
Abstract: A printing process for high-resolution transfer of all components for organic electronic devices on plastic substrates has been developed and demonstrated for pentacene (Pn), poly (3-hexylthiophene) and carbon nanotube (CNT) thin-film transistors (TFTs). The nanotransfer printing process allows fabrication of an entire device without exposing any component to incompatible processes and with reduced need for special chemical preparation of transfer or device substrates. Devices on plastic substrates include a Pn TFT with a saturation, field-effect mobility of 0.09cm2(Vs)−1 and on/off ratio approximately 104 and a CNT TFT which exhibits ambipolar behavior and no hysteresis.
TL;DR: The first organic complementary circuits on flexible substrates were reported in this article using pentacene and hexadecafluorocopperphthalocyanine (F/sub 16/CuPc) semiconductors.
Abstract: We report the first organic complementary circuits on flexible substrates. Organic thin-film transistors were fabricated using pentacene as the semiconductor for the p-channel devices and hexadecafluorocopperphthalocyanine (F/sub 16/CuPc) as the semiconductor for the n-channel devices. Both semiconductors were purchased from commercial sources and deposited by evaporation in vacuum. The pentacene layer was photolithographically patterned to simplify the circuit layout and reduce the circuit area. The transistors and circuits were manufactured on thin, transparent sheets of polyethylene naphthalate. Evaporated metals were used to define all contacts and interconnects, and a 50-nm-thick layer of solution-processed polyvinylphenol was used as the gate dielectric. Transistors and circuits operate at supply voltages as low as 8 V, and ring oscillators have a signal propagation delay as low as 8 /spl mu/s per stage. To our knowledge, these are the fastest organic complementary circuits reported to date.
TL;DR: It is found that the mobility tensor is highly anisotropic for three of the four considered polymorphs and the practical implication of this prediction on the technology of thin-film organic transistors is discussed.
Abstract: The band structure of the four known polymorphs of pentacene is computed from first principles using the accurate molecular orbitals of the isolated molecule as the basis for the calculation of the crystalline orbitals The computed bands are remarkably different for each polymorph, but their diversity can be easily rationalized using a simple analytical model that employs only three parameters The effect of the electronic structure on the hole mobility was evaluated using a simple model based on the constant relaxation time approximation It is found that the mobility tensor is highly anisotropic for three of the four considered polymorphs The practical implication of this prediction on the technology of thin-film organic transistors is discussed
TL;DR: In this article, the authors fabricated ultraflexible pentacene field effect transistors (FETs) with a mobility of 5 cm2 and an on/off ratio of 105, which are functional at the bending radius less than 1mm.
Abstract: We fabricated ultraflexible pentacene field-effect transistors (FETs) with a mobility of 05cm2∕Vs and an on/off ratio of 105, which are functional at the bending radius less than 1mm The transistors are manufactured on a 13-μm-thick polyimide film and covered by a 13-μm-thick poly-chloro-para-xylylene encapsulation layer so that transistors can be embedded at a neutral position This sandwiched structure can drastically suppress strain-induced changes in transistor characteristics Furthermore, the FETs show no significant change after bending cycles of 60 000 times on inward and outward bending stresses
TL;DR: In this article, the authors used time-resolved terahertz pulse spectroscopy to measure the transient photoconductivity of pentacene and functionalized Pentacene thin films.
Abstract: We observe bandlike transport in pentacene and functionalized pentacene thin films using time-resolved terahertz pulse spectroscopy. The measured transient photoconductivity exhibits fast $(l400\phantom{\rule{0.3em}{0ex}}\mathrm{fs})$ photogeneration of mobile charge carriers and reveals a transient carrier mobility that increases as the temperature decreases from 300 K down to 10 K, indicative of bandlike transport over subpicosecond time scales. A wavelength-independent photoconductive signal is observed. The transient photoconductivity in the thin-film samples exhibits a single-exponential decay, whereas a power-law decay is seen in single-crystal samples.
TL;DR: The photoreaction of the alpha-diketone precursor seemed to occur via the singlet excited state of pentacene, and the excited triplet state of the pentacenes generated singlet oxygen by sensitization, and it reacted with the ground-statepentacene to give the 6,13-endoperoxide.
Abstract: A novel alpha-diketone precursor of pentacene, 6,13-dihydro-6,13-ethanopentacene-15,16-dione, was prepared and converted successfully to pentacene in 74 % yield by photolysis of the precursor in toluene: Irradiation of the diketone solution in toluene with light of 460 nm under an Ar atmosphere caused the solution to change from yellow to fluorescent orange-pink within a few minutes, after which, purple precipitates appeared. After 35 min, the solution changed to colorless and the purple precipitates were filtered to give pentacene in 74 % yield. By contrast, in the presence of oxygen, the color of the solution changed from yellow to pale yellow, and only 6,13-endoperoxide of pentacene was quantitatively obtained. The rate of the reaction upon photolysis was measured by observing the decay of n-pi* absorption of the precursor at 460 nm, and was found to be similar in both the presence and absence of oxygen. Therefore, the photoreaction of the alpha-diketone precursor seemed to occur via the singlet excited state. Because the T-T absorption of pentacene was observed upon photolysis of the precursor in the nanosecond transient absorption measurement under an Ar atmosphere, the excited triplet state of the pentacene generated singlet oxygen by sensitization, and it reacted with the ground-state pentacene to give the 6,13-endoperoxide. The alpha-diketone deposited on glass was also converted successfully to pentacene film by photoirradiation. In addition, diketone precursors of a mixture of 2,8- and 2,9-dibromopentacene and 2,6-trianthrylene were also prepared and their photoconversion was performed.
TL;DR: In this article, the effect of air exposure on the electronic properties of pentacene single crystals was investigated and two competing mechanisms that modulated the electronic transport were identified. But neither of these mechanisms were considered in this paper.
Abstract: We report the effect of air exposure on the electronic properties of pentacene single crystals. Air can diffuse reversibly in and out of the crystals and influences the physical properties. We discern two competing mechanisms that modulate the electronic transport. The presence of oxygen increases the hole conduction, as in dark four O2 molecules introduce one charge carrier. This effect is enhanced by the presence of visible light. Contrarily, water, present in ambient air, is incorporated in the crystal lattice and forms trapping sites for injected charges.
TL;DR: In this article, the authors report on the latest improvements in organic field-effect transistors (OFETs) using ultra-thin anodized gate insulators, which are tested in transistors using pentacene and poly(triarylamine) as active layers.
Abstract: We report on our latest improvements in organic field-effect transistors (OFETs) using ultra-thin anodized gate insulators. Anodization of titanium (Ti) is an extremely cheap and simple technique to obtain high-quality, very thin (∼ 7.5 nm), pinhole-free, and robust gate insulators for OFETs. The anodized insulators have been tested in transistors using pentacene and poly(triarylamine) (PTAA) as active layers. The fabricated devices display low-threshold, normally “off” OFETs with negligible hysteresis, good carrier mobility, high gate capacitance, and exceptionally low inverse subthreshold slope. Device performance is improved via chemical modification of TiO2 with an octadecyltrichlorosilane (OTS) self-assembled monolayer (SAM). As the result of this combination of favorable properties, we have demonstrated OFETs that can be operated with voltages well below 1 V.