Biodegradable Materials for Transient Organic Transistors

Electronic devices that can physically disappear in a controlled manner without harmful by-products unveil a wide range of opportunities in medical devices, environmental monitoring, and next-generation consumer electronics. Their property of transience is indispensable for mitigating the global problem of electronic waste accumulation. Additionally, transient technologies that are biocompatible and can be biologically resorbed are of great potential for applications in temporary medical implants, since it eliminates the need for expensive device recovery surgery. Transistors are the key building blocks of modern electronics, and their fabrication using organic materials is beneficial due to their low cost, unprecedented flexibility and facile processing. This contribution reviews the technological application of biodegradable materials in four major classes of organic transistors, namely organic field-effect transistors (OFETs), organic synaptic transistors, electrolyte-gated OFETs, and organic electrochemical transistors. The fundamental biodegradation mechanism is discussed in detail, followed by a perspective of various biodegradable materials utilized as active semiconductors, dielectrics, electrolytes and substrates in the various types of organic transistor devices. This contribution comprehensively discusses the role and application of biodegradable materials in all of the key modern-day organic transistors, highlighting their unique properties that allow the fabrication of biodegradable, eco-friendly, and sustainable devices.     

Organic Field Effect Transistor-Based Photonic Synapses: Materials,Devices, and Applications

Photonic artificial synapses-based neuromorphic computing systems have been regarded as promising candidates for replacing von Neumann-based computing systems due to the high bandwidth, ultrafast signal transmission, low energy consumption, and wireless communication. Although significant progress has been made in developing varied device structures for synaptic emulation, organic field-effect transistors (OFETs) hold the compelling advantages of facile preparation, liable integration, and versatile structures. As a powerful and effective platform for photonic synapses, OFETs can fulfill not only the simulation of simple synaptic functions, but also complex photoelectric dual modulation and simulation of the visual system. Herein, an overview of OFET-based photonic synapses, including functional materials, device configurations, and innovative applications is provided. Meanwhile, rules for selecting materials, mechanism of photoelectric conversion, and fabrication techniques of devices are also highlighted. Finally, challenges and opportunities are all discussed, providing solid guidance for multilevel memory, multi-functional tandem artificial neural system, and artificial intelligence.     

A Review on Encapsulation Technology from Organic Light Emitting Diodes to Organic and Perovskite Solar Cells

Organic light emitting diodes (OLEDs) employing organic thin-film based emitters have attracted tremendous attention due to their widespread applications in lighting and as displays in mobile devices and televisions. The novel thin-film photovoltaic techniques using organic or organic–inorganic hybrid materials such as organic photovoltaics (OPVs) and perovskite solar cells (PSCs) have become emerging competitive candidates with regard to the traditional photovoltaic techniques on account of high-efficiency, low-cost, and simple manufacturing processing properties. However, OLEDs, OPVs, and PSCs are vulnerable to the undesired degradation induced by moisture and oxygen. To afford long-term stability, a robust encapsulation technique by employing materials and structures that possess high barrier performance against oxygen and moisture must be explored and employed to protect these devices. Herein, the recent progress on specific encapsulation materials and techniques for three types of devices on the basis of fundamental understanding of device stability is reviewed. First, their degradation mechanisms, as well as, influencing factors are discussed. Then, the encapsulation technologies and materials are classified and discussed. Moreover, the advantages and disadvantages of various encapsulation technologies and materials coupled with their encapsulation applications in different devices are compared. Finally, the ongoing challenges and future perspectives of encapsulation frontier are provided.     

Artificial Visual Systems With Tunable Photoconductivity Based on Organic Molecule-Nanowire Heterojunctions

The visual system, one of the most crucial units of the human perception system, combines the functions of multi-wavelength signal detection and data processing. Herein, the large-scale artificial synaptic device arrays based on the organic molecule-nanowire heterojunctions with tunable photoconductivity are proposed and demonstrated. The organic thin films of p-type 2,7-dioctyl[1]benzothieno[3,2-b][1] benzothiophene (C8-BTBT) or n-type phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) are used to wrap the InGaAs nanowire parallel arrays to configure two different type-I heterojunctions, respectively. Due to the difference in carrier injection, persistent negative photoconductivity (NPC) or positive photoconductivity (PPC) are achieved in these heterojunctions. The irradiation with different wavelengths (solar-blind to visible ranges) can stimulate the heterojunction devices, effectively mimicking the synaptic behaviors with two different photoconductivities. The long-term and multi-state light memory are also realized through synergistic photoelectric modulation. Notably, the arrays with different photoconductivities are adopted to build the hardware kernel for the visual system. Due to the tunable photoconductivity and response to multiple wavelengths, the recognition rate of neural networks can reach 100% with lower complexity and power consumption. Evidently, these photosynaptic devices are illustrated with retina-like behaviors and capabilities for large-area integration, which reveals their promising potential for artificial visual systems.     

Optoelectronic Synapses and Photodetectors Based on Organic Semiconductor/Halide Perovskite Heterojunctions: Materials,Devices, and Applications

Both photodetectors (PDs) and optoelectronic synaptic devices (OSDs) are optoelectronic devices converting light signals into electrical responses. Optoelectronic devices based on organic semiconductors and halide perovskites have aroused tremendous research interest owing to their exceptional optical/electrical characteristics and low-cost processability. The heterojunction formed between organic semiconductors and halide perovskites can modify the exciton dissociation/recombination efficiency and modulate the charge-trapping effect. Consequently, organic semiconductor/halide perovskite heterojunctions can endow PDs and OSDs with high photo responsivity and the ability to simulate synaptic functions respectively, making them appropriate for the development of energy-efficient artificial visual systems with sensory and recognition functions. This article summarizes the recent advances in this research field. The physical/chemical properties and preparation methods of organic semiconductor/halide perovskite heterojunctions are briefly introduced. Then the development of PDs and OSDs based on organic semiconductor/halide perovskite heterojunctions, as well as their innovative applications, are systematically presented. Finally, some prospective challenges and probable strategies for the future development of optoelectronic devices based on organic semiconductor/halide perovskite heterojunctions are discussed.     

Flexible,Conformal Organic Synaptic Transistors on Elastomer for Biomedical Applications

Bioelectronics in synaptic transistors for future biomedical applications, such as implanted treatments and human–machine interfaces, must be flexible with good mechanical compatibility with biological tissues. The rigid nature and high deposition temperature in conventional inorganic synaptic transistors restrict the development of flexible, conformal synaptic devices. Here, the dinaphtho[2,3‐b:2′,3′‐f]thieno[3,2‐b]‐thiophene organic synaptic transistor on elastic polydimethylsiloxane is demonstrated to avoid these limitations. The unique advantages of organic materials in low Young's modulus and low temperature process enable seamless adherence of organic synaptic transistors on arbitrary‐shaped objects. On 3D curved surfaces, the essential synaptic functions, such as potentiation/depression, short/long‐term synaptic plasticity, and spike voltage–dependent plasticity, are successfully realized. The time‐dependent surface potential characterization reveals the slow polarization of dipoles in the dielectric is responsible for hysteresis and synaptic behaviors. This work represents that organic materials offer a potential platform to realize the flexible, conformal synaptic transistors for the development of wearable and implantable artificial neuromorphic systems.     

2D Material Based Synaptic Devices for Neuromorphic Computing

The demand for computing power has been increasing exponentially since the emergence of artificial intelligence (AI), internet of things (IoT), and machine learning (ML), where novel computing primitives are required. Brain inspired neuromorphic computing systems, capable of combining analog computing and data storage at the device level, have drawn great attention recently. In addition, the basic electronic devices mimicking the biological synapse have achieved significant progress. Owing to their atomic thickness and reduced screening effect, the physical properties of 2D materials could be easily modulated by various stimuli, which is quite beneficial for synaptic applications. In this article, aiming at high-performance and functional neuromorphic computing applications, a comprehensive review of synaptic devices based on 2D materials is provided, including the advantages of 2D materials and heterostructures, various robust multifunctional 2D synaptic devices, and associated neuromorphic applications. Challenges and strategies for the future development of 2D synaptic devices are also discussed. This review will provide an insight into the design and preparation of 2D synaptic devices and their applications in neuromorphic computing.     

Innovation of Materials,Devices, and Functionalized Interfaces in Organic Spintronics

Organic spintronics has been attracting the interest of the scientific community because of its potential to complement the electronics industry by combining the spin and charge degrees of freedom. Synthesized organic materials with light elements have been widely applied in organic spintronics due to their intrinsic weak spin-orbit coupling and hyperfine interaction. Meanwhile, the prototypic devices in inorganic spintronics have been creatively utilized to fabricate analogous organic devices. The interfaces between organic materials and ferromagnetic electrodes in spintronic devices are diverse and can lead to many novel phenomena that influence the device performance. In this review, the novel organic materials, innovative devices, and functionalized interfaces in organic spintronics are comprehensively introduced. First, the fundamental concepts and parameters of organic spin devices are clarified. Subsequently, the organic materials applied in organic spintronics are classified, which include small molecules, polymers, and organic–inorganic hybrid perovskites. Moreover, several types of spin-related devices in this field are introduced and discussed. Thereafter, the functionalized interfaces of the spin-related devices are categorized and elaborated upon. Finally, a brief summary and future prospects are presented, which highlight the developments necessary in organic spintronics in the near future.     

Advanced Applications and Challenges of Electropolymerized Conjugated Microporous Polymer Films

Conjugated microporous polymers (CMPs) are a unique class of porous materials that have an integrated π-conjugated system and permanent intrinsic porosity. They are of great interest because of their outstanding performance in gas sorption, photoredox catalysis, energy storage, organic light-emitting diodes (OLEDs), and sensing applications. However, conventional chemically synthesized CMPs are solid powders and have poor solubility, which makes it difficult to process and integrate devices, and has become a bottleneck preventing their practical applications. Electropolymerization (EP) is a simple and efficient method for the preparation of CMP films, which simultaneously completes material synthesis and film processing. More importantly, the microstructure of the CMP films can be effectively controlled by electrochemical parameters. In this review, first, the basic synthetic principles and strategies of CMP films via EP are introduced, allowing for facile optimization of the structure and properties. Then, the recent progress of the EP CMP films is focused upon in organic electronics, energy storage, sensors, chemical capture and separation, and electrocatalysts. Finally, the challenges and outlook for EP CMP films are addressed.     

Retina-Inspired Artificial Synapses with Ultraviolet to Near-Infrared Broadband Responses for Energy-Efficient Neuromorphic Visual Systems

Neuromorphic visual system with image perception, memory, and preprocessing functions is expected to simulate basic features of the human retina. Organic optoelectronic synaptic transistors emulating biological synapses may be promising candidates for constructing neural morphological visual system. However, the sensing wavelength range of organic optoelectronic synaptic transistors usually limits their potential in artificial multispectral visual perception. Here, retina-inspired optoelectronic synaptic transistors that present broadband responses covering ultraviolet, visible, and near-infrared regions are demonstrated, which leverage the wide-range photoresponsive charge trapping layer and the heterostructure formed between PbS quantum dots and organic semiconductor. Simplified neuromorphic visual arrays are developed to simulate comprehensive image perception, memory, and preprocessing functions. Benefitting from the flexibility of the charge trapping and organic semiconductor layers, a flexible neuromorphic visual array can be fabricated, having an ultralow power consumption of 0.55 fJ per event under a low operating voltage of −0.01 V. More significantly, an accelerating image preprocessing effect can be observed in a wide wavelength range even beyond the perception range of the human visual system, due to the gate-adjustable synaptic plasticity. These devices are highly promising for implementing neuromorphic visual systems with broadband perception, increasing image processing efficiency, and promoting the development of artificial vision.     

Send a hologram [photorefractive materials]

As computers and the Internet become faster and faster, more and more information is transmitted, received, processed, and stored everyday. The demand for high-speed, large-capacity information systems is pushing scientists and engineers to explore all possible approaches, including electrical and optical means. Photorefractive materials and devices are becoming viable alternatives for information systems. Photorefractive materials, including traditional electrooptic photorefractive crystals as well as photopolymers and photosensitive glasses, have demonstrated their potential in information systems. In this article we describe several applications of various photorefractive materials in information storage, processing, and communication systems. Specifically, we briefly discuss the applications of the traditional electrooptic photorefractive crystals and photopolymers in volume holographic data storage (VHDS) and information processing. Then, we discuss our recent works on the applications of photopolymers, holographic polymer dispersed liquid crystals (H-PDLC), and photosensitive glasses in photonic devices for optical fiber communications.     

Spatially-correlated neuron transistors with ion-gel gating for brain-inspired applications

In this paper, ion-gel gated transistors based on solution-processed indium-gallium-zinc-oxide (In-Ga-Zn-O) semiconductors were fabricated. These transistors consisted of a spatial distribution configuration of multi-in-plane gates. Spike pluses applied on multi-in-plane gates are analogy to massive synaptic inputs from various dendritic positions. The basic neuromorphic functions, such as potentiation or depression behaviors, synaptic plasticity and frequency-dependent filtering, were demonstrated in these devices by applied a spiking on an in-plane gate. The output signal of neuromorphic devices is greatly relevant to the gate position-correlated input signal and the spatially-correlated information processing could advance the capacity of neuromorphic performance. Orientation selectivity was a broadly investigated phenomenon. More importantly, by using the spatial summation functions of dendritic integration, the orientation identification was successfully realized in our transistor with multi-in-plane gates. The spatially-correlated neuromorphic devices are exceedingly promising for the neural information processing and sensing.     

Ion-Gating Engineering of Organic Semiconductors toward Multifunctional Devices

Ion-gating engineering provides a new way to bridge electronics and ionics, and more importantly, bringing unprecedented opportunities for organic semiconductors (OSCs) based bioelectronics and solid-sate physics. Compared with conventional-dielectric gating, ion gating shows unique features in an extremely large electric field, high transconductance, low operating frequency, and ultrahigh carrier concentration. It therefore boosts the rapid development of different organic devices, including neuromorphic devices and amplifying transducers, and offers a powerful strategy to probe the charge transport, thermoelectric and even superconducting properties of organic materials at different scales. In this review, first, the fundamental mechanism of ion gating is discussed to enable multifunctional devices. The electrolyte materials and organic semiconductors are also summarized that are widely used in ion-gated devices and their associated properties are examined. Moreover, key concepts of manipulating ion–electron coupling are highlighted for opening up new frontiers in organic multifunctional electronics. Finally, the challenges and perspectives on the ion gating of OSCs are proposed to highlight the directions that deserve attention in this emerging interdisciplinary field.     

Blue Single‐Layer Organic Light‐Emitting Diodes Using Fluorescent Materials: A Molecular Design View Point

Since the beginning of organic light‐emitting diodes (OLEDs), blue emission has attracted the most attention and many research groups worldwide have worked on the design of materials for stable and highly efficient blue OLEDs. However, almost all the high‐efficiency blue OLEDs using fluorescent materials are multilayer devices, which are constituted of a stack of organic layers to improve the injection, transport, and recombination of charges within the emissive layer. Although the technology has been mastered, it suffers from real complexity and high cost and is time‐consuming. Simplifying the multilayer structure with a single‐layer one, the simplest devices made only of electrodes and the emissive layer have appeared as an appealing strategy for this technology. However, removing the functional organic layers of an OLED stack leads to a dramatic decrease of the performance and achieving high‐efficiency blue single‐layer OLEDs requires intense research especially in terms of materials design. Herein, an exhaustive review of blue emitting fluorophores that have been incorporated in single‐layer OLEDs is reported, and the links between their electronic properties and the device performance are discussed. Thus, a structure/properties/device performance relationship map is drawn, which is of interest for the future design of organic materials.     

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.     

Mechanochemical Processing of Highly Conducting Organic/Inorganic Composites Exhibiting Spin Crossover–Induced Memory Effect in Their Transport Properties

Bistable multifunctional materials have great potential in a large variety of devices, from sensors to information units. However, the direct exploitation of spin crossover (SCO) materials in electronic devices is limited due to their very high electrical resistance (insulators). Beyond their intrinsic properties, SCO materials may also work as probes to confer bistability as switchable components in hybrid materials, as controlled by external stimuli acting upon the SCO spin state. Low resistance conductors with memory effect may be obtained from the incorporation of SCO probes into a conducting organic polymer matrix. This strategy appeared to be limited by the strict synthetic conditions, since polymerization reactions are harsh enough to attack the redox-unstable SCO component. Because of this, just a few successful examples have been reported. Here a versatile processing protocol is introduced to obtain SCO/conducting polymer composites exploiting a post-synthetic mechanochemical approach that can be applied to any SCO component and any organic polymer. This new protocol allows highly conducting films of polypyrrole, polyaniline, and poly(3,4-ethylenedioxythiophene) (PEDOT) to be obtained, with bulk conductivities as high as 1 S·cm⁻¹, and exhibiting a thermal hysteresis in their electrical conductivity above room temperature.     

Small‐Molecule‐Based Organic Field‐Effect Transistor for Nonvolatile Memory and Artificial Synapse

With the incorporation of tailorable organic electronic materials as channel and storage materials, organic field‐effect transistor (OFET)‐based memory has become one of the most promising data storage technologies for hosting a variety of emerging memory applications, such as sensory memory, storage memory, and neuromorphic computing. Here, the recent state‐of‐the‐art progresses in the use of small molecules for OFET nonvolatile memory and artificial synapses are comprehensively reviewed, focusing on the characteristic features of small molecules in versatile functional roles (channel, storage, modifier, and dopant). Techniques for optimizing the storage capacity, speed, and reliability of nonvolatile memory devices are addressed in detail. Insight into the use of small molecules in artificial synapses constructed on OFET memory is also obtained in this emerging field. Finally, the strategies of molecular design for improving memory performance in view of small molecules as storage mediums are discussed systematically, and challenges are addressed to shed light on the future development of this vital research field.     

Template‐Assisted Synthesis of π‐Conjugated Molecular Organic Nanowires in the Sub‐100 nm Regime and Device Implications

π‐conjugated molecular organics such as rubrene, Alq₃, fullerene, and PCBM have been used extensively over the last few decades in numerous organic electronic devices, including solar cells, thin‐film transistors, and large‐area, low‐cost flexible displays. Rubrene and Alq₃, have emerged as promising platforms for spin‐based classical and quantum information processing, which has triggered significant research activity in the relatively new area of organic spintronics. Synthesis of these materials in a nanowire geometry, with feature sizes in the sub‐100 nm regime, is desirable as it often enhances device performance and is essential for development of high‐density molecular electronic devices. However, fabrication techniques that meet this stringent size constraint are still largely underdeveloped. Here, a novel, versatile, and reagentless method that enables growth of nanowire arrays of the above‐mentioned organics in the cylindrical nanopores of anodic aluminum oxide (AAO) templates is demonstrated. This method 1) allows synthesis of high‐density organic nanowire arrays on arbitrary substrates, 2) provides electrical access to the nanowire arrays, 3) offers tunability of the array geometry in a range overlapping with the relevant physical length scales of many organic devices, and 4) can potentially be extended to synthesize axially and radially heterostructured organic nanowires. Thus prepared nanowires are characterized extensively with an aim to identify their potential applications in diverse areas such as organic optoelectronics, photovoltaics, molecular nanoelectronics, and spintronics.     

Interface Engineering for Organic Electronics

The field of organic electronics has been developed vastly in the past two decades due to its promise for low cost, lightweight, mechanical flexibility, versatility of chemical design and synthesis, and ease of processing. The performance and lifetime of these devices, such as organic light‐emitting diodes (OLEDs), photovoltaics (OPVs), and field‐effect transistors (OFETs), are critically dependent on the properties of both active materials and their interfaces. Interfacial properties can be controlled ranging from simple wettability or adhesion between different materials to direct modifications of the electronic structure of the materials. In this Feature Article, the strategies of utilizing surfactant‐modified cathodes, hole‐transporting buffer layers, and self‐assembled monolayer (SAM)‐modified anodes are highlighted. In addition to enabling the production of high‐efficiency OLEDs, control of interfaces in both conventional and inverted polymer solar cells is shown to enhance their efficiency and stability; and the tailoring of source–drain electrode–semiconductor interfaces, dielectric–semiconductor interfaces, and ultrathin dielectrics is shown to allow for high‐performance OFETs.     

Green Lithography for Delicate Materials

A variety of unconventional materials, including biological nanostructures, organic and hybrid semiconductors, as well as monolayer, and other low-dimensional systems, are actively explored. They are usually incompatible with standard lithographic techniques that use harsh organic solvents and other detrimental processing. Here, a new class of green and gentle lithographic resists, compatible with delicate materials and capable of both top-down and bottom-up fabrication routines is developed. To demonstrate the excellence of this approach, devices with sub-micron features are fabricated on organic semiconductor crystals and individual animal's brain microtubules. Such structures are created for the first time, thanks to the genuinely water-based lithography, which opens an avenue for the thorough research of unconventional delicate materials at the nanoscale.