Gas Sensors Based on Organic Field-effect Transistors: Mechanisms, Advances, and Challenges

Modeling of bias-induced changes of organic field-effect transistor characteristics

An important application of organic semiconductors is to fabricate organic field-effect transistors (OFETs) which are essential building blocks for the next generation of organic circuits. In terms of molecular size or molecular weight, organic semiconductors can be divided into small-molecule and polymer semiconductors, and thus their corresponding OFETs can also be categorized into organic small molecule OFETs and polymer field-effect transistors (PFETs). On the basis of the main charge carriers transporting in OFET channels, organic semiconductors can be further divided into p-type, n-type, and ambipolar semiconducting materials. According to the characteristic of the organic semiconductors, the OFETs can be classified into two types: organic thin film transistors (OTFTs) and organic single crystal transistors. In any kind of OFET devices, organic semiconductor materials are the core; their properties determine the performance of the electronic devices. Therefore, the design and synthesis of high performance organic semiconductor materials are the basis and premise of the wide application of OFET devices. In the past few decades, great progress has been made in developing organic semiconductors. Besides organic semiconducting materials, there are many other factors influencing the performance of OFETs including device configuration, processing technique, and other devices physical factors, etc. In the following, a brief review of the development of p-type, n-type, ambipolar organic semiconductors and their field-effect properties is given. The history, mechanism, configuration, and fabrication methods of OFET devices and main performance influencing factors of OFETs are also introduced.

Organic field-effect transistors have found wide applications in flexible displays, smart cards, sensors and so on because they can be fabricated on flexible substrates with large scale and low cost. Though great progress has been made on p-channel organic field effect transistors, the development of n-channel organic field-effect transistors is largely lagged behind their p-channel counterparts. The lack of high performance n-channel organic semiconductors has become one of the bottlenecks for organic field-effect transistors. Low LUMO energy level is one of the characteristics of n- type organic semiconductors. The low LUMO level of organic semiconductors can not only reduce the electron injection barrier, in favor of electrons transferring from the electrode into the semiconductor layer, but also prevent the electron charge carriers being trapped by air and water and obtain ambient stability. Thiophene quinoidal organic semiconductors have attracted great attentions recently because they have low-lying LUMO energy levels and strong intermolecular interactions, and usually display high electron mobility. This review summarizes the application of the thiophene quinoidal organic semiconductors in organic transistors in recent years. According to the characteristic of the molecular structures, in the review, the thiophene quinoidal organic semiconductors are divided into two classes: the oligo- thiophene quinoidal organic semiconductors and fused-ring thiophene quinoidal organic semiconductors. Compared to the extensive studied aromatic diimide n-type organic semiconductors, the research on thiophene quinoidal organic semiconductors is not sufficient and the molecular species of thiophene quinoidal compounds is not rich. However, the excellent electron transport properties of thiophene quinoidal organic semiconductors support their potential applications as high performance solution processable n-channel organic semiconductors. In addition to the application in organic transistors, thiophene quinoidal compounds usually have reversible redox property, high electron affinity and other characteristics, and will have potential applications in organic solar cells, charge transfer salts, organic memories and sensors. We believe that the research of thiophene quinoidal organic semiconductors will make more progress.

Organic charge-modulated field-effect transistors (OCMFETs) have garnered significant interest as sensing platforms for diverse applications that include biomaterials and chemical sensors owing to their distinct operational principles. This study aims to improve the understanding of driving mechanisms in OCMFETs and optimize their device performance by investigating the correlation between organic field-effect transistors (OFETs) and OCMFETs. By introducing self-assembled monolayers (SAMs) with different functional groups on the AlOx gate dielectric surface, we explored the impact of the surface characteristics on the electrical behavior of both devices. Our results indicate that the dipole moment of the dielectric surface is a critical control variable in the performance correlation between OFET and OCMFET devices, as it directly impacts the generation of the induced floating gate voltage through the control gate voltage. The insights obtained from this study contribute to the understanding of the factors affecting OCMFET performance and emphasize their potential as platforms for diverse sensing systems.

Potentiometry on organic semiconductor devices

In this perspective article, we discuss the dynamic instability of charge carrier transport in a range of popular organic semiconductors. We observe that in many cases field-effect mobility, an important parameter used to characterize the performance of organic field-effect transistors (OFETs), strongly depends on the rate of the gate voltage sweep during the measurement. Some molecular systems are so dynamic that their nominal mobility can vary by more than one order of magnitude, depending on how fast the measurements are performed, making an assignment of a single mobility value to devices meaningless. It appears that dispersive transport in OFETs based on disordered semiconductors, those with a high density of localized trap states distributed over a wide energy range, is responsible for the gate voltage sweep rate dependence of nominal mobility. We compare such rate dependence in different materials and across different device architectures, including pristine and trap-dominated single-crystal OFETs, as well as solution-processed polycrystalline thin-film OFETs. The paramount significance given to a single mobility value in the organic electronics community and the practical importance of OFETs for applications thus suggest that such an issue, previously either overlooked or ignored, is in fact a very important point to consider when engaging in fundamental studies of charge carrier mobility in organic semiconductors or designing applied circuits with organic semiconductors.

The diketopyrrolopyrrole-based polymer (DPPT-TT) has been employed in organic power field effect transistors due to its exceptional off-state breakdown performance. The impact of organic semiconductor layer thickness on the breakdown performance has not been explored. In this study, we investigate the impact of DPPT-TT layer thickness on the breakdown voltage (BV) by fabricating organic field effect transistors (OFETs) with various DPPT-TT layer thicknesses. Our findings reveal that the devices' BV is a strong function of DPPT-TT layer thickness, and reducing the DPPT-TT layer thickness from 68 to 15 nm results in a decrease in BV from 291 to 86 V, attributed to the two-dimensional (2D) electric field crowding effect. An analytical model utilizing the 2D Poisson equation reveals an electric field at the DPPT-TT layer's surface. Thinner DPPT-TT layer exhibits larger electric field peak, leading to premature breakdown near the drain electrode. The relationship between breakdown electric field and DPPT-TT layer thickness was established by fitting the experimental data to the model, revealing an average BV error of only 8.8%. This phenomenon is validated to be ubiquitous in polymer based OFETs via DPPT-TT-based and P3HT-based devices. According to the proposed model, this 2D electric field crowding effect can be mitigated by adjusting the dielectric layer thickness (tD) and/or the dielectric material.

The majority of organic semiconductors developed are more easily oxidized (p-type) than reduced (n-type). However, n-type organic semiconductors are important for a range of optoelectronic applications including organic photovoltaic devices, light-emitting diodes and field effect transistors (FETs). In spite of this there has been significantly less development of n-type compounds. This project focused on developing novel n-type semiconductors for use in organic photovoltaic devices. To identify the organic acceptor candidates, a series of novel materials were synthesized and characterized. Amongst these materials, 2-[(7-{9,9-di-n-propyl-9H-fluoren-2-yl}benzo[c][1,2,5]thiadiazol-4-yl)methylene]malononitrile (K12) was identified as a lead candidate. K12 has the remarkable property in that it can be deposited from solution or by evaporation. K12 exhibits a tendency to order in the solid state at room temperature, and with mild heating the process can be accelerated. The extent of ordering in the films was followed by Polarized Optical Microscopy, Atomic Force Microscopy, and Wide Angle X-ray Diffraction. K12 was found to be a good electron acceptor candidate for bulk heterojunction organic solar cells and as an active channel in organic field effect transistors (OFETs). A 0.7% power conversion efficiency under AM1.5 light was achieved in a bulk heterojunction solar cell configuration with poly(3-n-hexylthiophene) as the donor. The thermal properties of K12 enable the film morphology to be controlled at easily accessible temperatures allowing the charge mobility to be tuned over two orders of magnitude. The electron mobility in the films was found to be independent of the initial processing conditions (solution or evaporation). The electron mobility measured in a FET configuration was of order 10-3 cm2/Vs for films prepared via either processing method whilst Photoinduced Charge Extraction in Linearly Increasing Voltage (PhotoCELIV) for annealed films gave a mobility of order 10-4 cm2/Vs. To further tune the properties of K12 several chemical modifications were made. To harvest more of the solar spectrum derivatives with electron donating groups such as thiophene (K12-Th) and diphenylamine (K12-Diph) were prepared to extend the conjugation length and form a Donor-Acceptor structure. Amongst the derivatives K12-Diph gave a greatly broadened absorption ranging between 200 nm and 800 nm. The dipole moment of K12 was found to play an important role in its morphology and hence K12b, which has a smaller dipole moment was prepared to investigate this effect further. It was found that the different dipole moments of K12 and K12b had dramatic effects on both the physical properties, reflected in their thermal properties and molecular packing. The morphological differences in films comprised of the each of the materials were observed to have a direct impact on the device performance in both OFET and OPV devices. The propensity of K12 to crystallize was found to give rise to better charge transport than K12b. Macromolecular structures K12-3 and K12-6 (with three and six K12 chromophores around a central benzene ring, respectively) were prepared. Although they had similar fundamental optoelectronic properties to K12 their physical properties were altered and although they could be solution processed did not form films that gave enhanced charge transport due to aggregation effects.

Organic field-effect transistors (OFETs) have been developed over the past few decades due to their potential applications in future electronics such as wearable and foldable electronics. As the electrical performance of OFETs has improved, patterning organic semiconducting crystals has become a key issue for their commercialization. However, conventional soft lithographic techniques have required the use of expensive processes to fabricate high-resolution master molds. In this study, we demonstrated a cost-effective method to prepare nanopatterned master molds for the fabrication of high-performance nanowire OFETs. We repurposed commercially available compact discs (CDs) as master molds because they already have linear nanopatterns on their surface. Flexible nanopatterned templates were replicated from the CDs using UV-imprint lithography. Subsequently, 6,13-bis-(triisopropylsilylethynyl) pentacene nanowires (NWs) were grown from the templates using a capillary force-assisted lithographic technique. The NW-based OFETs showed a high average field-effect mobility of 2.04 cm2 V−1 s−1. This result was attributed to the high crystallinity of the NWs and to their crystal orientation favorable for charge transport.

The organic field effect transistor (OFET) is a device where a thin film of an organic semiconductor (OS) bridges a channel between source and gate electrodes. The gate electrode, separated by a dielectric thin film from the organic semiconductor, controls the charge carrier density in the organic semiconductor by capacitive coupling. OFET responds with a current between source and drain to a voltage bias applied to the gate and drain electrodes, the source being grounded. Response of OFET is measured by a set of parameters that are extracted from the current voltage characteristics as a function of the gate voltage (transfer curves) or the drain voltage (output curves). The OFET has been studied for more than two decades because of its potential applications in flexible circuits, RFID tags, wearable electronics, and back-panel active matrix displays. Although organic electronics is the main technology driver, OFET plays a central role in the fundamental studies aimed to elucidate charge transport in organic semiconductors. OFET is widely used as an experimental gauge for probing charge mobility, carrier density, and doping levels in organic thin films and nanostructures. Despite the apparent simplicity of the architecture, the device physics is more subtle and elusive. OFET response cannot be simply elicited from molecular design and crystal packing, as it is dominated by the interactions of the OS with the device interfaces. At the OS/metal electrodes, charge injection/extraction occurs; at the OS/gate dielectric interface, charge carriers are capacitively accumulated, depleted, trapped, and transported; charge carriers cross organic semiconductor domain boundaries and are scattered/trapped by morphological/structural defects; the outer OS surface is exposed to the environment. The OFET response is extremely sensitive to any change occurring at these interfaces, happening either spontaneously (like in the case of charge trapping and bias stress), accidentally (a parasitic dopant), or by design (specific interaction of an analyte with species adsorbed or grafted at the interfaces). The effect of these interfaces is intertwined in the OFET response, and it is difficult to experimentally disentangle it. On one hand, the OFET inherent instability is detrimental to electronics applications; on the other hand, it makes the device interesting for exploring new paradigms of sensing and transduction. In the past five years, an increasing trend of publications with OFETs used as (bio-)sensors is observed [1]. Advantages of OFETs with respect to more robust and established devices as CMOS are: the ease of interface tailoring toward analytes, living cells, and tissues; the use of low-cost scalable fabrication (important for single shot sensing); technology transferrable on flexible substrates with tunable mechanical compliance; a library of biocompatible and biodegradable materials, the latter yet in nuce (both crucial features for implantable devices); fabrication of devices with minimal amounts of materials (semiconductors, conductors, dielectrics, recognition groups); upscaling and integration are simpler than for resistive or amperometric sensors. This chapter looks into the OFET as a low-dimensional device and how this distinctive feature can be exploited for

The use of polymer ferroelectric dielectrics in organic field-effect transistors (FETs) for nonvolatile memory application was demonstrated more than 15 years ago. The ferroelectric dielectric polyvinylidene fluoride (PVDF) and its copolymers are most widely used for such applications. In addition to memory applications, polymer ferroelectrics as a dielectric layer in organic FETs yield insights into interfacial transport properties. Advantages of polymer ferroelectric dielectrics are their high dielectric constant compared to other polymer dielectrics and their tunable dielectric constant with temperature. Further, the polarization strength may also be tuned by externally poling the ferroelectric dielectric layer. Thus, PVDF and its copolymers provide a unique testbed not just for investigating polarization induced transport in organic FETs, but also enhancing device performance. This article discusses recent developments of PVDF-based ferroelectric organic FETs and capacitors with a focus on tuning transport properties. It is shown that FET carrier mobilities exhibit a weak temperature dependence as long as the dielectric is in the ferroelectric phase, which is attributed to a polarization fluctuation driven process. The low carrier mobilities in PVDF-based FETs can be enhanced by tuning the poling condition of the dielectric. In particular, by using solution-processed small molecule semiconductors and other donor-acceptor copolymers, it is shown that selective poling of the PVDF-based dielectric layer dramatically improves FET properties. Finally, the prospects of further improvement in organic ferroelectric FETs and their challenges are provided.

Recent intensive research and development of organic field-effect transistors (FETs) [1–6] have been motivated by a new class of applications that cannot be easily realized by conventional electronics based on inorganic semiconductors. Organic transistors are mechanically flexible, thin, lightweight, and shock-resistant, because organic devices are manufactured on plastic films at low (ambient) temperature. Furthermore, manufacturing costs of organic transistor circuits would be inexpensive, even for large areas, when they are fabricated using printing technologies and/or roll-to-roll processes. There are two major applications for organic transistors. The first one is a flexible display. This new display includes a paper-like display or an electronic paper, where electric inks, electroluminescent (EL) devices, and liquid crystals or other mediums are powered by organic transistor active matrices [4, 7]. The second one is a radio frequency identification (RFID) tag [8, 9]. An organic transistor-based RFID tag may be printed on packages of products, resulting in an inexpensive and robust electronics. As another application of organic transistors, we demonstrated large-area flexible sensors. The first organic transistor-based large-area sensors are flexible pressure sensor matrices; organic transistor active matrices are used to read out pressure distributions over a large area from a 2-D array of pressure sensor cells. The new pressure sensor can be ideal for electronic artificial skin (e-skin) applications for future generations of robots. The mobility of pentacene that is known as a high-mobility low-molecular weight semiconductor is typically 1 cm2/Vs. This value is about two or three orders of magnitude lower than that of polyor single-crystalline silicon, respectively. Although flexible displays and/or RFID tags require usually high-electronic performance, the

Linear acenes are a widely studied class of materials in the field of Organic Electronics. Their aromatic system and the strong interaction of the π-electrons of neighbouring molecules in the solid state allow an efficient charge transport in these materials. The defined molecular structure of these small molecules and the possibility to tune their optical and electronic properties as well as the solid state packing through careful chemical design and synthesis have resulted in numerous applications of acenes in organic transistors and optoelectronic devices.
\nThis work focuses on the application of N-Heteroacenes and non-conjugated pentacene-based polymers as semiconductors in solution-processed organic field-effect transistors. These devices are used to evaluate the charge transport properties of the materials and derive structure-function relationships for the various compounds. To draw structure-function relationships from the studies described in this thesis, a myriad of characterisation techniques was employed to obtain an insight into the optical, electronic, electrical and morphological properties of each material. The effects of order, energetics and processing of the semiconductor on the transistor performance are all investigated.
\nWhile non-conjugated pentacene-based polymers offer an ease of processability, their amorphous nature inhibits efficient hole transport, resulting in a relatively poor transistor performance. The fashion in which the pentacene systems are connected to the polymer backbone changes their flexibility and therefore affecting the injection behaviour and charge transport properties.
\nThe nitrogen substitution in N-Heteroacenes results in an energetic stabilisation of the frontier molecular orbitals, allowing for an enhanced electron injection into these materials. For the symmetrical tetraazapentacene and two halogenated phenazine derivatives relatively high electron mobilities were achieved demonstrating their potential for future application as n-type semiconductors in organic field-effect transistors.
\nThe use of N-heteroacenes is not limited to electron transport only. Their N,N’-dihydro forms are electron rich compounds that exhibit good hole transport. This is demonstrated for differently substituted tetraazapentacenes as well as for a N,N’-dihydro diazahexacene and -heptacene. For these materials it is shown that the processing conditions not only affect the macroscopic transistor performance, but also influence the formation of polymorphs in thin films.
\nThe solid state packing of functionalised acenes is typically determined by their solubilising side chains. A norbornadienyl substitution at the side chain of the well-known 6,13-bis(triisopropylsiliylethynyl)pentacene and its tetraaza derivative was shown to result in an enhancement of the charge transport properties for the p-type derivatives and deterioration of the performance of the n-type transistors. These observations are related to changes in the charge transfer integrals, the film microstructure and the solid state packing.
\nIn conclusion, this work contributes to the development of guidelines for the design and synthesis of next generation N-heteroacenes to be applied in the future in state of the art organic electronic devices.

Xanthan gum biopolymer gated low voltage-operating photo synaptic organic field-effect transistor for neuromorphic electronics

Top contact organic thin-film field effect transistors based on regioregular poly(3-butylthiophene-2,5-diyl) (P3BT), poly(3-dodecylthiophene-2,5-diyl) (P3DDT) and poly(3-hexylthiophene-2,5-diyl) (P3HT) of similar concentration were fabricated by spin coating technique. The current-voltage (I-V) characteristics of these three different polymer based organic field-effect transistors (OFETs) were studied. The device performance parameters for each kind of OFET were estimated with the help of measured electrical characteristics and performance were compared to optimize the best polymer for FETs amongst these three polymers.