Layer In Organic Field-effect Transistors Research Articles

Isomerically Pure Anthra[2,3-b:6,7-b′]-difuran (anti-ADF), -dithiophene (anti-ADT), and -diselenophene (anti-ADS): Selective Synthesis, Electronic Structures, and Application to Organic Field-Effect Transistors

A new straightforward synthesis of isomerically pure anthra[2,3-b:6,7-b'] -difuran (anti-ADF), -dithiophene (anti-ADT), and -diselenophene (anti-ADS) from readily available 2,6-dimethoxyanthracene is described. The present successful synthesis makes it possible to overview the linear-shaped anti-acenedichalcogenophene compounds, that is, benzo[1,2-b:4,5-b']-, naphtho[2,3-b:6,7-b']-, and anthra[2,3-b:6,7-b']- difuran, -dithiophene, and -diselenophene. By comparing their electrochemical and photochemical properties, the electronic structures of acenedichalcogenophenes can be expressed as the outcome of balance between the central acene core and the outermost chalcogenophene rings. Among isomerically pure parent anti-anthradichalcogenophenes, anti-ADT and anti-ADS can afford crystalline thin films by vapor deposition, which acted as active layer in organic field-effect transistors with mobility as high as 0.3 cm(2) V(-1) s(-1) for ADT and 0.7 cm(2) V(-1) s(-1) for ADS. The mobility of isomerically pure anti-ADT is higher by several times than those reported for isomercally mixed ADT, implying that the isomeric purity could be beneficial for realizing the better FET mobility. We also tested the diphenyl derivatives of anti-ADF, -ADT, and -ADS as the active material for OFET devices, which showed high mobility of up to 1.3 cm(2) V(-1) s(-1).

ChemInform Abstract: Angular‐Shaped Naphthalene Tetracarboxylic Diimides for n‐Channel Organic Transistor Semiconductors.

A series of compounds based on the angular-shaped naphthalene tetracarboxylic diimide core have been synthesized, characterized and used as active layers of organic field-effect transistors (OFETs). The fabricated OFET devices exhibit n-type semiconducting characteristics, demonstrating the first examples of semiconductors based on angular-shaped naphthalene tetracarboxylic diimides.

Formation of inversion layers in organic field-effect transistors

An inversion current in unipolar organic field-effect transistors is not observed, which can be due to trapping of electrons or to negligible electron injection. Here, we distinguish between both cases by studying the depletion current of unipolar $p$-type transistors based on a deliberately doped organic semiconductor. For each doping level, the current can be completely pinched off, which unambiguously shows that no inversion layer is formed. Numerical calculations show that for electron injection barriers >1 eV, the transistor is thermodynamically not in equilibrium, such that a steady state is not reached in the time frame of the experiment.

Structure - Property Relationships Based on Phenyl-1H-Pyrrole End-Capped Thiophene Semiconductors

Thiophene oligomers end-capped with phenyl-1H-pyrrole were used as active layers of organic field-effect transistors. Two types of cores that vary in size and shape of α-linked thiophenes and fused-ring thiophenes were chosen to study the structure–property relationship. The crystal structures, optical, electrochemical and electronic properties were investigated, and the heterocyclic compounds exhibited clear p-type semiconducting behavior.

Direct patterning of poly(3-hexylthiophene) and its application to organic field-effect transistor

A direct photolithographic patterning method of regioregular poly(3-hexylthiophene) (rr-P3HT) as a semiconductive layer of organic field-effect transistors (OFETs) has been developed. The performance (0.092 cm2/Vs) of a bottom-contact type OFET using the patterned rr-P3HT as a semiconductive layer was almost the same as that (0.10 cm2/Vs) using the pristine rr-P3HT.

Angular-shaped naphthalene tetracarboxylic diimides for n-channel organic transistor semiconductors

A series of compounds based on the angular-shaped naphthalene tetracarboxylic diimide core have been synthesized, characterized and used as active layers of organic field-effect transistors (OFETs). The fabricated OFET devices exhibit n-type semiconducting characteristics, demonstrating the first examples of semiconductors based on angular-shaped naphthalene tetracarboxylic diimides.

Highly Dielectric and Photo-patternable Gate Insulators for Organic Field-effect Transistors

A highly dielectric and photo-patternable polymer material has been developed as a gate insulating layer in organic field-effect transistors (OFETs). The partially cyanomethylated poly(vinyl phenol) (PVPCM) was synthesized as a highly dielectric polymer matrix. The photosensitive PVPCM (PS-PVPCM) consisting of PVPCM (72 wt%), 4,4’-methylenebis[2,6-bis(hydroxymethyl)phenol] (25 wt%) as a cross-linker, and Irgacure PAG-103 (3 wt%) as a photoacid generator showed a good patterning performance such as high resolution of 6 μm, high sensitivity of 15.5 mJ/cm2, and good contrast of 3.3 when i-line lithography was employed. PVPCM and PS-PVPCM exhibited high dielectric constants (PVPCM = 4.51 and PS-PVPCM = 4.29). The OFET performance using poly(3-hexylthiophene) as a polymeric semiconductor and PS-PVPCM as a gate insulator showed the excellent charge mobility of 0.102 cm2/Vs and on/off ratio of 1.1 × 106.

Naphtho[2,3-b:6,7-b′]dichalcogenophenes: Syntheses, Characterizations, and Chalcogene Atom Effects on Organic Field-Effect Transistor and Organic Photovoltaic Devices

New linear-shaped naphtho[2,3-b:6,7-b′]-difurans (NDFs) and -selenophenes (NDSs) were synthesized selectively from 3,7-dibromo-2,6-dihydroxynaphthalene and evaluated as organic semiconductors in comparison to corresponding naphtho[2,3-b:6,7-b′]dithiophenes (NDTs). Evaluation of the electronic structures of the parent compounds by means of electrochemical and optical measurements clearly indicated that NDT and NDS are quite similar to each other, whereas only NDF has a marked different electronic structure. Thin film devices, including organic field-effect transistors (OFETs) and bilayer photovoltaics (OPVs) with C60 or C70 as an acceptor layer, were fabricated with the diphenyl derivatives using vacuum deposition. The thin films were found to be nicely crystalline with the edge-on molecular orientation both on Si/SiO2 (for OFETs) and ITO substrates (for OPVs). The thin films acted as active semiconducting layer in OFETs with mobility higher than 0.1 cm2 V–1 s–1 and as a donor layer in OPVs with power conversion efficiencies of up to 2.0%, indicating that the present naphthodichalcogenophenes are potential core structures for the development of new organic semiconductors.

Air-Stable and High-Mobility Organic Semiconductors Based on Heteroarenes for Field-Effect Transistors

Recent developments on heteroarene-based organic semiconductors applicable to high-performance, air-stable thin-film p-channel transistors are described. For the development of these new materials, including benzo[1,2-b:4,5-b']dichalcogenophenes (BDXs), [1]benzochalcogenopheno[3,2-b][1]benzochalcogenophenes (BXBXs), and dinaphtho[2,3-b:2',3'-f]chalcogenopheno[3,2-b]chalcogenophenes (DNXXs), new efficient synthetic methods are established. These materials are then evaluated as active layers in organic field-effect transistors (OFETs) fabricated by vacuum or solution processes. In the present work, molecular factors (molecular structures, energy levels and shapes of highest occupied molecular orbitals, molecular arrangements in the thin film) and the device performances are correlated and discussed to understand a structure-properties relationship. As a consequence of this approach, several air-stable and high-performance semiconductors for the OTFTs are successfully developed. For example, vapor-processable DNTT and solution-processable alkylated-BTBTs showing field-effect mobility as high as 3.0 cm V s and with 2.8 cm V s, respectively, are among the best for recently developed new materials. CONTENTS

Solution processable nanoparticles as high- k dielectric for organic field effect transistors

The need for low operating voltages in organic field effect transistors (OFETs) is a motivation for the development of low temperature solution processable synthesis of high dielectric constant materials. Specifically, the glycothermal synthesis of barium titanate nanoparticles (size 10–15 nm) is described here. The effect of reaction time, temperature and reactant ratio has been studied. An 85% yield of pure BaTiO 3 was obtained at a temperature of 220 °C and a reaction time of 16 h with the H 2O:propanol molar ratio fixed at 1:1. A suspension of these nanoparticles in ethanol was then used to fabricate the dielectric layers in OFETs. The roughness of the spun coated layers limited the performance of these transistors. The deposition of an overlayer of sol–gel silica on the nanoparticle films improved the charge transport properties within the pentacene organic thin films by up to two orders of magnitude. The combination of solution processable high dielectric constant BaTiO 3 nanoparticle layer and sol–gel silica is promising for low voltage organic electronics.

Novel Semiconducting Quinone for Air-Stable n-Type Organic Field-Effect Transistors

Quinones are promising moieties for n-type organic semiconductors due to their high electron affinity. Benzo[1,2-b:4,5-b']dithiophene-4,8-dione derivative with a quinone moiety have been synthesized, characterized, and used as active layer of organic field-effect transistors (OFETs). This derivative has deep LUMO level, leading to efficient charge-carrier injection and air stability. In addition, it forms a columnar structure with efficient intermolecular pi-pi and horizontal direction interactions, leading to high electron mobilities. In fact, OFET devices fabricated here showed good n-type characteristics, where the electron mobility was 0.15 cm(2) V(-1) s(-1) under vacuum conditions and above 0.1 cm(2) V(-1) s(-1) in air.

Three Structural Isomers of Dinaphthothieno[3,2-b]thiophenes: Elucidation of Physicochemical Properties, Crystal Structures, and Field-Effect Transistor Characteristics

Abstract In order to gain an insight into the relationship between the molecular structure and the semiconductor characteristics of highly π-extended heteroarene-based organic semiconductors, three structural isomers of dinaphthothieno[3,2-b]thiophenes with C2h symmetry were investigated. Of these, two isomers, dinaphtho[2,1-b:2′,1′-f ]thieno[3,2-b]thiophene (2) and dinaphtho[1,2-b:1′,2′-f ]thieno[3,2-b]thiophene (3), were newly synthesized, characterized, and utilized as active semiconducting layers in organic field-effect transistors (FETs). Detailed investigation of the physicochemical properties of 2 and 3, together with another isomer, dinaphtho[2,3-b:2′,3′-f ]thieno[3,2-b]thiophene (1), indicated that the electronic structures of the three isomers are fairly different from each other despite having the same molecular formula and the same aromatic constituents. Comparison of the molecular arrangements in the crystals elucidated by X-ray structural analysis implied that the molecular shape and the thus-induced favorable intermolecular interactions play important roles in determining the entire molecular arrangement. The characteristics of 2- and 3-based FETs with maximum field-effect mobilities (μFET s) of 10−3–10−2 cm2 V−1 s−1 were inferior to those of 1-based FETs with μFET s up to 3.0 cm2 V−1 s−1. The inferior characteristics of 2- and 3-based devices were due to film morphology as elucidated by atomic force microscopy (AFM) and supported by theoretical calculations of electronic structure in the solid state. Together, the results indicate that the molecular structure and shape, even for similar heteroarenes with the same molecular formula and symmetry, are important parameters to determine the solid-state properties of organic semiconductors.

Temperature dependent conductive behavior of pentacene in organic field-effect transistor

Impedance characterization at different temperature has been used to investigate the conductive behavior of pentacene as the semiconductor layer in organic field-effect transistor. It has been found that the pentacene exhibits an enhancement in conductivity by heating following an Arrhenius law with an activation energy transition from 0.004 to 0.018 eV at ∼323 K, which originates from band tail hopping that occurs around the Fermi edge.

Organic field effect transistors from ambient solution processed poly(triarylamine)–insulator blends

Two-component blends of amorphous poly(triarylamines) with selected amorphous and semi-crystalline polymeric binders are investigated as the semiconducting layer in organic field effect transistors (OFET). The blends can be deposited at room temperature from a good solvent and allow the fabrication of devices based on environmentally stable, low-cost organic semiconductors whose solution and solid state properties can be fine-tuned by careful binder selection making them suitable as the active layer in OFET based electronic nose arrays. A preliminary comparison of devices fabricated using drop casting and inkjet printing shows that the thin film morphology and the electronic properties of these blends are strongly dependent on the deposition method.

Anthraquinone derivatives affording n-type organic thin film transistors

New anthraquinone derivatives were prepared and used as active layers of organic field-effect transistors (OFETs); these devices showed good n-type characteristics and the quinones were found to be useful for organic semiconductors in OFETs.

High-performance solution-processed triisopropylsilylethynyl pentacene transistors and inverters fabricated by using the selective self-organization technique

To obtain selectively self-organized active layers of organic field-effect transistors (OFETs) and inverters from a solution-phased triisopropylsilylethynyl pentacene (TIPS-PEN) semiconductor, we locally patterned an oxide dielectric, covered with a hydrophobic fluoropolymer, using O2 plasma etching. Drop-cast TIPS-PEN molecules were selectively crystallized on the O2-plasma-etched area, where a hydrophilic oxide surface was produced. Modification of the patterned oxide dielectrics with hexamethyldisilazane led to a field-effect mobility of the TIPS-PEN OFETs of 0.185 cm2 V−1 s−1, a substhreshold swing of 0.738 V/decade and an on/off ratio of 107. Moreover, an inverter composed of two of these OFETs showed good device operation and inverter gain of 5.6.

Probing motion of electric dipoles and carriers in organic monolayers by Maxwell Displacement Current and optical second harmonic generation

Probing and modeling of surface polarization phenomena originating from dipoles and electrons in organic materials are studied. We use an electrical measurement that can directly probe orientational dipolar motion in monolayers by Maxwell Displacement Current, and show that a two-dimensional domain shape is dependent on electrostatic energy stored in the domain owing to the polar orientational alignment of permanent dipoles. Also, we demonstrate a novel optical second harmonic generation technique that allows carrier motion in active layers of organic field-effect transistors to be directly probed as the migration of electric field. We conclude that probing of electric field generated from moving charged particles, e.g., dipoles and electrons, at the interface is a common concept for studying dielectric polarization and its related two-dimensional interfacial phenomena in organic materials.

Synthesis, Structures, and Properties of Unsymmetrical Heteroacenes Containing Both Pyrrole and Furan Rings

Unsymmetrical heteroacenes, 11-phenylbenzofuro[3,2-b]carbazole (Ph-BFC) and its alkoxylated derivatives, were readily synthesized by palladium-catalyzed double N-arylation of arylamines. They characteristically form antiparallel cofacial pi-stacking arrangements, which may result from their unsymmetrical structures. Their physical properties show their potential for application as active layers in organic field-effect transistors.

Oriented growth of rubrene thin films on aligned pentacene buffer layer and its anisotropic thin-film transistor characteristics

Oriented growth of polycrystalline rubrene thin film on oriented pentacene buffer layer was investigated. The oriented pentacene buffer layer was created by thermal evaporation of pentacene on a rubbed polyvinylalcohol (PVA) surface. The pentacene layer in turn induced the oriented growth of rubrene crystals upon thermal deposition. The structures of successive layers were characterized by using grazing incidence X-ray diffraction (GIXD) and atomic force microscopy. Highly oriented rubrene crystallites with the a-axis aligning along the surface normal and the (002) plane preferentially oriented 45° away from the rubbing direction were found. In contrast, the rubrene thin film deposited on PVA or rubbed-PVA substrate without a pentacene buffer layer only gave amorphous phases. With the aligned pentacene/rubrene film as the active layer of organic field-effect transistor, anisotropic mobilities were observed. The highest field-effect mobility (0.105cm2/Vs) was observed along the direction 45° away from the rubbing direction and is ∼4 times higher than that for similar device prepared on unrubbed PVA. The direction was consistent with the GIXD observation that a large number of rubrene crystallites are having their [002] direction aligning in this direction. A favourable C–H⋯π interaction between an oriented pentacene layer and the rubrene layer on the control of molecular orientation in the conduction channel of the OFET is suggested.

Characterization of Vapor-Deposited Acenaphtho[1,2-k]fluoranthene Derivative Films as Active Layers of Organic Field-Effect Transistors

We have developed vapor-deposited films of acenaphtho[1,2-k] fluoranthene (AF) and their 7,14-dicarbalkoxy derivatives as active layers of organic field-effect transistors (OFETs). The AF- and 7,14-dicarbmethoxyacenaphtho[1,2-k] fluoranthene (DMAF)-based OFET devices did not work sufficiently. On the other hand, the 7,14-carbethoxyacenaphtho[1,2-k] fluoranthene (DEAF)-based OFET device showed typical p-channel FET responses with a field-effect mobility of 1.7×10−3 cm2 V−1 s−1 and a current on/off ratio of about 102. According to the X-ray diffraction measurements, the carbethoxy groups could largely improve the molecular packing that is favorable for high-performance OFETs.