Organic Tribotronic Transistor for Contact‐Electrification‐Gated Light‐Emitting Diode

Tribotronics is a new field about the devices fabricated using the electrostatic potential created by contact electrification as a “gate” voltage to tune/control charge carrier transport in semiconductors. In this paper, an organic tribotronic transistor is proposed by coupling an organic thin film transistor (OTFT) and a triboelectric nanogenerator (TENG) in vertical contact‐separation mode. Instead of using the traditional gate voltage for controlling, the charge carrier transportation in the OTFT can be modulated by the contact‐induced electrostatic potential of the TENG. By further coupling with an organic light‐emitting diode, a contact‐electrification‐gated light‐emitting diode (CG‐LED) is fabricated, in which the operating current and light‐emission intensity can be tuned/controlled by an external force–induced contact electrification. Two different modes of the CG‐LED have been demonstrated and the brightness can be decreased and increased by the applied physical contact, respectively. Different from the conventional organic light‐emitting transistor controlled by an electrical signal, the CG‐LED has realized the direct interaction between the external environment/stimuli and the electroluminescence device. By introducing optoelectronics into tribotronics, the CG‐LED has open up a new field of tribophototronics with many potential applications in interactive display, mechanical imaging, micro‐opto‐electro‐mechanical systems, and flexible/touch optoelectronics.     

Photo‐Stable Organic Thin‐Film Transistor Utilizing a New Indolocarbazole Derivative for Image Pixel and Logic Applications

Small molecule pentacene layer has been a representative among many organic thin‐film transistor (OTFT) channels with decent p‐type mobilities, but it is certainly light‐sensitive due to its relatively small highest occupied molecular orbital‐lowest unoccupied molecular orbital (HOMO‐LUMO) gap (1.85 eV). Although a few other small molecule‐based layers have been reported later, their photo‐stabilities or related device applications have hardly been addressed. Here, a new photostable organic layer is reported, heptazole (C₂₆H₁₆N₂), which has almost the same HOMO level as that of pentacene but with a higher HOMO‐LUMO gap (≈2.95 eV). This heptazole OTFT displays a decent mobility comparable to that of conventional amorphous Si TFTs, showing good photostability unlike pentacene OTFTs. An image pixel driving the photostable heptazole OTFT connected to a pentacene/Al Schottky photodiode is demonstrated. This heptazole OTFT also conveniently forms a logic inverter coupled with a pentacene OTFT, sharing Au for source/drain.     

Transparent Nanopaper‐Based Flexible Organic Thin‐Film Transistor Array

Eco‐friendly and low‐cost cellulose nanofiber paper (nanopaper) is a promising candidate as a novel substrate for flexible electron device applications. Here, a thin transparent nanopaper‐based high‐mobility organic thin‐film transistor (OTFT) array is demonstrated for the first time. Nanopaper made from only native wood cellulose nanofibers has excellent thermal stability (>180 °C) and chemical durability, and a low coefficient of thermal expansion (CTE: 5–10 ppm K^‐1). These features make it possible to build an OTFT array on nanopaper using a similar process to that for an array on conventional glass. A short‐channel bottom‐contact OTFT is successfully fabricated on the nanopaper by a lithographic and solution‐based process. Owing to the smoothness of the cast‐coated nanopaper surface, a solution processed organic semiconductor film on the nanopaper comprises large crystalline domains with a size of approximately 50–100 μm, and the corresponding TFT exhibits a high hole mobility of up to 1 cm²V^‐1 s^‐1 and a small hysteresis of below 0.1 V under ambient conditions. The nanopaper‐based OTFT also had excellent flexibility and can be formed into an arbitrary shape. These combined technologies of low‐cost and eco‐friendly paper substrates and solution‐based organic TFTs are promising for use in future flexible electronics application such as flexible displays and sensors.     

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.     

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.     

Stretchable Active Matrix Inorganic Light‐Emitting Diode Display Enabled by Overlay‐Aligned Roll‐Transfer Printing

An active matrix‐type stretchable display is realized by overlay‐aligned transfer of inorganic light‐emitting diode (LED) and single‐crystal Si thin film transistor (TFT) with roll processes. The roll‐based transfer enables integration of heterogeneous thin film devices on a rubber substrate while preserving excellent electrical and optical properties of these devices, comparable to their bulk properties. The electron mobility of the integrated Si‐TFT is over 700 cm² V^?1 s^?1, and this is attributed to the good interface between the Si channel and the thermally grown SiO₂ insulator. The light emission properties of the LED are of wafer quality. The resulting display stably operates under tensile strains up to 40%, over 200 cycles, demonstrating the potential of stretchable displays based on inorganic materials.     

Biocompatible and Biodegradable Organic Transistors Using a Solid‐State Electrolyte Incorporated with Choline‐Based Ionic Liquid and Polysaccharide

Biocompatible, biodegradable, and solid‐state electrolyte‐based organic transistors are demonstrated. As the electrolyte is composed of all edible materials, which are levan polysaccharide and choline‐based ionic liquid, the organic transistor fabricated on the electrolyte can be biocompatible and biodegrable. Compared to the other ion gel based electrolytes, it has superior electrical and mechanical properties, large specific capacitance (≈40 µF cm^?2), non‐volatility, flexibility, and high transparency. Thus, it shows mechanical reliability by maintaining electrical performances under up to 1.11% of effective bending strain, 5% of stretching, and have low operation voltage range when it is utilized in organic transistors. Moreover, the biodegradable electrolyte‐based organic transistors can be applied to bio‐integrated devices, such as electrocardiogram (ECG) recordings on human skin and the heart of a rat. The measured ECG signals from the transistors, compared to signals from electrode‐based sensors, has a superior signal‐to‐noise ratio. The biocompatible and biodegradable materials and devices can contribute to the development of many bioelectronics.     

Electrical Double‐Slope Nonideality in Organic Field‐Effect Transistors

The field effect transistor (FET) is arguably one of the most important circuit elements in modern electronics. Recently, a need has developed for flexible electronics in a variety of emerging applications. Examples include form‐fitting healthcare‐monitoring devices, flexible displays, and flexible radio frequency identification tags. Organic FETs (OFETs) are viable candidates for producing such flexible devices because they incorporate semiconducting π‐conjugated materials, including small molecules and conjugated polymers, which are intrinsically soft and mechanically compatible with flexible substrates. For OFETs to be industrially viable, however, they must achieve not only high charge carrier mobility, but also ideal and comprehensible electrical characteristics. Most recently, nonideal double‐slope characteristics in the transfer curves of OFETs (i.e., high slope at low gate voltage and low slope at high gate voltage), have stirred debate, which has led to different mechanistic rationales in the literature. This review focuses on the general observations, mechanistic understanding, and possible solutions associated with phenomena that result in FETs with double‐slope characteristics. By surveying and systematically summarizing in a single source relevant literature that deals with the issue of double slope, the experimental framework and theoretical basis for interpreting and avoiding this electrical nonideality in OFETs is provided.     

Organic Junction Field‐Effect Transistor

The realization and performance of a novel organic field‐effect transistor—the organic junction field‐effect transistor (JFET)—is discussed. The transistors are based on the modulation of the thickness of a depletion layer in an organic pin junction with varying gate potential. Based on numerical modeling, suitable layer thicknesses and doping concentrations are identified. Experimentally, organic JFETs are realized and it is shown that the devices clearly exhibit amplification. Changes in the electrical characteristics due to a variation of the intrinsic and the p‐doped layer thickness are rationalized by the numerical model, giving further proof to the proposed operational mechanism.     

An Organic Thin‐Film Transistor with a Photoinitiator‐Free Photosensitive Polyimide as Gate Insulator

This investigation deals with the synthesis and detailed study of a photoinitiator‐free photosensitive polyimide gate insulator for organic thin‐film transistors (OTFTs), one of the most important components of active‐matrix displays on plastic substrates. The photosensitive polyimide precursor poly(amic acid) is prepared from the aromatic dianhydride 3,3′,4,4′‐benzophenone tetracarboxylic dianhydride (BTDA) and the novel aromatic diamine 7‐(3,5‐diaminobenzoyloxy)coumarine (DACM). The photosensitivity of the poly(amic acid) film is investigated using a high‐pressure mercury lamp at 280–310 nm. The pattern resolution of the photocured film was about 50 μm. The surface morphology of the films before and after the photopatterning process is also investigated. In addition, we have fabricated pentacene OTFTs with the photoinitiator‐free photosensitive polyimide as gate insulator. The OTFT characteristics are discussed in more detail with respect to the electrical properties of the photosensitive polyimide thin film.     

Detailed Characterization of Contact Resistance,Gate‐Bias‐Dependent Field‐Effect Mobility,and Short‐Channel Effects with Microscale Elastomeric Single‐Crystal Field‐Effect Transistors

The organic field‐effect transistor (OFET) has proven itself invaluable as both the fundamental element in organic circuits and the primary tool for the characterization of novel organic electronic materials. Crucial to the success of the OFET in each of these venues is a working understanding of the device physics that manifest themselves in the form of electrical characteristics. As commercial applications shift to smaller device dimensions and structure/property relationships become more refined, the understanding of these phenomena become increasingly critical. Here, we employ high‐performance, elastomeric, photolithographically patterned single‐crystal field‐effect transistors as tools for the characterization of short‐channel effects and bias‐dependent parasitic contact resistance and field‐effect mobility. Redundant characterization of devices at multiple channel lengths under a single crystal allow the morphology‐free analysis of these effects, which is carried out in the context of a device model previously reported. The data show remarkable consistency with our model, yielding fresh insight into each of these phenomena, as well as confirming the utility of our FET design.     

A Review of Low‐Temperature Solution‐Processed Metal Oxide Thin‐Film Transistors for Flexible Electronics

Solution processing, including printing technology, is a promising technique for oxide thin‐film transistor (TFTs) fabrication because it tends to be a cost‐effective process with high composition controllability and high throughput. However, solution‐processed oxide TFTs are limited by low‐performance and stability issues, which require high‐temperature annealing. This high thermal budget in the fabrication process inhibits oxide TFTs from being applied to flexible electronics. There have been numerous attempts to promote the desired electrical characteristics of solution‐processed oxide TFTs at lower fabrication temperatures. Recent techniques for achieving low‐temperature (<350 °C) solution‐processed and printed oxide TFTs, in terms of the materials, processes, and structural engineering methods currently in use are reviewed. Moreover, the core techniques for both n‐type and p‐type oxide‐based channel layers, gate dielectric layers, and electrode layers in oxide TFTs are addressed. Finally, various multifunctional and emerging applications based on low‐temperature solution‐processed oxide TFTs are introduced and future outlooks for this highly promising research are suggested.     

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.     

Modulated Thermoelectric Properties of Organic Semiconductors Using Field‐Effect Transistors

Organic thermoelectric materials, which can transform heat flow into electricity, have great potential for flexible, ultra‐low‐cost and large‐area thermoelectric applications. Despite rapid developments of organic thermoelectric materials, exploration and investigation of promising organic thermoelectric semiconductors still remain as a challenge. Here, the thermoelectric properties of several p‐ and n‐type organic semiconductors are investigated and studied, in particular, how the electric field modulations of the Seebeck coefficient in organic field‐effect transistors (OFETs) compare with the Seebeck coefficient in chemically doped films. The extracted relationship between the Seebeck coefficient (S) and electrical conductivity (σ) from the field‐effect transistor (FET) geometry is in good agreement with that of chemically doped films, enabling the investigation of the trade‐off relationship among σ, S, carrier concentration, and charging level. The results make OFETs an effective candidate for the thermoelectric studies of organic semiconductors.     

A Single Transistor‐Level Direct‐Conversion Mixer for Low‐Voltage Low‐Power Multi‐band Radios

A CMOS direct‐conversion mixer with a single transistor‐level topology is proposed in this paper. Since the single transistor‐level topology needs smaller supply voltage than the conventional Gilbert‐cell topology, the proposed mixer structure is suitable for a low power and highly integrated RF system‐on‐a‐chip (SoC). The proposed direct‐conversion mixer is designed for the multi‐band ultra‐wideband (UWB) system covering from 3 to 7 GHz. The conversion gain and input P1dB of the mixer are about 3 dB and ?10 dBm, respectively, with multi‐band RF signals. The mixer consumes 4.3 mA under a 1.8 V supply voltage.     

Contact Resistance in Organic Field‐Effect Transistors: Conquering the Barrier

Organic semiconductors have sparked interest as flexible, solution processable, and chemically tunable electronic materials. Improvements in charge carrier mobility put organic semiconductors in a competitive position for incorporation in a variety of (opto‐)electronic applications. One example is the organic field‐effect transistor (OFET), which is the fundamental building block of many applications based on organic semiconductors. While the semiconductor performance improvements opened up the possibilities for applying organic materials as active components in fast switching electrical devices, the ability to make good electrical contact hinders further development of deployable electronics. Additionally, inefficient contacts represent serious bottlenecks in identifying new electronic materials by inhibiting access to their intrinsic properties or providing misleading information. Recent work focused on the relationships of contact resistance with device architecture, applied voltage, metal and dielectric interfaces, has led to a steady reduction in contact resistance in OFETs. While impressive progress was made, contact resistance is still above the limits necessary to drive devices at the speed required for many active electronic components. Here, the origins of contact resistance and recent improvement in organic transistors are presented, with emphasis on the electric field and geometric considerations of charge injection in OFETs.     

High‐Performance n‐ and p‐Type Field‐Effect Transistors Based on Hybridly Surface‐Passivated Colloidal PbS Nanosheets

Colloidally synthesized nanomaterials are among the promising candidates for future electronic devices due to their simplicity and the inexpensiveness of their production. Specifically, colloidal nanosheets are of great interest since they are conveniently producible through the colloidal approach while having the advantages of two‐dimensionality. In order to employ these materials, according transistor behavior should be adjustable and of high performance. It is shown that the transistor performance of colloidal lead sulfide nanosheets is tunable by altering the surface passivation, the contact metal, or by exposing them to air. It is found that adding halide ions to the synthesis leads to an improvement of the conductivity, the field‐effect mobility, and the on/off ratio of these transistors by passivating their surface defects. Superior n‐type behavior with a field‐effect mobility of 248 cm² V^?1 s^?1 and an on/off ratio of 4 × 10⁶ is achieved. The conductivity of these stripes can be changed from n‐type to p‐type by altering the contact metal and by adding oxygen to the working environment. As a possible solution for the post‐Moore era, realizing new high‐quality semiconductors such as colloidal materials is crucial. In this respect, the results can provide new insights which helps to accelerate their optimization for potential applications.     

High‐Resolution Monolithic Integrated Tribotronic InGaZnO Thin‐Film Transistor Array for Tactile Detection

Tactile detection is a crucial technology in many fields, such as electronic skin, touch screen control, human prostheses, and screen fingerprint identification. Tribotronics has demonstrated active mechanosensation from external mechanical stimuli, which greatly enriches the sensing mechanisms of tactile detection. In this work, a monolithic integrated indium‐gallium‐zinc‐oxide (InGaZnO or IGZO) thin‐film transistor (TFT) array is developed for high‐resolution tactile detection. By using the conventional semiconductor fabrication processes, each IGZO TFT cell in the array shows uniform electrical performance. In addition, the drain–source current can be individually tuned by the electrostatic potential generated by the contact electrification between a movable gate and the gate dielectric. The monolithic integrated array displays a relatively high resolution of 12 pixels per inch and can realize a millimeter‐level tactile perception and motion tracking. This work presents a facile and viable strategy toward micro/nano‐scale tribotronics, which can realize high‐resolution and large‐scale tactile detection.     

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.     

Network Structure Modification‐Enabled Hybrid Polymer Dielectric Film with Zirconia for the Stretchable Transistor Applications

Stretchable electronic devices should be enabled by the smart design of materials and architectures because their commercialization is limited by the tradeoff between stretchability and electrical performance limits. In this study, thin‐film transistors are fabricated using strategies that combine the unit process of a novel hybrid gate insulator and low‐temperature indium gallium tin oxide (IGTO) channel layer and a stress‐relief substrate structure. Novel hybrid dielectric films are synthesized and their molecular structural configurations are analyzed. These films consist of a polymer [poly(4‐vinylphenol‐co‐methylmethacrylate)], cross‐linkers having different binding structures [1,6‐bis(trimethoxysilyl)hexane (BTMSH), dodecyltrimethoxysilane, and poly(melamine‐co‐formaldehyde)], and an inorganic zirconia component (ZrO_x). The hybrid film with BTMSH cross‐linker and 0.2 M ZrO_x exhibits excellent insulating properties as well as mechanical stretchability. IGTO transistors fabricated on polyimide‐coated glass substrates are transferred to the rubber substrate to offer stretchability of the transistor pixelated thin‐film transistors. IGTO transistors fabricated on stretchable substrates using these strategies show promising electrical performance and mechanical durability. After 200 stretchability test cycles under uniaxial elongation of approximately 300%, the IGTO transistor still retains a high carrier mobility of 21.7 cm² V^?1 s^?1, a low sub‐threshold gate swing of 0.68 V decade^?1 and a high I_ON/OFF ratio of 2.0 × 10⁷.