Materials For Organic Field-effect Transistors Research Articles

Tailoring Functional Interlayers in Organic Field-Effect Transistor Biosensors.

This review aims to provide an update on the development involving dielectric/organic semiconductor (OSC) interfaces for the realization of biofunctional organic field-effect transistors (OFETs). Specific focus is given on biointerfaces and recent technological approaches where biological materials serve as interlayers in back-gated OFETs for biosensing applications. Initially, to better understand the effects produced by the presence of biomolecules deposited at the dielectric/OSC interfacial region, the tuning of the dielectric surface properties by means of self-assembled monolayers is discussed. Afterward, emphasis is given to the modification of solid-state dielectric surfaces, in particular inorganic dielectrics, with biological molecules such as peptides and proteins. Special attention is paid on how the presence of an interlayer of biomolecules and bioreceptors underneath the OSC impacts on the charge transport and sensing performance of the device. Moreover, naturally occurring materials, such as carbohydrates and DNA, used directly as bulk gating materials in OFETs are reviewed. The role of metal contact/OSC interface in the overall performance of OFET-based sensors is also discussed.

HOMO stabilisation in π-extended dibenzotetrathiafulvalene derivatives for their application in organic field-effect transistors.

Three new organic semiconductors, in which either two methoxy units are directly linked to a dibenzotetrathiafulvalene (DB-TTF) central core and a 2,1,3-chalcogendiazole is fused on the one side, or four methoxy groups are linked to the DB-TTF, have been synthesised as active materials for organic field-effect transistors (OFETs). Their electrochemical behaviour, electronic absorption and fluorescence emission as well as photoinduced intramolecular charge transfer were studied. The electron-withdrawing 2,1,3-chalcogendiazole unit significantly affects the electronic properties of these semiconductors, lowering both the HOMO and LUMO energy levels and hence increasing the stability of the semiconducting material. The solution-processed single-crystal transistors exhibit high performance with a hole mobility up to 0.04 cm(2) V(-1) s(-1) as well as good ambient stability.

ChemInform Abstract: Organic Semiconductors Based on [1]Benzothieno[3,2‐b][1]benzothiophene Substructure

AbstractReview: design, synthesis, and characterization; 54 refs.

Organic semiconductors based on [1]benzothieno[3,2-b][1]benzothiophene substructure.

The design, synthesis, and characterization of organic semiconductors applicable to organic electronic devices, such as organic field-effect transistors (OFETs) and organic photovoltaics (OPVs), had been one of the most important topics in materials chemistry in the past decade. Among the vast number of materials developed, much expectation had been placed on thienoacenes, which are rigid and planar structures formed by fusing thiophenes and other aromatic rings, as a promising candidate for organic semiconductors for high-performance OFETs. However, the thienoacenes examined as an active material in OFETs in the 1990s afforded OFETs with only moderate hole mobilities (approximately 0.1 cm(2) V(-1) s(-1)). We speculated that this was due to the sulfur atoms in the thienoacenes, which hardly contributed to the intermolecular orbital overlap in the solid state. On the other hand, we have focused on other types of thienoacenes, such as [1]benzothieno[3,2-b][1]benzothiophene (BTBT), which seem to have appropriate HOMO spatial distribution for effective intermolecular orbital overlap. In fact, BTBT derivatives and their related materials, including dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT), have turned out to be superior organic semiconductors, affording OFETs with very high mobilities. To illustrate some examples, we have developed 2,7-diphenyl BTBT (DPh-BTBT) that yields vapor-deposited OFETs having mobilities of up to 2.0 cm(2) V(-1) s(-1) under ambient conditions, highly soluble dialkyl-BTBTs (Cn-BTBTs) that afford solution-processed OFETs with mobilities higher than 1.0 cm(2) V(-1) s(-1), and DNTT and its derivatives that yield OFETs with even higher mobilities (>3.0 cm(2) V(-1) s(-1)) and stability under ambient conditions. Such high performances are rationalized by their solid-state electronic structures that are calculated based on their packing structures: the large intermolecular orbital overlap and the isotropic two-dimensional electronic structure are the key regardless of the molecular size and substituents on the BTBT and its related thienoacene cores. Along with the discovery of such attracting performances, versatile and practical methods for the synthesis of BTBT and its derivatives, and the π-extended derivatives including DNTT, dianthra[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DATT), and the thienoacenes with two thieno[3,2-b]thiophene moieties, have been developed. In addition, the materials have been recently utilized in sophisticated devices and circuits, including all-printed transistor arrays, flexible circuits on ultrathin plastic substrates, and biomedical applications, underscoring their promise as practical semiconductors for electronic device applications. These exciting results of the present BTBT-based materials are expected to open doors to new horizons of organic semiconductors in terms of practical application and the design and synthesis of far more superior materials.

A compact tetrathiafulvalene-benzothiadiazole dyad and its highly symmetrical charge-transfer salt: ordered donor π-stacks closely bound to their acceptors.

A compact and planar donor-acceptor molecule 1 comprising tetrathiafulvalene (TTF) and benzothiadiazole (BTD) units has been synthesised and experimentally characterised by structural, optical, and electrochemical methods. Solution-processed and thermally evaporated thin films of 1 have also been explored as active materials in organic field-effect transistors (OFETs). For these devices, hole field-effect mobilities of μFE = (1.3±0.5)×10(-3) and (2.7±0.4)×10(-3) cm(2) V s(-1) were determined for the solution-processed and thermally evaporated thin films, respectively. An intense intramolecular charge-transfer (ICT) transition at around 495 nm dominates the optical absorption spectrum of the neutral dyad, which also shows a weak emission from its ICT state. The iodine-induced oxidation of 1 leads to a partially oxidised crystalline charge-transfer (CT) salt {(1)2I3}, and eventually also to a fully oxidised compound {1I3}⋅1/2I2. Single crystals of the former CT compound, exhibiting a highly symmetrical crystal structure, reveal a fairly good room temperature electrical conductivity of the order of 2 S cm(-1). The one-dimensional spin system bears compactly bonded BTD acceptors (spatial localisation of the LUMO) along its ridge.

Employing microspectroscopy to track charge trapping in operating pentacene OFETs

Ultrathin pentacene films resemble benchmark and model materials for organic field-effect transistors (OFETs). We employ scanning transmission X-ray microspectroscopy (STXM) and confocal Raman microspectroscopy as highly resolving probes to obtain insight into the correlation of morphology and charge transport in pentacene OFETs. By combining the operation-induced intensity increase in Raman-active bands with micromorphology, we are able to visualize charge-induced effects, in particular charge trapping in pentacene OFETs during operation. The high sensitivity and specificity of Raman microscopy allows to distinguish between orientation and charge-induced effects and thus to locate the trapped charges at grain boundaries.

The application of pentacene and its derivatives in organic field-effect transistors

Organic field-effect transistors (OFETs), employing organic semiconductor as active layer, is a kind of electronic device in which the source-drain conductivity is controlled by electrical field. In comparison with conventional inorganic transistors, OFETs have attracted extensive attention due to their potential applications in large area, flexible and low-cost optoelectronic devices, such as organic memory elements, organic solar cell, flexible flat panel displays, and electronic papers. Acenes, by virtue of their tight solid state packing and superior properties, has been widely studied as semiconducting materials in OFETs. For example, pentacene has drawn great attention because it shows outstanding mobility as a p-type semiconducting material. Although the mobility of pentacene can even rival amorphous silicon, the poor solubility in organic solvents and low environmental stability indeed blocked its way for further application. A lot of modification works on acenes has been done in order to improve the stability, enhance the electrical performance, and increase the solubility. As a result, a series of pentacene derivatives has been demonstrated. In this paper, we covered a wide range of pentacene and its derivatives with focus on their applications in OFETs. We expect that this summary will provide guidelines for further research.

Hydrated bovine serum albumin as the gate dielectric material for organic field-effect transistors

Bovine serum albumin (BSA) is a natural protein with good hydration ability which contains acidic and basic amino acid residues of ca. 34% in total. In vacuum, pentacene organic field-effect transistors (OFETs) with BSA as the gate dielectric exhibits a field-effect mobility value (μFE,sat) of 0.3cm2V−1s−1 in the saturation regime and a threshold voltage (VTH) of ca. −16V. BSA is easy to be hydrated in air ambient. Electrical properties of BSA in vacuum and hydrated BSA in air ambient are characterized. Similar to polyelectrolyte, hydrated BSA may act the gate dielectric with the formation of electric double-layer capacitors (EDLCs) to improve the device performance. In a relative humidity of 47%, the μFE,sat value increases to 4.7cm2V−1s−1 and the VTH reduces to −0.7V. Generation of mobile ions in hydrated BSA and the formation of EDLCs are discussed.

Synthesis, Properties, and Structures of Functionalized peri-Xanthenoxanthene

Three types of alkylated peri-xanthenoxanthene (PXX) have been synthesized employing efficient synthetic routes. These heteroaromatic compounds exhibited different electronic and crystal structures according to UV-vis spectra, electrochemical measurements, and X-ray structural analyses. Among them, 1,7-DOPXX has been demonstrated as an active material for organic field-effect transistors with promising mobility and a high on/off ratio simultaneously.

High-performance n-channel thin-film transistors with acene-based semiconductors

Two acene-based semiconductors were investigated with respect to their performance as n-type materials in organic field-effect transistors. The partially fluorinated ditetracenes (Ditetracen is protected by copywrite through the Patent WO/2007/000268. The patent is property of the Dritte Patentportfolio Beteiligungsgesellschaft mbH & Co. KG.) (DT) were synthesized in a high yield with different degrees of fluorination, one with four and another with two fluorine substituents (named as DT-4F and DT-2F, respectively). Both materials exhibit high thermal stability, with decomposition temperatures above 500°C. Since both materials are supposed to have a lowered LUMO level compared to the non-fluorinated parent DT, n-type operation in thin-film transistors (TFTs) with gold source and drain contacts was expected. TFTs based on DT-2F, however, showed weak ambipolar transport only, which demonstrates insufficient fluorination to switch from hole-dominated to electron-dominated transport. On the other hand, high performance n-type TFTs have been achieved from DT-4F, with electron mobilities up to 1.0cm2/Vs. This result indicates that fluorinated DT material can act as excellent n-type semiconductor for applications in complementary circuits. This is demonstrated in a complementary inverter stage using DT-4F-based TFTs as n-type transistor and a non-fluorinated DT derivative-based TFT as p-type transistor.

Synthesis and characterization of a fluorinated oligosiloxane-containing encapsulation material for organic field-effect transistors, prepared via a non-hydrolytic sol–gel process

The operation of stable organic field-effect transistors (FETs) over long periods of time requires that organic FETs are encapsulated. We synthesized an inorganic–organic hybrid non-hydrolytic sol–gel material (TPDt) containing fluoroalkyl functional groups to encapsulate organic FETs. Fourier-transform infrared spectroscopy, atomic force microscopy, UV–Visible spectroscopy, and water contact angle measurements demonstrated that the TPDt films displayed smooth surfaces, good hydrophobicity, and optical transparency. The gas barrier properties of the TPDt films were tested by fabricating FETs using an organic semiconductor, poly[9,9-dioctylfluorenyl-2,7-diyl]-co-(bithiophene)]. The organic FETs were operated at 38°C in the presence of a 90% relative humidity air atmosphere. The field-effect mobility of the organic FET decreased only negligibly, even after 2500h operation under these conditions.

Investigation of polysilsesquioxane as a gate dielectric material for organic field-effect transistors

We fabricated pentacene field-effect transistors using polysilsesquioxane (PSQ) as a gate dielectric material that can be cured at sufficiently low temperatures for low cost plastic substrates. The surface roughness of the PSQ films were reduced by the introduction of 3-methacryloxypropyl groups, and their surface energy was controlled to match with that of the pentacene layers by the introduction of phenyl groups. Consequently, the carrier mobility was improved by two orders of magnitude and reached 0.38 cm2 V−1 s−1.

Albumen functions well as a low-cost dielectric material for organic field-effect transistors.

Albumen functions well as a low-cost dielectric material for organic field-effect transistors.

Oxygen bridged neutral annulenes: a novel class of materials for organic field-effect transistors

New, neutral, meso-substituted tetraoxa[22]annulene[2,1,2,1] aromatic macrocycles are synthesised and disclosed as new p-type semiconductors with reproducible bulk-like carrier mobility (as high as 0.40 cm(2) V(-1) s(-1)) on highly crystalline thin films deposited on octadecyltrichlorosilane modified SiO(2).

Theoretical Approach for the Development of Organic Semiconductors on the Basis of the MO Symmetry: Thienoacene as an Example

ABSTRACTWe have explored materials for organic field-effect transistors (OFETs) from the viewpoint of theoretical calculations. The herringbone structure, which realizes two-dimensional conduction, is investigated in detail. Transfer integrals (t) are calculated systematically as a function of the dihedral angle between the molecular planes (θ) and the displacement along the molecular long axis (D). Acenes, oligothiophenes, thienoacenes and tetrathiafulvalenes are investigated, and are discussed from the molecular orbital (MO) symmetry. Thienoacenes (nTAs) are particularly examined as a candidate of OFET materials from the calculations of transfer integrals and reorganization energies (λ) based on the energy levels and the MO symmetry. LUMO of nTAs have MO symmetry suitable for conduction, but these orbitals are usually not related to the conduction. We have investigated the electronic properties of the derivatives with dicarboximide moiety. nTA-tetracarboxydiimide is expected to show the herringbone structure and exhibit n-type transport from the properties of LUMO.

A Promising Approach to Obtain Excellent n-Type Organic Field-Effect Transistors: Introducing Pyrazine Ring

The effects of the pyrazine ring on the geometrical and electronic structures, molecular stacking motifs, carrier injection, and transport properties as well as electronic band structures for some typical molecules with pyrazines (such as tetracene, pentacene, and π-extended tetrathiafulvalene derivatives) were theoretically investigated by quantum chemical methods. The introduction of pyrazine does not affect the molecular planarity and in the meantime largely decreases the energies of the highest occupied molecular orbitals and the lowest unoccupied molecular orbitals and hence improves their stability in air and ability of electron injection. More important, it is very helpful for prompting the molecular π-stacking. Small electron reorganization energies and large electronic coupling originated from their dense π-stacking give rise to their excellent electron transport properties, which makes them become a class of promising candidates for excellent n-type organic field-effect transistor (OFET) materials. So introducing pyrazine is an effective approach to obtain the excellent n-type OFET materials.

Aceno[2,1,3]thiadiazoles for Field-Effect Transistors: Synthesis and Crystal Packing

An efficient synthetic approach to a series of aceno[2,1,3]thiadiazole derivatives is described. 2-TIPS and 2-TES molecules exhibited different crystal packings, and 2-TIPS show good device performances with hole mobility up to 0.4 cm(2) V(-1) s(-1) and an average mobility of 0.15 cm(2) V(-1) s(-1) as the active material for organic field-effect transistors. All of the results demonstrate these aceno[2,1,3]thiadiazole derivatives as promising materials for optoelectronic devices.

Thiophene–benzothiadiazole–thiophene (D–A–D) based polymers: effect of donor/acceptor moieties adjacent to D–A–D segment on photophysical and photovoltaic properties

New push–pull copolymers based on thiophene (donor) and benzothiadiazole (acceptor) units, poly[4,7-bis(3-dodecylthiophene-2-yl) benzothiadiazole-co-thiophene] (PT3B1) and poly[4,7-bis(3-dodecylthiophene-2-yl) benzothiadiazole-co-benzothiadiazole] (PT2B2), are designed and synthesized via Stille and Suzuki coupling routes respectively. Gel permeation chromatography shows the number average molecular weights are 31 100 and 8400 g mol−1 for the two polymers, respectively. Both polymers have shown absorption throughout a wide range of the UV-vis region, from 300 to 650 nm. A significant red shift of the absorption edge is observed in thin films compared to solution of the copolymers; the optical band gap is in the range of 1.7 to 1.8 eV. Cyclic voltammetry indicates reversible oxidation and reduction processes with HOMO energy levels calculated to be in the range of 5.2 to 5.4 eV. Upon testing both materials for organic field-effect transistors (OFETs), PT3B1 showed a hole mobility of 6.1 × 10−4 cm2 V−1 s−1, while PT2B2 did not show any field effect transport. Both copolymers displayed a photovoltaic response when combined with a methanofullerene as an electron acceptor. The best performance was achieved when the copolymer PT3B1 was blended with [70]PCBM in a 1 : 4 ratio, exhibiting a short-circuit current of 7.27 mA cm−2, an open circuit voltage of 0.85 V, and a fill factor of 41% yielding a power conversion efficiency of 2.54% under simulated air mass (AM) 1.5 global (1.5 G) illumination conditions (100 mW cm−2). Similar devices utilizing PT2B2 in place of PT3B1 demonstrated reduced performance with a short-circuit current of 4.8 mA cm−2, an open circuit voltage of 0.73 V, and a fill factor of 30% resulting in a power conversion efficiency of roughly 1.06%.

Nanoparticles of organic conductors: synthesis and application as electrode material in organic field effect transistors

Stabilization of TTF·TCNQ nanoparticles is studied by varying the ionic liquid nature and solvent medium. The best dispersion is obtained in an acetonitrile/acetone mixture and the smaller size by using [BMIM][BF4], as a stabilizing ionic liquid. Applications of well-dispersed TTF·TCNQ nanoparticles (mean diameter of about 35 nm) as electrode material in organic field-effect transistors are also reported.

Controlled polymerizations for the synthesis of semiconducting conjugated polymers

Conjugated polymers have been under active development since the 1970s as the active material in organic field-effect transistors (OFETs), photovoltaic devices and the emissive layer in light-emitting diodes (LEDs). Extensive work has been performed to investigate the physics and chemistry of these materials, and a variety of semiconducting polymers have been synthesized using a range of polymerization techniques. One of the most important key technologies is to obtain a well-controlled polymerization, which provides polymers with narrow polydispersities and defined molecular weights. In this paper, we describe the recent progress on the synthesis of semiconducting polymers as classified as polyphenylenes, polyphenylenevinylenes, polythiophenes, polyfluorene and their block copolymers by the use of controlled polymerizations.