Materials For Organic Field-effect Transistors Research Articles

Benzodicarbomethoxytetrathiafulvalene Derivatives as Soluble Organic Semiconductors

A series of new tetrathiafulvalene (TTF) derivatives bearing dimethoxycarbonyl and phenyl or phthalimidyl groups fused to the TTF core (6 and 15-18) has been synthesized as potential soluble semiconductor materials for organic field-effect transistors (OFETs). The electron-withdrawing substituents lower the energy of the HOMO and LUMO levels and increase the solubility and stability of the semiconducting material. Crystal structures of all new TTF derivatives are also described, and theoretical DFT calculations were carried out to study the potential of the crystals to be used in OFET. In the experimental study, the best performing device exhibited a hole mobility up to 7.5 × 10(-3) cm(2) V(-1) s(-1)).

Synthesis and Carrier Transport Properties of Triarylamine-based Amorphous Polymers for Organic Field-effect Transistors

Abstract Twenty-three kinds of triarylamine-based polymers, which are a new class of p-type semiconducting materials for organic field-effect transistors (OFETs), were synthesized by Pd-catalyzed amination of the corresponding aryldihalide and arylamine. All triarylamine-based polymers in this study exhibited p-channel behavior in OFETs, and field-effect hole mobilities ranged from 10−6 to 10−3 cm2 V−1 s−1.

Quinoid heteropentacenes as promising organic semiconductors for field-effect transistor applications

We report on a quinoid heteropentacene as p-type semiconductor in organic field-effect transistors. Both single crystal and thin-film transistors were fabricated with 7,14-diphenyl-chromeno[2,3-b]xanthene (DPCX). In this small molecule organic semiconductor the field-effect mobility is as high as 0.16 cm2/Vs in single-crystal devices and 0.01 cm2/Vs in thin-film devices. In addition, the devices show favorable properties such as near zero onset/threshold voltages and a small current hysteresis. X-ray diffraction experiments show the molecules to be arranged in slipped stacks and to have a flat backbone in the crystals. For thin films of DPCX the situation is complicated by the coexistence of a thin-film phase with the bulk phase. However, a comparison of DPCX thin films on octadecyltrichlorosilane (OTS)-treated and bare SiO2 gate dielectrics provides clear evidence that the OTS surface treatment leads to organic thin films with a better structural order. The low-cost synthesis and purification of DPCX along with the improved processability and the good electrical characteristics suggest that quinoid heteropentacenes are promising materials for organic field-effect transistors.

Downscaling of Organic Field‐Effect Transistors with a Polyelectrolyte Gate Insulator

A polyelectrolyte is used as gate insulator material in organic field-effect transistors with self-aligned inkjet printed sub–micrometer channels. The small separation of the charges in the electri ...

2,6-Bis(tributyltin)benzo[1,2-b:5,4-b’]dithiophene: a new synthon for organic semiconductors

A new tin-functionalized benzo[1,2-b:5,4-b’]dithiophene building block was synthesized that can provide access to a series of further extended π−conjugated materials for Organic Field-Effect Transistors (OFETs) via Stille coupling protocol, as exemplified in two cases. The synconfigured benzodithiophene scaffold complements the well-established (anti) benzo[1,2-b:4,5b’]dithiophene unit that has already been successfully employed in OFET devices. DFT-quantum theoretical calculations on a series of organic semiconductors support the selective modification of the band gap by either changing the HOMO, the LUMO, or both. The suitability of the tinfunctionalized synthon to efficiently access a variety of organic semiconductor materials with different properties was also confirmed by the study of their photophysical features.

Hexyl‐Substituted Oligoselenophenes with Central Tetrafluorophenylene Units: Synthesis, Characterisation and Application in Organic Field Effect Transistors

A series of selenophene oligomers incorporating conjugated fluorinated phenylene units have been synthesised as potential semiconductor materials for organic field-effect transistors (OFETs). X-ray crystallography shows that the molecules are held in close proximity by several short intermolecular contacts, making them ideal candidates for OFET applications.

Star-shaped conjugated systems derived from dithiafulvenyl-derivatized triphenylamines as active materials for organic solar cells

Hybrid conjugated systems consisting of a triphenylamine core substituted by three dithiafulvenyl moieties have been synthesized and tested as active materials in organic field-effect transistors and photovoltaic devices. UV–vis. spectroscopy studies demonstrate that, despite their amorphous character shown by X-ray diffraction and differential scanning calorimetry, strong interactions exist in these materials as an aggregative band is observed at low energies. A first evaluation of their potentialities as p semiconductor in organic field-effect transistors shows significant hole mobilities. For bilayer photovoltaic devices, a power conversion efficiency of 0.11% is observed and the external quantum efficiency of the cell under monochromatic irradiation shows a non-negligible contribution of the aggregative band.

Organic field-effect transistors based on tetrathiafulvalene derivatives

Abstract In recent years, tetrathiafulvalene (TTF) and its derivatives have been used as semiconducting materials for organic field-effect transistors (OFETs). In this review, we summarize the recent progress in the field of TTF-based OFETs. We introduce the structure and operation of OFETs, and focus on TTF derivatives used in OFETs. TTF derivatives used in OFETs can be divided into three parts by the semiconductor's morphology and the device fabrication technique: (1) TTF derivatives used for single-crystal OFETs, (2) TTF derivatives used for vacuum-deposited thin-film OFETs, and (3) TTF derivatives used for solution-processed thin-film OFETs. The single-crystal OFETs based on TTF derivatives were fabricated by drop-casting method and showed high performance, with the mobility up to 1.4 cm2/Vs. The vacuum-deposited thin-film OFETs based on TTF derivatives were well developed, some of which have shown high performance comparable to that of amorphous silicon, with good air-stability. Although the mobilities of most solution-processed OFETs based on TTF derivatives are limited at 10-2 cm2/Vs, the study on solution-processable TTF derivatives and their devices are promising, because of their low-cost, large-area-coverage virtues. The use of organic charge-transfer (OCT) compounds containing TTF or its derivatives in OFETs is also included in this review.

Tuning Orbital Energetics in Arylene Diimide Semiconductors. Materials Design for Ambient Stability of n-Type Charge Transport

Structural and electronic criteria for ambient stability in n-type organic materials for organic field-effect transistors (OFETs) are investigated by systematically varying LUMO energetics and molecular substituents of arylene diimide-based materials. Six OFETs on n+-Si/SiO2 substrates exhibit OFET response parameters as follows: N,N'-bis(n-octyl)perylene-3,4:9,10-bis(dicarboximide) (PDI-8): mu = 0.32 cm2 V(-1) s(-1), Vth = 55 V, I(on)/I(off) = 10(5); N,N'-bis(n-octyl)-1,7- and N,N'-bis(n-octyl)-1,6-dibromoperylene-3,4:9,10-bis(dicarboximide) (PDI-8Br2): mu = 3 x 10(-5) cm2 V(-1) s(-1), Vth = 62 V, I(on)/I(off) = 10(3); N,N'-bis(n-octyl)-1,6,7,12-tetrachloroperylene-3,4:9,10-bis(dicarboximide) (PDI-8Cl4): mu = 4 x 10(-3) cm2 V(-1) (s-1), Vth = 37 V, I(on)/I(off) = 10(4); N,N'-bis(n-octyl)-2-cyanonaphthalene-1,4,5,8-bis(dicarboximide) (NDI-8CN): mu = 4.7 x 10(-3) cm2 V(-1) s(-1), Vth = 28, I(on)/I(off) = 10(5); N,N'-bis(n-octyl)-1,7- and N,N'-bis(n-octyl)-1,6-dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDI-8CN2): mu = 0.13 cm2 V(-1) s(-1), Vth = -14 V, I(on)/I(off) = 10(3); and N,N'-bis(n-octyl)-2,6-dicyanonaphthalene-1,4,5,8-bis(dicarboximide) (NDI-8CN2): mu = 0.15 cm2 V(-1) s(-1), Vth = -37 V, I(on)/I(off) = 10(2). Analysis of the molecular geometries and energetics in these materials reveals a correlation between electron mobility and substituent-induced arylene core distortion, while Vth and I(off) are generally affected by LUMO energetics. Our findings also indicate that resistance to ambient charge carrier trapping observed in films of N-(n-octyl)arylene diimides occurs at a molecular reduction potential more positive than approximately -0.1 V (vs SCE). OFET threshold voltage shifts between vacuum and ambient atmosphere operation suggest that, at E(red1) < -0.1 V, the interfacial trap density increases by greater than approximately 1 x 10(13) cm(-2), while, for semiconductors with E(red1) > -0.1 V, the trap density increase is negligible. OFETs fabricated with the present n-type materials having E(red1) > -0.1 V operate at conventional gate biases with minimal hysteresis in air. This reduction potential corresponds to an overpotential for the reaction of the charge carriers with O2 of approximately 0.6 V. N,N'-1H,1H-Perfluorobutyl derivatives of the perylene-based semiconductors were also synthesized and used to fabricate OFETs, resulting in air-stable devices for all fluorocarbon-substituted materials, despite generally having E(red1) < -0.1 V. This behavior is consistent with a fluorocarbon-based O2 barrier mechanism. OFET cycling measurements in air for dicyanated vs fluorinated materials demonstrate that energetic stabilization of the charge carriers results in greater device longevity in comparison to the OFET degradation observed in air-stable semiconductors with fluorocarbon barriers.

Synthesis, characterization, and reactions of 6,13-disubstituted 2,3,9,10-tetrakis(trimethylsilyl)pentacene derivatives

Pentacene has been actively studied as relevant materials in organic field-effect transistors (OFETs) and organic light-emitting diodes (OLEDs). However, the low solubility and low stability of pentacene in common organic solvents have hindered its applications. When exposed to light or at high concentration, pentacene is found to dimerize easily. Many research groups are currently working on the design and synthesis of novel substituted pentacenes, but few of them systematically reported physical properties such as molecular spectroscopy and electronic properties, which might elucidate the influence of substituents on HOMO–LUMO gaps. Furthermore, the reactive nature of the central ring in pentacenes makes pentacenes good dienes for Diels–Alder reactions. In this paper, a series of soluble 6,13-disubstituted 2,3,9,10-tetrakis(trimethylsilyl)pentacenes were synthesized and characterized. Their reactions, structures, and physical properties were also studied. In addition, bulky o-carboranyl substituted pentacene derivative 15 and 6-chloro-2,3,9,10-tetrakis(trimethylsilyl)pentacene ( 16) were synthesized for the first time. Compound 16 possesses the largest dihedral angle (7.7° with two adjacent benzene rings) and shows a wave structure. Diels–Alder reactions with acceptable efficiency were carried out between 16 and various dienophiles.

Development of New Semiconducting Materials for Durable High-performance Air-stable Organic Field-effect Transistors

Abstract New thiophene- and selenophene-fused aromatic compounds were successfully developed as high-performance semiconducting materials for organic field-effect transistors (OFETs). Among them, the most advanced compounds designed on consideration of structure–function relationship showed durable air-stable FET performance. This article highlights the research strategy and development of these superior compounds as well as the molecular factors for the improvement of mobility, sensitivity, and stability, which are very informative for the design of practical semiconductors.

Polymer films filled with organic molecules by CO 2 laser evaporation in vacuum

A new p-type of material for organic field-effect transistors (OFETs) is 5,5′-di(9,9′-di-(butyl)-fluorene′)-2,2′-bithiophene (DBFBT). DBFBT films have been prepared by casting, by thermal evaporation in vacuum, and by means of CO 2 laser evaporation. The photon energy from a CO 2 laser is too small to break chemical bonds of the DBFBT molecule, but the heating rate is extremely high. Pressed pellets of powders composed of DBFBT and polychlorotrifluoroethylene (PCTFE) were used as targets for the fabrication of films. These targets were irradiated by a laser beam of 40 W cw-power and 0.5 cm diameter. The deposition rate was 30–50 nm/s. The films have been characterised by optical spectroscopy and by AFM. The structure of DBFBT films produced by casting from solution and thermal deposition is composed of elongated crystals, while the laser deposited layers show cluster structure. By FTIR the films show the absorption band of pure DBFBT, with intensity proportional to the DBFBT concentration in the PCTFE matrix. The absorption bands for a pure DBFBT film, for a film with 22 vol.% and with 34 vol.% of DBFBT in the PCTFE matrix are situated at 274, 298 and 402 nm, where the latter is the most intense one. No significant difference in the band positions of all samples has been found. We concluded that no interaction between chromophore and matrix occurs. The DBFBT molecules in the PCTFE matrix did not oxidize during storage under ambient condition over a period of a year.

Progresses in organic field-effect transistors and molecular electronics

In the past years, organic semiconductors have been extensively investigated as electronic materials for organic field-effect transistors (OFETs). In this review, we briefly summarize the current status of organic field-effect transistors including materials design, device physics, molecular electronics and the applications of carbon nanotubes in molecular electronics. Future prospects and investigations required to improve the OFET performance are also involved. © 2006 Higher Education Press.

IMPEDANCE ANALYSIS FOR CHARACTERIZATION OF MATERIALS USED IN ORGANIC TRANSISTORS

A method of impedance frequency response analysis (FRA) with template multilayer organic capacitor was studied to evaluate printable materials for all-printed organic field-effect transistors (OFETs). The new method allows us not only to characterize simultaneously dielectric and conductive behaviors of the materials but also to distinguish contributions to electrical conduction or to polarization from different sources such as dielectric layer, semiconductor layer, and different interfaces for OFETs. As a dielectric layer for OFETs, Urathan is preferred because of lower conductivity, nonmetallic conduction behavior at high temperature, and lower interfacial resistance. Conductivity and dielectric enhancement of Urathan can be realized by heating. The critical temperature for conductivity and dielectricity transition was calculated from FRA from temperature dependent experiments. The method could be a standard for fast screening and assessing printable materials for OFETs.

Star-Shaped Oligothiophenes for Solution-Processible Organic Electronics: Flexible Aliphatic Spacers Approach

The design, synthesis, and properties of a novel, solution-processible organic semiconductor based on star-shaped oligothiophenes linked through flexible aliphatic branches to a carbosilane core are described. This four-arm dendrimer-like molecule not only forms smooth thin films on large surfaces but also shows improved monolayer formation compared to its linear prototype, α,α‘-dihexylquaterthiophene (Hex-4T-Hex). For the new material, three different types of structural organization (nanowires and mono- and multilayers) were found, all of which are favorable for charge transport through the material in organic field-effect transistors. These processability and structural properties, in combination with high field-effect mobility, high current modulation, and a threshold voltage of ∼0 V, suggest that star-shaped oligothiophenes with carbosilane cores are a promising new class of semiconducting materials for use in flexible organic electronics.

Thiophene and Selenophene Copolymers Incorporating Fluorinated Phenylene Units in the Main Chain: Synthesis, Characterization, and Application in Organic Field-Effect Transistors

A series of thiophene oligomers bearing core phenylene and fluorinated phenylene units has been synthesized as potential semiconductor materials for organic field-effect transistors (OFETs). Polymerization of these compounds has been achieved using Stille and oxidative Coupling methods. Functionalization of the phenylene unit with fluorine atoms has a marked effect on the self-assembly and electronic properties of the parent materials: the optical band gaps and highest occupied molecular orbital levels are affected with the introduction Of fluorine atoms as a result of a combination of inductive effects and rigidification of the main chain. The design of these materials has focused on the self-assembly and solution processability of the materials. All the polymers are readily Soluble in common organic solvents. Self-assembly and planarization of the fluorinated materials in the solid state are identified by a combination of X-ray diffraction studies, absorption spectroscopy, and cyclic voltammetry. The organizational behavior of the films is in contrast to the conformational freedom observed in solution (absorption spectroscopy) and in the gas phase (computational Studies). Thin-film OFETs have been fabricated for the entire polymer series. Hole mobilities have been measured Lip to 10(-3) cm(2)/(V(.)s), with high current modulation (on/off ratios up to 10(5)) and low turn-on voltages (down to 2 V). For the Stille Coupled polymers, replacement of the bridging thiophene unit with selenophene generally increases the hole mobility of the polymers.

IMPEDANCE CHARACTERIZATION OF DIELECTRIC AND SEMICONDUCTING MATERIALS WITH ORGANIC CAPACITORS FOR ORGANIC TRANSISTORS

A new characterization method based on impedance frequency response analysis at different temperatures has been developed to assess and identify the dielectric and semiconductor materials for organic field-effect transistors (OFET). This method can not only characterize simultaneously dielectric and conductive behaviors of materials but also distinguish individual contributions to electrical conduction or to polarization from layers such as dielectric layer, semiconductor layer, and interfaces in OFET. Two kinds of materials, Urathan and DuPont 5018A as dielectric materials have been used to make a multilayer organic capacitor. It has been observed that Urathan, due to its lower conductivity, non-metallic conduction behavior at high temperature, and lower interfacial resistance, is more suitable as dielectric layer for OFET. Urathan appears an enhancement in conductivity by heating following an Arrhenius law with an activation energy transition from 0.002 to 0.24 eV at ~307 K, which originates from band tail hopping that occurs around the Fermi edge. At ~314 K, a dielectric transition also occurs, which is interpreted as a combination of electron polarization associated to the band tail hopping. The materials were used to fabricate OEFT, which performance was in agreement with that obtained from impedance analysis of the organic capacitor.

Comparative Study of the Field-Effect Mobility of a Copolymer and a Binary Blend Based on Poly(3-alkylthiophene)s

The performance of highly soluble regioregular poly[(3-hexylthiophene)-co-(3-octylthiophene)] (P3HTOT) as a semiconducting material in organic field-effect transistors (OFETs) is presented in comparison to that of the corresponding homopolymers. Transistors made from as-prepared layers of P3HTOT exhibit a mobility of ca. 7 × 10-3 cm2 V-1s-1, which is comparable to the performance of transistors made from as-prepared poly(3-hexylthiophene) (P3HT) and almost 6 times larger than the mobility of transistors prepared with poly(3-octylthiophene) (P3OT). On the other hand, the solubility parameter δp of P3HTOT is close to that of the highly soluble P3OT. Moreover, compared to a physical blend of poly(3-hexylthiophene) and poly(3-octylthiophene), the mobility of P3HTOT devices is almost twice as large and the performance does not degrade upon annealing at elevated temperatures. Therefore, the copolymer approach outlined here may be one promising step toward an optimum balance between a sufficient processability o...

Novel Dielectric Materials for Organic Electronics

AbstractWe present our latest results on the design and fabrication of very high capacitance dielectric materials for organic field-effect transistors. We will show that utilization of appropriate self-assembling siloxane building blocks and polymer matrices allows solution-processed, pinhole-free organic dielectrics. Electrical (MIS, TFT) data demonstrate that these insulators can be efficiently integrated into large TFT structures. These devices function for both p- and n-channel semiconductors, the molecular components of which exhibit greatly different core structures and substituent functionalities. Substantial TFT response is achieved at very low operational biases (&lt;1V), without serious leakage currents (&lt;10-8 A/cm at 1V).

Scanning probe investigations of morphology of new p-type material for organic field-effect transistors

Organic electronics is a fast growing fi eld of research and technological developments. First applications based on organic electronics have been available on the market for a couple of years, so high resolution colour displays based on organic light-emitting devices are used in digital cameras or mobile phones. Efforts are still going on to find better materials and for a better understanding structure property relationships of organic semiconductors. Although exemplary applications have been realized from polymers and amorphous layers of evaporable molecular materials, crystalline organic semiconductors receive intense attention because the charge-carrier mobilities in these materials are much higher than in polymers or amorphous layers. After optimizing the electronic properties of organic devices (e.g. organic field-effect transistors or organic light emitting diodes) and demonstration of fundamental devices, attention has turned to morphology and composition of the films. Understanding and controlling the morphology of organic films will have a signifi cant impact on the performance of organic-based devices, e.g. by minimizing the number of grain boundaries in the active region of an organic fieldeffect transistor (OFET)