Polyelectrolyte Dielectrics for Flexible Low‐Voltage Organic Thin‐Film Transistors in Highly Sensitive Pressure Sensing

Organic thin‐film transistors (OTFTs) can provide an effective platform to develop flexible pressure sensors in wearable electronics due to their good signal amplification function. However, it is particularly difficult to realize OTFT‐based pressure sensors with both low‐voltage operation and high sensitivity. Here, controllable polyelectrolyte composites based on poly(ethylene glycol) (PEG) and polyacrylic acid (PAA) are developed as a type of high‐capacitance dielectrics for flexible OTFTs and ultrasensitive pressure sensors with sub‐1 V operation. Flexible OTFTs using the PAA:PEG dielectrics show good universality and greatly enhanced electrical performance under a much smaller operating voltage of ?0.7 V than those with a pristine PAA dielectric. The low‐voltage OTFTs also exhibit excellent flexibility and bending stability under various bending radii and long cycles. Flexible OTFT‐based pressure sensors with low‐voltage operation and superhigh sensitivity are demonstrated by using a suspended semiconductor/dielectric/gate structure in combination with the PAA:PEG dielectric. The sensors deliver a record high sensitivity of 452.7 kPa^?1 under a low‐voltage of ?0.7 V, and excellent operating stability over 5000 cycles. The OTFT sensors can be built into a wearable sensor array for spatial pressure mapping, which shows a bright potential in flexible electronics such as wearable devices and smart skins.     

Pressure Control Organic Vapor Deposition Methods for Fabricating Organic Thin‐Film Transistors

In this letter, we report on the development progress of a pressure control organic vapor deposition (PCOVD) technology used to design and build a large area deposition system. We also investigate the growth characteristics of a pentacene thin film by PCOVD. Using the PCOVD method, the mobility and on/off current ratio of an organic thin‐film transistor (OTFT) on a plastic substrate are 0.1 cm²/Vs and 10⁶, respectively. The developed OTFT can be applied to a flexible display on a plastic substrate.     

Benzodithiophene Copolymer—A Low‐Temperature,Solution‐Processed High‐Performance Semiconductor for Thin‐Film Transistors

Poly(4,8‐didodecyl‐2,6‐bis‐(3‐methylthiophen‐2‐yl)‐benzo[1,2‐b:4,5‐b′]dithiophene) self‐assembled on appropriate substrates from solution and formed highly structured thin films at low temperatures. As an as‐prepared thin‐film semiconductor without thermal annealing, it exhibited excellent field‐effect transistor properties with mobility of ～ 0.15 cm² V^–1 s^–1 in thin‐film transistors.     

High‐Mobility Air‐Stable Solution‐Shear‐Processed n‐Channel Organic Transistors Based on Core‐Chlorinated Naphthalene Diimides

High charge carrier mobility solution‐processed n‐channel organic thin‐film transistors (OTFTs) based on core‐chlorinated naphthalene tetracarboxylic diimides (NDIs) with fluoroalkyl chains are demonstrated. These OTFTs were prepared through a solution shearing method. Core‐chlorination of NDIs not only increases the electron mobilities of OTFTs, but also enhances their air stability, since the chlorination in the NDI core lowers the lowest unoccupied molecular orbital (LUMO) levels. The air‐stability of dichlorinated NDI was better than that of the tetrachlorinated NDIs, presumably due to the fact that dichlorinated NDIs have a denser packing of the fluoroalkyl chains and less grain boundaries on the surface, reducing the invasion pathway of ambient oxygen and moisture. The devices of dichlorinated NDIs exhibit good OTFT performance, even after storage in air for one and a half months. Charge transport anisotropy is observed from the dichlorinated NDI. A dichlorinated NDI with ?CH₂C₃F₇ side chains reveals high mobilities of up to 0.22 and 0.57 cm² V^?1 s^?1 in parallel and perpendicular direction, respectively, with regard to the shearing direction. This mobility anisotropy is related to the grain morphology. In addition, we find that the solution‐shearing deposition affects the molecular orientation in the crystalline thin films and lowers the d(001)‐spacing (the out‐of‐plane interlayer spacing), compared to the vapor‐deposited thin films. Core‐chlorinated NDI derivatives are found to be highly suitable for n‐channel active materials in low‐cost solution‐processed organic electronics.     

Non‐volatile Ferroelectric Poly(vinylidene fluoride‐co‐trifluoroethylene) Memory Based on a Single‐Crystalline Tri‐isopropylsilylethynyl Pentacene Field‐Effect Transistor

A new type of nonvolatile ferroelectric poly(vinylidene fluoride‐co‐trifluoroethylene) (P(VDF‐TrFE)) memory based on an organic thin‐film transistor (OTFT) with a single crystal of tri‐isopropylsilylethynyl pentacene (TIPS‐PEN) as the active layer is developed. A bottom‐gate OTFT is fabricated with a thin P(VDF‐TrFE) film gate insulator on which a one‐dimensional ribbon‐type TIPS‐PEN single crystal, grown via a solvent‐exchange method, is positioned between the Au source and drain electrodes. Post‐thermal treatment optimizes the interface between the flat, single‐crystalline ab plane of TIPS‐PEN and the polycrystalline P(VDF‐TrFE) surface with characteristic needle‐like crystalline lamellae. As a consequence, the memory device exhibits a substantially stable source–drain current modulation with an ON/OFF ratio hysteresis greater than 10³, which is superior to a ferroelectric P(VDF‐TrFE) OTFT that has a vacuum‐evaporated pentacene layer. Data retention longer than 5 × 10⁴ s is additionally achieved in ambient conditions by incorporating an interlayer between the gate electrode and P(VDF‐TrFE) thin film. The device is environmentally stable for more than 40 days without additional passivation. The deposition of a seed solution of TIPS‐PEN on the chemically micropatterned surface allows fabrication arrays of TIPS‐PEN single crystals that can be potentially useful for integrated arrays of ferroelectric polymeric TFT memory.     

Reusable Cellulose‐Based Hydrogel Sticker Film Applied as Gate Dielectric in Paper Electrolyte‐Gated Transistors

A new concept for reusable eco‐friendly hydrogel electrolytes based on cellulose is introduced. The reported electrolytes are designed and engineered through a simple, fast, low‐cost, and eco‐friendly dissolution method of microcrystalline cellulose at low temperature using an aqueous LiOH/urea solvent system. The cellulose solution is combined with carboxymethyl cellulose, followed by the regeneration and simultaneous ion incorporation. The produced free standing cellulose‐based electrolyte films exhibit interesting properties for application in flexible electrochemical devices, such as biosensors or electrolyte‐gated transistors (EGTs), because of their high specific capacitances (4–5 µF cm^?2), transparency, and flexibility. Indium–gallium–zinc‐oxide EGTs on glass with laminated cellulose‐based hydrogel electrolytes (CHEs) as the gate dielectric are produced presenting a low working voltage (<2 V), showing an on–off current ratio (I _on/off) of 10⁶, a subthreshold swing lower than 0.2 V dec^?1, and saturation mobility (μ_Sat) reaching 26 cm² V^?1 s^?1. The flexible CHE‐gated transistors on paper are also demonstrated, which operate at switching frequencies up to 100 Hz. Combining the flexibility of the EGTs on paper with the reusability of the developed CHEs is a breakthrough toward biodegradable advanced functional materials allied with disposable/recyclable and low‐cost electronic devices.     

High‐Mobility Naphthalene Diimide and Selenophene‐Vinylene‐Selenophene‐Based Conjugated Polymer: n‐Channel Organic Field‐Effect Transistors and Structure–Property Relationship

Interdependence of chemical structure, thin‐film morphology, and transport properties is a key, yet often elusive aspect characterizing the design and development of high‐mobility, solution‐processed polymers for large‐area and flexible electronics applications. There is a specific need to achieve >1 cm² V^?1 s^?1 field‐effect mobilities (μ) at low processing temperatures in combination with environmental stability, especially in the case of electron‐transporting polymers, which are still lagging behind hole transporting materials. Here, the synthesis of a naphthalene‐diimide based donor–acceptor copolymer characterized by a selenophene vinylene selenophene donor moiety is reported. Optimized field‐effect transistors show maximum μ of 2.4 cm² V^?1 s^?1 and promising ambient stability. A very marked film structural evolution is revealed with increasing annealing temperature, with evidence of a remarkable 3D crystallinity above 180 °C. Conversely, transport properties are found to be substantially optimized at 150 °C, with limited gain at higher temperature. This discrepancy is rationalized by the presence of a surface‐segregated prevalently edge‐on packed polymer phase, dominating the device accumulated channel. This study therefore serves the purpose of presenting a promising, high‐electron‐mobility copolymer that is processable at relatively low temperatures, and of clearly highlighting the necessity of specifically investigating channel morphology in assessing the structure–property nexus in semiconducting polymer thin films.     

Electroless‐Plated Gold Contacts for High‐Performance,Low Contact Resistance Organic Thin Film Transistors

Deposition of metallic electrodes on a semiconductor medium is an indispensable factor in governing carrier injection, and a metal/semiconductor contact that can be formed via solution process is highly desired in printed electronics. However, fine‐patterning the solution processes of metallic electrodes without damaging the excellent electronic properties of organic semiconductors (OSCs) is still a challenge. In this work, electroless plating, a metal coating technique that involves auto‐catalytic reaction in an aqueous solution, is used to fabricate top‐contact organic thin‐film transistors (OTFTs). An electroless‐plated gold pattern with a spatial resolution of 10 micrometers is transferred and laminated on a monolayer of OSCs to serve as a hole‐injection electrode. The fabricated OTFTs exhibit reasonably high field‐effect mobility of up to 13 cm² V^?1 s^?1 and decent contact resistance as low as 120 Ω · cm, which implies that an ideal metal/semiconductor contact can be realized. This electroless plating technique can provide possibilities for practical mass production of organic integrated circuits because it is in principle cost‐effective, capable of covering large areas, high‐vacuum free, and environmentally friendly.     

Printable Organic‐Inorganic Nanoscale Multilayer Gate Dielectrics for Thin‐Film Transistors Enabled by a Polymeric Organic Interlayer

Here, a new approach to the layer‐by‐layer solution‐processed fabrication of organic/inorganic hybrid self‐assembled nanodielectrics (SANDs) is reported and it is demonstrated that these ultrathin gate dielectric films can be printed. The organic SAND component, named P‐PAE, consists of polarizable π‐electron phosphonic acid‐based units bound to a polymeric backbone. Thus, the new polymeric SAND (PSAND) can be fabricated either by spin‐coating or blade‐coating in air, by alternating P‐PAE, a capping reagent layer, and an ultrathin ZrOx layer. The new PSANDs thickness vary from 6 to 15 nm depending on the number of organic‐ZrOx bilayers, exhibit tunable film thickness, well‐defined nanostructures, large electrical capacitance (up to 558 nF cm^?2), and good insulating properties (leakage current densities as low as 10^?6 A cm^?2). Organic thin‐film transistors that are fabricated with representative p‐/n‐type organic molecular/polymeric semiconducting materials, function well at low voltages (<3.0 V). Furthermore, flexible TFTs fabricated with PSAND exhibit excellent mechanical flexibility and good stress stability, offering a promising route to low operating voltage flexible electronics. Finally, printable PSANDs are also demonstrated and afford TFTs with electrical properties comparable to those achieved with the spin‐coated PSAND‐based devices.     

Transistor Paint: Environmentally Stable N‐alkyldithienopyrrole and Bithiazole‐Based Copolymer Thin‐Film Transistors Show Reproducible High Mobilities without Annealing

New solution processable 4‐(2‐hexyldecan)‐4H‐bisthieno[2,3‐d:3′,2′‐b]pyrrole and 4,4′‐dialkyl‐2,2′‐bithiazole‐based copolymers (PBTzDTPs) are synthesized with excellent FET performance. These novel copolymers have considerable potential in printable electronics as they have high charge carrier mobilities, excellent air stability, good solution processibility, and no requirement for post‐deposition thermal annealing, all requirements for this field of application. The thin film transistors fabricated from PBTzDTPs achieve field effect mobilities as high as 0.14 cm² V^?1 s^?1 with current on/off ratios up to 10⁶ without thermal annealing. In addition, the devices exhibit stable performance in air, showing no significant degradation over 60 days. Moreover, the polymers described here provide an excellent example of the systems in which higher mobility performance does not require higher crystalline, long‐range ordered structures. Such a system appears to be particularly promising for rapid fabrication techniques, where kinetic conditions usually prevent the development of long‐range order.     

Cellulose‐Based Ionogels for Paper Electronics

A new class of biofriendly ionogels produced by gelation of microcellulose thin films with tailored 1‐ethyl‐3‐methylimidazolium methylphosphonate ionic liquids are demonstrated. The cellulose ionogels show promising properties for application in flexible electronics, such as transparency, flexibility, transferability, and high specific capacitances of 5 to 15 μF cm^?2. They can be laminated onto any substrate such as multilayer‐coated paper and act as high capacitance dielectrics for inorganic (spray‐coated ZnO and colloidal ZnO nanorods) and organic (poly[3‐hexylthiophene], P3HT) electrolyte‐gated field‐effect transistors (FETs), that operate at very low voltages (<2 V). Field‐effect mobilities in ionogel‐gated spray‐coated ZnO FETs reach 75 cm² V^?1 s^?1 and a typical increase of mobility with decreasing specific capacitance of the ionogel is observed. Solution‐processed, colloidal ZnO nanorods and laminated cellulose ionogels enable the fabrication of the first electrolyte‐gated, flexible circuits on paper, which operate at bending radii down to 1.1 mm.     

Boosting Direct X‐Ray Detection in Organic Thin Films by Small Molecules Tailoring

The attention focused on the application of organic electronics for the detection of ionizing radiation is rapidly growing among the international scientific community, due to the great potential of organic technology to enable large‐area conformable sensor panels. However, high‐energy photon absorption is challenging as organic materials are constituted of atoms with low atomic numbers. Here it is reported how, by synthesizing new solution‐processable organic molecules derived from 6,13‐bis(triisopropylsilylethynyl)pentacene (TIPS‐pentacene) and 2,8‐difluoro‐5,11‐bis(triethylsilylethynyl)anthradithiophene, with Ge‐substitution in place of the Si atoms to increase the material atomic number, it is possible to boost the X‐ray detection performance of organic thin films on flexible plastic substrates. Bis(triisopropylgermylethynyl)‐pentacene based flexible organic thin film transistors show high electrical performance with higher mobility (0.4 cm² V^?1 s^?1) and enhanced X‐ray sensitivity, up to 9.0 × 10⁵ µC Gy^?1 cm^?3, with respect to TIPS‐pentacene‐based detectors. Moreover, similar results are obtained for 5,11‐bis(triethylgermylethynyl)anthradithiophene devices, confirming that the proposed strategy, that is, increasing the atomic number of organic molecules by chemical tailoring to improve X‐ray sensitivity, can be generalized to organic thin film detectors, combining high X‐ray absorption, mechanical flexibility, and large‐area processing.     

Cover Picture: Bilayer Organic–Inorganic Gate Dielectrics for High‐Performance,Low‐Voltage,Single‐Walled Carbon Nanotube Thin‐Film Transistors,Complementary Logic Gates,and p–n Diodes on Plastic Substrates (Adv. Funct. Mater. 18/2006)

Low‐voltage, hysteresis‐free, flexible thin‐film‐type electronic systems based on networks of single‐walled carbon nanotubes and bilayer organic–inorganic nanodielectrics are detailed in work by Rogers and co‐workers reported on p. 2355. The cover image shows a schematic array of such thin‐film transistors (TFTs) on a plastic substrate. The structure of the bilayer nanodielectric, which consists of a film of HfO₂ formed by atomic layer deposition and an ultrathin layer of epoxy formed by spin‐casting, is also illustrated schematically. High‐capacitance bilayer dielectrics based on atomic‐layer‐deposited HfO₂ and spin‐cast epoxy are used with networks of single‐walled carbon nanotubes (SWNTs) to enable low‐voltage, hysteresis‐free, and high‐performance thin‐film transistors (TFTs) on silicon and flexible plastic substrates. These HfO₂–epoxy dielectrics exhibit excellent properties including mechanical flexibility, large capacitance (up to ca. 330 nF cm^–2), and low leakage current (ca. 10^–8 A cm^–2); their low‐temperature (ca. 150 °C) deposition makes them compatible with a range of plastic substrates. Analysis and measurements of these dielectrics as gate insulators in SWNT TFTs illustrate several attractive characteristics for this application. Their compatibility with polymers used for charge‐transfer doping of SWNTs is also demonstrated through the fabrication of n‐channel SWNT TFTs, low‐voltage p–n diodes, and complementary logic gates.     

Inkjet‐Printed Organic Electrodes for Bottom‐Contact Organic Field‐Effect Transistors

A graphite thin film was investigated as the drain and source electrodes for bottom‐contact organic field‐effect transistors (BC OFETs). Highly conducting electrodes (10² S cm^?1) at room temperature were obtained from pyrolyzed poly(l,3,4‐oxadiazole) (PPOD) thin films that were prepatterned with a low‐cost inkjet printing method. Compared to the devices with traditional Au electrodes, the BC OFETs showed rather high performances when using these source/drain electrodes without any further modification. Being based on a graphite‐like material these electrodes possess excellent compatibility and proper energy matching with both p‐ and n‐type organic semiconductors, which results in an improved electrode/organic‐layer contact and homogeneous morphology of the organic semiconductors in the conducting channel, and finally a significant reduction of the contact resistance and enhancement of the charge‐carrier mobility of the devices is displayed. This work demonstrates that with the advantages of low‐cost, high‐performance, and printability, PPOD could serve as an excellent electrode material for BC OFETs.     

Room‐Temperature Printing of Organic Thin‐Film Transistors with π‐Junction Gold Nanoparticles

Printing semiconductor devices under ambient atmospheric conditions is a promising method for the large‐area, low‐cost fabrication of flexible electronic products. However, processes conducted at temperatures greater than 150 °C are typically used for printed electronics, which prevents the use of common flexible substrates because of the distortion caused by heat. The present report describes a method for the room‐temperature printing of electronics, which allows thin‐film electronic devices to be printed at room temperature without the application of heat. The development of π‐junction gold nanoparticles as the electrode material permits the room‐temperature deposition of a conductive metal layer. Room‐temperature patterning methods are also developed for the Au ink electrodes and an active organic semiconductor layer, which enables the fabrication of organic thin‐film transistors through room‐temperature printing. The transistor devices printed at room temperature exhibit average field‐effect mobilities of 7.9 and 2.5 cm² V^?1 s^?1 on plastic and paper substrates, respectively. These results suggest that this fabrication method is very promising as a core technology for low‐cost and high‐performance printed electronics.     

High‐Performance Phototransistors Based on Organic Microribbons Prepared by a Solution Self‐Assembly Process

Oligoarenes as an alternative group of promising semiconductors in organic optoelectronics have attracted much attention. However, high‐performance and low‐cost opto‐electrical devices based on linear asymmetric oligoarenes with nano/microstructures are still rarely studied because of difficulties both in synthesis and high‐quality nano/microstructure growth. Here, a novel linear asymmetric oligoarene 6‐methyl‐anthra[2,3‐b]benzo[d]thiophene (Me‐ABT) is synthesized and its high‐quality microribbons are grown by a solution process. The solution of Me‐ABT exhibits a moderate fluorescence quantum yield of 0.34, while the microribbons show a glaucous light emission. Phototransistors based on an individual Me‐ABT microribbon prepared by a solution‐phase self‐assembly process showed a high mobility of 1.66 cm² V^?1 s^?1, a large photoresponsivity of 12 000 A W^?1, and a photocurrent/dark‐current ratio of 6000 even under low light power conditions (30 µW cm^?2). The measured photoresponsivity of the devices is much higher than that of inorganic single‐crystal silicon thin film transistors. These studies should boost the development of the organic semiconductors with high‐quality microstructures for potential application in organic optoelectronics.     

Inside Front Cover: High‐Performance and Stable Organic Thin‐Film Transistors Based on Fused Thiophenes (Adv. Funct. Mater. 3/2006)

A series of new organic semiconductors for organic thin‐film transistors using dithieno[3,2‐b:2′,3′‐d]thiophene as the core have been synthesized. In work reported by Liu, Zhu, and co‐workers on p. 426, the phenyl‐substituted compound exhibited a high mobility of 0.42 cm² V^–1 s^–1 and an on/off ratio of 5 × 10⁶. Weekly shelf‐life tests of the transistors based on the bis(diphenyl)‐substituted thiophene under ambient conditions showed that the mobility was almost unchanged after more than two months, demonstrating potential for applications in future organic electronics. A series of new organic semiconductors for organic thin‐film transistors (OTFTs) using dithieno[3,2‐b:2′,3′‐d]thiophene as the core are synthesized. Their electronic and optical properties are investigated using scanning electron microscopy (SEM), X‐ray diffraction (XRD), UV‐vis and photoluminescence spectroscopies, thermal gravimetric analysis (TGA), and differential scanning calorimetry (DSC). The compounds exhibit an excellent field‐effect performance with a high mobility of 0.42 cm² V^–1 s^–1 and an on/off ratio of 5 × 10⁶. XRD patterns reveal these films, grown by vacuum deposition, to be highly crystalline, and SEM reveals well‐interconnected, microcrystalline domains in these films at room temperature. TGA and DSC demonstrate that the phenyl‐substituted compounds possess excellent thermal stability. Furthermore, weekly shelf‐life tests (under ambient conditions) of the OTFTs based on the phenyl‐substituted compounds show that the mobility for the bis(diphenyl)‐substituted thiophene was almost unchanged for more than two months, indicating a high environmental stability.     

High‐Performance and Stable Organic Thin‐Film Transistors Based on Fused Thiophenes

A series of new organic semiconductors for organic thin‐film transistors (OTFTs) using dithieno[3,2‐b:2′,3′‐d]thiophene as the core are synthesized. Their electronic and optical properties are investigated using scanning electron microscopy (SEM), X‐ray diffraction (XRD), UV‐vis and photoluminescence spectroscopies, thermal gravimetric analysis (TGA), and differential scanning calorimetry (DSC). The compounds exhibit an excellent field‐effect performance with a high mobility of 0.42 cm² V^–1 s^–1 and an on/off ratio of 5 × 10⁶. XRD patterns reveal these films, grown by vacuum deposition, to be highly crystalline, and SEM reveals well‐interconnected, microcrystalline domains in these films at room temperature. TGA and DSC demonstrate that the phenyl‐substituted compounds possess excellent thermal stability. Furthermore, weekly shelf‐life tests (under ambient conditions) of the OTFTs based on the phenyl‐substituted compounds show that the mobility for the bis(diphenyl)‐substituted thiophene was almost unchanged for more than two months, indicating a high environmental stability.     

Inkjet Printed Large‐Area Flexible Few‐Layer Graphene Thermoelectrics

Graphene‐based organic nanocomposites have ascended as promising candidates for thermoelectric energy conversion. In order to adopt existing scalable printing methods for developing thermostable graphene‐based thermoelectric devices, optimization of both the material ink and the thermoelectric properties of the resulting films are required. Here, inkjet‐printed large‐area flexible graphene thin films with outstanding thermoelectric properties are reported. The thermal and electronic transport properties of the films reveal the so‐called phonon‐glass electron‐crystal character (i.e., electrical transport behavior akin to that of few‐layer graphene flakes with quenched thermal transport arising from the disordered nanoporous structure). As a result, the all‐graphene films show a room‐temperature thermoelectric power factor of 18.7 µW m^?1 K^?2, representing over a threefold improvement to previous solution‐processed all‐graphene structures. The demonstration of inkjet‐printed thermoelectric devices underscores the potential for future flexible, scalable, and low‐cost thermoelectric applications, such as harvesting energy from body heat in wearable applications.     

Nonvolatile Ferroelectric P(VDF‐TrFE) Memory Transistors Based on Inkjet‐Printed Organic Semiconductor

Nonvolatile ferroelectric poly(vinylidene fluoride‐co‐trifluoroethylene) memory based on an organic thin‐film transistor with inkjet‐printed dodecyl‐substituted thienylenevinylene‐thiophene copolymer (PC12TV12T) as the active layer is developed. The memory window is 4.5 V with a gate voltage sweep of ?12.5 V to 12.5 V. The field effect mobility, on/off ratio, and gate leakage current are 0.1 cm²/Vs, 10⁵, and 10^?10 A, respectively. Although the retention behaviors should be improved and optimized, the obtained characteristics are very promising for future flexible electronics.