Organic Thin Film Transistor Applications Research Articles

Preparation of Aluminum Oxide Layer for Gate Insulator Application in Organic Thin-Film Transistors

The characteristics of organic thin-films transistors (OTFTs) with the anodized aluminum oxide insulator have been investigated. The OTFT with the barrier-type aluminum oxide insulator exhibited the mobility of 0.09 cm2/Vs, the subthreshold slope of 1.3 V/decade, the threshold voltage of −2.2 V, and the on/off ratio of 7.7 × 104, which are superior to those for the device with the porous-type insulator. The smooth surface of the barrier-type film is found to contribute to a long range hopping of charge carriers in the conducting channel by decreasing the activation energy for the conduction of charge carriers.

An Amorphous Polythiophene as a Binder Material for Organic Thin-Film Transistor Channel Applications

Blending of crystalline low molecular-weight organic semiconductors, e.g., 6,13-bis (triisopropylsilylethynyl)pentacene (TIPS-Pentacene), and binder polymers has recently been known to be an effective way to achieve high performance organic thin-film transistors (OTFTs). However, only limited number of examples has been reported as binder materials for OTFT applications, e.g., poly(triarylamine), poly(α-methylstyrene), and etc. In this work, an amorphous polythiophene derivative, poly(3-(2-ethylhexyl)thiophene) (P3EHT), was prepared by introducing 2-ethylhexyl group as a side-chain to the polythiophene. The P3EHT was synthesized by Grignard metathesis (GRIM) polymerization to give a molecular weight (Mn) of 4,100 (PDI = 3.4), and it was readily soluble in common organic solvents, such as chloroform, tetrahydrofuran, toluene, and etc. An amorphous nature of the P3EHT was confirmed from differential scanning calorimetry (DSC) and ultraviolet-visible spectroscopy results. A mobility of 2.5 × 10−4 cm2/Vs, an on/off ratio of 0.57 × 103 and a threshold voltage of 1.6 V have been obtained with OTFTs using a P3EHT/TIPS-Pentacene blend.

Azine- and Azole-Functionalized Oligo´ and Polythiophene Semiconductors for Organic Thin-Film Transistors

In the organic electronics research field, several strategies have been used to modulate the transport properties of thiophene-derived semiconductors via sequential functionalization of their π-conjugated cores. This review summarizes the major design and synthetic strategies for tuning thiophene-containing small molecule and polymer properties by introducing electron-deficient nitrogen-containing azine and azole moieties. Several examples are presented which elucidate the structural, optical, and electronic consequences of incorporating these electron-deficient fragments in the conjugated skeletons, particularly relating to applications in organic thin-film transistors.

Threshold voltage adjustment of organic thin film transistor by introducing a polysilicon floating gate

The structure of organic thin film transistors (OTFTs) is optimized by introducing a floating gate into the gate dielectric to reduce the threshold voltage of OTFTs. Then the optimized device is simulated, and the simulation results show that the threshold voltage of optimized device is reduced by about 10 V. The reduction of the threshold voltage is helpful and useful for the application of OTFTs in many areas. In addition, this way of reducing the threshold voltage of OTFT is compatible with traditional silicon technology and can be used in manufacturing.

Thiophene-rich fused-aromatic thienopyrazine acceptor for donor–acceptor low band-gap polymers for OTFT and polymer solar cell applications

Thiophene enriched fused-aromatic thieno[3,4-b]pyrazine systems were designed and employed to produce low band gap polymers (Eg = 1.0–1.4 eV) when copolymerized with fluorene and cyclopentadithiophene. The copolymers are mainly investigated for organic thin film transistor and organic photovoltaic applications. Molecular packing in the thin films of these polymers was investigated using Grazing incidence X-ray Scattering. Although both fluorene and cyclopentadithiophene polymers follow similar face to face π–π stacking, the latter polymers show much smaller lamellar d-spacings due to side-chain interdigitation between the lamellae. This lead to the higher charge carrier mobilities in cyclopentadithiophene polymers (up to 0.044 cm2/V.s) compared to fluorene polymers (up to 8.1 × 10−3 cm2/V.s). Power conversion efficiency of 1.4% was achieved using fluorene copolymer in solar cells with a fullerene derivative as an acceptor. Although the cyclopentadithiophene polymers show lower band gaps with higher absorption coefficients compared to fluorene copolymers, but the power conversion efficiencies in solar cells of these polymers are low due to their low ionization potentials.

Tuning surface properties in photosensitive polyimide. Material design for high performance organic thin-film transistors

A photosensitive polyimide (PSPI) is synthesized to serve as the gate dielectric in organic thin-film transistors (OTFTs). A method to control the charge density within the channel of the OTFT using the PSPI on a silicon dioxide gate dielectric is proposed. The surface energy, surface carriers, and capacitance of gate dielectrics can be tuned by varying irradiation doses of UV light on the PSPI surface to create ultrahigh performance pentacene-based OTFTs with an average field-effect mobility (μ) of above 6.0 cm2/Vs. The correlation between the surface energy of the PSPI films and the μ value of OTFT devices is investigated. Moreover, the impact of gate-dielectric properties on the structural properties of pentacene films is closely related to the electrical characteristics of OTFTs. The results of contact angle, X-ray diffraction, electrostatic force microscopy, and micro-Raman spectroscopy measurements indicate that the surface energy and doping carriers coming from the PSPI film play important roles in the carrier transport of pentacene film for OTFT applications.

Toward strain resistant flexible organic thin film transistors

We suggest a molecular design for small molecules as polycrystalline organic semiconductors in flexible organic thin film transistor applications, providing an improved stability during substrate bending. A stable operation of alkyl-substituted sexithiophens was obtained under strain up to ±2.5%. The stability relates to the flexibility of the alkyl chain substitution, scales with their chain length, and outperforms those of pentacene devices. These findings were independent from the underlying dielectric layer.

Comparative Study of MDMO-PPV Thin-Film Transistors Using Thermal SiO2 and PECVD Silicon Oxynitride as Insulator

We demonstrate that PECVD SiOxNy with good dielectric properties can replace SiO2 in Organic Thin-Film Transistors (OTFT) applications. It can be used on ITO-covered glass or even flexible substrates thanks to the low deposition temperatures. Poly [2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) is used as the active layer, due to its wide range of applications such as solar cells, light-emitting diodes and light-emitting transistors. We show also that charge carrier mobility can be at least two times higher for silicon oxynitride on p+-Si substrate than silicon dioxide, ≈ 1.1×10-4 cm2.V-1.s-1, which could be further improved by surface treatments.

Synthesis of Acenaphthyl and Phenanthrene Based Fused-Aromatic Thienopyrazine Co-Polymers for Photovoltaic and Thin Film Transistor Applications

Dithiophene and fluorene co-polymers containing fused aromatic thieno[3,4-b]pyrazine moieties were synthesized for organic thin film transistor (OTFT) and organic photovoltaic (OPV) applications. Suzuki and Stille polycondensation reactions were used for the polymerization. The band gap (Eg) of the polymers was tuned in the range of 1.15−1.6 eV to match the solar spectrum. Density functional theory calculations were carried out to rationalize the low band gaps. These polymers showed field effect mobility (μ) as high as 0.2 cm2/(V·s) with an on/off ratio as high as 106 in OTFT devices. Interestingly, one polymer in this class also showed ambipolar charge transport. Power conversion efficiency (PCE) up to 1.3% was achieved in bulk heterojunction solar cells, indicating that these materials are promising for OPV applications.

Synthesis and characterization of a series of bis(dimethyl- n-octylsilyl)oligothiophenes for organic thin film transistor applications

A series of bis-dimethyl- n-octylsilyl end-capped oligothiophenes consisting of two to six thiophene units has been synthesized and characterized to develop novel organic semiconductor materials. The UV–vis spectral data indicate that these silyl end-capped oligothiophenes have longer conjugation lengths as evidenced by the higher λ max values than the corresponding unsubstituted thiophene oligomers. The thermal analyses indicate that the bis-silylated oligothiophenes show lower melting point ( DSi-4T = 80 °C; DSi-5T = 115 °C; DSi-6T = 182 °C) than the corresponding dialkylated thiophene oligomers by 100 °C and hexamer DSi-6T exhibits a liquid crystalline mesophase at 143 °C. The α,ω-bis(dimethyl- n-octylsilyl)oligothiophenes ( DSi-6T) have a remarkably high solubility in chloroform which are comparable to the corresponding α,ω-dihexyloligothiophenes. The remarkably increased solubility by these silyl end groups leads bis-silylated oligothiophenes to be applicable to solution processable devices for thin film transisitor (TFT) by utilizing a spin-coating technique. α,ω-Bis(dimethyl- n-octylsilyl)sexithiophene can be deposited as active semiconducting layer in thin film transistors, either by vacuum evaporation or by spin-coating. A high charge-carrier mobility has been obtained for both deposition techniques, μ = 4.6 × 10 −2 and 1.4 × 10 −2 cm 2 V −1 s −1, respectively.

Fabrication of High Performance Organic Thin Film Transistor Arrays and Application to 5-inch Flexible Displays

AbstractWe have demonstrated a 5-inch flexible color liquid crystal display (LCD) and organic light emitting display (OLED) driven by low-voltage operation organic TFT. In order to achieve high-quality and high-resolution moving images, OTFTs with high performances such as a high mobility, high ON/OFF ratio, low sub-threshold slope (SS) and low operating voltage, are developed. We fabricated pentacene-based low-voltage operation OTFT with a Ta2O5 gate dielectric prepared at a low temperature process. The resulting OTFT array showed a high mobility of 0.3-0.4 cm2/Vs, ON/OFF ratio over 107, VTH=2.7V, and low SS=0.3 V/decade. OTFTs with solution-processable materials such as fluoropolymer gate dielectric and liquid-crystalline semiconducting polymers, PBTTT, were also investigated. Electrical characteristics and stabilities of these devices will be discussed. In the final section, we will demonstrate OTFT-driven flexible displays. Both of the flexible LC device and the OLED device were successfully integrated on the pentacene-based OTFT arrays. Printing and lamination techniques were introduced to assemble the flexible LC device. Phosphorescent polymer materials, which can be patterned by ink-jet printing, were used for emitting layer of OLED. Color moving images were successively shown on the resulting 5-inch displays using an active-matrix driving technique of the OTFT at a low driving voltage of 15V.

Attractive Tetraceno[2,3-b]thiophene Derivatives for Organic Transistor Applications: A Theoretical Analysis

We propose new semiconductor candidates based on recently synthesized tetraceno[2,3-b]thiophene for p-type organic transistor applications. The proposed model semiconductor organics are 5,12-bis(trimethylsilylethynyl)-tetraceno[2,3-b]thiophene and 5,12-bis(triethylsilylethynyl)-tetraceno-[2,3-b]thiophene. The suitability of these proposed model organics as p-type semiconductor materials for the organic thin-film transistor applications were tested theoretically by studying their electronic parameters that are essential for ideal semiconductor candidates. For this purpose, density functional theory calculations were made to calculate their band gaps, excitation energies, and hole-reorganization energies. Eleven other p-type semiconductor organics including the well-known, and popular alkynyl pentacenes and anthradithiophenes were also studied. The results derived in the present study clearly indicate that these proposed organics show better oxidative stability and have band gaps lower than those of many of...

A new approach to the surface modification of polymeric gate insulators for organic thin-film transistor applications

Surface modification of the gate insulators is a key factor in improving the performance of organic thin-film transistors (OTFTs). We have developed a novel method for modifying the surface of a gate-insulating polymer containing an unsaturated double bond. A polyimide (PI) gate insulator containing an unsaturated double bond as a side chain was synthesized using the monomers pyromellitic dianhydride (PMDA), 4,4′-methylenedibenzenamine (MDA), and ( E)-3,5-diaminobenzyl-3-(biphenyl-4-yl)acrylate (BIP). Octadecanethiol (ODT) was successfully reacted with the unsaturated double bond of this PI gate insulator under solvent-free conditions, thus modifying its surface and making it more hydrophobic. The modified hydrophobic surface strongly affected the initial growth mechanism of pentacene and, subsequently, the performance of the corresponding OTFTs. The devices fabricated with the modified PI gate insulators showed superior performance compared to those prepared with nonmodified insulators and exhibited a carrier mobility of 0.41 cm 2/(V s), an on/off current ratio of about 8.6 × 10 5, and a subthreshold slope of 2.31 V/dec.

Plasma‐Polymerized Multistacked Organic Bipolar Films: A New Approach to Flexible High‐κ Dielectrics

Organic-based flexible dielectric films with high permittivity (e) are desirable for future applications of organic thin-film transistors, such as smartcards and radio frequency identification (RFID) tags, that concurrently desire flexible or conformable substrates. Advances in the development of all-polymer field-effect transistors (PFETs) have been particularly significant for their potential in a flexible form. Major improvements have been reported for both the semiconductor and the dielectric components of PFETs. For example, with respect to semiconductor considerations, noteworthy gains have been achieved in increasing carrier mobility and in identifying lower-cost production technology, however, PFET operational voltages tend to be too high for the intended portable applications. But the voltages could be significantly lowered if a suitable dielectric with a high dielectric constant (k), compatible with the PFET process and in a flexible form factor with thicknesses that is controllably thin, was identified. This Communication focuses on the organic dielectric film component of PFETs, required for all-polymer PFETs by tailoring high-k polymer films. The much-needed advances in this area pose difficult challenges in terms of materials science considerations. One such problem centers on the fact that organic films are inherently low-k; having k values significantly less than those of ceramic (inorganic) materials. Additionally, leakage currents through the film also pose potential problems given the generally porous nature of organic based polymeric films. On the other hand, organic films offer far more favorable

Phthalocyanato Tin(IV) Dichloride: An Air‐Stable, High‐Performance, n‐Type Organic Semiconductor with a High Field‐Effect Electron Mobility

Phthalocyanato tin(IV) dichloride, an axially dichloriniated MPc, is an air-stable high performance n-type organic semiconductor with a field-effect electron mobility of up to 0.30 cm2 V−1 s−1. This high mobility together with good device stability and commercial availability make it a most suitable n-type material for future organic thin-film transistor applications.

Thiophene Polymer Semiconductors for Organic Thin‐Film Transistors

Printed organic thin-film transistors (OTFTs) have received great interests as potentially low-cost alternative to silicon technology for application in large-area, flexible, and ultra-low-cost electronics. One of the critical materials for TFTs is semiconductor, which has a dominant impact on the transistor properties. We review here the structural studies and design of thiophene-based polymer semiconductors with respect to solution processability, ambient stability, molecular self-organization, and field-effect transistor properties for OTFT applications. We show that through judicial monomer design, delicately controlled pi-conjugation, and strategically positioned pendant side-chain distribution, novel solution-processable thiophene polymer semiconductors with excellent self-organization ability to form extended lamellar pi-stacking orders can be developed. OTFTs using semiconductors of this nature processed in ambient conditions have provided excellent field-effect transistor properties.

Development of Thiophene-based Polymeric Semiconductors for Printed Thin-Film Transistors

Recently, organic thin-film transistors (OTFTs) have received great interests as potentially low-cost alternative to hydrogenated α-Si transistors for applications in large-area, flexible, and ultra-low-cost electronics. The development of high mobility, solution- processable, and air-stable organic semiconductor is the key to achieve this transition. In this paper, we review structural studies and design of thiophene-based polymer semiconductors with respect to solution processability, ambient stability, molecular self-organization, and field-effect transistor properties for OTFT applications. We show that through delicately controlled pi-conjugation, strategically positioned pendant side-chain distribution, and carefully tailored side-chain length, novel solution-processable thiophene-based polymer semiconductors with mobility similar to the hydrogenated α-Si transistors and good ambient stability were developed.

Determination of the Crystal Structure of Substrate-Induced Pentacene Polymorphs in Fiber Structured Thin Films

It is widely recognized that the intrinsic charge transport properties in organic thin-film transistors (OTFTs) depend strongly on the crystal structure of the organic semiconductor layer. Pentacene, showing one of the highest charge carrier mobilities among organic semiconductors, is known to crystallize in at least four polymorphs, which can be distinguished by their layer periodicity d(001). Only two polymorphs grow as single crystals, and their detailed crystal structure has been solved. The substrate-induced 15.4 A polymorph is the most relevant for OTFT applications; however, its crystal structure has remained incomplete as it only grows as a fiber structured thin film. Here we extend the crystal truncation rod X-ray scattering technique to fiber structured thin films. We determined the complete crystal structure of this polymorph grown on technologically relevant substrates. We found that the molecular arrangement within the unit cell is substrate dependent, which may lead to a controlled fine-tuni...

Novel organic inverters with dual-gate pentacene thin-film transistor

Abstract We report on the fabrication and characterization of dual-gate pentacene organic thin-film transistors (OTFTs) with plasma-enhanced atomic-layer-deposited (PEALD) 150 nm thick Al2O3 as a bottom-gate dielectric and PEALD 200 nm thick Al2O3 as a top-gate dielectric. The Vth of dual-gate OTFT has changed systematically with the application of voltage bias to top-gate electrode. When voltage bias from −10 V to 10 V is applied to top gate, Vth changes from 1.95 V to −9.8 V. Two novel types of the zero drive load logic inverter with dual-gate structure have been proposed and fabricated using PEALD Al2O3 gate dielectrics. Because the variation of Vth due to chemical degradation and the spatial variation of Vth are inherent in OTFTs, the compensation technology by dual-gate structure can be essential to OTFT applications.

Functionalized interfaces by plasma treatments on silicon and silicon dioxide substrates

Plasma-enhanced chemical vapour deposition has been applied to the fabrication of high-quality SiO 2/Si interfaces and to the functionalization of the silicon dioxide surfaces for organic thin film transistor applications. The advantage of the method herein reported resides in the possibility of activating the substrate, depositing and functionalizing high-quality SiO 2 films in a single-run process, at low temperature. The structural properties of silicon dioxide samples have been studied by infrared spectroscopy, angle resolved and depth profiling X-ray photoelectron spectroscopy. The electronic properties have been retrieved from the leakage current values and the Fowler–Nordheim current plots.