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     

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     

Stability and 2,4-dinitrotoluene response of organic field effect transistors based on π-conjugated thiophene oligomers

This paper reports on organic field effect transistors (OFETs) based on two π-conjugated oligomers derived from thiophenes and their use as sensors for the detection of 2,4-dinitrotoluene (DNT). The detection mechanism relies on donor-acceptor interactions between the π-conjugated system (donor) and the nitrated molecule (acceptor). An important feature of sensors is the stability under operation, so, a large part of this work will be dealing with the behavior of OFETs under bias stress experiments as well as with the influence of temperature during operation. Most of results reported here are concerning hexyl capped tetra Thienylene–Vinylene (denominated 4-TV). Some preliminary results on the promising hexyl capped quinquethiophene derived from 3,4-ethylenedioxythiophene (denominated TETET) are also reported. Under a DNT contaminated air atmosphere ( 7 ppm), 4-TV based OFETs exhibit an increase of the drain current when DNT is present in the atmosphere as expected.     

Research Progress of Organic Field-Effect Transistor Based Chemical Sensors

Due to their high sensitivity and selectivity, chemical sensors have gained significant attention in various fields, including drug security, environmental testing, food safety, and biological medicine. Among them, organic field-effect transistor (OFET) based chemical sensors have emerged as a promising alternative to traditional sensors, exhibiting several advantages such as multi-parameter detection, room temperature operation, miniaturization, flexibility, and portability. This review paper presents recent research progress on OFET-based chemical sensors, highlighting the enhancement of sensor performance, including sensitivity, selectivity, stability, etc. The main improvement programs are improving the internal and external structures of the device, as well as the organic semiconductor layer and dielectric structure. Finally, an outlook on the prospects and challenges of OFET-based chemical sensors is presented.     

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.     

Organic thin-film transistors for chemical and biological sensing

Organic thin-film transistors (OTFTs) show promising applications in various chemical and biological sensors. The advantages of OTFT-based sensors include high sensitivity, low cost, easy fabrication, flexibility and biocompatibility. In this paper, we review the chemical sensors and biosensors based on two types of OTFTs, including organic field-effect transistors (OFETs) and organic electrochemical transistors (OECTs), mainly focusing on the papers published in the past 10 years. Various types of OTFT-based sensors, including pH, ion, glucose, DNA, enzyme, antibody-antigen, cell-based sensors, dopamine sensor, etc., are classified and described in the paper in sequence. The sensing mechanisms and the detection limits of the devices are described in details. It is expected that OTFTs may have more important applications in chemical and biological sensing with the development of organic electronics.     

Probing of contact formation via light emission from organic field-effect transistors

The recent progress on ambipolar organic light-emitting field-effect transistors has opened new possibilities for characterizing the basic physical processes in organic field-effect transistors (OFETs). Here, a way of investigating the contact formation from the light emission of OFETs is presented. In general, in the ambipolar transport regime a spatially controllable recombination zone can be observed in the channel of the transistor by light emission. In the unipolar regimes no light emission is expected due to the fact that only one charge carrier type is accumulated in the channel region and, thus, charge carriers cannot recombine. However, our results indicate that light emission in the unipolar transport regimes is possible due to thermal injection of charge carries at the contacts. It will be demonstrated that emission in the unipolar regime is present in devices providing ohmic contacts and that it can be totally suppressed utilizing non-ohmic contacts which can be achieved by the use of suited metals. Further, employing a double layer of different color emitting acenes in the transistor channel, the vertical movement of the recombination zone can be probed by a color change in the light emission when passing from the unipolar to the ambipolar regime. Both the dependence of the light emission on the utilized contact metal and the color change in the double layer device clearly demonstrate that different conditions apply for the light emission in the unipolar and ambipolar regimes.     

High-performance triisopropylsilylethynyl pentacene transistors via spin coating with a crystallization-assisting layer

The effects of spin speed and an amorphous fluoropolymer (CYTOP)-patterned substrate on the crystalline structures and device performance of triisopropylsilylethynyl pentacene (TIPS-PEN) organic field-effect transistors (OFETs) were investigated. The crystallinity of the TIPS-PEN film was enhanced by decreasing the spin speed, because slow evaporation of the solvent provided a sufficient time for the formation of thermodynamically stable crystalline structures. In addition, the adoption of a CYTOP-patterned substrate induced the three-dimensional (3D) growth of the TIPS-PEN crystals, because the patterned substrate confined the TIPS-PEN molecules and allowed sufficient time for the self-organization of TIPS-PEN. TIPS-PEN OFETs fabricated at a spin speed of 300 rpm with a CYTOP-patterned substrate showed a field-effect mobility of 0.131 cm(2) V(-1) s(-1), which is a remarkable improvement over previous spin-coated TIPS-PEN OFETs.     

Flexible high capacitance nanocomposite gate insulator for printed organic field-effect transistors

Ceramic-polymer nanocomposite dielectric consisting of an epoxy solution with propylene glycol methyl ether acetate as the solvent and barium titanate nanoparticles with capacitance in excess of 60 pF/mm² was developed and utilized as the gate insulator for organic field-effect transistors (OFETs). The high relative permittivity (κ = 35), bimodal nanocomposite utilized had two different filler particle sizes 200 nm and 1000 nm diameter particles. Bottom gate organic filed-effect transistors were demonstrated using a commercially available printing technology for material deposition. A metal coated plastic film was used as the flexible gate substrate. Solution processable, p-type arylamine-based amorphous organic semiconductor was utilized as the active layer. Fabricated OFETs with the solution processed nanocomposite dielectric had a high field-induced current and a low threshold voltage; these results suggest that the low operating voltage was due to the high capacitance gate insulator. In this paper, we review the characteristics of the nanocomposite dielectric material and discuss the processing and performance of the printed organic devices.     

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.     

Patterning technology for solution-processed organic crystal field-effect transistors

Organic field-effect transistors (OFETs) are fundamental building blocks for various state-of-the-art electronic devices. Solution-processed organic crystals are appreciable materials for these applications because they facilitate large-scale, low-cost fabrication of devices with high performance. Patterning organic crystal transistors into well-defined geometric features is necessary to develop these crystals into practical semiconductors. This review provides an update on recentdevelopment in patterning technology for solution-processed organic crystals and their applications in field-effect transistors. Typical demonstrations are discussed and examined. In particular, our latest research progress on the spin-coating technique from mixture solutions is presented as a promising method to efficiently produce large organic semiconducting crystals on various substrates for high-performance OFETs. This solution-based process also has other excellent advantages, such as phase separation for self-assembled interfaces via one-step spin-coating, self-flattening of rough interfaces, and in situ purification that eliminates the impurity influences. Furthermore, recommendations for future perspectives are presented, and key issues for further development are discussed.     

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).     

Recent progress in photoactive organic field-effect transistors

Recent progress in photoactive organic field-effect transistors (OFETs) is reviewed. Photoactive OFETs are divided into light-emitting (LE) and light-receiving (LR) OFETs. In the first part, LE-OFETs are reviewed from the viewpoint of the evolution of device structures. Device performances have improved in the last decade with the evolution of device structures from single-layer unipolar to multi-layer ambipolar transistors. In the second part, various kinds of LR-OFETs are featured. These are categorized according to their functionalities: phototransistors, non-volatile optical memories, and photochromism-based transistors. For both, various device configurations are introduced: thin-film based transistors for practical applications, single-crystalline transistors to investigate fundamental physics, nanowires, multi-layers, and vertical transistors based on new concepts.     

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.     

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.     

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.     

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.     

Fabrication of organic field effect transistor by directly grown poly(3 hexylthiophene) crystalline nanowires on carbon nanotube aligned array electrode

We fabricated organic field effect transistors (OFETs) by directly growing poly (3-hexylthiophne) (P3HT) crystalline nanowires on solution processed aligned array single walled carbon nanotubes (SWNT) interdigitated electrodes by exploiting strong π-π interaction for both efficient charge injection and transport. We also compared the device properties of OFETs using SWNT electrodes with control OFETs of P3HT nanowires deposited on gold electrodes. Electron transport measurements on 28 devices showed that, compared to the OFETs with gold electrodes, the OFETs with SWNT electrodes have better mobility and better current on-off ratio with a maximum of 0.13 cm(2)/(V s) and 3.1 × 10(5), respectively. The improved device characteristics with SWNT electrodes were also demonstrated by the improved charge injection and the absence of short channel effect, which was dominant in gold electrode OFETs. The enhancement of the device performance can be attributed to the improved interfacial contact between SWNT electrodes and the crystalline P3HT nanowires as well as the improved morphology of P3HT due to one-dimensional crystalline nanowire structure.     

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.     

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.