Three-Dimensional, Inkjet-Printed Organic Transistors and Integrated Circuits with 100% Yield, High Uniformity, and Long-Term Stability

From Printed Devices to Vertically Stacked, 3D Flexible Hybrid Systems

The pursuit of miniaturized Si electronics has revolutionized computing and communication. During recent years, the value addition in electronics has also been achieved through printing, flexible and stretchable electronics form factors, and integration over areas larger than wafer size. Unlike Si semiconductor manufacturing which takes months from tape-out to wafer production, printed electronics offers greater flexibility and fast-prototyping capabilities with lesser resources and waste generation. While significant advances have been made with various types of printed sensors and other passive devices, printed circuits still lag behind Si-based electronics in terms of performance, integration density, and functionality. In this regard, recent advances using high-resolution printing coupled with the use of high mobility materials and device engineering, for both in-plane and out-of-plane integration, raise hopes. This paper focuses on the progress in printed electronics, highlighting emerging printing technologies and related aspects such as resource efficiency, environmental impact, integration scale, and the novel functionalities enabled by vertical integration of printed electronics. By highlighting these advances, this paper intends to reveal the future promise of printed electronics as a sustainable and resource-efficient route for realizing high-performance integrated circuits and systems.

Oxide semiconductor based deep-subthreshold operated read-out electronics for all-printed smart sensor patches

The ability to fabricate an entire smart sensor patch with read-out electronics using commercial printing techniques may have a wide range of potential applications. Although solution-processed oxide thin film transistors (TFTs) are capable of providing high mobility electron transport, resulting in large ON-state current and power output, there is hardly any literature report that uses the printed oxide TFTs at the sensor interfaces. Here, printed amorphous indium-gallium-zinc oxide (a-IGZO)-based deep-subthreshold operated TFTs that comprise signal amplifiers and analog-to-digital converters (ADCs) that can successfully digitalize the analog sensor signals up to a frequency range of 1 kHz are reported. In addition, exploiting the high current oxide TFTs, a current drive circuit placed after the ADC unit has been found useful in producing easy-to-detect visual recognition of the sensor signal at a predefined threshold crossover. Notably, the entire smart sensor patch is demonstrated to operate at a low supply voltage of ≤2 V, thereby ensuring that it can be an on-chip energy source compatible and standalone detection unit.

Improving both performance and stability of n-type organic semiconductors by vitamin C

Organic semiconductors (OSCs) are one of the most promising candidates for flexible, wearable and large-area electronics. However, the development of n-type OSCs has been severely held back due to the poor stability of their most candidates, that is, the intrinsically high reactivity of negatively charged polarons to oxygen and water. Here we demonstrate a general strategy based on vitamin C to stabilize n-type OSCs, remarkably improving the performance and stability of their device, for example, organic field-effect transistors. Vitamin C scavenges reactive oxygen species and inhibits their generation by sacrificial oxidation and non-sacrificial triplet quenching in a cascade process, which not only lastingly prevents molecular structure from oxidation damage but also passivates the latent electron traps to stabilize electron transport. This study presents a way to overcome the long-standing stability problem of n-type OSCs and devices.

Vertically Integrated Monolithic Neuromorphic Nanowire Device for Physiological Information Processing

This study demonstrates the conceptual design and fabrication of a vertically integrated monolithic (VIM) neuromorphic device. The device comprises an n-type SnO2 nanowire bottom channel connected by a shared gate to a p-type P3HT nanowire top channel. This architecture establishes two distinct neural pathways with different response behaviors. The device generates excitatory and inhibitory postsynaptic currents, mimicking the corelease mechanism of bilingual synapses. To enhance the signal processing efficiency, we employed a bipolar spike encoding strategy to convert fluctuating sensory signals to spike trains containing positive and negative pulses. Utilizing the neuromorphic platform for synaptic processing, physiological signals featuring bidirectional fluctuations, including electrocardiogram and breathing signals, can be classified with an accuracy of over 90%. The VIM device holds considerable promise as a solution for developing highly integrated neuromorphic hardware for healthcare and edge intelligence applications.

Low-dimensional nanostructures for monolithic 3D-integrated flexible and stretchable electronics

Flexible/stretchable electronics, which are characterized by their ultrathin design, lightweight structure, and excellent mechanical robustness and conformability, have garnered significant attention due to their unprecedented potential in healthcare, advanced robotics, and human-machine interface technologies. An increasing number of low-dimensional nanostructures with exceptional mechanical, electronic, and/or optical properties are being developed for flexible/stretchable electronics to fulfill the functional and application requirements of information sensing, processing, and interactive loops. Compared to the traditional single-layer format, which has a restricted design space, a monolithic three-dimensional (M3D) integrated device architecture offers greater flexibility and stretchability for electronic devices, achieving a high-level of integration to accommodate the state-of-the-art design targets, such as skin-comfort, miniaturization, and multi-functionality. Low-dimensional nanostructures possess small size, unique characteristics, flexible/elastic adaptability, and effective vertical stacking capability, boosting the advancement of M3D-integrated flexible/stretchable systems. In this review, we provide a summary of the typical low-dimensional nanostructures found in semiconductor, interconnect, and substrate materials, and discuss the design rules of flexible/stretchable devices for intelligent sensing and data processing. Furthermore, artificial sensory systems in 3D integration have been reviewed, highlighting the advancements in flexible/stretchable electronics that are deployed with high-density, energy-efficiency, and multi-functionalities. Finally, we discuss the technical challenges and advanced methodologies involved in the design and optimization of low-dimensional nanostructures, to achieve monolithic 3D-integrated flexible/stretchable multi-sensory systems.

Joint Experimental and Theoretical Study on Deposition Morphologies in Polymer Sessile Droplets

Although past experimental and theoretical research has made substantial progress in understanding evaporation behaviors in various suspensions, the fundamental mechanism for polymer sessile droplets is still lacking. One critical effect is the molecular weight on the evaporation behaviors. Here, systematic experiments are carried out to investigate the evaporation behavior of polymer droplets under the effects of polymer concentration, evaporation rate, and especially molecular weight. We obtain polymer films with various morphologies with molecular weights ranging from 2 orders of magnitude to 4 orders of magnitude and polymer concentration across 4 orders of magnitude. We further develop a theoretical model based on the Onsager principle to explain the evaporation mechanism from a dynamic perspective. Analysis indicates that increasing molecular weight or polymer concentration enhances the contact angle hysteresis and slows down the evaporation, resulting in the transition from multiring to coffee ring and eventually to uniform films. The findings offer a guideline for achieving the desired deposition patterns via droplet processing techniques.

Topology-Mediated Molecule Nucleation Anchoring Enables Inkjet Printing of Organic Semiconducting Single Crystals for High-Performance Printed Electronics

Printable organic semiconducting single crystals (OSSCs) offer tantalizing opportunities for next-generation wearable electronics, but their development has been plagued by a long-standing yet inherent problem─spatially uncontrolled and stochastic nucleation events─which usually causes the formation of polycrystalline films and hence limited performance. Here, we report a convenient approach to precisely manipulate the elusive molecule nucleation process for high-throughput inkjet printing of OSSCs with record-high mobility. By engineering curvature of the contact line with a teardrop-shaped micropattern, molecule nucleation is elegantly anchored at the vertex of the topological structure, enabling formation of a single nucleus for the subsequent growth of OSSCs. Using this approach, we achieve patterned growth of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene single crystals, yielding a breakthrough for an organic field-effect transistor array with a high average mobility of 12.5 cm2 V-1 s-1. These findings not only provide keen insights into controlling molecule nucleation kinetics but also offer opportunities for high-performance printed electronics.

Solution-Processed Polymer Dielectric Interlayer for Low-Voltage, Unipolar n-Type Organic Field-Effect Transistors

The integration of organic electronic circuits into real-life applications compels the fulfillment of a range of requirements, among which the ideal operation at a low voltage with reduced power consumption is paramount. Moreover, these performance factors should be achieved via solution-based fabrication schemes in order to comply with the promise of cost- and energy-efficient manufacturing offered by an organic, printed electronic technology. Here, we propose a solution-based route for the fabrication of low-voltage organic transistors, encompassing ideal device operation at voltages below 5 V and exhibiting n-type unipolarization. This process is widely applicable to a variety of semiconducting and dielectric materials. We achieved this through the use of a photo-cross-linked, low-k dielectric interlayer, which is used to fabricate multilayer dielectric stacks with areal capacitances of up to 40 nF/cm2 and leakage currents below 1 nA/cm2. Because of the chosen azide-based cross-linker, the dielectric promotes n-type unipolarization of the transistors and demonstrated to be compatible with different classes of semiconductors, from conjugated polymers to carbon nanotubes and low-temperature metal oxides. Our results demonstrate a general applicability of our unipolarizing dielectric, facilitating the implementation of complementary circuitry of emerging technologies with reduced power consumption.

Tunable Organic Active Neural Probe Enabling Near-Sensor Signal Processing

Neural recording systems have significantly progressed to provide an advanced understanding and treatment for neurological diseases. Flexible transistor-based active neural probes exhibit great potential in electrophysiology applications due to their intrinsic amplification capability and tissue-compliant nature. However, most current active neural probes exhibit bulky back-end connectivity since the output is current, and the development of an integrated circuit for voltage output is crucial for near-sensor signal processing at the abiotic/biotic interface. Here, inkjet-printed organic voltage amplifiers are presented by monolithically integrating organic electrochemical transistors and thin-film polymer resistors on a single, highly flexible substrate for in vivo brain activity recording. Additive inkjet printing enables the seamless integration of multiple active and passive components on the somatosensory cortex, leading to significant noise reduction over the externally connected typical configuration. It also facilitates fine-tuning of the voltage amplification and frequency properties. The organic voltage amplifiers are validated as electrocorticography devices in a rat in vivo model, showing their ability to record local field potentials in an experimental model of spontaneous and epileptiform activity. These results bring organic active neural probes to the forefront in applications where efficient sensory data processing is performed at sensor endpoints.

Flexible organic field-effect transistors-based biosensors: progress and perspectives

Organic field-effect transistors (OFETs) have been proposed beyond three decades while becoming a research hotspot again in recent years because of the fast development of flexible electronics. Many novel flexible OFETs-based devices have been reported in these years. Among these devices, flexible OFETs-based sensors made great strides because of the extraordinary sensing capability of FET. Most of these flexible OFETs-based sensors were designed for biological applications due to the advantages of flexibility, reduced complexity, and lightweight. This paper reviews the materials, fabrications, and applications of flexible OFETs-based biosensors. Besides, the challenges and opportunities of the flexible OFETs-based biosensors are also discussed.

Impact of Planar and Vertical Organic Field-Effect Transistors on Flexible Electronics

The development of flexible and conformable devices, whose performance can be maintained while being continuously deformed, provides a significant step toward the realization of next-generation wearable and e-textile applications. Organic field-effect transistors (OFETs) are particularly interesting for flexible and lightweight products, because of their low-temperature solution processability, and the mechanical flexibility of organic materials that endows OFETs the natural compatibility with plastic and biodegradable substrates. Here, an in-depth review of two competing flexible OFET technologies, planar and vertical OFETs (POFETs and VOFETs, respectively) is provided. The electrical, mechanical, and physical properties of POFETs and VOFETs are critically discussed, with a focus on four pivotal applications (integrated logic circuits, light-emitting devices, memories, and sensors). It is pointed out that the flexible function of the relatively newer VOFET technology, along with its perspective on advancing the applicability of flexible POFETs, has not been reviewed so far, and the direct comparison regarding the performance of POFET- and VOFET-based flexible applications is most likely absent. With discussions spanning printed and wearable electronics, materials science, biotechnology, and environmental monitoring, this contribution is a clear stimulus to researchers working in these fields to engage toward the plentiful possibilities that POFETs and VOFETs offer to flexible electronics.

Dual-gate thin film transistor lactate sensors operating in the subthreshold regime

Organic thin-film transistors (TFTs) with an electrochemically functionalized sensing gate are promising platforms for wearable health-monitoring technologies because they are light, flexible, and cheap. Achieving both high sensitivity and low power is highly demanding for portable or wearable devices. In this work, we present flexible printed dual-gate (DG) organic TFTs operating in the subthreshold regime with ultralow power and high sensitivity. The subthreshold operation of the gate-modulated TFT-based sensors not only increases the sensitivity but also reduces the power consumption. The DG configuration has deeper depletion and stronger accumulation, thereby further making the subthreshold slope sharper. We integrate an enzymatic lactate-sensing extended-gate electrode into the printed DG TFT and achieve exceptionally high sensitivity (0.77) and ultralow static power consumption (10 nW). Our sensors are successfully demonstrated in physiological lactate monitoring with human saliva. The accuracy of the DG TFT sensing system is as good as that of a high-cost conventional assay. The developed platform can be readily extended to various materials and technologies for high performance wearable sensing applications.

Vertically Integrated Electronics: New Opportunities from Emerging Materials and Devices

Vertical three-dimensional (3D) integration is a highly attractive strategy to integrate a large number of transistor devices per unit area. This approach has emerged to accommodate the higher demand of data processing capability and to circumvent the scaling limitation. A huge number of research efforts have been attempted to demonstrate vertically stacked electronics in the last two decades. In this review, we revisit materials and devices for the vertically integrated electronics with an emphasis on the emerging semiconductor materials that can be processable by bottom-up fabrication methods, which are suitable for future flexible and wearable electronics. The vertically stacked integrated circuits are reviewed based on the semiconductor materials: organic semiconductors, carbon nanotubes, metal oxide semiconductors, and atomically thin two-dimensional materials including transition metal dichalcogenides. The features, device performance, and fabrication methods for 3D integration of the transistor based on each semiconductor are discussed. Moreover, we highlight recent advances that can be important milestones in the vertically integrated electronics including advanced integrated circuits, sensors, and display systems. There are remaining challenges to overcome; however, we believe that the vertical 3D integration based on emerging semiconductor materials and devices can be a promising strategy for future electronics.

Vertically stacked, low-voltage organic ternary logic circuits including nonvolatile floating-gate memory transistors

Multi-valued logic (MVL) circuits based on heterojunction transistor (HTR) have emerged as an effective strategy for high-density information processing without increasing the circuit complexity. Herein, an organic ternary logic inverter (T-inverter) is demonstrated, where a nonvolatile floating-gate flash memory is employed to control the channel conductance systematically, thus realizing the stabilized T-inverter operation. The 3-dimensional (3D) T-inverter is fabricated in a vertically stacked form based on all-dry processes, which enables the high-density integration with high device uniformity. In the flash memory, ultrathin polymer dielectrics are utilized to reduce the programming/erasing voltage as well as operating voltage. With the optimum programming state, the 3D T-inverter fulfills all the important requirements such as full-swing operation, optimum intermediate logic value (~V_DD/2), high DC gain exceeding 20 V/V as well as low-voltage operation (< 5 V). The organic flash memory exhibits long retention characteristics (current change less than 10% after 10⁴s), leading to the long-term stability of the 3D T-inverter. We believe the 3D T-inverter employing flash memory developed in this study can provide a useful insight to achieve high-performance MVL circuits.

High-density information processing without increasing the circuit complexity is highly desired in electronics. Here, Im et al. demonstrate a low-voltage organic ternary logic circuit vertically integrated with the nonvolatile flash memory, increasing the information density by a factor of 3.

Electrically Reconfigurable Organic Logic Gates: A Promising Perspective on a Dual-Gate Antiambipolar Transistor

Electrically reconfigurable organic logic circuits are promising candidates for realizing new computation architectures, such as artificial intelligence and neuromorphic devices. In this study, multiple logic gate operations are attained based on a dual-gate organic antiambipolar transistor (DG-OAAT). The transistor exhibits a Λ-shaped transfer curve, namely, a negative differential transconductance at room temperature. It is important to note that the peak voltage of the drain current is precisely tuned by three input signals: bottom-gate, top-gate, and drain voltages. This distinctive feature enables multiple logic gate operations with "only a single DG-OAAT," which are not obtainable in conventional transistors. Five logic gate operations, which correspond to AND, OR, NAND, NOR, and XOR, are demonstrated by adjusting the bottom-gate and top-gate voltages. Moreover, varying the drain voltage makes it possible to reversibly switch two logic gates, e.g., NAND/NOR and OR/XOR. In addition, the DG-OAATs show a high degree of stability and reliability. The logic gate operations are observed even months later. The hysteresis in the transfer curves is also negligible. Thus, the device concept is promising for realizing multifunctional logic circuits with a simple transistor configuration. Hence, these findings are expected to surpass the current limitations in complementary metal-oxide-semiconductor devices.

Fabrication and electrical properties of printed three-dimensional integrated carbon nanotube PMOS inverters on flexible substrates

The low resolution of current printing technology (usually 10-100 μm) limits the number of printed thin film transistors (TFTs) per processable area, resulting in the low integration of printed circuits. In this work, we developed a three-dimensional (3D) integration technology to increase the integration of printed TFTs and firstly achieved printed 3D single-walled carbon nanotube (SWCNT) PMOS inverter arrays on the flexible substrates. The flexible 3D PMOS inverter consists of a bottom-gate SWCNT TFT (i.e., a driving TFT) and a top-gate SWCNT TFT (i.e., a load TFT). Printed SWCNT TFTs exhibited good electrical properties with high carrier mobility (up to 9.53 cm² V^-1 s^-1), high I_on/I_off ratio (10⁵-10⁶), low hysteresis, and small subthreshold swing (SS) (70-80 mV dec^-1). As-prepared 3D PMOS inverters exhibited rail-to-rail voltage output characteristics, high voltage gain (10) at a low operating voltage (V_DD < 1 V), and good mechanical flexibility. Furthermore, the printed 3D PMOS inverters could be utilized to detect ammonia gases, exhibiting satisfactory stability and recovery rate. It is crucial for realizing high-density, multi-functional printed carbon-based electronic devices and circuits for wearable electronics and the Internet of Things (IoT).

Electrohydrodynamic-Jet-Printed Phthalimide-Derived Conjugated Polymers for Organic Field-Effect Transistors and Logic Gates

A π-conjugated polymer semiconductor, PBDTTTffPI, was synthesized for use as an organic semiconductor suitable for electrohydrodynamic (EHD) jet printing technology. Bulky alkylation of the polymer gave PBDTTTffPI good solubility in several organic solvents. EHD jet printing using PBDTTTffPI ink produced direct patterns of polymer semiconductors while maintaining smooth surface morphologies and crystal structures similar to those of spin-coated PBDTTTffPI films. EHD-jet-printed PBDTTTffPI was appropriate for use as a semiconductor layer in organic field-effect transistors (OFETs) and logic gates. OFETs that used EHD-jet-printed PBDTTTffPI had better electrical characteristics than devices that used spin-coated semiconductor films. When a dielectric material (Al₂O₃) with a high dielectric constant was introduced, the jet-printed PBDTTTffPI operated well at low voltages. Integrated devices such as inverters, NAND gates, and NOR gates were fabricated by printing PBDTTTffPI patterns and showed good switching behaviors. Therefore, the use of printable PBDTTTffPI provides an advance toward fabrication of practical integrated arrays in next-generation devices.

Spatiotemporal Measurement of Arterial Pulse Waves Enabled by Wearable Active-Matrix Pressure Sensor Arrays

Wearable pressure sensors have demonstrated great potential in detecting pulse pressure waves on the skin for the noninvasive and continuous diagnosis of cardiac conditions. However, difficulties lie in positioning conventional single-point sensors on an invisible arterial line, thereby preventing the detection of adequate signal amplitude for accurate pulse wave analysis. Herein, we introduce the spatiotemporal measurements of arterial pulse waves using wearable active-matrix pressure sensors to obtain optimal pulse waveforms. We fabricate thin-film transistor (TFT) arrays with high yield and uniformity using inkjet printing where array sizes can be customizable and integrate them with highly sensitive piezoresistive sheets. We maximize the pressure sensitivity (16.8 kPa^-1) and achieve low power consumption (10¹ nW) simultaneously by strategically modulating the TFT operation voltage. The sensor array creates a spatiotemporal pulse wave map on the wrist. The map presents the positional dependence of pulse amplitudes, which allows the positioning of the arterial line to accurately extract the augmentation index, a parameter for assessing arterial stiffness. The device overcomes the positional inaccuracy of conventional single-point sensors, and therefore, it can be used for medical applications such as arterial catheter injection or the diagnosis of cardiovascular disease in daily life.

Hybrid Thin-Film Materials Combinations for Complementary Integration Circuit Implementation

Beyond conventional silicon, emerging semiconductor materials have been actively investigated for the development of integrated circuits (ICs). Considerable effort has been put into implementing complementary circuits using non-silicon emerging materials, such as organic semiconductors, carbon nanotubes, metal oxides, transition metal dichalcogenides, and perovskites. Whereas shortcomings of each candidate semiconductor limit the development of complementary ICs, an approach of hybrid materials is considered as a new solution to the complementary integration process. This article revisits recent advances in hybrid-material combination-based complementary circuits. This review summarizes the strong and weak points of the respective candidates, focusing on their complementary circuit integrations. We also discuss the opportunities and challenges presented by the prospect of hybrid integration.

Layout-to-Bitmap Conversion and Design Rules for Inkjet-Printed Large-Scale Integrated Circuits

Digital inkjet printing (IJP) can greatly reduce the manufacturing cost and waste of flexible large-area electronics by adding micro-fine patterns onto plastic foils. Advanced system design using IJP has been limited by the lack of an electronic design automation (EDA) approach. An EDA approach based on a vector-based layout drawing requires parameterized IJP design rules. This study proposes a layout-to-bitmap (L2B) conversion procedure and line-based design rules that leverage the existing circuit layout EDA tools for advanced IJP designs. The L2B conversion is accomplished by optimizing the parameters of the horizontal and vertical lines by varying the drop spacings and platen temperatures. Next, the line-based layouts are converted to bitmap files which are used as IJP input data for printing multiple metal layers. This study systematically investigated the development of an IJP process employing Ag nanoparticles. The physical characteristics of the proposed process were evaluated based on theories concerning inkjet-printed bead formation. The design rules for fabricating printed thin-film transistor (TFT) circuits were documented. Documentation is the first step in creating an IJP process design kit for advanced electronics design. Using the optimized L2B conversion procedure and the design rules, a 10 × 10 array of printed organic TFTs was fabricated to demonstrate the reliability of the developed process. Additionally, the fabricated printed organic TFTs indicated that the proposed process could be extended to large-scale system designs.

Layer-By-Layer Printing Strategy for High-Performance Flexible Electronic Devices with Low-Temperature Catalyzed Solution-Processed SiO2

Additive printing techniques have been widely investigated for fabricating multilayered electronic devices. In this work, a layer-by-layer printing strategy is developed to fabricate multilayered electronics including 3D conductive circuits and thin-film transistors (TFTs) with low-temperature catalyzed, solution-processed SiO₂ (LCSS) as the dielectric. Ultrafine, ultrasmooth LCSS films can be facilely formed at 90 °C on a wide variety of organic and inorganic substrates, offering a versatile platform to construct complex heterojunction structures with layer-by-layer fashion at microscale. The high-resolution 3D conductive circuits formed with gold nanoparticles inside the LCSS dielectric demonstrate a high-speed response to the transient voltage in less than 1 µs. The TFTs with semiconducting single-wall carbon nanotubes can be operated with the accumulation mode at a low voltage of 1 V and exhibit average field-effect mobility of 70 cm²  V^-1  s^-1 , on/off ratio of 10⁷ , small average hysteresis of 0.1 V, and high yield up to 100% as well as long-term stability, high negative-gate bias stability, and good mechanical stability. Therefore, the layer-by-layer printing strategy with the LCSS film is promising to assemble large-scale, high-resolution, and high-performance flexible electronics and to provide a fundamental understanding for correlating dielectric properties with device performance.

A comprehensive nano-interpenetrating semiconducting photoresist toward all-photolithography organic electronics

Owing to high resolution, reliability, and industrial compatibility, all-photolithography is a promising strategy for industrial manufacture of organic electronics. However, it receives limited success due to the absence of a semiconducting photoresist with high patterning resolution, mobility, and performance stability against photolithography solution processes. Here, we develop a comprehensive semiconducting photoresist with nano-interpenetrating structure. After photolithography, nanostructured cross-linking networks interpenetrate with continuous phases of semiconducting polymers, enabling submicrometer patterning accuracy and compact molecular stacking with high thermodynamic stability. The mobility reaches the highest values of photocrosslinkable organic semiconductors and maintains almost 100% after soaking in developer and stripper for 1000 min. Owing to the comprehensive performance, all-photolithography is achieved, which fabricates organic inverters and high-density transistor arrays with densities up to 1.1 × 10⁵ units cm^-2 and 1 to 4 orders larger than conventional printing processes, opening up a new approach toward manufacturing highly integrated organic circuits and systems.

Suppressing bias stress degradation in high performance solution processed organic transistors operating in air

Solution processed organic field effect transistors can become ubiquitous in flexible optoelectronics. While progress in material and device design has been astonishing, low environmental and operational stabilities remain longstanding problems obstructing their immediate deployment in real world applications. Here, we introduce a strategy to identify the most probable and severe degradation pathways in organic transistors and then implement a method to eliminate the main sources of instabilities. Real time monitoring of the energetic distribution and transformation of electronic trap states during device operation, in conjunction with simulations, revealed the nature of traps responsible for performance degradation. With this information, we designed the most efficient encapsulation strategy for each device type, which resulted in fabrication of high performance, environmentally and operationally stable small molecule and polymeric transistors with consistent mobility and unparalleled threshold voltage shifts as low as 0.1 V under the application of high bias stress in air.

Electrical instability of organic field-effect transistors (OFETs) during operation remains a challenge that limits the device’s real-world technological viability. Here, the authors report a method for diagnosing and suppressing bias stress in solution-processed OFETs operated in air.

A Flexible 3D Organic Preamplifier for a Lactate Sensor

Organic transistors are promising platforms for wearable biosensors. However, the strategies to improve signal amplification have yet to be determined, particularly regarding biosensors that generate very weak signals. In this study, an organic voltage amplifier is presented for a lactate sensor on flexible plastic foil. The preamplifier is based on a 3D complementary inverter, which is achieved by vertically stacking complementary transistors with a shared gate between them. The shared gate is extended and functionalized with a lactate oxidase enzyme to detect lactate. The sensing device successfully detects the lactate concentration in the human sweat range (20-60 mm) with high sensitivity (6.82 mV mm^-1 ) due to high gain of its amplification. The 3D integration process is cost-effective as it is solution-processable and doubles the number of transistors per unit area. The device presented in this study would pave the way for the development of high-gain noninvasive sweat lactate sensors that can be wearable.

Hofmeister-Effect-Guided Ionohydrogel Design as Printable Bioelectronic Devices

Bioelectronic platforms convert biological signals into electrical signals by utilizing biocatalysts that provide tools to monitor the activity of cells and tissues. Traditional conducting materials such as solid conductors and conducting polymers are confronted with a great challenge in sophisticated production processes and mismatch at biological tissues-machine interfaces. Furthermore, the biocatalyst, the key functional component in the electron-transfer reaction for bio-signal detection denatures easily in an ionic conductive solution. Herein, a bionic strategy is elaborately developed to synthesize an ionohydrogel bioelectronic platform that possesses extracellular-matrix-like habitat by employing hydrated ionic liquids (HILs) as ionic solvent and bioprotectant. This strategy realizes an integration of ionic and enzymatic electronic circuits and minimization of the disparities between tissues and artificial machines. The Hofmeister effect of HILs on enzyme proteins and polymer chains ensures the high bioactivity of the enzymes and greatly improves the mechanical properties of the ionohydrogels. Moreover, hydrogen bonds formed by ILs, water, and polymer chains greatly improve the water-retention of the ionohydrogel and give it more practical significance. Consequently, the promising ionohydrogel is partly printed and fabricated into wearable devices as a pain-free humoral components monitor and a wireless motion-sensor.

Flexible Bottom-Gated Organic Field-Effect Transistors Utilizing Stamped Polymer Layers from the Surface of Water

The facile sequential deposition of functional organic thin films by solution processes is critical for the development of a variety of high-performance organic devices without restriction in terms of materials and processes. Herein, we propose a simple fabrication process that entails stacking multiple layers of functional polymers to fabricate organic field-effect transistors (OFETs). The process involves stamping organic semiconducting layers formed on the surface of water onto a commonly used polymeric dielectric layer. Our scheme makes it possible to independently optimize organic semiconductor films by controlling the solvent evaporation time during the process of film formation on the surface of water. This approach eliminates the need to be concerned about any interference with adjacent layers. Utilizing this process, the fabrication of high-performance bottom-gated OFETs is demonstrated on a glass and a flexible substrate. The OFETs consist of a vertically stacked diketopyrrolopyrrole-based polymer semiconducting layer on the poly(methyl methacrylate) film with a maximum hole mobility of 0.85 cm²/V s.

Solution-Processed Centimeter-Scale Highly Aligned Organic Crystalline Arrays for High-Performance Organic Field-Effect Transistors

Solution-printed organic single-crystalline films hold great potential for achieving low-cost manufacturing of large-area and flexible electronics. For practical applications, organic field-effect transistor arrays must exhibit high performance and small device-to-device variation. However, scalable fabrication of highly aligned organic crystalline arrays is rather difficult due to the lack of control over the crystallographic orientation, crystal uniformity, and thickness. Here, a facile solution-printing method to fabricate centimeter-sized highly aligned organic crystalline arrays with a thickness of a few molecular layers is reported. In this study, the solution shearing technique is used to produce large-area, organic highly crystalline thin films. Water-soluble ink is printed on the hydrophobic surface of organic crystalline films, to selectively protect it, followed by etching. It is shown that the addition of a surfactant dramatically changes the fluid drying dynamics and increases the contact line friction of the aqueous solution to the underlying nonwetting organic crystalline film. As a result, centimeter-scale highly aligned organic crystalline arrays are successfully prepared on different substrates. The devices based on organic crystalline arrays show good performance and uniformity. This study demonstrates that solution printing is close to industrial application and also expands its applicability to various printed flexible electronics.

Organic-based inverters: basic concepts, materials, novel architectures and applications

The review article covers the materials and techniques employed to fabricate organic-based inverter circuits and highlights their novel architectures, ground-breaking performances and potential applications.

Photo-Induced Doping in a Graphene Field-Effect Transistor with Inkjet-Printed Organic Semiconducting Molecules

In this work, we report a novel method of maskless doping of a graphene channel in a field-effect transistor configuration by local inkjet printing of organic semiconducting molecules. The graphene-based transistor was fabricated via large-scale technology, allowing for upscaling electronic device fabrication and lowering the device's cost. The altering of the functionalization of graphene was performed through local inkjet printing of N,N'-Dihexyl-3,4,9,10-perylenedicarboximide (PDI-C6) semiconducting molecules' ink. We demonstrated the high resolution (about 50 µm) and accurate printing of organic ink on bare chemical vapor deposited (CVD) graphene. PDI-C6 forms nanocrystals onto the graphene's surface and transfers charges via π-π stacking to graphene. While the doping from organic molecules was compensated by oxygen molecules under normal conditions, we demonstrated the photoinduced current generation at the PDI-C6/graphene junction with ambient light, a 470 nm diode, and 532 nm laser sources. The local (in the scale of 1 µm) photoresponse of 0.5 A/W was demonstrated at a low laser power density. The methods we developed open the way for local functionalization of an on-chip array of graphene by inkjet printing of different semiconducting organic molecules for photonics and electronics.

High-Speed Photothermal Patterning of Doped Polymer Films

Organic semiconductors (OSCs) offer a new avenue to the next-generation electronics, but the lack of a scalable and inexpensive nanoscale patterning/deposition technique still limits their use in electronic applications. Recently, a new lithographic etching technique has been introduced that uses molecular dopants to reduce semiconducting polymer solubility in solvents and a direct-write laser to remove dopants locally, enabling rapid OSC etching with diffraction limited resolution. Previous publications postulated that the reaction that enables patterning is a photochemical reaction between photoexcited dopants with neutral solvent molecules. In this work, we analyze the photoinduced dissolution kinetics of F4TCNQ doped P3HT films using time-resolved in situ optical probing. We find two competing mechanisms that control de-doping and dissolution: the first is the photochemical reaction posited in the literature, and the second involves direct heating of the polymer by the laser, inducing increased solubility for both the polymer and dopant. We show that the wavelength-specific photochemical effect is dominant in low photon doses while the photothermal effect is dominant with high excitation rates regardless of laser wavelength. With sufficiently high optical intensity input, the photothermal mechanism can in principle achieve a high writing speed up to 1 m/s. Our findings bring new insights into the mechanisms behind laser direct writing of OSCs based on dopant induced solubility control and enable ultraprecise fabrications of various device configurations in large-scale manufacturing.

Electrohydrodynamic-Jet (EHD)-Printed Diketopyrrolopyroole-Based Copolymer for OFETs and Circuit Applications

We report the employment of an electrohydrodynamic-jet (EHD)-printed diketopyrrolopyrrole-based copolymer (P-29-DPPDTSE) as the active layer of fabricated organic field-effect transistors (OFETs) and circuits. The device produced at optimal conditions showed a field-effect mobility value of 0.45 cm²/(Vs). The morphologies of the printed P-29-DPPDTSE samples were determined by performing optical microscopy, X-ray diffraction, and atomic force microscopy experiments. In addition, numerical circuit simulations of the optimal printed P-29-DPPDTSE OFETs were done in order to observe how well they would perform in a high-voltage logic circuit application. The optimal printed P-29-DPPDTSE OFET showed a 0.5 kHz inverter frequency and 1.2 kHz ring oscillator frequency at a 40 V supply condition, indicating the feasibility of its use in a logic circuit application at high voltage.

Flexible Pressure-Sensitive Contact Transistors Operating in the Subthreshold Regime

Organic thin-film transistor (TFT)-based pressure sensors have received huge attention for wearable electronic applications such as health monitoring and smart robotics. However, there still remains a challenge to achieve low power consumption and high sensitivity at the same time for the realization of truly wearable sensor systems where minimizing power consumption is significant because of limited battery run time. Here, we introduce a flexible pressure-sensitive contact transistor (PCT), a new type of pressure-sensing device based on organic TFTs for next-generation wearable electronic skin devices. The PCT consists of deformable S/D electrodes integrated on a staggered TFT. The deformable S/D electrodes were fabricated by embedding conducting single-walled carbon nanotubes on the surface of microstructured polydimethylsiloxane. Under pressure loads, the deformation of the electrodes on an organic semiconductor layer leads modulation of drain current from variation in both the channel geometry and contact resistance. By strategic subthreshold operation to minimize power consumption and increase the dominance of contact resistance because of gated Schottky contact, the PCT achieves both ultralow power consumption (order of 10¹ nW) and high sensitivity (18.96 kPa^-1). Finally, we demonstrate a 5 × 5 active matrix PCT array on a 3 μm-thick parylene substrate. The device with ultralow power consumption and high sensitivity on a biocompatible flexible substrate makes the PCT promising candidate for next-generation wearable electronic skin devices.

Immobilization of planktonic algal spores by inkjet printing

The algal cell immobilization is a commonly used technique for treatment of waste water, production of useful metabolites and management of stock culture. However, control over the size of immobilized droplets, the population of microbes, and production rate in current techniques need to be improved. Here, we use drop-on-demand inkjet printing to immobilize spores of the alga Ecklonia cava within alginate microparticles for the first time. Microparticles with immobilized spores were generated by printing alginate-spore suspensions into a calcium chloride solution. We demonstrate that the inkjet technique can control the number of spores in an ejected droplet in the range of 0.23 to 1.87 by varying spore densities in bioink. After the printing-based spore encapsulation, we observe initial sprouting and continuous growth of thallus until 45 days of culture. Our study suggest that inkjet printing has a great potential to immobilize algae, and that the ability to control the number of encapsulated spores and their microenvironments can facilitate research into microscopic interactions of encapsulated spores.

Highly stacked 3D organic integrated circuits with via-hole-less multilevel metal interconnects

Multilevel metal interconnects are crucial for the development of large-scale organic integrated circuits. In particular, three-dimensional integrated circuits require a large number of vertical interconnects between layers. Here, we present a novel multilevel metal interconnect scheme that involves solvent-free patterning of insulator layers to form an interconnecting area that ensures a reliable electrical connection between two metals in different layers. Using a highly reliable interconnect method, the highest stacked organic transistors to date, a three-dimensional organic integrated circuits consisting of 5 transistors and 20 metal layers, is successfully fabricated in a solvent-free manner. All transistors exhibit outstanding device characteristics, including a high on/off current ratio of ~10⁷, no hysteresis behavior, and excellent device-to-device uniformity. We also demonstrate two vertically-stacked complementary inverter circuits that use transistors on 4 different floors. All circuits show superb inverter characteristics with a 100% output voltage swing and gain up to 35 V per V.

Though large-scale integration of organic transistors into integrated circuits via 3D stacking is a promising approach, reliable methods of device fabrication are still needed. Here, the authors report a metal interconnect scheme for reliable fabrication of 3D integrated organic transistor circuits.

Fully Solution-Processed Transparent Artificial Neural Network Using Drop-On-Demand Electrohydrodynamic Printing

Artificial neural networks (ANN), deep learning, and neuromorphic systems are exciting new processing architectures being used to implement a wide variety of intelligent and adaptive systems. To date, these architectures have been primarily realized using traditional complementary metal-oxide-semiconductor (CMOS) processes or otherwise conventional semiconductor fabrication processes. Thus, the high cost associated with the design and fabrication of these circuits has limited the broader scientific community from applying new ideas, and arguably, has slowed research progress in this exciting new area. Solution-processed electronics offer an attractive option for providing low-cost rapid prototyping of neuromorphic devices. This article proposes a novel, wholly solution-based process used to produce low-cost transparent synaptic transistors capable of emulating biological synaptic functioning and thus used to construct ANN. We have demonstrated the fabrication process by constructing an ANN that encodes and decodes a 100 × 100 pixel image. Here, the synaptic weights were configured to achieve the desired image processing functions.

Recent Progress in Inkjet-Printed Thin-Film Transistors

Drop-on-demand inkjet printing is one of the most attractive techniques from a manufacturing perspective due to the possibility of fabrication from a digital layout at ambient conditions, thus leading to great opportunities for the realization of low-cost and flexible thin-film devices. Over the past decades, a variety of inkjet-printed applications including thin-film transistors (TFTs), radio-frequency identification devices, sensors, and displays have been explored. In particular, many research groups have made great efforts to realize high-performance TFTs, for application as potential driving components of ubiquitous wearable electronics. Although there are still challenges to enable the commercialization of printed TFTs beyond laboratory-scale applications, the field of printed TFTs still attracts significant attention, with remarkable developments in soluble materials and printing methodology. Here, recent progress in printing-based TFTs is presented from materials to applications. Significant efforts to improve the electrical performance and device-yield of printed TFTs to match those of counterparts fabricated using conventional deposition or photolithography methods are highlighted. Moreover, emerging low-dimension printable semiconductors, including carbon nanotubes and transition metal dichalcogenides as well as mature semiconductors, and new-concept printed switching devices, are also discussed.

Three-dimensional monolithic integration in flexible printed organic transistors

Direct printing of thin-film transistors has enormous potential for ubiquitous and lightweight wearable electronic applications. However, advances in printed integrated circuits remain very rare. Here we present a three-dimensional (3D) integration approach to achieve technology scaling in printed transistor density, analogous to Moore's law driven by lithography, as well as enhancing device performance. To provide a proof of principle for the approach, we demonstrate the scalable 3D integration of dual-gate organic transistors on plastic foil by printing with high yield, uniformity, and year-long stability. In addition, the 3D stacking of three complementary transistors enables us to propose a programmable 3D logic array as a new route to design printed flexible digital circuitry essential for the emerging applications. The 3D monolithic integration strategy demonstrated here is applicable to other emerging printable materials, such as carbon nanotubes, oxide semiconductors and 2D semiconducting materials.

Parylene-Based Double-Layer Gate Dielectrics for Organic Field-Effect Transistors

We demonstrate high-performance and stable organic field-effect transistors (OFETs) using parylene-based double-layer gate dielectrics (DLGDs). DLGDs, consisting of parylene C as the upper layer and F as the lower layer, are designed to simultaneously provide good interface and bulk gate dielectric properties by exploiting the advantages of each gate dielectric. The structural effects of DLGDs are systematically investigated by evaluating the electrical characteristics and dielectric properties while varying the thickness ratio of each gate dielectric. The OFET with the optimized DLGD exhibits high performance and operational stability. This systematic approach will be useful for realizing practical electronic applications.

Printed Organic Complementary Inverter with Single SAM Process Using a p-type D-A Polymer Semiconductor

The demonstration of the complementary integrated circuit using printing processes is indispensable for realizing electronic devices using organic thin film transistors. Although complementary integrated circuits have advantages such as low power consumption and a wide output voltage range, complementary integrated circuits fabricated by the printing method have problems regarding driving voltage and performance. Studies on fabrication processes of electronic circuits for printing technology, including optimization and simplification, are also important research topics. In this study, the fabrication process of the printed complementary integrated circuit was simplified by applying a p-type donor-acceptor (D-A) polymer semiconductor, which is not strongly affected by the electrode work function. An inverter circuit and the ring oscillator circuit were demonstrated using this process. The fabricated ring oscillator array showed excellent performance, with low voltage operation and low performance variation.

Multi-Valued Logic Circuits Based on Organic Anti-ambipolar Transistors

Multivalued logic circuits, which can handle more information than conventional binary logic circuits, have attracted much attention as a promising way to improve the data-processing capabilities of integrated circuits. In this study, we developed a ternary inverter based on organic field-effect transistors (OFET) as a potential component of high-performance and flexible integrated circuits. Key elements are anti-ambipolar and n-type OFETs connected in series. First, we demonstrate an organic ternary inverter that exhibits three distinct logic states. Second, the operating voltage was greatly reduced by taking advantage of an Al₂O₃ gate dielectric. Finally, the operating voltage was finely tuned by the designing of the device geometry. These results are achievable owing to the flexible controllability of the device configuration, suggesting that the organic ternary inverter plays an important role with regard to high-performance organic integrated circuits.

Inkjet-Spray Hybrid Printing for 3D Freeform Fabrication of Multilayered Hydrogel Structures

Here, a new bioprinting process by combining drop-on-demand inkjet printing with a spray-coating technique, which enables the high-resolution, high-speed, and freeform fabrication of large-scale cell-laden hydrogel structures is reported. Hydrogel structures with various shapes and composed of different materials, including alginate, cellulose nanofiber, and fibrinogen, are fabricated using the inkjet-spray printing. To manufacture cell-friendly hydrogel structures with controllable stiffness, gelatine methacryloyl is saponified to stabilize jet formation and is subsequently mixed with sodium alginate to prepare blend inks. The hydrogels crosslinked from the blend inks are characterized by assessing physical properties including the microstructure and mechanical stiffness and cellular responses including the cell viability, metabolic activity, and functionality of human dermal fibroblasts within the hydrogel. Cell-laden hydrogel structures are generated on a large scale and collagen type I secretion and spreading of cells within the hydrogels are assessed. The results demonstrate that the inkjet-spray printing system will ensure the formation of a cell-laden hydrogel structure with high shape fidelity in a rapid and reliable manner. Ultimately, the proposed printing technique and the blend bioink to be used to fabricate 3D laminated large-scale tissue equivalents that potentially mimic the function of native tissues is expected.

Inkjet Etching of Polymers and Its Applications in Organic Electronic Devices

Inkjet printing techniques for the etching of polymers and their application to the fabrication of organic electronic devices are reviewed. A mechanism is proposed for the formation of via holes in polymer layers through inkjet printing with solvent, and recent achievements in the fabrication with inkjet etching of various three-dimensional microstructures (i.e., microwells, microgrooves, hexagonal holes, and concave structures) are discussed. In addition, organic electronic devices are presented that use inkjet-etched subtractive patterns as platforms for the selective depositions of an emissive material, a liquid crystal, an organic conductor, an organic insulator, and an organic semiconductor, and as an optical waveguide.

High-Resolution Transfer Printing of Graphene Lines for Fully Printed, Flexible Electronics

Pristine graphene inks show great promise for flexible printed electronics due to their high electrical conductivity and robust mechanical, chemical, and environmental stability. While traditional liquid-phase printing methods can produce graphene patterns with a resolution of ∼30 μm, more precise techniques are required for improved device performance and integration density. A high-resolution transfer printing method is developed here capable of printing conductive graphene patterns on plastic with line width and spacing as small as 3.2 and 1 μm, respectively. The core of this method lies in the design of a graphene ink and its integration with a thermally robust mold that enables annealing at up to ∼250 °C for precise, high-performance graphene patterns. These patterns exhibit excellent electrical and mechanical properties, enabling favorable operation as electrodes in fully printed electrolyte-gated transistors and inverters with stable performance even following cyclic bending to a strain of 1%. The high resolution coupled with excellent control over the line edge roughness to below 25 nm enables aggressive scaling of transistor dimensions, offering a compelling route for the scalable manufacturing of flexible nanoelectronic devices.

Selective Conversion from p-Type to n-Type of Printed Bottom-Gate Carbon Nanotube Thin-Film Transistors and Application in Complementary Metal-Oxide-Semiconductor Inverters

The fabrication of printed high-performance and environmentally stable n-type single-walled carbon nanotube (SWCNT) transistors and their integration into complementary (i.e., complementary metal-oxide-semiconductor, CMOS) circuits are widely recognized as key to achieving the full potential of carbon nanotube electronics. Here, we report a simple, efficient, and robust method to convert the polarity of SWCNT thin-film transistors (TFTs) using cheap and readily available ethanolamine as an electron doping agent. Printed p-type bottom-gate SWCNT TFTs can be selectively converted into n-type by deposition of ethanolamine inks on the transistor active region via aerosol jet printing. Resulted n-type TFTs show excellent electrical properties with an on/off ratio of 10⁶, effective mobility up to 30 cm² V^-1 s^-1, small hysteresis, and small subthreshold swing (90-140 mV dec^-1), which are superior compared to the original p-type SWCNT devices. The n-type SWCNT TFTs also show good stability in air, and any deterioration of performance due to shelf storage can be fully recovered by a short low-temperature annealing. The easy polarity conversion process allows construction of CMOS circuitry. As an example, CMOS inverters were fabricated using printed p-type and n-type TFTs and exhibited a large noise margin (50 and 103% of 1/2 V_dd = 1 V) and a voltage gain as high as 30 (at V_dd = 1 V). Additionally, the CMOS inverters show full rail-to-rail output voltage swing and low power dissipation (0.1 μW at V_dd = 1 V). The new method paves the way to construct fully functional complex CMOS circuitry by printed TFTs.

3D-Printed OFETs of the 1,4-bis(3-phenylquinoxalin-2-yl)benzene-based polymer semiconductors

In this study, we polymerized a 1,4-bis(3-phenylquinoxalin-2-yl)benzene unit with DPP and isoindigo units to produce four new polymers and deeply investigated the influence of DPP and isoindigo units on the semiconductor characteristics, band gap, and orientation properties of these polymers.