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     

25th Anniversary Article: Key Points for High‐Mobility Organic Field‐Effect Transistors

Remarkable progress has been made in developing high performance organic field‐effect transistors (OFETs) and the mobility of OFETs has been approaching the values of polycrystalline silicon, meeting the requirements of various electronic applications from electronic papers to integrated circuits. In this review, the key points for development of high mobility OFETs are highlighted from aspects of molecular engineering, process engineering and interface engineering. The importance of other factors, such as impurities and testing conditions is also addressed. Finally, the current challenges in this field for practical applications of OFETs are further discussed.     

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.     

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.     

Current Trends in Shrinking the Channel Length of Organic Transistors Down to the Nanoscale

In this Review article, we highlighted current trends in shrinking the channel length of organic field effect transistors (OFETs) down to the nanoscale in three systems where sophisticated device fabrication has been integrated into the development of different electrodes with nanoscale gaps. The design principle is the combination of molecular design freedom and flexible molecular self‐assembly with state‐of‐the‐art device fabrication to realize organic field effect nano‐transistors where molecular materials themselves behave as pivotal elements. Three different types of nanoscale electrodes are used for OFETs: metals, single‐walled carbon nanotubes (SWCNTs), and graphenes. These electrodes are made by e‐beam lithography as well as other complementary methods (shadow deposition, underetching, nanoimprinting, rubber stamping, and microcontact printing).     

Controlled Deposition of Crystalline Organic Semiconductors for Field‐Effect‐Transistor Applications

The search for low‐cost, large‐area, flexible devices has led to a remarkable increase in the research and development of organic semiconductors, which serve as one of the most important components for organic field‐effect transistors (OFETs). In the current review, we highlight deposition techniques that offer precise control over the location or in‐plane orientation of organic semiconductors. We focus on various vapor‐ and solution‐processing techniques for patterning organic single crystals in desired locations. Furthermore, the alignment of organic semiconductors via different methods relying on mechanical forces, alignment layers, epitaxial growth, and external magnetic and electric fields are surveyed. The advantages, limitations, and applications of these techniques in OFETs are also discussed.     

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.     

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: Organic Field‐Effect Transistors: The Path Beyond Amorphous Silicon

Over the past 25 years, organic field‐effect transistors (OFETs) have witnessed impressive improvements in materials performance by 3–4 orders of magnitude, and many of the key materials discoveries have been published in Advanced Materials. This includes some of the most recent demonstrations of organic field‐effect transistors with performance that clearly exceeds that of benchmark amorphous silicon‐based devices. In this article, state‐of‐the‐art in OFETs are reviewed in light of requirements for demanding future applications, in particular active‐matrix addressing for flexible organic light‐emitting diode (OLED) displays. An overview is provided over both small molecule and conjugated polymer materials for which field‐effect mobilities exceeding > 1 cm² V^–1 s^–1 have been reported. Current understanding is also reviewed of their charge transport physics that allows reaching such unexpectedly high mobilities in these weakly van der Waals bonded and structurally comparatively disordered materials with a view towards understanding the potential for further improvement in performance in the future.     

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.     

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.     

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.     

Gas Sensors Based on Nano/Microstructured Organic Field‐Effect Transistors

Benefiting from the advantages of organic field‐effect transistors (OFETs), including synthetic versatility of organic molecular design and environmental sensitivity, gas sensors based on OFETs have drawn much attention in recent years. Potential applications focus on the detection of specific gas species such as explosive, toxic gases, or volatile organic compounds (VOCs) that play vital roles in environmental monitoring, industrial manufacturing, smart health care, food security, and national defense. To achieve high sensitivity, selectivity, and ambient stability with rapid response and recovery speed, the regulation and adjustment of the nano/microstructure of the organic semiconductor (OSC) layer has proven to be an effective strategy. Here, the progress of OFET gas sensors with nano/microstructure is selectively presented. Devices based on OSC films one dimensional (1D) single crystal nanowires, nanorods, and nanofibers are introduced. Then, devices based on two dimensional (2D) and ultrathin OSC films, fabricated by methods such as thermal evaporation, dip‐coating, spin‐coating, and solution‐shearing methods are presented, followed by an introduction of porous OFET sensors. Additionally, the applications of nanostructured receptors in OFET sensors are given. Finally, an outlook in view of the current research state is presented and eight further challenges for gas sensors based on OFETs are suggested.     

Mobility Deception in Nanoscale Transistors: An Untold Contact Story

Mobility is a critical parameter that is routinely used for benchmarking the performance of field‐effect transistors (FETs) based on novel nanomaterials. In fact, mobility values are often used to champion nanomaterials since high‐performance devices necessitate high mobility values. The current belief is that the contacts can only limit the FET performance and hence the extracted mobility is an underestimation of the true channel mobility. However, here, such misconception is challenged through rigorous experimental effort, backed by numerical simulations, to demonstrate that overestimation of mobility occurs in commonly used geometries and in nanomaterials for which the contact interface, contact doping, and contact geometry play a pivotal role. In particular, dual‐gated FETs based on multilayer MoS₂ and WSe₂ are used as case studies in order to elucidate and differentiate between intrinsic and extrinsic contact effects manifesting in the mobility extraction. The choice of 2D layered transition metal dichalcogenides (TMDCs) as the semiconducting channel is motivated by their potential to replace and/or coexist with Si‐based aging FET technologies. However, the results are equally applicable to nanotube‐ and nanowire‐based FETs, oxide semiconductors, and organic‐material‐based thin‐film 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.     

含氟有机场效应晶体管材料的研究进展

在空气中稳定的n型有机半导体材料是当前有机场效应晶体管(OFET)研究的重要方向.分子中引入氟取代基和三氟甲基/全氟烷基可能会将有机半导体材料从p型转变成n型,同时还可提高有机半导体材料的稳定性.从分子结构的角度出发介绍了含氟类有机半导体材料的最新进展及其在场效应晶体管中的应用,并进一步提出了该领域的研究前景.     

Band-like electron transport in organic transistors and implication of the molecular structure for performance optimization

The Hall effect and an increase of field-effect mobility with decreasing temperature is observerd in n-channel single-crystal organic field-effect transistors (OFETs). A quantitative analysis of these findings, together with results on different p-channel transistors, indicate the importance of the semiconductor molecular polarizability and the structure of the charge transport layers in the crystal for the observation of band-like transport in OFETs.     

Decoupling the Effects of Self‐Assembled Monolayers on Gold,Silver, and Copper Organic Transistor Contacts

In bottom‐contact organic field‐effect transistors (OFETs), the functionalization of source/drain electrodes leads to a tailored surface chemistry for film growth and controlled interface energetics for charge injection. This report describes a comprehensive investigation into separating and correlating the energetic and morphological effects of a self‐assembled monolayers (SAMs) treatment on Au, Ag, and Cu electrodes. Fluorinated 5,11‐bis(triethylsilylethynyl) anthradithiophene (diF‐TES‐ADT) and pentafluorobenzenethiol (PFBT) are employed as a soluble small‐molecule semiconductor and a SAM material, respectively. Upon SAM modification, the Cu electrode devices benefit from a particularly dramatic performance improvement, closely approaching the performance of OFETs with PFBT‐Au and PFBT‐Ag. Ultraviolet photoemission spectroscopy, polarized optical microscopy, grazing‐incidence wide‐angle X‐ray scattering elucidate the metal work function change and templated crystal growth with high crystallinity resulting from SAMs. The transmission‐line method separates the channel and contact properties from the measured OFET current–voltage data, which conclusively describes the impact of the SAMs on charge injection and transport behavior.     

Characteristics of organic inverters using vertical- and lateral-type organic transistors

The characteristics of vertical-type organic static induction transistors (OSITs) were compared with those of lateral-type organic field effect transistors (OFETs). From these experiments, it was confirmed that the OSITs can operate at a voltage one order less than that required for OFETs. We also fabricated two types of organic inverter based on OSITs and OFETs and investigated their transfer characteristics. These results demonstrate that it is possible to decrease the operational voltage of organic inverters from ± 20 V to ± 2 V by using two OSITs with higher on/off ratios.