2D Molecular Crystal Bilayer p–n Junctions: A General Route toward High‐Performance and Well‐Balanced Ambipolar Organic Field‐Effect Transistors

Ambipolar organic field‐effect transistors (OFETs) are vital for the construction of high‐performance all‐organic digital circuits. The bilayer p–n junction structure, which is composed of separate layers of p‐ and n‐type organic semiconductors, is considered a promising way to realize well‐balanced ambipolar charge transport. However, this approach suffers from severely reduced mobility due to the rough interface between the polycrystalline thin films of p‐ and n‐type organic semiconductors. Herein, 2D molecular crystal (2DMC) bilayer p–n junctions are proposed to construct high‐performance and well‐balanced ambipolar OFETs. The molecular‐scale thickness of the 2DMC ensures high injection efficiency and the atomically flat surface of the 2DMC leads to high‐quality p‐ and n‐layer interfaces. Moreover, by controlling the layer numbers of the p‐ and n‐type 2DMCs, the electron and hole mobilities are tuned and well‐balanced ambipolar transport is accomplished. The hole and electron mobilities reach up to 0.87 and 0.82 cm² V^?1 s^?1, respectively, which are the highest values among organic single‐crystalline double‐channel OFETs measured in ambient air. This work provides a general route to construct high‐performance and well‐balanced ambipolar OFETs based on available unipolar materials.     

Charge‐Trapping‐Induced Non‐Ideal Behaviors in Organic Field‐Effect Transistors

Organic field‐effect transistors (OFETs) with impressively high hole mobilities over 10 cm² V^?1 s^?1 and electron mobilities over 1 cm² V^?1 s^?1 have been reported in the past few years. However, significant non‐ideal electrical characteristics, e.g., voltage‐dependent mobilities, have been widely observed in both small‐molecule and polymer systems. This issue makes the accurate evaluation of the electrical performance impossible and also limits the practical applications of OFETs. Here, a semiconductor‐unrelated, charge‐trapping‐induced non‐ideality in OFETs is reported, and a revised model for the non‐ideal transfer characteristics is provided. The trapping process can be directly observed using scanning Kelvin probe microscopy. It is found that such trapping‐induced non‐ideality exists in OFETs with different types of charge carriers (p‐type or n‐type), different types of dielectric materials (inorganic and organic) that contain different functional groups (? OH, ? NH₂, ? COOH, etc.). As fas as it is known, this is the first report for the non‐ideal transport behaviors in OFETs caused by semiconductor‐independent charge trapping. This work reveals the significant role of dielectric charge trapping in the non‐ideal transistor characteristics and also provides guidelines for device engineering toward ideal OFETs.     

Precise Patterning of Laterally Stacked Organic Microbelt Heterojunction Arrays by Surface‐Energy‐Controlled Stepwise Crystallization for Ambipolar Organic Field‐Effect Transistors

Ambipolar organic field‐effect transistors (OFETs) combining single‐crystalline p‐ and n‐type organic micro/nanocrystals have demonstrated superior performance to their amorphous or polycrystalline thin‐film counterparts. However, large‐area alignment and precise patterning of organic micro/nanocrystals for ambipolar OFETs remain challenges. Here, a surface‐energy‐controlled stepwise crystallization (SECSC) method is reported for large‐scale, aligned, and precise patterning of single‐crystalline laterally stacked p–n heterojunction microbelt (MB) arrays. In this method, the p‐ and n‐type organic crystals are precipitated via a stepwise process: first, the lateral sides of prepatterned photoresist stripes provide high‐surface‐energy sites to guide the aligned growth of p‐type organic crystals. Next, the formed p‐type crystals serve as new high‐surface‐energy positions to induce the crystallization of n‐type organic molecules at their sides, thus leading to the formation of laterally stacked p–n microbelts. Ambipolar OFETs based on the p–n heterojunction MB arrays exhibit balanced hole and electron mobilities of 0.32 and 0.43 cm² V^?1 s^?1, respectively, enabling the fabrication of complementary‐like inverters with large voltage gains. This work paves the way toward rational design and construction of single‐crystalline organic p–n heterojunction arrays for high‐performance organic, integrated circuits.     

Organic Light‐Emitting Transistors: Materials,Device Configurations,and Operations

Organic light‐emitting transistors (OLETs) represent an emerging class of organic optoelectronic devices, wherein the electrical switching capability of organic field‐effect transistors (OFETs) and the light‐generation capability of organic light‐emitting diodes (OLEDs) are inherently incorporated in a single device. In contrast to conventional OFETs and OLEDs, the planar device geometry and the versatile multifunctional nature of OLETs not only endow them with numerous technological opportunities in the frontier fields of highly integrated organic electronics, but also render them ideal scientific scaffolds to address the fundamental physical events of organic semiconductors and devices. This review article summarizes the recent advancements on OLETs in light of materials, device configurations, operation conditions, etc. Diverse state‐of‐the‐art protocols, including bulk heterojunction, layered heterojunction and laterally arranged heterojunction structures, as well as asymmetric source‐drain electrodes, and innovative dielectric layers, which have been developed for the construction of qualified OLETs and for shedding new and deep light on the working principles of OLETs, are highlighted by addressing representative paradigms. This review intends to provide readers with a deeper understanding of the design of future OLETs.     

25th Anniversary Article: Microstructure Dependent Bias Stability of Organic Transistors

Recent studies of the bias‐stress‐driven electrical instability of organic field‐effect transistors (OFETs) are reviewed. OFETs are operated under continuous gate and source/drain biases and these bias stresses degrade device performance. The principles underlying this bias instability are discussed, particularly the mechanisms of charge trapping. There are three main charge‐trapping sites: the semiconductor, the dielectric, and the semiconductor‐dielectric interface. The charge‐trapping phenomena in these three regions are analyzed with special attention to the microstructural dependence of bias instability. Finally, possibilities for future research in this field are presented. This critical review aims to enhance our insight into bias‐stress‐induced charge trapping in OFETs with the aim of minimizing operational instability.     

Fast Deposition of Aligning Edge‐On Polymers for High‐Mobility Ambipolar Transistors

Fast deposition of aligning ambipolar polymers for high‐performance organic field‐effect transistors (OFETs) and inverter circuits are highly desired for both scientific studies and industry applications. Here, large‐area and ordered polymer films are prepared by a bar‐coating method at a rate of 120 mm s^?1 in air. Atomic force microscopy and grazing‐incidence wide‐angle X‐ray scattering analysis indicate uniform edge‐on poly(fluoroisoindigo‐difluorobithiophene‐fluoroisoindigo‐bithiophene) (PFIBI‐BT) in 11.7 ± 1 nm film (≈5 layers). The elongated, uniformly oriented grains can reduce the adverse effects of the grain boundaries and facilitate charge transport in polymers. Furthermore, OFETs based on parallel film show high hole/electron mobilities up to 5.5/4.5 cm² V^?1 s^?1, which are approximately nine times of the devices prepared by spin‐coating. The gain of the inverter is as high as 174, which is one of the highest values in polymer inventers currently. These results demonstrate that the excellent bipolar performance of few‐layer PFIBI‐BT can be ensured while achieving the compatibility of the experimental process with industrial preparation.     

Organic FETs: Functional Organic Field‐Effect Transistors (Adv. Mater. 40/2010)

Functional organic field‐effect transistors (OFETs) have attracted increasing attention in the past few years due to their wide variety of potential applications. Research on functional OFETs underpins future advances in organic electronics. In this review, different types of functional OFETs including organic phototransistors, organic memory FETs, organic light emitting FETs, sensors based on OFETs and other functional OFETs are introduced. In order to provide a comprehensive overview of this field, the history, current status of research, main challenges and prospects for functional OFETs are all discussed     

Recent Progress in High‐Mobility Organic Transistors: A Reality Check

Over the past three decades, significant research efforts have focused on improving the charge carrier mobility of organic thin‐film transistors (OTFTs). In recent years, a commonly observed nonlinearity in OTFT current–voltage characteristics, known as the “kink” or “double slope,” has led to widespread mobility overestimations, contaminating the relevant literature. Here, published data from the past 30 years is reviewed to uncover the extent of the field‐effect mobility hype and identify the progress that has actually been achieved in the field of OTFTs. Present carrier‐mobility‐related challenges are identified, finding that reliable hole and electron mobility values of 20 and 10 cm² V^?1 s^?1, respectively, have yet to be achieved. Based on the analysis, the literature is then reviewed to summarize the concepts behind the success of high‐performance p‐type polymers, along with the latest understanding of the design criteria that will enable further mobility enhancement in n‐type polymers and small molecules, and the reasons why high carrier mobility values have been consistently produced from small molecule/polymer blend semiconductors. Overall, this review brings together important information that aids reliable OTFT data analysis, while providing guidelines for the development of next‐generation organic semiconductors.     

Functional Organic Field‐Effect Transistors

Functional organic field‐effect transistors (OFETs) have attracted increasing attention in the past few years due to their wide variety of potential applications. Research on functional OFETs underpins future advances in organic electronics. In this review, different types of functional OFETs including organic phototransistors, organic memory FETs, organic light emitting FETs, sensors based on OFETs and other functional OFETs are introduced. In order to provide a comprehensive overview of this field, the history, current status of research, main challenges and prospects for functional OFETs are all discussed     

Organic Semiconductors based on Dyes and Color Pigments

Organic dyes and pigments constitute a large class of industrial products. The utilization of these compounds in the field of organic electronics is reviewed with particular emphasis on organic field‐effect transistors. It is shown that for most major classes of industrial dyes and pigments, i.e., phthalocyanines, perylene and naphthalene diimides, diketopyrrolopyrroles, indigos and isoindigos, squaraines, and merocyanines, charge‐carrier mobilities exceeding 1 cm² V^?1 s^?1 have been achieved. The most widely investigated molecules due to their n‐channel operation are perylene and naphthalene diimides, for which even values close to 10 cm² V^?1 s^?1 have been demonstrated. The fact that all of these π‐conjugated colorants contain polar substituents leading to strongly quadrupolar or even dipolar molecules suggests that indeed a much larger structural space shows promise for the design of organic semiconductor molecules than was considered in this field traditionally. In particular, because many of these dye and pigment chromophores demonstrate excellent thermal and (photo‐)chemical stability in their original applications in dyeing and printing, and are accessible by straightforward synthetic protocols, they bear a particularly high potential for commercial applications in the area of organic electronics.     

Charge Injection in Solution‐Processed Organic Field‐Effect Transistors: Physics,Models and Characterization Methods

A high‐mobility organic semiconductor employed as the active material in a field‐effect transistor does not guarantee per se that expectations of high performance are fulfilled. This is even truer if a downscaled, short channel is adopted. Only if contacts are able to provide the device with as much charge as it needs, with a negligible voltage drop across them, then high expectations can turn into high performances. It is a fact that this is not always the case in the field of organic electronics. In this review, we aim to offer a comprehensive overview on the subject of current injection in organic thin film transistors: physical principles concerning energy level (mis)alignment at interfaces, models describing charge injection, technologies for interface tuning, and techniques for characterizing devices. Finally, a survey of the most recent accomplishments in the field is given. Principles are described in general, but the technologies and survey emphasis is on solution processed transistors, because it is our opinion that scalable, roll‐to‐roll printing processing is one, if not the brightest, possible scenario for the future of organic electronics. With the exception of electrolyte‐gated organic transistors, where impressively low width normalized resistances were reported (in the range of 10 Ω·cm), to date the lowest values reported for devices where the semiconductor is solution‐processed and where the most common architectures are adopted, are ～10 kΩ·cm for transistors with a field effect mobility in the 0.1–1 cm²/Vs range. Although these values represent the best case, they still pose a severe limitation for downscaling the channel lengths below a few micrometers, necessary for increasing the device switching speed. Moreover, techniques to lower contact resistances have been often developed on a case‐by‐case basis, depending on the materials, architecture and processing techniques. The lack of a standard strategy has hampered the progress of the field for a long time. Only recently, as the understanding of the rather complex physical processes at the metal/semiconductor interfaces has improved, more general approaches, with a validity that extends to several materials, are being proposed and successfully tested in the literature. Only a combined scientific and technological effort, on the one side to fully understand contact phenomena and on the other to completely master the tailoring of interfaces, will enable the development of advanced organic electronics applications and their widespread adoption in low‐cost, large‐area printed circuits.     

Investigation of Electrode Electrochemical Reactions in CH3NH3PbBr3 Perovskite Single‐Crystal Field‐Effect Transistors

Optoelectronic devices based on metal halide perovskites, including solar cells and light‐emitting diodes, have attracted tremendous research attention globally in the last decade. Due to their potential to achieve high carrier mobilities, organic–inorganic hybrid perovskite materials can enable high‐performance, solution‐processed field‐effect transistors (FETs) for next‐generation, low‐cost, flexible electronic circuits and displays. However, the performance of perovskite FETs is hampered predominantly by device instabilities, whose origin remains poorly understood. Here, perovskite single‐crystal FETs based on methylammonium lead bromide are studied and device instabilities due to electrochemical reactions at the interface between the perovskite and gold source–drain top contacts are investigated. Despite forming the contacts by a gentle, soft lamination method, evidence is found that even at such “ideal” interfaces, a defective, intermixed layer is formed at the interface upon biasing of the device. Using a bottom‐contact, bottom‐gate architecture, it is shown that it is possible to minimize such a reaction through a chemical modification of the electrodes, and this enables fabrication of perovskite single‐crystal FETs with high mobility of up to ≈15 cm² V^?1 s^?1 at 80 K. This work addresses one of the key challenges toward the realization of high‐performance solution‐processed perovskite FETs.     

When Flexible Organic Field-Effect Transistors Meet Biomimetics: A Prospective View of the Internet of Things

The emergence of flexible organic electronics that span the fields of physics and biomimetics creates the possibility for increasingly simple and intelligent products for use in everyday life. Organic field-effect transistors (OFETs), with their inherent flexibility, light weight, and biocompatibility, have shown great promise in the field of biomimicry. By applying such biomimetic OFETs for the internet of things (IoT) makes it possible to imagine novel products and use cases for the future. Recent advances in flexible OFETs and their applications in biomimetic systems are reviewed. Strategies to achieve flexible OFETs are individually discussed and recent progress in biomimetic sensory systems and nervous systems is reviewed in detail. OFETs are revealed to be one of the best systems for mimicking sensory and nervous systems. Additionally, a brief discussion of information storage based on OFETs is presented. Finally, a personal view of the utilization of biomimetic OFETs in the IoT and future challenges in this research area are provided.     

Organic High Electron Mobility Transistors Realized by 2D Electron Gas

A key breakthrough in inorganic modern electronics is the energy‐band engineering that plays important role to improve device performance or develop novel functional devices. A typical application is high electron mobility transistors (HEMTs), which utilizes 2D electron gas (2DEG) as transport channel and exhibits very high electron mobility over traditional field‐effect transistors (FETs). Recently, organic electronics have made very rapid progress and the band transport model is demonstrated to be more suitable for explaining carrier behavior in high‐mobility crystalline organic materials. Therefore, there emerges a chance for applying energy‐band engineering in organic semiconductors to tailor their optoelectronic properties. Here, the idea of energy‐band engineering is introduced and a novel device configuration is constructed, i.e., using quantum well structures as active layers in organic FETs, to realize organic 2DEG. Under the control of gate voltage, electron carriers are accumulated and confined at quantized energy levels, and show efficient 2D transport. The electron mobility is up to 10 cm² V^?1 s^?1, and the operation mechanisms of organic HEMTs are also argued. Our results demonstrate the validity of tailoring optoelectronic properties of organic semiconductors by energy‐band engineering, offering a promising way for the step forward of organic electronics.     

Wafer‐Scale Fabrication of High‐Performance n‐Type Polymer Monolayer Transistors Using a Multi‐Level Self‐Assembly Strategy

Wafer‐scale fabrication of high‐performance uniform organic electronic materials is of great challenge and has rarely been realized before. Previous large‐scale fabrication methods always lead to different layer thickness and thereby poor film and device uniformity. Herein, the first demonstration of 4 in. wafer‐scale, uniform, and high‐performance n‐type polymer monolayer films is reported, enabled by controlling the multi‐level self‐assembly process of conjugated polymers in solution. Since the self‐assembly process happened in solution, the uniform 2D polymer monolayers can be facilely deposited on various substrates, and theoretically without size limitations. Polymer monolayer transistors exhibit high electron mobilities of up to 1.88 cm² V^?1 s^?1, which is among the highest in n‐type monolayer organic transistors. This method allows to easily fabricate n‐type conjugated polymers with wafer‐scale, high uniformity, low contact resistance, and excellent transistor performance (better than the traditional spin‐coating method). This work provides an effective strategy to prepare large‐scale and uniform 2D polymer monolayers, which could enable the application of conjugated polymers for wafer‐scale sophisticated electronics.     

Organic semiconductors for organic field-effect transistors

Abstract

The advantages of organic field-effect transistors (OFETs), such as low cost, flexibility and large-area fabrication, have recently attracted much attention due to their electronic applications. Practical transistors require high mobility, large on/off ratio, low threshold voltage and high stability. Development of new organic semiconductors is key to achieving these parameters. Recently, organic semiconductors have been synthesized showing comparable mobilities to amorphous-silicon-based FETs. These materials make OFETs more attractive and their applications have been attempted. New organic semiconductors resulting in high-performance FET devices are described here and the relationship between transistor characteristics and chemical structure is discussed.     

Halogenated Tetraazapentacenes with Electron Mobility as High as 27.8 cm2 V−1 s−1 in Solution‐Processed n‐Channel Organic Thin‐Film Transistors

Molecular engineering of tetraazapentacene with different numbers of fluorine and chlorine substituents fine‐tunes the frontier molecular orbitals, molecular vibrations, and π–π stacking for n‐type organic semiconductors. Among the six halogenated tetraazapentacenes studied herein, the tetrachloro derivative (4Cl‐TAP) in solution‐processed thin‐film transistors exhibits electron mobility of 14.9 ± 4.9 cm² V^?1 s^?1 with a maximum value of 27.8 cm² V^?1 s^?1, which sets a new record for n‐channel organic field‐effect transistors. Computational studies on the basis of crystal structures shed light on the structure–property relationships for organic semiconductors. First, chlorine substituents slightly decrease the reorganization energy of the tetraazapentacene whereas fluorine substituents increase the reorganization energy as a result of fine‐tuning molecular vibrations. Second, the electron transfer integral is very sensitive to subtle changes in the 2D π‐stacking with brickwork arrangement. The unprecedentedly high electron mobility of 4Cl‐TAP is attributed to the reduced reorganization energy and enhanced electron transfer integral as a result of modification of tetraazapentacene with four chlorine substituents.     

π‐Conjugated Polymers Incorporating a Novel Planar Quinoid Building Block with Extended Delocalization and High Charge Carrier Mobility

Two novel conjugated polymers incorporating quinoidal thiophene are successfully synthesized. By combining 1D nuclear magnetic resonance (NMR) and 2D nuclear Overhauser effect spectroscopy analyses, the isomeric form of the major quinoid monomer is clearly identified as the asymmetric Z, E‐configuration. The quinoidal polymers are synthesized via Stille polymerization with thiophene or bithiophene. Both quinoidal polymers exhibit the low band gap of 1.45 eV and amphoteric redox behavior, indicating extended conjugation owing to the quinoidal backbone. These quinoidal polymers show ambipolar behaviors with high charge carrier mobilities when applied in organic field‐effect transistors. In addition, the radial alignment of polymer chains achieved by off‐center spin‐coating leads to further improvement of device performance, with poly(quinoidal thiophene–bithiophene) exhibiting a high hole mobility of 8.09 cm² V^?1 s^?1, which is the highest value among the quinoidal polymers up to now. Microstructural alteration via thermal annealing or off‐center spin‐coating is found to beneficially affect charge transport. The enhancement of crystallinity with strong π–π interactions and the nanofibrillar structure arising from planar well‐delocalized quinoid units is considered to be responsible for the high charge carrier mobility.     

MoS2/Rubrene van der Waals Heterostructure: Toward Ambipolar Field‐Effect Transistors and Inverter Circuits

2D transition metal dichalcogenides are promising channel materials for the next‐generation electronic device. Here, vertically 2D heterostructures, so called van der Waals solids, are constructed using inorganic molybdenum sulfide (MoS₂) few layers and organic crystal – 5,6,11,12‐tetraphenylnaphthacene (rubrene). In this work, ambipolar field‐effect transistors are successfully achieved based on MoS₂ and rubrene crystals with the well balanced electron and hole mobilities of 1.27 and 0.36 cm² V^?1 s^?1, respectively. The ambipolar behavior is explained based on the band alignment of MoS₂ and rubrene. Furthermore, being a building block, the MoS₂/rubrene ambipolar transistors are used to fabricate CMOS (complementary metal oxide semiconductor) inverters that show good performance with a gain of 2.3 at a switching threshold voltage of ?26 V. This work paves a way to the novel organic/inorganic ultrathin heterostructure based flexible electronics and optoelectronic devices.     

25th Anniversary Article: Progress in Chemistry and Applications of Functional Indigos for Organic Electronics

Indigo and its derivatives are dyes and pigments with a long and distinguished history in organic chemistry. Recently, applications of this ‘old’ structure as a functional organic building block for organic electronics applications have renewed interest in these molecules and their remarkable chemical and physical properties. Natural‐origin indigos have been processed in fully bio‐compatible field effect transistors, operating with ambipolar mobilities up to 0.5 cm²/Vs and air‐stability. The synthetic derivative isoindigo has emerged as one of the most successful building‐blocks for semiconducting polymers for plastic solar cells with efficiencies > 5%. Another isomer of indigo, epindolidione, has also been shown to be one of the best reported organic transistor materials in terms of mobility (～2 cm²/Vs) and stability. This progress report aims to review very recent applications of indigoids in organic electronics, but especially to logically bridge together the hereto independent research directions on indigo, isoindigo, and other materials inspired by historical dye chemistry: a field which was the root of the development of modern chemistry in the first place.