Fabrication Of Organic Thin-film Transistors Research Articles

Direct-Written Silver Electrodes for All-Solution-Processed Low-Voltage Organic Thin Film Transistors Towards Flexible Electronics Applications

Recent progress inprinted electronics has offered the possibility of fabricatingvarious organic-based electronic devices,such as organic light-emitting diodes (OLEDs) and organic thin film transistors (OTFTs).As one of the important deposition methods inprinted electronics, an inkjet printing technique offers the deposition of solution-processable materials onto a variety of substrates using simpler fabrication steps at lower processing temperatures, which is suitable for flexible electronic applications. Despite being the leading choice in OTFT fabrication,the clogging issues that frequently occurred at the printhead nozzle havenot only limitedthe material selection but also restrained the efforts to bring organic electronic devices to the market. Apart from that, although remarkable progress has been made to enhance the performance of the OTFT, the high operating voltage resulting from the low gate capacitance density of the inorganic oxide-based dielectric layer remains a critical limitation that hinders the practical application of the OTFT. Hence, in this paper, we propose a simple solution-based method to develop a low-voltageOTFT. This work utilized a direct-write printing technique to print silver source/drain and gate electrodes incorporated into a bottomgate bottom contact OTFT structure, a spin-coating deposition method to deposit both small molecule TIPS-pentacene organic semiconducting layer and high-kPVA dielectric layer. Notably, the proposed OTFT achieved a micrometre channel length with a saturation mobility of 4.49 × 10-1cm2/Vs, a threshold voltage of -1.5V, an on/off current ratio of 108, and a subthreshold swing of 66.8mV/dec whilethe overall fabrication temperature and operating voltage arekept below 150 °Cand -15 V, respectively.The direct ink writing technology incorporated into the high-kdielectric layer provides a new strategy to fabricate organic-related components,particularly the OFTFs at lower manufacturing cost and temperature towardsflexible and low-operating voltage electronic devices.

Visualizing the 2nd Layer: 4,4´-Bipyridine Adsorption on Sb(111) from EMImBF4

Room temperature ionic liquids (RTIL) have had a lot of attention in modern chemical research. Due to their high stability under applied electrode potential, the wide electrochemical window and dual usability as a solvent and an electrolyte RTILs have become very attractive in the field of applied electrochemistry and modern energetics [1]. Molecular self-assembly at solid surfaces, resulting in the formation of the nanostructures with well-controlled properties and functionality reveals fascinating perspectives in science and technology at nanoscale [2]. For instance, the smart tailoring of the structural properties of the functionalized electrodes enables SAMs to be used as protective coatings and for the fabrication of organic thin-film transistors and sensors, and as triggers of specific electrochemical processes.In this study the in situ STM and impedance spectroscopy methods have been applied to study the structure and properties of the electrochemically polished Sb(111) single crystal electrode | EMImBF4, and EMImBF4 + 1% 4,4´-bipyridine, interface.Using in situ scanning tunnelling microscopy, the adsorption/desorption of 4,4’-bipyridine was demonstrated and a dense underlying structure, formed below a sparse self-assembled monolayer, was visualized. The detection of two separate adsorbed layers indicates that the ordering of organic molecules could extend well beyond the monolayer on the electrode’s surface. These insights are of fundamental and practical importance in the development of nanoelectronics.

Improvement of the comprehensive properties of polymer templates via a novel dry blending method for fabricating OTFT electrodes

Polymer template is widely used in the fabrication of organic thin-film transistor (OTFT) electrodes. However, the mechanical/thermal properties of the polymer template are generally poor owing to their low Young’s modulus (E) and high coefficient of thermal expansion (CTE), resulting in the formation of massive wrinkles and cracks on the OTFT electrodes, as well as the electrodes incomplete transfer, which seriously affects the fabrication of the electrodes. Here, a novel dry blending method is described in which SiO2 nanoparticles are filled into a grooved silicon template, followed by permeation of polydimethylsiloxane (PDMS) into the SiO2 nanoparticle gaps. The SiO2 nanoparticles in the groove are extracted by curing and peeling off PDMS, affording a PDMS/SiO2 composite template with a nanoparticle content of 83.8 wt% and good transparency. The composite template has a Young’s modulus of 16.58 MPa and a CTE of 96 ppm/°C, which constitutes an increase of 91.56 % and a reduction of 69.23 %, respectively, compared with the original PDMS template. Finally, the high performance OTFT electrodes were fabricated using the composite template with good comprehensive properties.

Thin-Film Engineering of Solution-Processable n-Type Silicon Phthalocyanines for Organic Thin-Film Transistors.

Metal and metalloid phthalocyanines are an abundant and established class of materials widely used in the dye and pigment industry as well as in commercial photoreceptors. Silicon phthalocyanines (SiPcs) are among the highest-performing n-type semiconductor materials in this family when used in organic thin-film transistors (OTFTs) as their performance and solid-state arrangement are often increased through axial substitution. Herein, we study eight axially substituted SiPcs and their integration into solution-processed n-type OTFTs. Electrical characterization of the OTFTs, combined with atomic force microscopy (AFM), determined that the length of the alkyl chain affects device performance and thin-film morphology. The effects of high-temperature annealing and spin coating time on film formation, two key processing steps for fabrication of OTFTs, were investigated by grazing-incidence wide-angle X-ray scattering (GIWAXS) and X-ray diffraction (XRD) to elucidate the relationship between thin-film microstructure and device performance. Thermal annealing was shown to change both film crystallinity and SiPc molecular orientation relative to the substrate surface. Spin time affected film crystallinity, morphology, and interplanar d-spacing, thus ultimately modifying device performance. Of the eight materials studied, bis(tri-n-butylsilyl oxide) SiPc exhibited the greatest electron field-effect mobility (0.028 cm2 V-1 s-1, a threshold voltage of 17.6 V) of all reported solution-processed SiPc derivatives.

New Indolo[3,2-b]indole based small organic molecules for Organic Thin Film Transistors (OTFTs): A combined experimental and DFT Study

Synthesis of new indolo[3,2-b]indoles (5a- 5j) in presence of Ag-doped ZnO and (Diacetoxyiodo) benzene system under visible-light have been reported. All the new fused linear heterocyclic indolo[3,2-b]indole systems (5a- 5j) thoroughly characterized by spectroscopic methods like mass, UV-visible, NMR and C, H, N elemental analysis. Further, their photophysical properties were carried out by combined experimental and theoretical studies. Thermogravimetric studies are carried out to confirm the thermal stability of molecules. The frontier molecular orbitals of molecules are characterized with the help of cyclic voltammetry. Additionally, the compounds of series 5 were used for the fabrication of organic thin-film transistors, which indicated the hole mobilities in the range of 0.11 – 0.85 cm2/Vs and with on/off ratio 105 on ODTS-SiO2 substrate at 50 °C and are also supported by DFT studies.

Wettability Control of Interfaces for High-Performance Organic Thin-Film Transistors by Soluble Insulating Polymer Films.

Organic small-molecule semiconductors have higher carrier mobility compared to polymer semiconductors, while the actual performances of these materials are susceptible to morphological defects and misalignment of crystalline grains. Here, a new strategy is explored to control the crystallization and morphologies of a solution-processed organic small-molecule semiconductor 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) using soluble polymer films to control the wettability of substrates. Different from the traditional surface modification method, the polymer layer as a modification layer is soluble in the semiconductor solution during the fabrication of organic thin-film transistors (OTFTs). The dissolved polymer alters the state of the semiconductor solution, which in turn, changes the crystallographic morphologies of the semiconductor films. By controlling the solubility and thickness of the polymer modification layers, it is possible to regulate the grain boundary and domain size of C8-BTBT films, which determine the performances of OTFTs. The bottom-gate transistors modified by a thick PS layer exhibit a mobility of >7 cm2/V·s and an on/off ratio of >107. It is expected that this new modification method will be applicable to high-performance OTFTs based on other small molecular semiconductors and dielectrics.

All-Printed Piezoresistive Sensor Matrix with Organic Thin-Film Transistors as a Switch for Crosstalk Reduction

A generation of piezoresistive sensors (force or deformation) fully processed by printing technologies is increasingly being implemented in applications due to advantages as a large-area application, simple device, integration, and high flexibility. This work reports the development of a fully printed piezoresistive (5 × 5 sensor) matrix in which an organic thin-film transistor (OTFT) is placed in each sensor to allow the readout of each sensor independently and thus reducing the crosstalk between individual sensors. The manufacturing was carried out using inkjet printing for the deposition of materials in a thin layer stacked on top of each other to obtain functional OTFTs. The piezoresistive nanocomposite sensors, based on multiwalled carbon nanotubes within an elastomeric styrene-ethylene-butadiene-styrene (SEBS) polymer matrix, were fabricated by screen printing. The fabrication and characterization of both OTFT and sensors are presented and discussed in detail. The inkjet-printed OTFTs (width/length channel ratio of ∼130) show a drain-source current (IDS) of 150 μA with a gate-source voltage of −40 V. Gauge factors of up to 5.9 were obtained for the sensors, resulting in a current variation of 1.5 μA. This corresponds to about 0.7% of the total IDS in a deformation cycle.

Damage-free Metal Electrode Transfer to Monolayer Organic Single Crystalline Thin Films

Solution-processed organic thin film transistors (OTFTs) are an essential building block for next-generation printed electronic devices. Organic semiconductors (OSCs) that can spontaneously form a molecular assembly play a vital role in the fabrication of OTFTs. OTFT fabrication processes consist of sequential deposition of functional layers, which inherently brings significant difficulties in realizing ideal properties because underlayers are likely to be damaged by application of subsequent layers. These difficulties are particularly prominent when forming metal contact electrodes directly on an OSC surface, due to thermal damage during vacuum evaporation and the effect of solvents during subsequent photolithography. In this work, we demonstrate a simple and facile technique to transfer contact electrodes to ultrathin OSC films and form an ideal metal/OSC interface. Photolithographically defined metal electrodes are transferred and laminated using a polymeric bilayer thin film. One layer is a thick sacrificial polymer film that makes the overall film easier to handle and is water-soluble for dissolution later. The other is a thin buffer film that helps the template adhere to a substrate electrostatically. The present technique does not induce any fatal damage in the substrate OSC layers, which leads to successful fabrication of OTFTs composed of monolayer OSC films with a mobility of higher than 10 cm2 V−1 s−1, a subthreshold swing of less than 100 mV decade−1, and a low contact resistance of 175 Ω⋅cm. The reproducibility of efficient contact fabrication was confirmed by the operation of a 10 × 10 array of monolayer OTFTs. The technique developed here constitutes a key step forward not only for practical OTFT fabrication but also potentially for all existing vertically stacked organic devices, such as light-emitting diodes and solar cells.

One-Step Coating Processed Phototransistors Enabled by Phase Separation of Semiconductor and Dielectric Blend Film.

Fabrication of organic thin-film transistors (OTFTs) via high throughput solution process routes have attracted extensive attention. Herein, we report a simple one-step coating method for vertical phase separation of the poly(3-hexylthiophene-2,5-diyl) (P3HT) and poly(methyl methacrylate) (PMMA) blends as semiconducting and dielectric layers in OTFTs. These OTFTs can be used as phototransistors for ultraviolet (UV) light detection, where the phototransistors exhibited great photosensitivity of 597.6 mA/W and detectivity of 4.25 × 1010 Jones under 1 mW/cm2 UV light intensity. Studies of the electrical properties in these phototransistors suggested that optimized P3HT contents in the blend film can facilitate the improvement of film morphology, and therefore form optimized vertical phase separation of the PMMA and P3HT. These results indicate that the simple one-step fabrication method creates possibilities for realizing high throughput phototransistors with great photosensitivity.

One-Volt, Solution-Processed Organic Transistors with Self-Assembled Monolayer-Ta2O5 Gate Dielectrics.

Low-voltage, solution-processed organic thin-film transistors (OTFTs) have tremendous potential to be key components in low-cost, flexible and large-area electronics. However, for these devices to operate at low voltage, robust and high capacitance gate dielectrics are urgently needed. Herein, the fabrication of OTFTs that operate at 1 V is reported. These devices comprise a solution-processed, self-assembled monolayer (SAM) modified tantalum pentoxide (Ta2O5) as the gate dielectric. The morphology and dielectric properties of the anodized Ta2O5 films with and without n-octadecyltrichlorosilane (OTS) SAM treatment have been studied. The thickness of the Ta2O5 film was optimized by varying the anodization voltage. The results show that organic TFTs gated with OTS-modified tantalum pentoxide anodized at 3 V (d ~7 nm) exhibit the best performance. The devices operate at 1 V with a saturation field-effect mobility larger than 0.2 cm2 V−1 s−1, threshold voltage −0.55 V, subthreshold swing 120 mV/dec, and current on/off ratio in excess of 5 × 103. As a result, the demonstrated OTFTs display a promising performance for applications in low-voltage, portable electronics.

Thermally deposited silk fibroin as the gate dielectric layer in organic thin-film transistors based on conjugated polymer

This study investigated the morphology, structural transformation, and electric properties of organic thin-film transistors (OTFTs) with silk fibroin as their dielectric layer at different annealing temperatures. Silk fibroin was employed because it can simplify the OTFT fabrication process and decrease fabrication cost. We successfully fabricated OTFTs based on poly(3-hexylthiophene) (P3HT) dissolved in chlorobenzene (CB) or 1,2,4-trichlorobeneze (TCB). The P3HT had fiber-like network structures when TCB was used and isolated spherical domains when CB was employed. The nanowire morphology obtained when TCB was used may have enabled efficient charge transport. The silk fibroin layer annealed at 40 °C had the smallest particles and least aggregate. The calculated field-effect mobility was 2.06 × 10−3 cm2 (Vs)−1 (when TCB was used), and the highest on/off ratio was 103, obtained for the silk fibroin annealed at 40 °C. The OTFT with the 40 °C-annealed-layer had the best performance because of the smooth surface of the α-helix structure of this silk fibroin layer, with such smoothness enabling favorable P3HT crystalline formation. Biological materials such as silk protein are low cost and environmentally friendly and require low temperature in manufacturing. This study indicates the suitability of silk fibroin for use in various electronic devices.

Contact effects analyzed by a parameter extraction method based on a single bottom-gate/top-contact organic thin-film transistor

Contact effects in organic thin-film transistors (OTFTs) were examined by using our previously proposed parameter extraction method from the electrical characteristics of a single staggered-type device. Gate-voltage-dependent contact resistance and channel mobility in the linear regime were evaluated for bottom-gate/top-contact (BGTC) pentacene TFTs with active layers of different thicknesses, and for pentacene TFTs with contact-doped layers prepared by coevaporation of pentacene and tetrafluorotetracyanoquinodimethane (F4TCNQ). The extracted parameters suggested that the influence of the contact resistance becomes more prominent with the larger active-layer thickness, and that contact-doping experiments give rise to a drastic decrease in the contact resistance and a concurrent considerable improvement in the channel mobility. Additionally, the estimated energy distributions of trap density in the transistor channel probably reflect the trap filling with charge carriers injected into the channel regions. The analysis results in this study confirm the effectiveness of our proposed method, with which we can investigate contact effects and circumvent the influences of characteristic variations in OTFT fabrication.

P‐123: Carrier Transport Mechanism and Device Application of Polymer‐Type Organic Thin‐Film Transistors

The p‐type organic thin‐film transistors (OTFTs) were fabricated and investigated in this study. Solution and photolithography technology were used for organic thin film formation and patterning which is compatible with the existing a‐Si:H TFT production. Furthermore, the process temperature is below 120 °C which enabling the fabrication of OTFTs on either glass or flexible substrates. In addition, Carrier transport mechanism of polymer‐type OTFT was demonstrated to be a heat‐activated hopping transport.

Insights Into Interface Treatments in p-Channel Organic Thin-Film Transistors Based on a Novel Molecular Semiconductor

Organic thin-film transistors (OTFTs) were fabricated using a novel small molecule, C6-NTTN, as the semiconductor layer in several different architectures. The C6-NTTN layer was deposited via both vacuum evaporation at different substrate temperatures and via solution-processing, which yield maximum hole mobilities of 0.16 and 0.05 cm2/ ${\mathrm{ V}}\cdot {\mathrm{ s}}$ , respectively. Surface treatments of the substrate, insulator, and metal contacts used for OTFT fabrication employing polymer films and different self-assembled monolayers were investigated. In particular, in bottom-gate devices, the insulator surface hydrophobicity was optimized by the deposition of poly(methyl methacrylate) or hexamethyldisilazane, while in the top-gate geometry, pentafluorobenzenethiol was efficiently used to modify the substrate surface energy and to change the contact work function. Atomic force microscopy analysis was exploited to understand the relationship between the semiconductor thin-film morphology and the device electrical performance. The results shown here indicate an inverse proportionality between the mobility and the interface trap density, with parameters depending especially on semiconductor–insulator interfacial properties, and a correlation between the threshold voltage and the characteristics of the semiconductor–metal interface.

Simulation and characterization of short-channel organic thin-film transistors fabricated using ink-jet printing and an imprint process

The authors fabricated short-channel organic thin-film transistors (OTFTs) on a polyethersulphone substrate using ink-jet printing combined with an imprint method. 6,13-bis(triisopropylsilylethynyl)pentacene and polyvinyl alcohol were used as an active material and a gate insulator, respectively, in the OTFT fabrication. The channel length was reduced remarkably by interfacing ink-jet printing with imprint technology. The authors obtained an OTFT mobility of approximately 0.25 cm2/V s in the saturation region, and the OTFT threshold voltage was approximately −2.5 V. The results were compared with the simulated results to understand the variations in the device structure and material properties.

Ultra-high-resolution printing of flexible organic thin-film transistors

ABSTRACTFully printed electronics on plastic have attracted considerable interest owing to their high compatibility and ease of integration. Here, an ultra-high-resolution printing technique based on parallel vacuum ultraviolet patterning that can produce high-contrast wettability regions on flexible substrates was developed. This technique was used to selectively deposit a functional ink with a 1 µm feature size, thereby allowing the large-scale fabrication of organic thin-film transistors with channels as short as 1 µm under an ambient atmosphere. Moreover, in short-channel devices, hole injection barriers can be tuned by printing the optimum gate overlaps associated with selectively doping semiconductor/electrode interfaces, resulting in a marked reduction in contact resistance from 20 to 1.5 kΩ cm, and an elevation of the charge carrier mobility to a record high of 0.3 cm2 V−1 s−1 in a 1-µm-channel device. The results indicate that the developed technique is promising for the fabrication of large-area, high-resolution, low-cost electronics.

A new biodegradable gate dielectric material based on keratin protein for organic thin film transistors

By virtue of the biocompatibility, environmental benignity, and sustainability, as well as low cost of keratin protein's source herein, we report its application as a gate dielectric material for organic electronic devices. Keratin protein, which is a biodegradable material was directly extracted from poultry chicken feathers (CFs). Solution-processed regioregular poly(3-hexylthiophene) (P3HT) organic thin film transistors (OTFTs) with keratin dielectric thin film exhibited enhanced charge mobility of 2.293 × 10−3 cm2 V−1 s−1 (saturation regime), high on-off current ratio of 105 and low threshold voltage, -1V as compared to conventional SiO2 dielectric. All the fabrication processes were performed below 100 °C. A detailed semiconductor-dielectric interface study has revealed that the high content of β-sheet structure in keratin protein has guided the P3HT polymer chains, through supramolecular forces of interaction to form 2D nanoribbons of large crystallite size (150 nm) over keratin thin film. This has led to the reduced trapping sites at the semiconductor/dielectric interface and hence the enhanced electrical performance of OTFTs was observed. Atomic force microscopy (AFM) and Grazing Incidence X-ray diffraction (GI-XRD) study was further employed to explore the mechanism of formation of 2D nanoribbons of P3HT. The primary and secondary structure of keratin protein as analyzed by AFM, Transmission electron microscopy (TEM) and Fourier transform spectroscopy (FTIR) is also provided in this study. The water compatible nature of keratin has helped to overcome the common issue of dielectric layer washing that occurs mostly during fabrication of OTFTs by sol-gel method. The biodegradable nature of keratin protein has also been demonstrated here by providing it as feed to fishes in an aquarium.

Triacetate cellulose gate dielectric organic thin-film transistors

Here, we report on the performance and the characterization of all solution-processable top-contact organic thin-film transistors (OTFTs) consisting of a natural-resourced triacetate cellulose gate dielectric and a representative hole-transport poly[2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene] (pBTTT) semiconductor layer on rigid or flexible substrates. The bio-based triacetate cellulose layer has an important role in the OTFT fabrication because it provides the pBTTT semiconducting polymer with highly suitable gate dielectric properties including a low surface roughness, hydrophobic surface, appropriate dielectric constant, and low leakage current. The triacetate cellulose gate dielectric-based pBTTT OTFTs exhibit an average filed-effect mobility of 0.031 cm2/Vs similar to that obtained from a SiO2 gate dielectric-based OTFT device in ambient conditions. Even after a bending stimulation of 100 times and in an outward bending state, the flexible triacetate cellulose gate pBTTT OTFT device still showed excellent electrical device performance without any hysteresis.

Plasma-enhanced chemical vapor-deposited organic dielectric layer for low voltage, flexible organic thin-film transistor

We investigated the electronic properties of pentacene-based organic thin-film transistors (OTFTs) incorporated with an organic dielectric layer (ODL) fabricated using a homemade, cold plasma–enhanced chemical vapor deposition system. Ar-carried trimethylaluminum monomer was introduced into an evaporator mixer controlled at 130°C, and an ODL was deposited on Si- or indium tin oxide (ITO)–coated plastic substrate under N2 plasma. High radio frequency power (100W)-deposited ODL (dielectric constant κ=1.6) supported the fabrication of pentacene-based OTFT on Si substrate at low voltages (<1.5V). The decreased deposition time (10min) reduced the film thickness of the ODL (95nm), thereby increasing its dielectric constant to 3.3. We also deposited ODL on ITO–coated plastic substrate to fabricate a flexible OTFT with a mobility of 2.1cm2/Vs and an on/off current ratio (Ion/Ioff) of 103.

Anisotropic charge-carrier mobilities of liquid crystalline conjugated polymers on photo-aligned PVCN dielectric insulators

We fabricated organic thin-film transistors (OTFTs) with photo-reactive poly(vinylcinnamate) (PVCN) as a dielectric layer and liquid crystalline poly(2,5-bis(3-dodecylthiophene-2-yl)thieno[3,2-b]thiophene) (PBTTT) as a semiconducting layer. The PVCN layer plays an important role in the fabrication of OTFTs with solution processable PBTTT, because it provides orientation of the liquid crystalline polymer chains as well as gate insulating characteristics. The electrical properties of PBTTT-OTFTs with the UV-cured PVCN film were investigated for the films prepared with linearly polarized ultraviolet light irradiation along parallel and perpendicular to the electrode channel direction.