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FET device performance, morphology and X-ray thin film structure of unsubstituted and modified quinquethiophenes
Abstract
Bottom contact FET devices were realized, using vacuum evaporated thin films of unsubstituted quinquethiophene and two modified derivatives deposited at different substrate temperatures. X-ray diffraction (XRD) and atomic force microscopy (AFM) data on thin films deposited in the same conditions are reported and related to the electrical characteristics of the devices.
... Similar behavior is observed in the polymers, where the high temperature allows the recrystallization of the amorphous part. 30,31 Probably phase I, since it is characterized by the presence of a long interplanar distance, adopts a waferlike structure as phase II, and hence the alkyl chains are disordered at RT, while, by increasing the temperature, they are able to move and reach more ordered positions. We focused our attention on the pattern at 190°C, which shows sharp and well resolved peaks, and we indexed it with a monoclinic C (base-centered) cell (a = 25.220(2) ...
The crystallization of the 2,2'-(5,5'-(Ethyne-1,2-diyl)bis(thiophene-5,2-diyl))bis(5-hexyl-4H-thieno[2,3-c]pyrrole-4,6(5H)-dione), hereafter NTA, in different solvents and/or condition yielded to the identification of five different polymorphs which are characterized by impressive differences in the photophysical behavior. Phase I, phase II, phase III and phase IV are characterized by a yellow, green, red and orange emission under illumination with 365 nm light. The crystal structures of phase II and III were determined by SCXRD. Phase II is characterized by the elongated molecules which lie parallels to each other in layers and hence by the presence of a long axis in the cell parameters. The two thienoimide-thiophene systems do not lay in the same plane which is consistent with the higher energy emission. In phase III, the NTA molecules pile up and form columns with strong π-π interactions. The packing is consistent with the presence of red emission and the absence of a long axis in the cell parameters. The thermal characterization revealed that phase I, II, III converts into phase IV, which, upon melting, recrystallizes in the elusive phase V. Quantum-chemical studies were carried out to investigate optical and charge transport properties. Promising charge transport properties with a dominant n-type character were predicted however, the role of internal conformational disorder could influence the efficiency of charge transport.
... 738 Furthermore, addition of fluorine was found to increase the solubility and stability and lower the reduction potential. Ostoja et al. 739 compared OFET device performance of unsubstituted and modified quinquethiophenes including 369 (Chart 62). XRD results of vacuumdeposited thin film showed the formation of highly ordered and crystalline thin films at different temperatures (30, 90, and 140°C). ...
Conjugated poly-ynes and poly(metalla-ynes) constitute an important class of new materials with potential application in various domains of science. The key factors responsible for the diverse usage of these materials is their intriguing and tunable chemical and photophysical properties. This review highlights fascinating advances made in the field of conjugated organic poly-ynes and poly(metalla-ynes) incorporating group 4–11 metals. This includes several important aspects of conjugated poly-ynes viz. synthetic protocols, bonding, electronic structure, nature of luminescence, structure–property relationships, diverse applications, and concluding remarks. Furthermore, we delineated the future directions and challenges in this particular area of research.
... The possibility of fabricating large area, mechanically flexible devices through lowcost processes has promoted the engineering of a variety of material structures, both polymeric and molecular ones, as well as the possible device architectures and functionalities. 1,2 On these lines, the use of organic semiconductors and conductors in biosensors is rising interest for advanced bioelectronic applications. [3][4][5][6][7] Due to their redox properties, 8 high chemical stability, thermal durability and photostability 9 perylene based materials are among the most employed systems. ...
We report the design, synthesis and structure properties investigation of a new perylene diimide material (PDI-Lys) bearing lysine end substituents. Water processed films of PDI-Lys were prepared and their self-assembly, morphology and electrical properties in both inert and air environment were theoretically and experimentally investigated. With the aim of evaluating the potential of PDI-Lys as biocompatible and functional neural interface for organic bioelectronics applications, its electrochemical impedance as well as the adhesion and viability properties of primary neurons on the PDI-Lys films were studied. By combined theoretical calculations and electrical measurements we show that due to conversion between neutral and zwitterionic anion, the PDI-Lys film conductivity increased significantly on passing from air to inert atmosphere, reaching a maximum value of 6.3 S x m-1. We also show that the PDI-Lys film allows neural cells adhesion and neuron differentiation and decreases up to 5 times the electrode/solution impedance in comparison to a naked gold electrode. The present study introduces an innovative, water processable conductive film usable in organic electronics and as putative neural interface.
... The uniformity and domain size of organic thin films directly affect the performance of some organic devices such as organic photovoltaics (OPV) [1,2] and organic field effect transistors (OFET) [3,4]. The softness, flexibility, and easy sampling of organic materials have both advantages and disadvantages over inorganic devices. ...
The effect of peripheral halogenation is examined based on analytical transmission electron microscopy and thermal analyses of two chemical family structures, specifically the vanadyl-phthalocyanine family (VOPcX: X = H16, F14.5) and the copper-phthalocyanine family (CuPcX: X = H16, F16, Cl16, Cl8Br8), focusing on the process of molecular changes and crystalline disintegrations. To clarify the molecular transformations, electron energy-loss spectroscopy (EELS) is applied to two fluorinated phthalocyanines (VOPcF14.5 and CuPcF16), by monitoring mass changes as well as energy loss near edge structures (ELNES). The elemental mass of both VOPcF14.5 and CuPcF16 remain constant up to 0.5 C x cm(-2), except in the case of mass reduction attributed to oxygen loss occurring in VOPcF14.5. It is expected that the released oxygen will induce higher radiation damage in VOPcF14.5. Although mass variation is not observed in CuPcF16, it is found from ELNES that the pi resonant system of nitrogen is more radiation sensitive than that of carbon. These results imply that the electron sensitivity in VOPcX is triggered by eliminated oxygen or, thus, an induced larger empty space, whereas the sensitivity of CuPcX is dominated only by a large intermolecular empty space resulting in the following bond alterations. It is also found that the decomposition temperature (Td) measured by thermal analyses and the characteristic dose (D1/e) are exponentially correlated to the "effective molecular occupancy" (Oe) evaluated as a volume function of molecules in unit cells. By measuring Td and/or Oe, we discuss the durability of peripheral halogenation with respect to the radiation damage.
Organic field-effect transistors (OFETs) are key enabling devices for plastic electronics technology, which has a potentially disruptive impact on a variety of application fields, such as health, safety and communication. Despite the tremendous advancements in understanding the OFET working mechanisms and device performance, further insight into the complex correlation between the nature of the charge injecting contacts and the electrical characteristics of devices is still necessary. Here, an in-depth study of the metal-organic interfaces that provides a direct correlation to the performance of OFET devices is reported. The combination of synchrotron x-ray spectroscopy, atomic force microscopy, electron microscopy, and theoretical simulations on two selected electron transport organic semiconductors with tailored chemical structures allows to gain insight on the nature of the injecting contacts. This multiple analysis repeated at the different stages of contact formation provides a clear picture on the synergy between organic/metal interactions, interfacial morphology and structural organization of the electrode. The simultaneous synchrotron x-ray experiments and electrical measurements of OFETs in operando uncovers how the nature of the charge injecting contacts has a direct impact on the injection potential of OFETs.
As fabrication and positioning of micro- and nanocrystals grow in importance in a wide range technological applications, there is an increasing requirement for the development of a shared technological platform that is able to process materials from solutions on large areas. Here, the application of lithographically controlled wetting (LCW) for the manipulation and positioning of single crystals directly on devices is reported. In the LCW, a stamp, consisting of a metallic grid, is positioned in contact with a liquid thin film spread on a substrate; under these conditions, the capillary forces pin the solution to the stamp protrusions, thus splitting the continuous film in separated droplets or channels. As the solvent evaporates and the solution reaches the saturation, the solute precipitates onto the substrate within the menisci, giving rise to a structured thin film, in correspondence of the protrusion of the stamp. The possibility to achieve a patterning of crystals of conjugated molecules with a defined shape and with controlled size directly on device is demonstrated and pattern of crystals are investigated using polarized optical microscopy, atomic force microscopy, X-ray diffraction; they are also electrically characterized. The application of lithographically controlled wetting on the manipulation and positioning of single crystals directly on devices. It is demonstrated that it is possible to achieve patterning of crystals of conjugated molecules with a defined shape and with controlled size directly on a device.
This chapter focuses on the development of organic semiconductors, with particular emphasis on the design strategy of novel semiconductors with a high mobility and stability. The organic semiconductor is a key component of an organic field-effect transistor (OFET). In terms of molecular weight, organic semiconductors can be subdivided into small molecules and polymers while, on the basis of the main charge carriers transporting in OFET channels, organic semiconductors can be further divided into p-type, n-type, and bipolar semiconducting materials. The details of some polymer semiconductors are described in the chapter. As most organic semiconductors possess symmetrical molecular structures, the general synthetic techniques employed in this field will not involve complicated chiral syntheses. The organic semiconductors obtained from multistep syntheses always require to be further purified, using a variety of techniques include recrystallization, column chromatography, physical vapor deposition (PVD) and soxhlet extraction.Controlled Vocabulary TermsOFETs; organic semiconductors; semiconductor doped polymers
This chapter reviews the achievements in the development of molecular and polymeric semiconductors for charge transport in thin-film transistors (TFTs). In particular, it introduces the basic concepts of organic semiconductor structure and organic thin-film transistor (OTFT) operation and then focuses on initial studies and works. Organic semiconductors for OTFTs must possess two essential structural features for their successful implementation in printed electronics. The first feature is a π-conjugated core/chain composed of linked unsaturated units. The second feature is core functionalization with solubilizing substituents, which is essential for inexpensive manufacture by solution methods as well as for enhancing solid-state core interactions. There are several advantages in using polymeric versus molecular p-conjugated semiconductors. Isoindigo has become a popular conjugated moiety in polymer semiconductor design because of its strong electron-withdrawing character. Polymeric p-channel TFTs have reached new heights, with hole mobilities unthinkable only few years back and surpassing 10 cm2V−1 s−1.
1,4-Bis(phenylethynyl)benzene (BPB) derivatives with alkyl or alkoxy terminal chains were synthesized, and the mesomorphic and charge-carrier-transport properties were investigated. The BPB derivatives exhibited high charge-carrier mobilities in the crystal mesophase or crystal phase.
A novel theoretical tool for some new low-band-gap copolymers has been developed on the basis of density functional theory (DFT) quantum chemical calculations to model their photophysical properties. These copolymers are constituted of thiophene and benzothiadiazole (PTBT) units essentially as well as their derivatives leading to donor (D)-acceptor (A) structure-types. Firstly, we study the insertion of thiophene as spacer unit into PTBT backbone to reach polydithiophene-alt-benzothiadiazole (PDTBT) copolymer, secondly, bithiophene usual unit was replaced by cyclopentadithiophene entity to obtain poly (cyclopentadithiophene-alt-benzothiadiazole) (PCPDTBT) copolymer and finally, ethynyl and vinylene spacers were added to obtain a novel oligomer models, denoted poly(cyclopentadithiophene-ethynyl-benzothiadiazole) (PCPDTEBT) and poly(cyclopentadithiophene-vinylene-benzothiadiazole) (PCPDTVBT), respectively, and to investigate their effect into the main backbone on various properties by examining structural and electronic properties.PCPDTEBT copolymer presents the optimal properties to be used as an active layer in organic solar cell (OSC). It shows unique combination of properties: a moderate gap (1.55 eV), high extinction coefficients (ϵ = 3.11 × 105 mol−1 L cm−1), a low oxidation potential associated with high thermal stability and environmental sustainability. Thus, this copolymer is then blended with [6,6]-phenyl-C61- butyric acid methyl ester (PCBM) in bulk-heterojunction solar cell. A model band diagram was established for the proposed solar structure, simulating the energy behavior of such active layer.
Over the last two decades, ever increasing interest has been focused on π-conjugated triple-bond-containing systems, namely, (poly)aryl acetylenes, as a very promising class of semiconducting materials, owing to the availability of flexible/efficient synthetic protocols and the new conception of their conformational and steric advantages. In this review, the major design/synthetic strategies used to obtain molecular aryl acetylene semiconductors are discussed. A brief discussion of their key properties as well as their performance in organic field-effect transistors and photovoltaic cell applications is also included.
Four novel organic semiconductor phenyl-oligothiophene derivatives containing acetylenic spacers, 5-(phenylethynyl)-2,2′:5′,2″-terthiophene, 5-(phenylethynyl)-2,2′:5′,2″:5″,2‴-quaterthiophene, 4-([2,2′:5′,2″-terthiophen]-5-ylethynyl)aniline, 4-((5′-iodo-[2,2′-bithiophen]-5-yl)ethynyl)aniline were synthesized and studied with respect to their thermal, photophysical, electrochemical and morphological properties. TGA and DSC results show all the oligomers have good thermal stability. Electrochemical measurements suggest the compounds can be used as photovoltaic materials. UV–vis spectra confirm all the molecules adopt close side-to-side packing, indicating H-aggregation. 4-([2,2′:5′,2″-Terthiophen]-5-ylethynyl)aniline exhibits excellent fluorescence properties when applied in an optoelectric field. XRD analysis indicates that the four compounds have a high degree of lamellar ordering and crystallinity. Film morphology properties reveal a clearly single-crystalline monoclinic morphology with lamellae parallel to the substrate, exhibiting that adjacent molecules have a strong intermolecular interaction. The potential liquid crystalline property is illustrated by DSC, XRD and polarization optical microscopy. These properties demonstrate that these oligomers could be promising candidates for the organic thin film materials and photovoltaic devices.
The synthesis of two new thieno(bis)imide (TBI, N) end functionalized oligothiophene semiconductors is reported. In particular, trimer (NT3N) and pentamer (NT5N) have been synthesized and characterized. Two different synthetic approaches for their preparation were tested and compared namely conventional Stille cross coupling and direct arylation reaction via C–H activation. Theoretical calculations, optical and electrochemical characterization allowed us to assess the role of the π-conjugation extent, i.e., of the oligomer size on the optoelectronic properties of these materials. In both TBI ended compounds, due to the strong localization of the LUMO orbital on the TBI unit, the LUMO energy is almost insensitive to the oligomer size, this being crucial for the fine-tailoring of the energy and the distribution of the frontier orbitals. Surprisingly, despite its short size and contrarily to comparable TBI-free analogues, NT3N shows electron charge transport with mobility up to μN = 10−4 cm2 V−1 s−1, while increasing the oligomer size to NT5N promotes ambipolar behavior and electroluminescence properties with mobility up to μN = 0.14 cm2 V−1 s−1 and to μP = 10−5 cm2 V−1 s−1.
3′-methyl-(5,5′′-bis[3-ethyl-3-(6-phenyl-hexyloxymethyl)-oxetane])-2,2′:5′,2′′-terthiophene (5T(Me)Ox) is a solution processable small molecule semiconductor displaying smectic-C and nematic liquid crystal phases. The pendant oxetane group can be polymerized in situ in the presence of a suitable photoacid at concentrations ≥1% by weight. Spin-coated films of pure 5T(Me)Ox and 5T(Me)Ox doped with the soluble photoacid were characterized by absorption and photoluminescent spectroscopy. Thick pristine films showed absorption and emission from a crystalline phase. Thin monolayer (
Phenylethynyl, butylphenylethynyl and pentafluorophenylethynyl end-capped terthiophene oligomers (PATTP, BPATTP and FPATTP) have been synthesized and characterized. Their physical, optical and electrochemical properties varied with the end substituents were investigated. DSC results revealed that three compounds were crystalline in the film state and thermally stable by 200°C under nitrogen atmosphere. The oligothiophenes have exhibited H-aggregation, which was verified by significant blue-shift in UV–vis absorption spectra in the films, indicating a close side-to-side molecular packing. X-ray diffraction measurement on the films has revealed molecular edge-on substrate growth orientation. Top-contact field-effect transistors were demonstrated by spin-coating or ink-jetting the solution of the oligomers. The preliminary results showed that PATTP and BPATTP can serve as p-type carriers with hole-motility as 3×10−4 and 1×10−2cm2/(Vs), respectively, while pentafluorophenylacetylene afforded FPATTP as n-type carriers with electron-motility around 10−5cm2/(Vs).
Thin films based on the akylphenyl-substituted quaterthiophenes 5,5'''-bis(4-methylphenyl)- 2,2': 5', 2": 5', 2'''-quaterthiophene ( 1), 5,5'''-bis(4-n-propylphenyl)-2,2': 5', 2": 5", 2"'-quaterthiophene ( 2), and 5,5'"-bis( 4-hexylphenyl)- 2,2': 5', 2": 5", 2'"-quaterthiophene (3) were grown by vacuum deposition on thermally grown SiO2 substrates and characterized in a thin film transistor (TFT) configuration. Atomic force microscopy and specular (theta-2 theta) X-ray diffraction (XRD) revealed films with small, crystalline grains, in which the oligothiophenes were oriented " end-on" with respect to the SiO2 substrate. Interplanar spacing increased (1 = 28.0 angstrom, 2 = 29.5 angstrom, 3 = 37.7 angstrom), consistent with increasing alkyl tail length. Grazing incidence X-ray diffraction (GIXD) of the films (ca. 350 angstrom thick) revealed nearly equivalent in-plane unit-cell areas ( 1 = 44.1 angstrom(2), 2 = 44.4 angstrom(2), 3 = 43.8 angstrom(2)). The films functioned as p-channel semiconductors in a TFT configuration, exhibiting nearly equivalent hole mobilities (mu approximate to 0.05 +/- 0.01, 0.04 +/- 0.01, 0.06 +/- 0.01 cm(2)/Vs for 1, 2, and 3, respectively). Variable-temperature measurements demonstrated that the activation energy of the mobility for thin films of 3 was similar to 55 meV. The increasing alkyl chain length does not appear to improve molecular ordering; however, the addition of a phenyl end-substituent appears to greatly improve the on-to-off ratio in TFTs.
A series of poly(oligothienylene vinylenes) (PTmVs, m = 2–4) with a varying number of consecutively bound thienylene rings are successfully prepared in thin films by chemical vapor deposition polymerization (CVDP) using the corresponding bis(halomethyl)thiophenes as starting materials. The chemical and electronic structures are studied spectroscopically and also by cyclic voltammetry. Top-gate field-effect transistors are fabricated by two consecutive CVDP cycles of PTmV and poly(p-xylylene) followed by the deposition of a Au gate electrode. In the case of a PT3V active layer, a field-effect mobility value of 0.5 × 10–4 cm2 V–1 s–1 is obtained.
The organic semiconductor α-quinquethiophene (T5) is used as the active layer in organic field-effect transistors. We have investigated the adsorption of T5 on the (110) surface of copper and on the CuO nanotemplate formed by the high-temperature exposure of Cu(110) to molecular oxygen. The results were obtained with high-resolution scanning tunneling microscopy (STM) under ultra-high-vacuum (UHV) conditions. The adsorption of T5 on copper is an important model system because it mimics the active-layer–electrode interface in organic devices. The molecules were observed to adsorb onto both the pristine Cu(110) surface and the CuO nanotemplate, showing a greater affinity for the pristine copper surface. Surprisingly, however, the T5 molecules assembled with a much higher degree of long-range order on the oxygen-passivated portion of the surface.
In the last few years, thiophene-based materials, which are semiconductor and fluorescent compounds, have become a highly interdisciplinary field of research, with diverse studies ranging from the fabrication of electronic and optoelectronic devices to the selective detection of biopolymers. This article presents a survey of the papers published in the last two years and concludes with the authors' views on future developments.
In this article, we present an overview and prospective of inhomogeneous thin deposits of functional materials, which are usually unusable for applications as deposited. We show how common unwanted morphologies, such as crystallites, dewetting and fibrils can be transformed by driving their self-organization with external tools. In the first part we consider self-organization leading to thin deposits from compounds molecularly dispersed in a solution focusing on the relationship between the microscopic morphology appearing upon aggregation onto surfaces and the functional properties of these materials. In particular we considered the formation of crystallites, fibrils and droplets. In the second part we show how the inhomogeneous (and often unwanted) morphologies can be "corrected" by unconventional lithography in their self-organization becoming functional for several devices.
Ethynylene-containing benzo[1,2-b:4,5-b′]dithiophene derivatives 1a–c (BPEBDT, BTEBDT and BHPEBDT) were designed and synthesized. Their physicochemical properties were studied by absorption spectra and electrochemistry. 1a–c displayed high field-effect transistors performance, a mobility up to 1.17 cm2 V−1 s−1 with on/off current ratio of 107 was achieved.
A series of thiophene-phenylene semiconductors containing –CC– bonds were designed and synthesized. Their thermal, optical, electrochemical and FET properties were fully characterized. The crystal structure of 5,5′-bis(phenylethynyl)-2,2′-bithiophene (1c) revealed that the introduction of carbon-carbon triple bonds efficiently eliminated the steric repulsions between adjacent aromatic rings. TGA, UV-vis spectra and electrochemical measurements showed that all compounds had good thermal and environmental stability. FET devices based on these materials exhibited high performance and stability. All the results suggested the introduction of carbon-carbon triple bonds provided an efficient route to high performance organic semiconductors.
For use in high performance thin film transistors, the semiconducting material must have high carrier mobility, form crystalline films whose conductive axes re aligned between the source and drain electrodes and be processable. Changes at the molecular level significantly affect all of these parameters, giving the chemist an unprecedented degree of flexibility in materials design. This review introduces a number of different materials systems into thin film transistor structures. Focus is on the fabrication of thin film transistor structures and the characterization methods used for both devices and thin films. A wide range of molecular and oligomeric organic compounds in transistor structures are also covered, highlighting the advances that have been made in achieving high mobility materials.
Both the periodic and non-periodic structures of perchlorocoronene (C(24)Cl(12)) crystals were characterized by high-resolution transmission electron microscopy (HRTEM), electron diffraction (ED), electron energy-loss spectroscopy (EELS), and energy-filtered transmission electron microscopy (EFTEM). The HRTEM images at the boundary of the C(24)Cl(12) crystals exhibit the flexibility of defect structures, where molecules align to compensate for the discontinuity between two different domains. Emphasized by the filtered images, it was found that the non-periodic regions are created everywhere with a small electron beam irradiation (∼ 10(6)electrons nm(-2)) and then spread over the entire regions to completely destroy the periodic structures after a higher electron dose (∼ 2 × 10(6)electrons nm(-2)). The effect of the electron beam irradiation was monitored by ED, EELS, and EFTEM, where periodic structures and content elements are well preserved up to 10(6)electrons nm(-2), but chlorine atoms decreased with a much higher electron dose. This is explained by the breakage of the C-Cl bond to detach chlorine atoms, confirmed by energy-loss near the edge structures (ELNES) of carbon π * peaks and chlorine loss at the edge of the specimen, as well as by theoretical simulation. The detachment of chlorine is localized at the peripheral edge around a hole confirmed by core-loss EFTEM imaging.
Two thiophene-phenylene semiconductors, bis(2-phenylethynyl) end-substituted oligothiophenes (diPhAc-nTs, n = 2, 3), were synthesized and studied with respect to their optical, electrochemical, structural and electrical properties. The optical and electrochemical properties of the oligomers in solution were investigated by UV-vis absorption and photoluminescence spectroscopies, and cyclic voltammetry. High vacuum evaporated thin films were investigated by optical absorption, X-ray diffraction and AFM, and implemented as p-type semiconducting layers into organic thin-film transistors (OTFTs). A comparative study in solution and in the solid state with distyryl-oligothiophenes (DSnTs, n = 2, 3) reveals the great influence of acetylenic (-C[triple bond]C-) vs. olefinic (-C=C-) spacers in thiophene-phenylene derivatives on electronic structure, physical properties, and device efficiencies. Substituting olefinic for acetylenic pi-spacers in terthiophene-based conjugated semiconductors leads to one of incontrovertible attributes of OTFTs for low cost applications, a high mobility at low substrate temperature (T(sub)) i.e. typically 25 degrees C. Fine-tuning in the HOMO/LUMO levels by reducing the HOMO level introduces increased air-oxidation strength of thin films where OTFTs provide exactly the same hole mobility value after 100 days in air. All the results suggested that introduction of carbon-carbon triple bonds provided an efficient route to highly air-stable organic thin film transistors.
The effect of semiconductor crystal structure on the performance of organic field-effect transistors is investigated through the fabrication and characterization of devices based on single porphyrin crystals with different center metals. The field-effect mobility of the transistors is found to increase with decreasing intermolecular distance, attributable to greater overlap of pi orbitals among close-packed molecules and the consequent promotion of charge transport. (c) 2007 American Institute of Physics.
The organic semiconductor alpha-quinquethiophene (T5) is used as the active layer in organic field-effect transistors. We have investigated the adsorption of T5 on the (110) surface of copper and on the CuO nanotemplate formed by the high-temperature exposure of Cu(110) to molecular oxygen. The results were obtained with high-resolution scanning tunneling microscopy (STM) under ultra-high-vacuum (UHV) conditions. The adsorption of T5 on copper is an important model system because it mimics the active-layer-electrode interface in organic devices. The molecules were observed to adsorb onto both the pristine Cu(110) surface and the CuO nanotemplate, showing a greater affinity for the pristine copper surface. Surprisingly, however, the T5 molecules assembled with a much higher degree of long-range order on the oxygen-passivated portion of the surface.
Single crystals of unsubstituted sexithiophene (6T) were produced by a sublimation technique. X-ray structure shows that the molecule has a quasi-planar all-trans configuration, the torsional angles between adjacent rings being lower than their respective standard deviation. The unit cell belongs to space group P2(1)/n and presents the herringbone packing common to a great deal of planar molecules. Owing to the well-defined orientation of the molecules in single crystals, the polarization of vibration modes and coupling due to crystalfield effects could be unambiguously determined from polarized light IR spectroscopy. Raman spectra of the single crystals are also presented and compared to that of polycrystalline evaporated films.
The structures of the series of oligothienyls (T4-T5-T6) have been determined and refined from X-ray powder diffraction data using the Rietveld full-profile analysis. Unlike polythiophene, all the molecules in the isomorphous series display no internal symmetry, being only approximately planar within 12°. Packing is always achieved through a common molecular disposition, which is usually defined as a herringbone arrangement, with an angle between mean planes of adjacent molecules which spans from 60° to 70°.
A symmetrical alpha,omega-substituted sexithiophene derivative containing thermally removable branched ester solubilizing groups has been prepared. These oligomers can be solution cast into thin films and then thermolyzed to remove the solubilizing group, leaving short pendant alkene groups on the oligomer. Device testing of thin film transistors shows an increase in hole mobility from 1 x 10-5 cm2/(V s) with on/off ratios of approximately 100 before thermolysis to 5 x 10-2 cm2/(V s) with on/off ratios >105 after thermolysis. This method offers an attractive route to easily processed and highly performing thiophene oligomers.
Hole mobility in organic ultrathin film field-effect transistors is studied as a function of the coverage. For layered sexithienyl films, the charge carrier mobility rapidly increases with increasing coverage and saturates at a coverage of about two monolayers. This shows that the first two molecular layers next to the dielectric interface dominate the charge transport. A quantitative analysis of spatial correlations shows that the second layer is crucial, as it provides efficient percolation pathways for carriers generated in both the first and the second layers. The upper layers do not actively contribute either because their domains are smaller than the ones in the second layer or because the carrier density is negligible.
Organic thin-film transistors (OTFTs) have lived to see great improvements in recent years. This review presents new insight into conduction mechanisms and performance characteristics, as well as opportunities for modeling properties of OTFTs. The shifted focus in research from novel chemical structures to fabrication technologies that optimize morphology and structural order is underscored by chapters on vacuum-deposited and solution-processed organic semiconducting films. Finally, progress in the growing field of the n-type OTFTs is discussed in ample detail. The Figure, showing a pentacene film edge on SiO2, illustrates the morphology issue.
Organic thin-film transistors (OTFTs) have lived to see great improvements in recent years. This review presents new insight into conduction mechanisms and performance characteristics, as well as opportunities for modeling properties of OTFTs. The shifted focus in research from novel chemical structures to fabrication technologies that optimize morphology and structural order is underscored by chapters on vacuum-deposited and solution-processed organic semiconducting films. Finally, progress in the growing field of the n-type OTFTs is discussed in ample detail. The Figure, showing a pentacene film edge on SiO2, illustrates the morphology issue.
Roughness scaling properties of sexithienyl thin films are investigated by scanning force microscopy as a function of the substrate deposition temperature. The correlation length of the surface fluctuations follows the behavior of the average domain size, increasing exponentially with temperature from submicron to micron scales. Self-affinity is exhibited on 3 orders of magnitude of the spatial frequencies when the morphology changes from grain aggregates to lamellae. The decay with temperature of the roughness scaling exponent alpha from 1 to 0.7 suggests a transition from diffusion-limited growth to a strong adsorption regime.
Organic thin-film transistors (OTFTs) have lived to see great improvements in recent years. This review presents new insight into conduction mechanisms and performance characteristics, as well as opportunities for modeling properties of OTFTs The shifted focus in research from novel chemical structures to fabrication technologies that optimize morphology and structural order is underscored by chapters on vacuum-deposited and solution-processed organic semiconducting films. Finally, progress in the growing field of the n-type OTFTs is discussed in ample detail. The Figure, showing a pentacene film edge on SiO 2, illustrates the morphology issue.
α-hexathienyl (α–6T) is a highly promising material for application in thin film transistor devices. Recently, record high mobilities, together with record high current on/off ratios, have been reported.1 Thus far, structural information on this exciting material is sketchy. The crystal structures of several such hexamers have been investigated, but only with powder samples, since the crystal growth has proven exceedingly difficult.2-5 Powder Rietveld refinements on these materials are severely hampered by the large number of overlapping reflections, preferred orientation, ambiguities in symmetry, etc. Here, we present a crystal structure of the high-temperature polymorph of α–6T (α–6T/HT), as determined from a single-crystal structure analysis. In this polymorph, the hexamer crystallizes in the smallest unit cell so far reported for this material, but the molecule is flat. Extended Hückel theory (EHT) band structure calculations show that α–6T/HT is an indirect gap semiconductor, with the conduction band minimum at Y and the valence band maximum at Γ. The conduction and valence bands both show a remarkable degree of dispersion along X and Y for a molecular crystal. The electronic band structure of this material is strikingly similar to that of the two-dimensional organic superconductors based on bis(ethylenedithio)tetrathiafulvalene (ET), such as κ−(ET)2 Cu(NCS)2.
In order to analyze the correlation between charge transport and structural properties in conjugated oligomers, sexithiophene, 6T, was substituted by hexyl groups, both on the terminal alpha positions (alpha,omegaDH6T) and as pendant groups in the beta position (beta,beta'DH6T). Structural characterizations by X-ray diffraction show that vacuum-evaporated thin films of 6T and alpha,omegaDH6T consist of layered structures in a monoclinic arrangement, with all-trans planar molecules standing on the substrate. When compared to 6T, alpha,omegaDH6T is mainly characterized by a very large increase of molecular organization at the mesoscopic level, evidenced by a much longer range ordering. Electrical characterizations indicate that the conductivity of alpha,omegaDH6T is largely anisotropic, with a ratio of 120 in favor of the conductivity parallel to the substrate plane, i.e. along the stacking axis. The charge carrier mobility, determined on field-effect transistors fabricated from these conjugated oligomers, also shows an increase by a factor of 25 when passing from 6T to alpha,omegaDH6T, reaching a value of 5 x 10(-2) cm2 V-1 s-1. In contrast, beta,beta'DH6T presents very low conductivity and mobility, the latter being below detection limit. These results are attributed to the self-assembly properties brought by alkyl groups in the alpha,omega position.
Ionization energies, attachment energies, and electrochemical reduction potentials of thiophene oligomers (n {le} 5) have been determined experimentally (ultraviolet photoelectron and electron transmission spectroscopies and cyclic voltammetry) and theoretically (ionization and attachment energies by MINDO/3). The geometrical parameters of the most stable conformation of 2,2{prime}-bithienyl have been computed at the ab initio STO-3G level with complete relaxation. A short extrapolation of the energy data to the polymer provided accurate and reliable values for important properties of (gas phase) polythiophene, namely, ionization energy (6.9 eV), valence bandwidth (3.2 eV), electron affinity (0.9-1.1 eV), HOMO-LUMO energy gap (5.9 eV), and {lambda}{sub max} (2.7 eV).
Organic field effect transistors (OFETs) made from quaterthiophene (4T) and quinquethiophene (5T) have been fabricated with greatly enhanced field-effect mobilities. It is shown that the problem seems to lie in achieving efficient charge injection rather than in poor charge transport within the material. Dopping of the drain and source contacts with a thin layer of the electron acceptor TCNQ is demonstrated to improve charge injection in 4T devices. Other factors affecting performance are considered briefly.
Communication: The efficiency of organic field-effect transistors (OFETs) based on high-purity alpha-octithiophene (alpha-ST) and its alkylated derivative DH alpha-8T (see Figure) has been investigated, particular attention being paid to the influence of oligomer length, material purity, and film ordering on OFET performance, especially field effect mobility. The question of whether the mobility of the oligothiophenes keeps on increasing with chain length is discussed.
A new pentacene derivative, 2,3,9,10-tetramethyl-pentacene (Me4PENT), has been synthesized, characterized, and tested in a field-effect transistor (FET) device (see Figure). A bottom-contact-mode FET device fabricated with Me4PENT was shown to exhibit a high charge-transport mobility of 0.31 cm(2) V-1 s(-1) when fabricated at a deposition substrate temperature of 85 degreesC.
Thin films based on the tolyl-substituted oligothiophenes 5,5′′-bis(4-methylphenyl)-2,2′:5′,2′′-terthiophene (1), 5,5′′′-bis(4-methylphenyl)-2,2′:5′,2′′:5′′,2′′′-quaterthiophene (2) and 5,5′′′′-bis(4-methylphenyl)-2,2′:5′,2′′:5′′,2′′′:5′′′,2′′′′-quinqethiophene (3) exhibit hole-transport behavior in a thin-film transistor (TFT) configuration, with reasonable mobilities and high current on/off (Ion/Ioff) ratios. Powder X-ray diffraction (PXRD) reveals that these films, grown by vacuum deposition onto the thermally grown silicon oxide surface of a TFT, are highly crystalline, a characteristic that can be attributed to the general tendency of phenyl groups to promote crystallinity. Atomic force microscopy (AFM) reveals that the films grow layer by layer to form large domains, with some basal domain areas approaching 1000 μm2. The PXRD and AFM data are consistent with an “end-on” orientation of the molecules on the oxide substrate. Variable-temperature current–voltage (I–V) measurements identified the activation regime for hole transport and revealed shallow level traps in thin films of 1 and 2, and both shallow and deep level traps in thin films of 3. The activation energies for thin films of 1, 2, and 3 were similar, with values of Ea = 121, 100, and 109 meV, respectively. The corresponding trap densities were Ntrap/Nv = 0.012, 0.023, and 0.094, where Ntrap is the number of trap states and Nv is the number of conduction states. The hole mobilities for the three compounds were similar (μ ≃ 0.03 cm2 V–1 s–1), and the Ion/Ioff ratios were comparable with the highest values reported for organic TFTs, with films of 2 approaching Ion/Ioff = 109 at room temperature.
Relationship between molecular structure and electrical performance of oligothiophene organic thin film transistor (TFT) was studied. The performance of TFT's with ultra-thin self-assembled octadecyltrichlorosilane (OTS) monolayer gate dielectrics deposited by evaporation was found to be more sensitive to the choice of semiconductors. The long side chains lead to an increase in the ethyl-substituted sexithiophene in the effective thickness of the gate dielectric, with reduced gate currents and current mobilities.
A study of the solution and solid-state electrochemical properties of defined oligomers of the α-thiophene and the p-phenylene series with chain lengths 2 ⩽ N ⩽ 16 is presented. Upon p-doping of the oligomers in the solid state their solid-state polymerization on the electrode was observed. The investigations clearly demonstrate that the important steps of the electropolymerization of conducting polymers take place exclusively in the solid state on the electrode. The degree of polymerization strongly depends on the anodic electrode potential. At low potentials polymers with long, ideally-linked chains are formed, whereas at high potentials strongly cross-linked polymers are generated. A new charge storage mechanism for conjugated oligomers and polymers is presented taking into account the morphology of the materials. We propose a two-step mechanism to explain hysteresis effects observed in voltammograms for charging processes on these materials. In the first stage of the charging process we assume that the originally twisted segments become more planar, enabling a better delocalization of charges along the chain (intramolecular stabilization). This is the prerequisite for the second step, an intermolecular stabilization through interactions between neighbouring charged segments, leading to the delocalization of the excess charges over several segments and, therefore, to further stabilization of the system.
The observation of n-channel conduction in a DCMT-based organic thin-film transistor (OTFT) with a saturation electronmobility of 0.2 cm2/Vs was reported. It was shown that devices with DCMT films grown at higher temperatures can be ambipolar, i.e., they can be n-channel or p-channel depending on the sign of th gate bias. Temperature measurement son high n-channel mobility DCMT OTFTs revealed that transport was activated in the temperature range 260 to 80 K, with a small activation energy of 35±10 meV.
We show that thin films grown by vacuum sublimation, or formed by melted powders, of semiconductor alpha-quinquethiophene (T5) exhibit a hierarchical self-affinity organization that spans scales from tens of nanometers to hundreds of micrometers. T5 organization was investigated by X-ray diffraction (XRD), atomic force microscopy (AFM), and optical microscopy. XRD showed that vacuum-evaporated T5 films were characterized by a preferred orientation of the h00 planes parallel to the glass substrate. Melting of the films followed by rapid quenching to room temperature led to the formation of micrometer-sized, single-crystal-like structures, characterized by uniaxially aligned stripes. XRD proved that the melting-quenching process enhanced molecular ordering and increased the size of domains with the molecule's long axes tilted by about 65 degrees with respect to the substrate plane and piled up side-by-side along parallel columns. AFM measurements on the melt-quenched structures showed that a hierarchical architecture was built by reiteration across multiple length scales of the same recurring motif. Because of the tendency of T5 to form highly crystalline vacuum-evaporated thin films, a field-effect hole mobility comparable to state-of-the-art FET mobility of alpha-sexithiophene films was reached, without any attempt to optimize deposition conditions.
A new family of perfluoroarene-modified thiophene semiconductors 1-3 has been synthesized to assess the influence of perfluoroarene introduction and regiochemistry on molecular and thin-film transistor properties. Compound 1 is an n-type semiconductor with a mobility approaching 0.1 cm2V -1s-1 whereas 2 and 3 exhibit p-type behavior. These results show that the origin of n-type carrier mobility is not solely a consequence of solution/film LUMO and HOMO energies.
Grazing incidence X-ray diffraction reveals that a pentacene monolayer, grown on an amorphous SiO2 substrate that is commonly used as a dielectric layer in organic thin film transistors (OTFTs), is crystalline. A preliminary energy-minimized model of the monolayer, based on the GIXD data, reveals that the pentacene molecules adopt a herringbone arrangement with their long axes tilted slightly from the substrate normal. Although this arrangement resembles the general packing features of the (001) layer in single crystals of bulk pentacene, the monolayer lattice parameters and crystal structure differ from those of the bulk. Because carrier transport in pentacene OTFTs is presumed to occur in the semiconductor layers near the dielectric interface, the discovery of a crystalline monolayer structure on amorphous SiO2 has important implications for transport in OTFTs.
The facile synthesis of poorly soluble unsubstituted and modified alpha-quinque- and sexithiophenes under microwave irradiation in the liquid phase is described. The use of microwave irradiation allowed these compounds to be prepared in a few minutes and at high yields by means of the Suzuki cross-coupling reaction. Unsubstituted sexithiophene was obtained in 10 min via the one-pot borylation/Suzuki reaction, purified according to a very simple procedure, and isolated in 84% yield. The efficient synthesis of two new methylated quinque- and sexithiophenes displaying liquid crystalline properties is reported. A new microwave-assisted methodology for the conversion of aldehyde-terminated quinque- and sexithiophenes into the corresponding cyano derivatives is also described. The use of microwaves was extended to the Sonogashira coupling reaction and found to be very effective in the preparation of a quinquethiophene containing acetylenic spacers. The electronic and optical characterization of this compound is reported and discussed in relation to that of unsubstituted quinquethiophene.
- A Facchetti
- M H Yoon
- C L Stern
- H E Katz
- T J Marks
A. Facchetti, M.H. Yoon, C.L. Stern, H.E. Katz, T.J. Marks, Angew
Chem. Int. Ed. 42 (2003) 3900–3903.
- D V Kelley
- P F Muyres
- T P Baude
- T D Smith
- H Jones
- M Meng
- G Bendikov
- R Mitchell
- F Helgeson
- Z Wudl
- T Bao
- C Siegrist
- C H Kloc
- Chen
(a) T.W. Kelley, D.V. Muyres, P.F. Baude, T.P. Smith, T.D. Jones,
Mater. Res. Soc. Symp. Proc. 771 (2003) L6.5.1–L6.5.11;
(b) H. Meng, M. Bendikov, G. Mitchell, R. Helgeson, F. Wudl,
Z. Bao, T. Siegrist, C. Kloc, C.H. Chen, Adv. Mater. 15 (2003)
1090–1093;
(c) S.E. Fritz, S.M. Martin, C.D. Frisbie, M.D. Ward, M.F. Toney,
J. Am. Chem. Soc. 126 (2004) 4084–4085.
- F Garnier
- A Yassar
- R Hajlaoui
- G Horowitz
- F Deloffre
- B Servet
- S Ries
- P Alnot
F. Garnier, A. Yassar, R. Hajlaoui, G. Horowitz, F. Deloffre, B.
Servet, S. Ries, P. Alnot, J. Am. Chem. Soc. 115 (1993) 8716–8721.
- A R Murphy
- J M J Fréchet
- P Chang
- J Lee
- V Subramanian
A.R. Murphy, J.M.J. Fréchet, P. Chang, J. Lee, V. Subramanian, J.
Am. Chem. Soc. 126 (2004) 1596–1597.
- R Hajlaoui
- G Horowitz
- F Garnier
- A Arcebrouchet
- L Laigre
- A Elkassmi
- F Demanze
- F Kouki
(a) R. Hajlaoui, G. Horowitz, F. Garnier, A. ArceBrouchet, L. Laigre,
A. ElKassmi, F. Demanze, F. Kouki, Adv. Mater. 9 (1997) 389–393;
(b) R. Hajlaoui, D. Fichou, G. Horowitz, B. Nessakh, M. Constant,
F. Garnier, Adv. Mater. 9 (1997) 557–561.
- G Antolini
- F Horowitz
- F Kouki
- Garnier
[12] (a) L. Antolini, G. Horowitz, F. Kouki, F. Garnier, Adv. Mater. 10
(1998) 382–385;
- M Melucci
- M Gazzano
- G Barbarella
- M Cavallini
- F Biscarini
- P Maccagnani
- P Ostoja
(a) M. Melucci, M. Gazzano, G. Barbarella, M. Cavallini, F. Biscarini, P. Maccagnani, P. Ostoja, J. Am. Chem. Soc. 125 (2003)
10266–10274;
(b) M. Melucci, G. Barbarella, M. Zambianchi, P. Di Pietro, A.
Bongini, J. Org. Chem. 2004 (ASAP article).
- M Halik
- H Klauk
- U Zshieschang
- G Schmid
- S Ponomarenko
- S Kyrchmeyer
M. Halik, H. Klauk, U. Zshieschang, G. Schmid, S. Ponomarenko,
S. Kyrchmeyer, Adv. Mater. 15 (2003) 917–922.
- Siegrist
- Antolini